TECHNOLOGY - IIairwalkbooks.com/images/pdf/pdf_102_1.pdf(a) Single point cutting tools. (b) Multi point cutting tools. A single point cutting tool has a wedge like action and are used

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As Per R-2017 Syllabus of Anna University

FOR B.E IV SEMESTER MECHANICAL ENGINEERING STUDENTSFOR B.E IV SEMESTER MECHANICAL ENGINEERING STUDENTS

R - 2017

Dr. S. Ramachandran, M.E., Ph.D.,

Professor - Mech

Sathyabama Institute of Science and Technology

Chennai - 119

MANUFACTURING TECHNOLOGY - II

Dr. T. Varunkumar, M.E., Ph.D.,

Professor and Head

Department of Mechanical Engineering

P. A. College of Engineering & Technology

Pollachi

Dr. R. Senthil Kumar, M.E., Ph.D.,

Professor and Head

Department of Mechanical Engineering

D haanish Ahmed College of Engineering

Padappai

thSeventh Edition: 12 December 2018

ISBN:978-93-88084-20-8

350/-

978-93-88084-20-8

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Unit – I

THEORY OF METAL CUTTING

Mechanics of chip formation, single point cutting tool, forces inmachining, Types of chip, cutting tools - nomenclature, orthogonal metalcutting, thermal aspects, cutting tool materials, tool wear, tool life, surfacefinish, cutting fluids and Machinability

1.1 INTRODUCTION - METAL REMOVAL PROCESSESMetal removal process is a manufacturing process by which a

workpiece is given (i) a desired shape (ii) a desired size and (iii) a desiredsurface finish.

To achieve one or all of these, the excess material from the workpieceis removed in the form of chips with the help of some properly shaped andsized tools. The metal removal processes are chip forming processes.

1.1.1 Classification of Metal Removal Processes

Metal removal processes are broadly classified into two categories.

(i) Chip forming (Metal Cutting / Removal) Processes:Examples are Turning, Boring, Shaping, Planing, Slotting,Drilling, Reaming, Milling, Broaching, Thread Cutting, Grinding,Honing, Gear cutting etc.,

(ii) Chipless Forming Processes: Examples are Rolling, Spinning,Forging, Extrusion, Stamping etc.,

Chip forming processes are manufacturing processes in which thedesired shape, size and surface finish of workpiece is obtained by separatinglayer from parent workpiece in the form of chips, whereas in chiplessforming processes no chips are formed.

Explain the types and application of different types of cutting tool.[AU. Nov/Dec 2011]

1.2 CUTTING TOOLSIn metal cutting process chip removal is performed either by cutting

tools having distinct cutting edges or by abrasives used in grinding wheels,abrasive sticks, abrasive cloth etc.

Metal cutting tools are broadly classified as:

(a) Single point cutting tools.

(b) Multi point cutting tools.

A single point cutting tool has a wedge like action and are used inlathe, shaping and slotting machines.

Two or more single point cutting tools when arranged together as aunit in a specific manner forms a multipoint cutting tool and are used inmilling machine, broaching machine, etc.

Single point cutting tools are used for operations like turning, facing,chamfering, thread cutting and general purpose operations.

Multi point cutting tools are used for special operations likeknurling, milling, drilling, reaming, etc.

1.2.1 Classification of cutting tools (single point cutting tool)

Single point cutting tool can be classified according to

(i) Method of manufacture

(a) Forged tool

(b) Brazed tool tip with carbon steel shank

(c) Fastened tool tip on to the carbon steel shank

(ii) Holding method

(a) Solid tool

(b) Tool bit inserted in tool holder

(iii) Method of operation

(a) Turning tool (b) Forming tool

(c) Chamfering tool (d) Boring tool

(e) Thread cutting tool (f) Internal thread cutting tool

(g) Facing tool (h) Parting-off tool

(i) Grooving Tool

1.2 Manufacturing Technology-II - www.airwalkbooks.com

(iv) Method of applying feed

(a) Left hand tool

(b) Right hand tool

(c) Round nose tool

A single point cutting tool is a tool with one face and one continuouscutting edge that removes metal from a workpiece being machined in lathe.

1.2.2 Factors affecting cutting Tool efficiency

The accuracy with which several angles have been ground oncutting tool.

Shape of cutting edge of tool.

Type of cutting tool material.

Heat-treatment of cutting tool.

Condition of machine.

Type and efficiency of coolants.

Correct speed and feed.

1.2.3 Types of tools

(i) Forged tool (solid tool)

It is a single piece toolforged entirely of a cuttingtool material which may beeither high speed steel orhigh carbon steel.

Round shape to the tool isgiven by forging and theaccurate tool geometry isimparted by grinding a toolon cutter grinder.

A forged tool is shown inFig. 1.1.

ToolB lank

Tool B it

Face

Nose Base

Fig.1.1. Forged Tool.

www.srbooks.org Theory of Metal Cutting 1.3

(ii) Brazed tip tool

Brazing is method of joiningtwo or more metals bymeans of a fusible alloy ormetal called spelter.

Tools made of cementedcarbide, stellite, etc. are verycostly and hence made in theform of tool tips in differentform.

These tool tips are brazed tothe end of carbon steel shankas shown in Fig. 1.2.

(iii) Fastened tip tool (Mechanical fastening)

Brazed tool does not havesufficient rigidity, sosometimes the tips(cemented carbide, ceramictips, etc.) are clamped tothe tool shank by means ofclamp and bolt. (Fig. 1.3)

Tungsten carbide tippedtools are commonly used tomodern lathes with highspeed and power.

(iv) Tool bit and Tool holders

A tool bit is a very shortshank cutting materialwhich is inserted in aforged carbon steel holder and clamped in position by bolt orscrew.

A tool tip may be solid tool or a tipped tool.

H igh speed s tee l tip

C opper b raz ing

p iece

C arbon stee l shank

Fig.1.2. Brazing Process.

Fig.1.3 M echanically Fastened Tipped Tool

C lam ping screw

C lam p

Shank


Tool holders are made ofdifferent designs accordingto shape and purpose ofcutting tool.

A typical tool holder isshown in Fig. 1.4 with aHSS tool bit.

The slots are broachedeither parallel to the

bottom or at 15 angle.HSS tool bits may be usedin either type whilecarbide tipped tool bitshave only a small reliefangle so they must alwaysbe used with horizontallyslotted holders.

A tool holder with acarbide or ceramic insertcutting tool and a chipbreaker is shown in Fig1.5. The chip breaker hasa bevelled edge and is clamped on the top of the cutting tool.

Once the carbide insert is wornout, it is thrown away and replacedwith a new one.

Advantages of tool bit over solid tools

Tool bit can be easily adjusted with respect to work.

Less expensive than solid tools.

Regrinding is easy since only one cutting edge need to begrounded.

Replacing is easy.

Fig.1.4. Tool Bit and Tool Holder.

W ork C lam pingscrew

Too l ho lderang le 15 o

Too l b it

Fig. 1.5 Tool holder with chip breaker

Shank

C ap screw

D am p

C hipbreaker

lip

H old


Disadvantages

Less rigidity

Non uniform heat transfer compared to solid tool.

(v) Tools for method of operation

In lathe different operations require different types of tools and arediscussed below.

Turning Tool

There are two types of turning tools-rough turning tool and finishturning tool

Rough turning tool is used to remove most of the metal in orderto bring the size of the workpiece to the desired size (near by).Rough turning saves time and leaves only small amount of metalto be removed by the finishing cut (Fig 1.6(a))

Finish turning is carried out after the rough turning operation isdone. For fine finish a finishing tool is used. Finishing toolremoves only very slight material. Tool geometry is such that verysmooth and accurate surface finish is obtained. Sometimescemented carbide tipped tool or stellite tipped turning tools areused. (Fig 1.6 (b))

(b) Chamfer Tool

A simple straight tool can perform chamfering with cutting edgeset at the desired chamfer angle.

Fig.1.6(a) Rough turn ing tool Fig.1.6(b) Finish turning tool


For large number ofchamfer operations,special chamfer toolwith the side cuttingedge angle ground tothe chamfer angle isused. (Fig 1.7)

(c) Boring Tool

A boring bar is made ofmild steel with slots or holes cut into it to accomdate the tool bitwhich may be locked by an allen screw. The amount of projectionof the cutting edge of the tool from the centre of bar determinesthe finished hole diameter of work.

The tool bit may be a HSS or cemented carbide tip. Fig 1.8 showsa HSS bit for boring bar.

In mass production, boring cutters are fitted on the boring bar.

Fig 1.7 Cham fering operation

Blind ho le operation

Tw o cu tting edge for opera tionquick

Fig 1.8 (a) Boring cutter on boring bars

10 -12 oo

10 -12 oo 10 -12 oo

10 -12oo

20o

10o Fig 1.8 (b) H.S.S. tool bit for boring bar


A tool bit with both sides cutting edge is used for quickmachining.

(d) External Thread Cutting Tool

Single point cutting tool with metric B.S.W or American ‘V’threads grounded cutting edges are used for thread cuttingoperation.

The included angle at the nose of tool determines the angle of

thread and may be 60 for metric thread and 55 for B.S.W.

The size and cross section of the cutting edge of tool dependsupon the pitch of the thread. Fig 1.9 shows a HSS thread cuttingtool.

(e) Internal Thread Cutting

Internal thread cutting tool may be forged type or bit type andholded in a boring bar.

The cutting edge is similar to that of external thread tool withlarge front clearance angle.

(f) Facing Tool

Facing tool removes metal by its side cutting edges and hence notop rake angle is necessary in a facing tool. Fig 1.10 shows aHSS facing tool.

Fig 1.9 H.S .S. Thread cutting tool


(g) Grooving tool

A grooving tool has thecutting edges in the form ofsquare, round or ‘V’ shapeaccording to the type ofgroove to be cut.

A grooving tool is similar tothat of a parting tool. (Fig1.11).

(h) Parting Tool

A cut off or parting tool isused for cutting grooves orfor cutting off stock. Cutoff tool bit may bemounted in a cut off toolholder.

Some cut off tools aredesigned with bevelledsides which provide siderelief on each side.

A HSS parting tool isshown in Fig 1.12

10 -12 ’’o

12 -14 ’’o

10 -12o o

10 -20o o

12’’

8o

Fig. 1.11 Under cutting tool

6o

34o

54o

8 o

6 o

Fig 1.10 H .S.S . Facing tool

2o

2o

5 to 6o o

1o

2 o

2 o

Fig 1.12 H .S.S Parting off tool


(i) Right Hand Tool

When a tool is fed from right to left hand end of the lathe bed,it is called right hand tool.

It is used for operation like turning, thread cutting etc.

A right hand tool has its cutting edge on its left hand end whenviewed from top with its nose pointing away from operator [Fig1.13(a)].

(j) Left Hand Tool

When a tool is fed from head stock to tail stock end of the lathe,it is called left hand tool.

A left hand tool is used for left hand threading operation andturning operation with a shoulder.

A left hand tool has its cutting edge on its right hand end whenviewed from top with its nose left hand [Fig 1.13(b)].

(k) Round Nose Tool

A round nose tool has its nose completly rounded and can be fedfrom either side left to right or right to left end of lathe bed. [Fig1.13(c)]

They have no back rake and side rake angle. It is used for finishturning operation.

Fig 1.13(a) Right hand tool

Fig Left hand tool1.13(b) Fig Round

nose tool1.13(c)


1.3 SINGLE POINT CUTTING TOOL

The various parts of a single point cutting tool are shank, neck, face,base, heel, cutting edge or lip flank, point, height and width. A single pointtool is shown in Fig 1.14(a).

Parts of Single Point Cutting Tool

Shank : Shank is the main body of the tool at one end of which thecutting portion is formed.

Neck : The portion which is reduced in section to form necessarycutting edges and angles is called the neck.

Face : Face of the tool is the surface across which the chips travelas they are formed and is visible to the operator when lookingdown at the top from above.

Base : Base is the surface on which the tool rests.

Face Shank

End Cutting Edge Angle

N ose

Side C u tting Edge

Side C u tting Edge Ang leBack R ake Ang le

S ide R ake Angle

Shank

L ip Angle

End C learance Angle

S ide R e li fe

End Reli fe

Base

H eel

Fig.1.14 (a) Parts of Tool


Heel : It is the curved portion at the bottom of the tool where thebase and flank meet.

Cuttingedge

: Cutting edge or lip is the portion of the face edge along whichthe chip is separated from the workpiece.

NOMENCLATUREStandard Angles of Single Point Cutting Tool depend upon the shape

of tool. These are described below.

(i) Side rake angle: Side rake angle is theangle by which the face of the tool is inclinedside-ways whereas the back rack angle is theangle by which the face of the tool is inclinedtowards back.

The side rake angle is the angle betweenthe tool face and a line parallel to its base andmeasured in a plane right angles to the base andat right angles to the centre line of the point ofthe tool (side cutting edge).

s ide rake angle

End cu tting edge angle

N ose

Shank

cutting edge

Top rake ang le

s ide c lea rance ang le

s ide cu tting edge angle

End clea rance angle

Fig 1.14 (b) Tool parts

H eight

S ide R e lie f Ang le

S ide R ake Angle

w id th

Fig. 1.15


The side rack angle of a tool determines the tool thickness behind thecutting edge.

(ii) Back rake angle: Backrake angle is the angle betweenthe face of the tool and a lineparallel to the base of the shankin a plane parallel to the centreline of the point (or parallel to theside cutting edge) and at rightangles to the base.

If the inclination of facebackwards is downwards, theback rake angle is positive, and ifthe slope is upwards, then the angle is negative.

This angle helps in turning the chip away from the workpiece.

Back rake angle affects the direction of chip flow. Tool life increasesand cutting force is reduced by increasing back rake angle.

Increasing the rake angle facilitates easy flow of chip which increasestool life, improves surface finish and reduces cutting force. Increasing rakeangle also minimizes size and effect of built up edges, cutting temperature,cutting force and power consumption. As a result better surface is obtained.Higher rake angle makes the point weak which may induce tool chatter.

(iii) End relief angle: End relief angle is provided on tool to provideclearance between the workpiece and the tool so as to prevent the rubbingof workpiece with end flake of tool. It is the angle between the surface ofthe flank immediately below the point and a line drawn from the pointperpendicular to the base.

Excessive relief angle reduces the strength of tool, therefore, it should

not be too large. Generally its value varies from 6 to 10

(iv) Side relief angle: Side relief angle is provided on the tool to provideclearance between its flank and the workpiece surface. It is the angle betweenthe surface of the flank immediately below the point and a plane at rightangles to the centre line of the point of the tool. This angle must be large

Face Back Rake A ng le

FlankL ip Angle

BaseHeel

C learance Ang le

End Relie f Ang le

Po in t

Fig. 1.16


enough for turning operations to allow for feed helix angle on the shoulderof workpiece.

(v) End cutting edge angle:It provides clearance betweenthe tool cutting edge andworkpiece, and the side cuttingedge angle is responsible forturning the chip away from thefinished surface. Side cuttingedge angle is the angle betweenthe straight cutting edge on theside of the tool and side of thetool shank. It provides themajor cutting action and should,therefore, be kept as sharp as possible. Too much of this angle causes chatter.

(vi) Nose Angle: It is the angle between the side cutting edge and the endcutting edge.

(vii) Nose radius: It is provided to remove the fragile corner of the tool.It increases the tool life and improves surface finish.

(viii) Clearance angle: It is the angle between the portion of the flankadjacent to the base and the plane perpendicular to the base. This angleprovides free-cutting action, minimises tool forces and decreases cuttingtemperature. Excessive clearance angle may cause chatter and excessive toolwear.

(ix) Lip Angle: It is the angle between the tool face and the ground end

surface of flank. It is usually between 60 and 80

1.3.1 Tool Signature

What do you understand by cutting tool signature? [AU-April/May 2017]

Tool signature is numerical method of identification of toolstandardized by American Standards Association (ASA) according to whichthe seven elements comprising signature of a single point tool are alwaysstated in the following order:

Fig. 1.17

End C utting Edge Angle

N ose Angle

S ide cuttingEdge AngleFaceC utting

Edge

N eck Shank


(i) Back rake angle

(ii) Side rake angle

(iii) End relief angle

(iv) Side relief angle

(v) End cutting edge angle

(vi) Side cutting edge angle, and

(vii) Nose radius.

Symbols of degrees of angles and units for nose radius are omittedand only numerical values of those components are indicated.

Tool signature for a single point cutting tool is a sequence of numberlisting the various angles in degrees and the nose radius. A typical toolsignature given by American Standard Association is given below.

Example: A tool specified with the following as per ASA8-16-7-7-8-16-6 has the following angles.

8 Back rake angle, 16 side rake, 7 end relief, 7 side relief, 8 end

cutting edge, 16 side cutting edge angles and 6 mm nose radius.


1.3.2 Influence of Tool angles in machining

1. Rake Angle

Briefly explain the effect of rake angle during cutting. [AU Nov/Dec 2010]

Rake angle has the following functions.

Helps in flow of chip in convenient direction.

Reduces cutting force and helps to increase tool life and reducepower consumption.

Improves surface finish.

Amount of Rake angle to be given depends upon the following parameters.

Type of material being cut: Small rake angle is given for hardermaterial and large rake angle is given for soft material.

Type of Tool Material used: High speed tools (eg. cementedcarbide) are given minimum or negative rake angle to increasetool strength.

Depth of Cut: Higher the depth of cut lower should be rakeangle. Smaller depth of cut have high rake angle tools.

Rigidity of the tool holder and condition of machine: Animproperly supported tool and old machine should have tool withlarge rake angle to reduce cutting pressure.

Rake angle may be positive, zero or negative as shown in Fig. 1.19

A tool has positive rake when the face of tool slopes away from thecutting edges and slants towards the back or side of the tool.

R

T

R

TT

Fig. 1.19. Positive, zero and negative rake R-R ake, T-Thrust


A tool has zero rake when the face of tool has no slope and in thesame plane or parallel to upper surface of shank. Turning brass usually havezero rake tools. Zero rake increases strength of tool and prevents cutting edgefrom digging into the workpiece.

When will negative rake angles be used? [AU-May/June 2013]

A tool has negative rake angle when the face of the tool slopes awayfrom the cutting edge and slants upwards towards the back or side of tool.It is used in turning metal with cemented carbide tipped tool in massproduction.

Advantages of negative rake angle

Point of application of cutting force is changed from weak tostronger section.

Can work at very high speed.

Increases tool life and reduces tool wear.

Increases lip angle and hence permits higher depth of cut.

2. Clearance angle

Clearance angle prevents the flank from rubbing against the surface ofwork allowing only cutting edge to come in contact with the workpiece.

Front clearance angle prevents front flank of tool from rubbingworkpiece. It is large for large workpiece diameter.

Side clearance angle prevents the side of the tool from rubbingworkpiece when longitudinal feed is given. Larger feed requires large sideclearance angle.

3. Nose radius

Nose radius clears feed marks caused by previous shearing action.

It increases strength of cutting edge and hence increase tool life.

High heat dissipation.

4. Side cutting edge angle

Increases tool life and force distribution on wider surface.

Helps in greater cutting speed.


Improves surface finish and quickly dissipates heat

Usually its value is 15

4. End Cutting edge Angle

It is given to prevent the trailing front cutting edge of tool from

rubbing against workpiece. Its value varies between 8 to 15. High value ofthis, weakens tools.

5. Lip Angle

Lip angle influences the strength of cutting edge. Lip angle directlydepends upon rake and clearance angle. Large lip angle helps in machiningharder metals, giving high depth of cut, increases tool life and improvesdissipation of heat.

1.4 THEORY OF METAL CUTTING - MECHANICS OF CHIP FORMATION

Describe the mechanism of chip formation. [AU - April/May 2017]

Describe the mechanism of metal cutting. [AU. Apr/May 2011] [AU. May/June 2014]

1.4.1 Mechanics of Metal Cutting and Chip formation

Any metal cutting process involves workpiece, tool (including holdingdevices), chips and cutting fluid. For removing the metal, a wedge shapedtool is considered stationary and the workpiece moves to the right. The area

III

III

Shear P laneC h ip Too l

R ake Angle( )

C learance A ng le ( )Shear

A ng le ( )

Setting Angle

D epth o f C u t

W ork

A

B

Fig. 1.20 (a) Position of tool in relation to work.


of metal in front of tool gets compressed causing high temperature shear.The stress in workpiece just ahead of the cutting tool reaches ultimate strengthand particles shears to form chip elements. Fig 1.20 (a) shows position oftool in relation to work in order to cut metal. There are three basic anglesof importance-rake angle, clearance angle and setting angle.

The outward or shearing movement of each successive is arrested bywork hardening and the movement is transferred to the next element. Theprocess is continuous and repetitive to give continuous chip which iscompressed, burnished and slightly serrated top side caused by shearingaction.

The place of element shearing is called shear plane. Thus chip isformed by plastic deformation of grain structure of metal along the shearplane. The deformation occurs along a narrow band across the shear plane.

The structure begins elongating alongAB below shear plane and continue elongatingtill it completely deforms along the line CDabove shear plane as shown in Fig 1.20(b) andchip is born. The region between AB and CDis called shear zone or primary deformationzone.

Actually lines AB and CD are notparallel and may produce wedge-shape whichis thicker near the tool face at the right than at the left.

Because of this, Curling of the chip occurs in metal cutting. Also thenon uniform distribution of the forces at the chip-tool interface and on theshear plane, the shear plane is curved slightly downward causing curling ofthe chip from the cutting face of tool.

Observations in any cutting operation

Metal is cut by removal of chips either continuous ribbon ordiscontinuous chips. Chip is thicker than the actual depth of cutand correspondingly shortened.

Hardness of chip is greater than the hardness of parent material

There is no flow of metal at right angles to direction of flow.

BA

DC

C h ip

Too l

Fig.1.20 (b) Shear zone during metal cutting


Flow lines on side and back of chip indicates shearing mechanism.Front surface is smooth due to burnishing action.

Lot of heat is generated in the process of cutting due to frictionbetween the chip and tool. Friction can be reduced by using sharpcutting edge, good tool finish, good tool geometry, using cuttingfluid etc.

In front of cutting tool point, generally no crack is observed. Dueto strain hardening, the hardness of metal in chip, the built upedge and near the finished surface is usually greater than that forthe metal.

Sometimes a built up edge is formed at the tip of the tool and itsignificantly alters the cutting process. It deteriorates the surfacefinish and rate of tool wear is increased.

1.4.2 Chip Formation

Chip formation has already been explained in mechanism of metalcutting. All machining processes involve formation of chips by deforming thework material on the surface of the job with the help of a cutting tool. Theextent of deformation that the material suffers not only determines the typeof the chip but also determines the quality of the machined surface, cuttingforces, temperature developed and dimensional accuracy of the job.Depending upon the tool geometry, cutting conditions and work material, alarge variety of chip shapes and sizes are produced during different machiningoperations.

1.5 METHODS OF METAL CUTTING PROCESSES

Metal Cutting processes are generally classified into two types.

(i) Orthogonal cutting process (Two dimensional)

(ii) Oblique cutting process (Three dimensional)


What is meant by orthogonal cutting? [AU Apr/May 2010]

What is orthogonal rake system? [AU May/June 2014]

1.6 ORTHOGONAL METAL CUTTINGOrthogonal cutting process is one in which the cutting face of the tool is

90 to the line of action or path of the tool. In other words, the edge of toolis perpendicular to the cutting velocity vector as shown in Fig. 1.21(a)

What is meant by oblique cutting? [AU Apr/May 2010]

1.7 OBLIQUE METAL CUTTINGOblique cutting process is one in which the cutting face is inclined at an

angle less than 90 to the path of the tool, the cutting action is known asoblique as shown in Fig 1.21(b)

Fig 1.22 shows the chip flow in orthogonal and oblique cutting. Inorthogonal cutting the chip coils in a tight, flat spiral where as in obliquecutting the chip flows sideways in a long curl. Angle’s i and nc are of

importance in oblique cutting. In orthogonal cutting i 0 & nc 0. Orthogonal

cutting is used for knife turning, broaching and slotting where as bulkmachining is done by oblique cutting.

Feed

90o

Rake

Knife edge

Feed

60oRake

Roughing

Depth o f cu t

(a) Orthogonal (b) Oblique

Fig. 1.21. Orthogonal and Oblique cutting


1.7.1 Differences between orthogonal and oblique cutting.

Compare orthogonal and oblique cutting. [AU Apr/May 2011]

Differentiate between orthogonal and oblique cutting.[AU April/May 2016] [AU April/May 2015] [FAQ}]

S.No. Orthogonal Cutting Oblique Cutting

1. The cutting edge of the tool

remains at 90 to the direction offeed (of the tool or the work)

The cutting edge of the toolremains inclined at an acuteangle to direction of feed.

2. The chip flows in a directionnormal to the cutting edge of thetool.

The chip flow is not normal but

at an angle to the normal tothe cutting edge.

3. The cutting edge clears the widthof the workpiece on either ends.

The cutting edge may or may notclear the width of the workpiece.

4. Only two components of cuttingforce which are perpendicular toeach other are acting on tool.

Three components of cuttingforce perpendicular to each otheracts on the tool.

5. Maximum chip thickness occursat the middle.

Maximum chip thickness may notoccur at middle.,

v

c

o

vw

ork

Chi

p

o

ab

c

di

c

(c) Oblique

Fig. 1.22. Direction of chip flow in orthogonal and oblique cutting.




6. The shear force acts on a smallerarea, so shear force per unit areais more.

The shear force acts on a largearea, hence shear force per unitarea is smaller.

7. Tool life is smaller than that inoblique cutting.

Tool life is higher thanorthogonal cutting.

8. The cutting edge is bigger thanthe width of cut.

The cutting edge is smaller thanthe width of cut.

Discuss the various types of chips produced during metal machining.[AU Apr/May 2010] [AU Nov/Dec 2010] [AU Apr/May 2011]

How are the chips classified? [AU Apr/May 2017]

1.8 TYPES OF CHIPS

The chips are broadly classified into three categories:

(i) Continuous Chip

(ii) Continuous chip with built up edges.

(iii) Discontinuous Chip

Variables affecting the type of chip

The type of chip produced in a particular operation depends upon thefollowing variables.

Properties of material being cut. (i.e ductile or brittle)

Cutting speed.

Depth of cut.

Feed rate.

Rake angle.

Type and way of application of cutting fluid.

Surface roughness of the tool face.

Coefficient of friction between the chip and tool interface.

Temperature of the chip on the tool face.

Nature of cutting (i.e. continuous or intermittent).


With the help of neat sketch explain continuous chips and continuous chipswith build up edges. [AU - Nov/Dec 2011]

1.8.1 Continuous Chips

During the cutting of ductile materials like low carbon steel, copper,brass, aluminium alloys etc., a continuous ribbon type chip is produced. Thepressure of tool makes the material ahead of the cutting edge deformplastically. It undergoes compression and shear. The material then slides overthe tool rake face for some distance and then leaves the tool. Friction betweenthe chip and tool may produce secondary deformation on chip. The plasticzone ahead of the tool edge is called the Primary Zone of deformation andthe deformation Zone on the rake face is usually called Secondary Zone ofdeformation as shown in Fig. 1.23. Both these zones and the sliding of chipon rake face produce heat. The extent of primary zone deformation dependupon.

(i) Cutting speed (ii) Rake angle of tool (iii) Friction on rake face(iv) Work material characteristics.

With large rake angle tools, the chip formation is gradual and materialsuffers less overall deformation. Cutting forces are also low. With small ornegative rake angle tools, the material suffers more severe deformation withlarge cutting forces.

At high cutting speed, the thickness of the primary zone of deformationshrinks i.e it becomes narrower.

Prim ary zone of deform ation

Secondary zone o f deform ation

W ork p iece

Tool

Ch ip

Fig.1.23 Continuous chip


Specify the condition under which chips are formed. [AU Apr/May 2017]

Conditions favorable for continuous chip are

(i) Ductile Material.

(ii) Large rake angle.

(iii) High cutting speed.

(iv) Small depth of cut.

(v) Small feed rate.

(vi) Efficient way of applying cutting fluid to prevent built up edge.

(vii) Low coefficient of friction at chip tool interface.

(viii) Polished face of the cutting tool.

(ix) Use of material having low coefficient friction as cutting tool,(Ex) cemented carbide.

Continuous chips pose difficulty while machining, it gets wrapped overthe machined portion of workpiece if not quickly disposed. So duringmachining a device known as “Chip Breaker” is attached over the tool post(near the tool nose) which breaks the chip into smaller fragments.

1.8.2 Continuous Chips with Built Up Edges (BUE).

The temperature is high at the interface between the chip and the toolduring cutting. As the chip moves over the tool face due to the high normalload on the tool face, high temperature and high coefficient of frictionbetween chip and tool interface, a portion of chip gets welded on the tool

W ork p iece

Bu ilt up edge

Fragem ents o f B U E

Fig.1.24 C ontinuous chip with B U E


C hip

Too l

W ork p iece

In itiation o f B U E

(a) n itiation o f B U E I

C hip

Too l

W ork p iece

G row th of B U E

(b) Growth of B U E

C hip Too l

W ork p iece

Fragm ents of B U E

(c) Breaking of B U E

Fig. 1.25. Formation of Built up Edge and Fragmentation


face forming the embryo of built up edge (BUE). The strain hardened chipis so hard that now it becomes part of the cutting edge and starts cutting thematerial. Since this built up edge is irregular in shape, the surface producedbecomes rough. As the machining continues, more and more chip materialgets welded on the embryo built up edge, this increases its size and ultimately,it becomes unstable and gets sheared off. This cycle is repeated. During theunstable stage, some fragments of the built up edge are carried along theunder surface of chip while some escape along the flank thus worsening thesurface finish of the machined surface. [Fig. 1.24]

However there is a remedy. Increasing in cutting speed, increases theinterface temperature which softens the built up edge. As a result, the criticalsize of the built up edge completely disappears. Fig 1.25 shows the formativecycle of built up edge. After the embryo of built up edge reaches the finalstage, it is sheared off. Again the embryo is formed and the whole cycle isrepeated.

1.8.3 Discontinuous Chips

Discontinuous chips are produced during the cutting of brittle materiallike cast iron, brasses etc containing higher % of Zinc. The chip formationmechanism is different from that of ductile material. A slight plasticdeformation produced by a small advance of the cutting tool edge into thejob leads to a crack formation in the deformation Zone. With further advanceof the cutting tool, the crack travels and a small lump of material startsmoving up the rake face as shown in Fig. 1.26.

W ork p iece

Too l

C h ips

Fig.1 .25 (d) Discontinuous chip


The force and constraints of motion acting on the lump make the crackpropagate towards the surface, and thus a small fragment of chip getsdetached. As the tool moves further, this sequence is repeated.

Specify the conditions under which Discontinuous chips are formed[AU - Apr/May 17]

Following are conditions at which discontinuous chips are formed

Use of brittle material.

Smaller negative rake angle.

Large chip thickness i.e. large depth of cut and high feed rate.

Low cutting speeds.

Dry cutting (i.e. cutting without use of cutting fluid).

1.8.4 Chip Breakers

Chip breakers are important components of tool design particularlywhen tool has to cut ductile materials like low carbon steels, copper,aluminium, low zinc brasses etc. These materials produce long continuouschips which are difficult to handle and occupy large volumes. Such chipsfouls the tool, clutter up the machine and work place and are difficult toremove. These chips are to be broken into small pieces for ease of handlingand to prevent it from becoming hazardous. Hence chip breakers are used tobreak this continuous chips into small pieces. The general types of chipbreakers are

Fig. 1.26. Formation of Discontinuous Chip.

Too l Too l Too l

W ork p iece W ork p iece W ork p iece

(a) (b) (c)

In itia ldefo rm ation C rack

Form ation

C h ipsegm ent


(i) Step type.

(ii) Groove type.

(iii) Clamp type.

These types are shown in Fig.1.27

In general shop practice, thechips are broken by the followingmethods.

(i) By a stepped type breakerin which a step is groundon the face of the toolalong the cutting edge.

(ii) By clamping a piece ofsheet metal in the path ofthe coil.

(iii) By a clamp type breaker inwhich a thin carbide plateis brazed or screwed onthe face of tool.

(iv) By a groove type breaker in which a small groove is groundbehind the cutting edge.

Step type Groove type Clam p typeFig. 1.27 (a) Chip Breakers

Fig 1.27(b) Types of ch ip breaker

(b) Grooved

(a) Stepped

(c) Clam ped - on


1.8.5 Geometry of Chip Formation

When a wedge shaped tool is pressed against the workpiece, chip isproduced by deformation of material ahead of cutting edge because ofshearing action taking place in a zone known as shear plane. This shear planeseparates the deformed and undeformed material.

The Geometry of chip formation is shown in the Fig. 1.28

Considering the Geometry of chip formation we have the following.

Vc : Velocity of tool against workpiece (Cutting Velocity).

AB : Shear plane.t : Depth of Cut (Feed in turning operation)tc : Chip thickness

Vt : Velocity of chip relative to tool acting along tool face.

Vs : Velocity of chip relative to workpiece along shear plane

Considering the principles of kinematics, the three velocity vectors(Vt, Vc, Vs) form a closed velocity triangle ABD as shown in Fig. 1.29. Also

t

B Too l

V c

tcV S

-G A

F

V t

Fig.1.28. Geometry of Chip formation

C hip


from the kinematics, the vector sum of cutting velocity Vc and chip velocity

Vt is equal to the shear velocity vector Vs

Velocity Relationships

Refer [Fig. 1.29]

From Right Angle triangle ACD, BDC we have

DC Vc sin ; DC Vt cos

From above relation, we have

Vc sin Vt cos

So, Vt Vc sin

cos ...(1.1)

Similarly, from right angle triangle AED, AEB we have

AE Vc cos ; AE Vs cos

Vc cos Vs cos

So, Vs Vc cos

cos ...(1.2)

VC

Vt

B

C

VS

A

E

D

Fig.1.29 C utting velocities triangle


1.8.6 Shear plane angle and chip thickness ratio r

The chip thickness ratio is defined as the ratio of depth of cut t to

the chip thickness tc

Chip thickness ratio r ttc

From the Geometry of Fig. 1.29 we see that AE perpendicular to toolchip interface represents tc i.e. Chip Thickness.

From right angle triangles ABG & ABE we have

AB t

sin , AB

tccos AE tc

dividing the above two equations we have

ABAB

t/sin

tc/cos

i.e. ttc

sin

cos r

. . . r

ttc

r ttc

sin

cos cos sin sin

. . . cos cos cos sin sin

r cos cos sin sin sin

r cos cos sin sin

sin 1

r cos tan

r sin 1

r cos tan

1 r sin

. . . tan r cos

1 r sin ...(1.3)


(or) Shear Angle tan 1

r cos 1 r sin

K

2t

tan 1 .

cos K sin

...(1.3(a))

(Here the term 1r

is termed as chip reduction coefficient or chip

compression factor and is denoted by K)

The cutting ratio or chip thickness ratio is always less than unity andcan be evaluated by measuring chip thickness and depth of cut. But it isdifficult to measure chip thickness precisely due to roughness on back surfaceof chip.

The chip reduction coefficient can also be estimated in a different

manner by measuring the length of the chip (lc

Volume of metal removed Volume of Chip.

So, t b l tc bc lc c ...(1.4)

(Here t, b, l, being thickness or depth, width, length and density ofmetal cut and ‘c’ standing suffix for chip).

If width of chip is same as workpiece i.e b bc, and density is same

for both (i.e) c we have

t l tc lc

ttc

lcl

We know ttc

r so, r ttc

lcl

[chip thickness ratio or cutting ratio]

Also density of metal can be used to find the chip reduction coefficient

rc t b

m ...(1.5)

where m is Weight per unit length of metal.


1.9 FORCES IN MACHININGFig 1.30 shows a turning operation with oblique cutting. In this the

cutting edge ab makes an angle with the direction of feed. The metal beingcut undergoes cutting forces. These forces are resolved in three mutuallyperpendicular direction as shown in Fig. 1.30.

The three forces are

(i) Feed Force Fd: It is horizontal component of the cutting force, acting

in the direction of feed of the tool. It is acting tangent to the generatedsurface.

(ii) Thrust force Fr: It is reaction force between the tool and the

workpiece acting in radial direction perpendicular to feed direction.

(iii) Main cutting force Fc: It is the vertical component of the cutting

force acting in vertical direction.

The resultant force R Fd2 Fr

2 Fc2

1.9.1 Forces in Orthogonal Metal Cutting

Fig 1.31 shows an orthogonal cutting process. In this process, the

cutting force has two components only, one in the feed direction Fd and

other in vertical direction - cutting force Fc

c

a

b

Fd

F r

Feed

Fo

F y

F z

Fx

R

Fig.1.30. Forces in oblique Turning


The two components of forces Fd, Fc and forces acting on chip are

shown in Fig. 1.31(a).

As the cutting tool moves along the feed direction, the metal getsplastically deformed along the shear plane and the chip moves along the rake

surface of tool and due to roughness of chip, frictional Force F is actingon the tool.

Fc

Fa

F d

Feed

Fs

F nF cF d

Too l

Feed

W orkp iece

N

F

Chip

Fig. 1.31. Orthogonal Turning.

(a) (b)

F s

F N

R

F

R �

N

Chip

Fig. 1.31 (c) Free body diagram


Following are the forces developed.

Force F : It is the Frictional resistance of chip acting on tool.

Force N : It is reaction provided by the tool.

Force Fs : It is shear force on metal.

Force Fn : It is normal to shear plane and it is backing up force causingcompressive stress on the shear plane.

Fig 1.31 (c) shows the free body diagram of forces acting on chip.

Here the Resultant

R Fn2 Fs

2 ; R F2 N2

Both R and R are equal in magnitude and opposite in direction andare collinear since chip is in equilibrium.

Describe the merchant’s model for orthogonal cutting. [AU Apr/May 2015]

Explain merchant force circle along with assumptions. [AU Apr/May 2010]

1.10 MERCHANT CIRCLE DIAGRAM AND THEORYFrom a fixed geometry of the cutting tool, there exists a definite

relationship among the above mentioned forces (section 1.9)

The components of forces could be measured by a dynamometer andall the forces could be calculated.

Merchant represented these forces in a circle, known as Merchantscircle diagram shown in Fig 1.32.

Following are the assumptions made in merchants to workout forcerelations.

(i) Tool is perfectly sharp and there is no contact along theclearance face.

(ii) The shear surface is a plane extending upward from the cuttingedge.

(iii) The cutting edge is a straight line.

(iv) The chip does not flow to either side.

(v) The depth of cut is constant.


(vi) Width of the tool is greater than that of workpiece.

(vii) The work moves relative to tool with uniform velocity.

(viii) A continuous chip is produced with no built up edge.

(ix) Plain strain condition exists i.e width of chip remains equal towidth of the workpiece.

In the Fig 1.32 we have

back rake angle

shear angle

angle of friction ;

Forces Fd and Fc can be measured by dynamometer

Shear angle can be measured by photomicrograph or by measuringthickness of chip and depth of cut. (discussed earlier).

Fig. 1.32 Merchant Circle diagram


Once the Fd, Fc, and are known, all the other components of forces

acting on the chip can be determined by the geometry shown in Fig 1.32.We can draw the following figures from Fig 1.32 and find relations.

From the Fig 1.33(a) we have from the geometry

Fs AB AC BC ; Fc AD ; Fn BE

Fs AB Fc cos Fd sin ...(1.6)

Fn BE Fc sin Fd cos ...(1.6(a))

Again from Fig 1.33(a) we have

Fc R cos [From le ADE]

Fs R cos [From le ABE]

So R Fs

cos ...(1.7)

Substituting R in Fc we get

Fc Fs

cos cos ...(1.8)

Fd

Fc

c

e

d

f

F

N

b

a

R

o

(b)

-

Fd

F s

Fc

Fn

D

C

B

A

R

(a)E

Fig. 1.33. Geometry of Forces


or Fs Fc cos

cos ...(1.9)

From Fig 1.33(b) we have.

N ab oe od de

N Fc cos Fd sin

Since F ao be

ef fb cd fb

F Fc sin Fd cos ...(1.10)

Let coefficient of friction, then we have

F N

Coefficient of friction

FN

Fc sin Fd cos Fc cos Fd sin

...(1.11)

dividing the numerator and denominator by cos we get

Coefficient of friction Fc tan Fd

Fc Fd tan ...(1.12)

1.10.1 Condition For maximum cutting force

From the equation (1.8) we have

Fc Fs

cos cos

where Fs shear force

Fs shear stress Area of shear plane

Fs s b tsin

...(1.13)


Substituting 1.13 in 1.8 we get

Fc s b tsin

cos

cos ...(1.14)

For maximum Fc, we have

d Fc

d 0

d Fc

d

dd

s b t

sin

cos cos

0

s b t cos dd

1sin cos

0

d Fc

d s b t cos

cos cos sin sin

sin cos 2

0

(or) cos cos sin sin 0

cos [ ] 0

[. . . cos A B sin A sin B cos A cos B]

cos 2 0 cos /2[. . . cos /2 0]

2 /2 [Machinining constant Cm](1.15)

or Shear Angle 4

2

2

...(1.15 (a))

The above relationship is based on Earnest Merchant Theory and alsocalled as “Modified Merchant Theory”, which makes the followingconclusions.

1. The stress is maximum at the shear plane and it remainsconstant.

2. The shear takes place in a direction in which the energy requiredfor shearing is minimum.


Merchant modified the relationship desired by Earnest - Merchant, byassuming that the shear stress along the shear plane varies linearly withnormal stress. It is given as

s 0 K n ...(1.16)

Where s Shear stress

0 Static stress

n Normal stress

K constant

Equation 1.14 becomes,

Fc 0 K n b tsin

cos

cos

For maximum Fc, we have

d Fc

d 0, we get cos 2 K

or 2 cos 1 K

Shear Angle cos 1 K

2 2

2 ...(1.17)

1.10.2 Lee and Shaffer Theory

According to Lee and Shaffer Theory, the shear occurs on a singleplane. So, for a cutting process according to this theory,

(i) The Material ahead of the cutting tool, behave as ideal plasticmaterial.

(ii) The chip does not get hardened.

(iii) The chip and parent work-material are separated by shear plane.


According to Lee and Shaffer Theory

4 ...(1.18)

The relation was further modified by taking into account factor basedon changes due to built up edge formation.

4 ...(1.19)

1.10.3 Power and workdone in cutting process

Let Pc Horse Power (HP) in kW required for cutting.

Pm Gross Horse Power HP in kW of the motor.

PI Idle Horse power i.e., Horse Power consumed while runningidle in kW

Vc Cutting Velocity

Work done in cutting W Fc Vc in Nm/s or Watt ...(1.20)

Where Fc Cutting Force in N

Work done in shear Ws Fs Vs ...(1.21)

Where Fs Shear force and

Vs Velocity of chip relative to work in m/s.

Work done in friction Wf Ff V

Ff Frictional force,

V Velocity of chip relative to cutting tool in m/s.

Now Total work done in cutting W Ws Wf

Fc Vc Fs Vs Ff V ...(1.22)

Also cutting Power in Pc Fc Vc

60 75 1.36 kW

...(1.23)


Here Fc Vc is workdone in kgm/min.

Or Force of Cutting Fc Pc 6120

Vc...(1.24)

Here Fc is in kg, Vc in m/min, Pc in kW

Also we have Pc Pm PI ...(1.25)

Mechanical Tool efficiency (tool Pc

Pm...(1.26)

1.10.4 Stress and Strain in Chip

Let avg Average Shear Stress on Shear plane

As Area of Shear Plane

w Width of the chip

t Thickness of chip

We have Shear Stress s Fs

As where Fs Shear force

We know that As w.t

sin

s Fs sin

w t...(1.27)

From the equation 1.6 we have

Fs Fc cos Fd sin

s [Fc cos Fd sin ] sin

w.t

Shear Stress s Fc cos sin Fd sin2

w.t...(1.28)


1.10.5 Shear Strain in Cutting

Let us consider the chip consists of a large number of element asshown in Fig. 1.34

Let x Thickness of each element

s Displacement of each element through shear plane

e Strain

We know that Strain e s

x

ACx

AB BC

x

e ABx

BCx

x tan 90

x x tan

x

e tan tan 90 ...(1.29)

e tan cot

Too l

xs

x

s

-

A B C D

90-

O

x

s

Fig.1.34. Shear Strain.


sin cos

cos sin

e sin sin cos cos

sin cos

e sin [sin cos cos sin ] cos [cos cos sin sin ]

sin cos

. . . sin A B sin A cos B cos A sin B

cos A B cos A cos B sin A sin B

e sin2 cos sin cos sin cos2 cos cos sin sin

sin cos

e cos [sin2 cos2 ]

sin cos

strain e cos

sin cos ... (1.30)

From the equation 1.2 we have

VS VC cos

cos

Substituting (1.2) in 1.30

Strain e Vs

Vc sin

Vs e Vc sin ... (1.31)

1.10.6 Energy in cutting

Total energy consumed per unit time in cutting

Energy E Fc Vc ... (1.32)

Total energy consumed per unit volume of metal removed

Em E

Vc w t

Fc Vc

Vc w t

Fc

w t... (1.33)


The total energy required per unit volume of metal removed is

ETot Es Ef Ea Em

where Es Shear energy per unit volume

Ef Specific friction energy

Ea Surface energy per unit volume (negligible)

Em Momentum energy per unit volume (negligible)

Es s Vs

Vc sin and Ef

Fw tc

... (1.34)

Practically all the energy required in metal cutting is consumed in theplastic deformation on the shear plane and the friction between chip and tool.

1.11 THERMAL ASPECTS

When cutting tool is subjected to various forces, it should have

(i) Thermal shock resistance

(ii) Low coefficient of friction

(iii) High thermal conductivity and

(iv) Low coefficient of thermal expansion

The above properties are discussed in the next section.

Enumerate the factors that affect the cutting temperature during machining.(AU. Nov/Dec 2011)

Factors affecting the cutting temperature during machining are:

(a) Cutting tool geometry (various angles like rake angle, cuttingedge angle, relief angle, lip angle, nose radius).

(b) workpiece and tool material.

(c) Machine tool rigidity.

(d) Cutting condition (speed, feed and depth of cut)

(e) Microstructure of work piece and tool material

(f) Nose Radius of the cutting tool.


1.12 CUTTING TOOL MATERIALSThe materials having certain specific properties and characteristics are used

as tool materials. Tool material is harder than the material to be cut. Type ofcutting tool material to be used depends upon.

(i) Physical and chemical properties of metal to be cut

(ii) Type of manufacturing process i.e., either Turning, Milling,Grinding etc.

(iii) Rate of production & volume of production.

(iv) Condition of the machine tool.

(v) Complexity of tool and material to be cut.

State the Desirable characteristics of a cutting tool material.[AU - Nov/Dec 2011]

What are the main requirements of cutting materials?[AU - April/May 2015]

Enumerate the essential requirements of a tool material.[AU - May/June 2013]

1.12.1 Desirable Properties of Cutting Tools

The various and important properties of cutting tools are

(i) Hot Hardness

(ii) Wear resistance

(iii) Mechanical and Thermal shock resistance

(iv) Toughness

(v) Friction properties between tools & workpiece

(vi) Chemical reactivity between tool and workpiece

(vii) Ease of availability and manufacture

(viii) High thermal conductivity

(ix) Low coefficient of thermal expansion

(x) Cost of tool.

The most important properties of tool material are hot hardness, wearresistance and toughness


(i) Hot Hardness

Hot Hardness is a measure of the ability of a tool material to retainits hardness even at elevated temperature without loosing its cutting edge. Inmetal cutting, heat is generated during the process due to which the hardnessof the cutting material reduces and consequently the cutting ability of thetool (or the cutting edge of the tool) will reduce. Therefore, it is a veryimportant factor for any materials to be used as a cutting material. In practice,the hardness is increased by adding element like chromium, molybdenum,vanadium, tungsten.

(ii) Wear Resistance

Wear means loss of material. Wear of tools is caused by abrasion,adhesion and diffusion. Abrasive action is because of flow of chip over therake face under high pressure and rubbing action of the machined surfacewith tool flank. Adhesion is gradual loss of tool material when its particlesadhere to the chip or machined surface and get torn away. Diffusion wear isdue to transfer of atoms of hard alloy constituents of tool material into workor chip materials resulting in heating of tool.

A wornout tool will have following effects

(i) Poor surface finish dimensional tolerence on workpiece.

(ii) Increase in cutting force and thus increase in power consumption.

(iii) Increase in temperature and vibration.

Therefore tools must have high wear resistance.

(iii) Toughness

Toughness is the ability of a material to absorb deformation energybefore fracture. Tougher the material, higher the ability of material to absorbimpact loads and intermittent cuts. It is however observed from experiencethat materials which are wear resistant and have high hot hardness are alsomore brittle and therefore less tough.

(iv) Mechanical and Thermal Shock Resistance

If a material has high hardness, its resistance to wear is more. Butincrease in hardness, renders it to shock, because it loses toughness andfracture under impact load easily. There is shock load to the tool when itjust engages with the work and at regular interval if the cutting is intermittent.


Also, if the temperature of the cutting tool and workpiece is increasedsuddenly, thermal shock will affect the cutting tool. Therefore, the toolmaterial should have high mechanical and thermal shock resistance.

(v) Friction

There should be low friction between the tool and workpiece since thefriction generates heat. The coefficient of friction between the tool andworkpiece should be as low as possible.

(vi) Chemical reaction/affinity between the Tool and Workpiece

If there is a high affinity of work material with tool material, the toolwill wear out easily and hence the tool material should have less affinity orno affinity with work material.

(vii) Availability and Manufacture

A tool material with the above mentioned properties must be easilyavailable or can be easily manufactured. If its manufacture is very hard, itmay not be of much use to the machining.

(viii) High Thermal Conductivity

Tool material should have high thermal conductivity so that the heatgenerated during cutting is easily removed from the chip-tool interface.

(ix) Coefficient of Thermal expansion

Tool material should have low coefficient of thermal expansion toavoid distortion during heat treatment.

(x) Tool Cost

The cost of material is also an important factor for its selection astool material. Tool material should be of low cost.

1.12.2 Types of Cutting Tool Materials

Discuss cutting tool materials used in metal cutting.[AU-May/June 2016] [AU-May/June 2014]

List the various tool materials used in industry. State the optimumtemperature of each of the tool materials. [AU - Apr/May 2011]

List the important characteristics of a cutting tool material.[AU - May/June 2014]


The various types of cutting tool materials are:

(i) Carbon tool steels or carbon steels.

(ii) Medium alloy steels or Alloy tool steels

(iii) High Speed Steels (HSS)

(iv) Cast alloys (or) Stellites

(v) Cemented Carbide tool Materials

(vi) Oxide or Ceramic tool Materials

(vii) Diamond

(i) CARBON TOOL STEELS OR CARBON STEELS

The composition of general carbon steels are Carbon 0.8 to 1.3%,Manganese - 0.1 to 0.4% and Silicon - 0.1 to 0.4%. Few alloying elementsare added to improve properties of Carbon Steels. These are Vanadium andChromium. The composition of Carbon-Vanadium Steels and CarbonChromium Steels are:

(i) Carbon Steels : 0.8 to 1.3% C, 0.1 to 0.4% Mn, 0.1 to0.4% Si.

(ii) Carbon-Vanadium Steels : 0.8 to 1.3% C, 0.1 to 0.4% Mn, 0.1 to0.4% Si, 0.15-0.25% V.

(iii) Carbon-Chromium Steels : 0.8 to 1.3% C, 0.1 to 0.4% Mn, 0.1 to0.4% Si, 0.40-0.60% Cr.

Characteristics of Carbon Steels

Carbon Steels have low hot hardness and poor hardenability. They

can be worked upto 200 to 250C. At higher temperature, theyloose hardness rapidly.

Carbon Steels are used for Cutting soft materials like Wood,Plastic, Aluminium, Copper etc.,

Carbon steels are used for making Taps and Core drills formachining soft materials and for making wood working tools.


Effect of alloying element:

Tungsten increases the wear resistance

Chromium and Manganese improves hardenability.

Vanadium increases toughness by giving heat treatment.

(ii) MEDIUM ALLOY STEELS

In medium alloy steels, alloying elements like Tungsten, Chromium,Molybdenum are added to improve hardenability. The carbon content in thesealloy steel is around 1.2 to 1.3%. Higher Carbon content increases hardness

and wear resistance. Tools of these material can work between 250C to

300C and speed is 20 to 40% more than carbon steels. These steels materialsare used in making drills, taps and reamers.

(iii) HIGH SPEED STEELS (HSS)

The composition of High Speed Steel is 18% Tungsten, 5.5%Chromium, 0.7%. Carbon and small amount of Manganese, Vanadium andSilicon. This HSS steel was developed by Fredenck W.Taylor and M.White.

It can work upto 600C at 40 m/min.

HSS is of three types:

(i) High Tungsten HSS

(ii) High Molybdenum HSS

(iii) Tungsten-Molybdenum HSS.

The composition of the above HSS is given below:

(i) High Tungsten HSS : 18% W, 4% Cr, 1% V, 0.6% C &Balance Fe

(ii) High Molybdenum HSS : 1% W, 4.5% Cr, 1.5% V, 8.5% Mo,0.8% C and Balance Fe.

(iii) Tungsten-Molybdenum HSS : 6% W, 4% Cr, 2% V, 6% Mo andBalance Fe

Characteristics of HSS

High Tungsten HSS is the best of the above three for all purposetool steels.


Tungsten and Molybdenum increase the hot hardness.

Vanadium iron Carbide tools are very hard constituents of HSSand imparts high wear resistance to tool at all temperatures.

To increase the cutting efficiency, 2 to 5% of Cobalt is added.One of the composition 2% W, 4% Cr, 2% V, 12% Cobalt arecalled Super high Speed Steels. Because of heavy cost, it is usedfor heavy cut operations only.

HSS hot hardness is quite high so it retains the cutting ability

upto 600C at 40 m/min.

HSS has high wear resistance and good hardenability.

Uses: HSS is used in Drill, Broaches, Reamers, Milling Cutters,Taps, Lathe Cutting Tools, Gear hobs etc.

(iv) CAST ALLOYS (OR) STELLITES

Stellites or cast alloys are non-ferrous alloy containing Tungsten,Chromium, Cobalt and Carbon used for cutting tools. These alloys containno iron and hence cannot be shaped because they cannot be heat treated.They are casted into final shape. They are casted from a temperature about

1300C. The Chemical Composition of these cast alloys are 12 to 17% W,30 to 35% Cr, 45 to 55% Co, 2 to 4% C.

Characteristics of Stellites

Cast alloys are not hard at room temperature but becomes very

hard above 1000F (hardness more than HSS)

Cast alloys are very brittle hence not widely used.

Cast alloys have less toughness but more wear resistance than HSSand allow cutting speed thrice than that of HSS.

Uses: Used in manufacture of Valve seats, Push rod sheets andErosion shield of steam turbine etc.


(v) CEMENTED CARBIDE TOOLS

Discuss advantages and limitations of cemented carbide tools.[AU - Nov/Dec 2011]

The main constituents of cemented carbide tools is tungsten carbide(WC). This material was discovered by Moissan. Tungsten carbide materialsare produced by powder metallurgy by pressing and bonding. Cementedcarbide tools are of three types.

1. Straight Cemented Carbides: Containing tungsten carbide held inmatrix of Cobalt. These are more ductile and less brittle.

2. Titanium-Tungsten Cemented Carbides: Consisting of solid grains,solid solution of tungsten carbide in carbide of titanium and surplus grainsof tungsten carbide all bonded by cobalt in cobalt matrix.

Symbolically given by WC Co WC TiC. These are very brittle.

3. Titanium-Tantalum-Tungsten Cemented Carbide: Consists of grainsof solid solution of carbide of titanium, tantalum and tungsten and surplusgrains of tungsten carbide cemented together by Cobalt Symbolically:

WC Co WC TiCTaC

Characteristics of Cemented Carbide Tools

These tools have / are

High hardness, heat resistance, wear resistance, high hot hardness.

These tools can be used upto 1000C

High thermal conductivity and low thermal expansion comparedto steel.

No plastic flow to stress upto 3500 N/mm2

Low impact resistance.

Very expensive.

Operate at cutting speed upto 45 to 360 m/min.

These are very brittle and hence rigidly supported and have lowshock resistance.

Uses: Used to machine cast iron, non-ferrous and light metal andalloys, non-metallic materials like rubber, glass, plastics, plastics


carbon electrodes, in machining unhardened carbon and alloysteels, heat resistance steels and super alloys workpieces.

Generally cutting tools are six inches in length and have squarecross sections, but carbide tools consists of shank made in steeland at one end it has cemented carbide piece called bits and aredivided into 2 groups namely brazed tip carbide tools and throwaway inserts.

(vi) CERAMIC TOOLS

Ceramic tools are also called cemented oxides. The main constituentof ceramic tools are aluminium, tauxite (a dehydrated alumina) converted intocrystalline form called alpha aluminium. Fine grains are obtained from theprecipitation of alumina (in powder form) and tool tips are produced by hot

or cold pressing of the powder. (sintering process at 1600 1700C). Certainamount of magnesium oxide or titanium oxide are used along with somebinder.

Characteristics of ceramic tools

They have very high compressive strength. It is quite brittle.

Low heat conductivity, so no coolant is required during machining.

Have high strength and hot hardness upto 1200C.

Have low coefficient of friction and hence low heat generated.

Have 2 to 5 times more cutting speed than other tools.

Advantages

Very high cutting speed so low machining time.

High tool life with large depth of cuts.

Low wear rate and hence high dimensional accuracy with highsurface finish.

Low cost of production.

Disadvantages

High initial cost-40 to 200% more than carbide tools.

High rigidity of machine tools is required.


More power required since high speed and feed rate.

Tools are brittle so proper tool geometry, holding devices are tobe used.

Application

Turning, boring and facing at high speeds, used for finishingoperation on non-ferrous and ferrous metals, machining of castingand hard steels.

Cermets are ceramic metal combinations of Iron, Chromium,Titanium and other metals, added to aluminium oxide and boroncarbide. The brittleness of the ceramic tools is considerablyreduced.

(vii) DIAMOND CUTTING TOOLS

Diamond is the hardest known material today. They are used in cuttingtools. Diamond is of four classes-carbons, ballar, boarts and ornamentalstones. Cutting tools are made from boarts which are single crystal, less clearand fault free.

Characteristics of diamond

They are very hard, hence very brittle.

They are abrasion resistant with low coefficient of friction andlow thermal coefficient of expansion.

They burn to CO2 at 800C

They cannot take shock loads.

High heat conductivity and poor electrical conductor.

Advantages

Very high production rate with close tolerance, high surface finish.

Small depth of cut can be given (0.215 micron).

Cost of grinding is reduced.

Chances of built up edge formation is nil.


Disadvantages

Very high cost.

Interrupted cut machining is not possible.

Machine tool should have high rigidity.

Cannot be used for machining beyond 800C.

Exclusively used for shallow cuts.

Applications

Used for machining non metals like rubber, ceramic, graphite andplastic.

Used for machining precious metals like Platinum, Gold andSilver, Soft metals like Copper, Brass, Zinc alloys.

1.13 TOOL WEARA new or newly ground tool has sharp cutting edges and smooth flanks.

During machining operation it is subjected to cutting forces, temperatures,sliding action, mechanical and thermal shocks. Under these severe conditions,the tools gradually wear out and even fractures, necessitating a tool change.

This tool wear causes the following effects

The cutting forces increases.

The dimensional accuracy of the work decreases.

The surface roughness of work increases.

Increase in the temperature between tool and workpiece.

The tool-work-machine starts vibrating.

The workpiece/tool may get damaged.

Loss of production and increase in cost.

Hence the study of tool wear is very important. The tool wear occursat two places on a cutting tool.

(i) At the cutting edge and the principal flank of the tool.

(ii) At the rake face of the tool. Refer Fig 1.35.

The wear at the flank is called flank wear and the wear at the rakeface is called crater wear.


Explain the mechanisms associated with progressive tool wear.[AU-April/May 2015] [AU-April/May 2017]

Describe mechanism and pattern of cutting tool wear.[AU - Apr/May 2018]

1.13.1 Tool Wear Mechanisms

Some of the important tool wear mechanisms of a hard tool are:

How do you classify tool wear? [AU - Apr/May 2010]

Classify the tool wear. [AU-May/June 2013]

(i) Shearing at High Temperature

(ii) Diffusion Wear.

(iii) Adhesive Wear (Attrition Wear)

(iv) Abrasive Wear

(v) Fatigue Wear

C ra te r w ear C ra te r w id th

Flank w earFlank w earheigh t

(a)

Fig. 1.35. Tool W ear

A

B B ��B ��B �

C ra te r w ear

Flank w ear

ABC -O rig inalcross-section

AB B B C -C ross-sectiono f w orn ou t too l

� ��

(b)

C


(vi) Electrochemical effect

(vii) Oxidation effect

1. Shearing at High Temperature

The strength of hard metal decreases at high temperatures. The shearyield stress becomes smaller at high temperature than at room temperature.Though the metal sliding over it has lower yield stress, nevertheless, the chipmay got so much work hardened as to be able to exert frictional stresssufficient to cause yielding by shear of the hard tool metal. The higher thetemperature at the interface, the greater is the effect as shown in Fig. 1.36.

2. Diffusion Wear

When a sliding metal is in contact with another metal, the temperatureis very high and the alloying atom from harder metal starts diffusing into thesofter matrix, thereby increasing the hardness and abrasiveness of the softmaterial. Also atoms from softer material diffuse into the harder medium thusweakening the surface layer of the tool. Diffusion process is highly dependentupon the temperature. Diffusion process doubles for an increase of

temperature of order of 20C in machining using HSS tools. Fig 1.37 showsdiffusion process.

C hip m otion

Shear s tress dueto chip

C h ip

Too l

Shearing ofa ridge

M ach inedsurface

Fig. 1.36. Wear by P lastic Yield ing and Shear.


3. Adhesive Wear (Attrition Wear)

When a soft metal slide over a hard metal such that it always presentsa newly formed surface to the same portion of the hard metal. Due to friction,

Stee l ch ip

C h ip

Too l

Fig. 1.37. Diffusion Wear Process

C

H SS

H SS Too l

.

C h ip

C h ip

Too l

Fig. 1.38. Adhesive W ear Mechanism.

W eld

W eld

Too l

Too l partic lew e lded to chip


high temperature and pressure, particles of soft material adhere to a few highspots of the hard metal as shown in Fig 1.38. As a result, flow of the softermetal over the surface of the hard metal becomes irregular or less laminarand contact between the two becomes less continuous. More particles join upto form “Built up edge”. These Built up edges when grow up are torn outfrom the surface. This process continues and appears as if the surface of hardmetal is nibbled and looks uneven.

4. Abrasive Wear

The softer metal sliding over the surface of the harder metal maycontain appreciable concentrations of hard particles (Eg). Casting may havesand particles. Under such condition, hard particles act as small cutting edgeslike those of a grinding wheel on the surface of a hard metal which in duecourse, is wornout through abrasion (Fig. 1.39). Also the particles of the hardtool metal, which intermittently get torn out from its surface are draggedalong the tool surface or rolled over. These particles plough grooves into thesurface of the hard tool metal.

5. Fatigue Wear

Asperities are formed when two surface slides in contact with eachother under pressure. These asperities interlocks with each other. Due to

Chip

Chip

Too l

Fig. 1.39. Abrasive Wear M echanism .

Tool

Hard partic lein chip& m achined surface

M achined surface


friction, compressive stress is developed on one side of asperity and tensilestress on the other side (Fig 1.40). After the asperities of a given pair hasmoved over or through each other, the above stresses are relieved. New pairof asperities are soon formed and the stress cycle is repeated. Thus thematerial of the hard metal near the surface undergoes cyclic stresses. Thisphenomenon causes surface cracks and ultimately crumbling of hard metal.The variable thermal stresses due to high temperature also contribute tofatigue wear.

6. Electrochemical Effect

Due to the high temperatures existing on tool chip interface, athermoelectric EMF is set up in closed circuit due to formation of junctionat the chip tool interface assisting the tool wear.

7. Oxidation Effect

Grooves and notches are formed at rake face and flank due to thereaction of sliding portion of chip and machined surface with atmosphericoxygen to form abrasive oxides causing wear.

Chip

Ch ip Flow

Tool

Fig. 1.40. Fatigue Wear Mechanism .

Tool

Tension

Compression

Force by chip


8. Chemical decomposition

Local chemical reaction may occur that weaken the tool materialthrough formation of weak compounds or dissolution of the bond betweenthe binder and the hard constituents of carbide tool. These weakened particlesare easily torn away by the aspirities of the chip or on machined surface.

1.13.2 Main Types of Tool wear (Damage) in Cutting

The main types of Tool Wear / Damage are

(i) Flank Wear (ii) Crater Wear

(iii) Groove formation

With the help of a neat sketch, show crater wear and flank wear on acutting tool. [AU - Nov/Dec 2011]

1. Flank Wear

(Refer Fig 1.35)

C ra te r w ear C ra te r w id th

Flank w earFlank w earheigh t

(a)

Fig. 1.35. Tool W ear

A

B B ��B ��B �

C ra te r w ear

Flank w ear

ABC -O rig inalcross-section

AB B B C -C ross-sectiono f w orn ou t too l

� ��

(b)

C


The wear at the side and end of flank of tool is called Flank wear.Flank wear is caused by the rubbing action of the machined surface. Theworn out region is called wear land. Wear land is not of uniform width. Itis widest at a point farthest from the nose. When diffusion becomespredominant wear mode on the flank, a critical wear land is formed andaccelerating wear rate takes place and then rapid wear. It is advisable tochange the tool well before the on-set of the rapid wear in order to avoidcatastrophic tool failure. A typical wear curve for cutting is shown in Fig1.41.

2. Crater Wear

Crater Wear occurs on the rake face of tool in the form of a pit calledcrater. It is formed at a distance from the cutting edge. It is a temperaturedependent phenomenon caused by diffusion, adhesion etc. Fig 1.42 shows the

radius of curvature Rc, depth of crater KT, width of crater KB KM and the

distance of the start of the crater from the tool tip KM change with time.

The crater significantly reduces the strength of the tool and may lead to totalfailure.

Tim e

Wid

th o

f fla

nk w

ear

In itial rapid w ear

C onstant rate w ear region

R apid w earC

O C T

B

Fig. 1.41. A Typical Wear Curve for a Cutting Tool.


1.13.3 Tool Failure

Tool failure is said to have occurred when a tool is unable to producedesired shape, size and finish on the workpiece. A tool failure can occur dueto any one of the following.

(i) Loss of form stability due to high temperature and stresses.

(ii) Mechanical breakage of tool.

(iii) By the process of gradual wear on flank.

A

KB

K M

K B

K T

R C

KT

R C

(a)

Time

Val

ue o

f Cha

ract

ristic

A

(b)

Fig.1.42. Progress of Crater Wear


1.13.4 Measurement of Wear

Tool wear can be measured by any one of the following methods withdifferent degree of accuracy and convenience.

(i) Measurement of height of Wear land.

(ii) Measurement of Volume (or depth).

(iii) Measurement of loss of weight of the tool.

(iv) Diamond Indentor technique.

(v) Radioactive technique.

1.14 TOOL LIFE

Define Tool life. [AU Apr/May 2010] [AU Apr/May 2011]

Tool life is defined as the time elapsed between two successivegrinding of tool (or) the time for which a cutting edge or a cutting tool canbe usefully employed without grinding (in case of HSS tools) or replacement(in the case of throwaway carbide or oxide inserts) is called as tool life.

The other ways of expressing tool life are

(i) Machine time: Tool life is the total time of operation of thismachine tool.

(ii) Actual cutting time: The tool life is the time elapsed duringwhich the tool is actually cutting, between two successivegrindings.

(iii) Volume of metal: Once a certain volume of metal is removed,the life of the tool is assumed to be over.

1.14.1 Tool failure Criterion

The various criterion for judging tool failure are:

(i) Complete Failure

A tool is continued to be used until it can cut the workpiece. So whena tool fails to cut, then the tool has to be ground.

(ii) Flank Failure

The wear on the flank causes the reduction in depth of cut. Theworkpiece becomes taper if the cutting is continued. Therefore, if the wear


on flank reaches certain height, the tool is removed and reground. This ismost general criterion of tool failure.

Flank wear is measured in Maker’s microscope.

(iii) Finish Failure

When the surface roughness of the workpiece reaches a certain highvalue, then the cutting of the tool is discontinued and regrinding is done.This criterion becomes specially important when close fitting is requiredbetween the mating surfaces. Due to rough and uneven surfaces, the fittingmay not be very close.

(iv) Size Failure

A tool is said to be failed when there is a change in the dimensionof the finished workpiece by a certain specified value.

(v) Cutting Force Failure

If the cutting forces are increased by certain amount, the tool is saidto be failed and regrounded.

State the parameters that influence tool life and discuss.[AU Apr/May 2010]

1.14.2 Factors affecting Tool Life

The various factors which affect the tool life are

(i) Cutting Speed

(ii) Depth of Cut

(iii) Feed rate

(iv) Tool material properties

(v) Tool geometry

(vi) Work material properties

(vii) Type of cutting fluid and method of application

(viii) Rigidity of machine-tool-workpiece system

(ix) Nature of cutting.


(i) Cutting Speed

Cutting speed is one of the important factor which affects the tool life.The temperature increases with the increase in the cutting speed whichreduces the hardness of tool and increases the flank and crater wears therebyreducing the tool life.

Frederick W.Taylor conducted number of experiments and derived anempirical relationship between tool life and the cutting speed given by

VTn C ...(1.35)

Where V Cutting speed in m/min

T Tool life in min

n Tool life index [depending upon tool andwork material and cutting environments]

C Constant

In equation if T 1 then V C

Here the constant C can be physically interpreted as the cutting speedfor which the tool life is one minute.

In Taylor’s equation the tool life equation becomes straight line on

log-log scale as shown in Fig. 1.43 i.e log V n log T log C ...(1.36)

The values of n for different tool materials are:

Too l life T (m in )

Cut

ting

spee

d V

(m/m

in)

Log

V

L og T+n Log V = Log C

Log T

Fig.1.43. Tool life Vs Cutting speed


n 0.2 to 0.25 for HSS

0.25 to 0.45 for Carbide Tools

0.4 to 0.55 for Ceramic Tools

Equation 1.35 may be generalized to include the effects of feed f anddepth of cut d.

VTn f n1 dn2 C1

Where n, n1, n2, C1 are constants depending upon tool and work

material, tool geometry and type of coolant used etc.

(ii) Effect of Feed rate and depth of cut

With the increase in the feed rate and depth of cut the tool lifedecreases. The life of the cutting tool is influenced by the amount of metalremoved by the tool per minute which in turn depends upon the feed rateand depth of cut.

The effect of feed and depth of cut on tool life for cemented carbidetool and low carbon steel combination is given by:

V T0.2 260

f0.35 t0.08

Where V Cutting Speed in m/min

T Tool life in min

f Feed in mm/min

t Depth of cut in mm

(iii) Effect of Tool Material

Fig 1.44 shows the tool life variation against cutting speed for differenttool materials. The tool life is greatest for ceramic tools and lowest for HSS.

(iv) Effect of work material hardness and microstructure

A general emphirical relationship between the hardness and cuttingspeed for a given tool is given as


VH1.7 Constant

Where

V Permissible cutting speedH Brinell hardness number

% reduction in size

If hardness is more,corresponding velocity shouldbe less as given by theexpression.

Micro structure of workmaterial affects the tool life. As percentage of pearlite increase, the tool lifedecreases at any and every cutting speed.

(v) Effect of Cutting Fluid

As the tool cuts the workpiece, a lot of heat is generated due to frictionand rubbing. Heat produced during metal cutting is carried away from thetool and workpiece by means of cutting fluid. It also reduces the frictionbetween the chip tool interface and increases the tool life.

An empirical relationship between tool life and temperatures of chiptool interface has been established and is given as

T n K

Where T Tool life in min

Interface Temperature in Cn An exponent indexK Constant

(vi) Tool Geometry

Tool geometry having various angles influences the life of the tool.

Back rake angle affects the shear angle, shear strain and cuttingforce.

High back rake reduces cutting force but makes the wedge thinnerand rise in temperature consequently more wear rate and lowertool life.

30

60

150

300

90

1 2 3 5 10 20 30 50 100

Too l life,T, m in

Fig. 1.44 Effect Tool material Cutting Speed on Tool life

Ideal

Ceram ic too lCarb ide too l

H igh speed s tee l too l


Negative back rake increases cutting force but the wedge becomesmore stronger.

Therefore optimum back rake angle should be used and its range

is 5 to 10.

Principle cutting edge angle also affects the tool life.

For the tool with 90 cutting edge angle (orthogonal cutting), thecutting edge is impact loaded over a small area and hence thecutting force is very high there by reduces the life of tool.

For the tool with less cutting edge angle (oblique cutting) the toolexperiences cutting force gradually and over a larger area andhence tool is safer and has more life.

(vii) Rigidity of Workpiece-Machine tool System

If the rigidity of workpiece-machine tool system is low, higher thevibration of the system and higher the chances of tool failure. The vibrationinduces chipping of tool (specifically brittle tools), because of impact loadingon the tool due to intermittent cutting. Its rigidity is very high then thedamping is more and vibration is less and less chatter and more life. Chattercauses fatigue or catastrophic failure of tool.

(viii) Nature of Cutting

Sometimes the job is such that cutting edge has to frequently enterand exit from the cut as for example in turning a work piece havinglongitudinal slots (Intermittent Cutting). Each entrance and exit gives animpact on the cutting edge that can shorten the tool life, especially if thetool material is hard or brittle.

(ix) Effect of nose radius of tool

Nose radius of the tool improves tool life and surface finish of theworkpiece. A relationship between cutting speed, tool life and nose radius isgiven below.

VT0.09 300 R0.25

Where R Nose radius in mm

T Tool life in min


V Cutting speed in m/min

Nose radius has an optimum value at which tool life is maximumbeyond which the tool life reduces. Larger nose radius means more contactarea which inturn increases friction there by reducing life of tool.

1.14.3 Machining Cost

Cost of machining involves the following cost

(i) Machining cost (cutting cost or machine/operating cost)

(ii) Tool cost (Tool cost and Grinding Cost)

(iii) Idle cost (or) non productive cost.

Total cost per piece CTot Cm CI CT CG

Where CM Machining cost C1 D L

1000 VS

Where C1 Direct labor cost Over head cost in Rs/min

D Diameter of work piece machined in mm

L Length of machining in mm

V Cutting speed in m/min

S Feed in mm/rev

CI Idle cost C1 Idle time per piece

CT Tool changing cost

C1 Tool failure per workpiece T1.

Where T1 Tool changing time.

CG Tool grinding cost per piece

Tool cost per gr ind No. of failures per piece.

Optimum Tool life for minimum cost is

Topt 1n

1 C1 T1 C2

C1


Where C2 Tool cost per grind

also VTn Constant (Taylor equation)

1.15 SURFACE FINISH

A surface can be characterised by its topography and microstructure.The topography describes its micro geometrical properties or texture in termsof roughness, waviness and lay. Microstructure describes the depth and natureof the altered material zone just below the surface.

A

(a )

Lay

Inclus ion

B low ho le

B

C ut o ff leng th

Valleys M ean line

B M agn ified Peaks

R oughness spacing

Waviness spacing

(b)

(c)

A M agn ified

Fig. 1.45. Elem ents of Surface Texture

Waviness he ight


Surface finish (or surface texture) refers to the following properties ofa machined surface as shown in Fig. 1.45.

Roughness: Roughness consists of relatively close-spaced or fine surfaceirregularities, mainly in the form of feed marks left by cutting tool on themachined surface. The mean height or depth is measured over a 1 mm cutoff length or roughness sampling length.

Waviness: It consists of all surface irregularities whose spacing is greaterthan the roughness sampling length. Vibration, chatter and tool or workpiecedeflections due to cutting loads and cutting temperature may cause waviness.

Lay: Lay denotes the predominate direction of the surface irregularities. Thelay is usually specified with respect to an edge called the reference edge ofworkpiece.

Surface flaws: These are random spaced irregularities i.e those which occurat some particular location on the surface or at widely varying intervals. Flawscould be due to inherent defects such as inclusions, cracks, blow-holes etc.

1.15.1 Factors affecting surface finish

The factors which affects the surface finish are:

(i) Cutting tool geometry

(ii) Workpiece geometry

(iii) Machine tool rigidity

(iv) Workpiece material

(v) Cutting condition (speed, feed and depth of cut)

(vi) Tool material.

(i) Cutting Tool Geometry

The various angles rake, relief, cutting edge and nose radius directlyaffects the surface finish on the workpiece.

(ii) Workpiece Geometry

Long slender workpiece have low stiffness against both static anddynamic forces. As a result waviness effects are more in long workpiece thansmall workpieces


(iii) Machine Tool Rigidity

A sufficient high rigid machine produce less vibration which inturnreduces the waviness in workpiece and produce high surface finish.

(iv) Workpiece Material

Chemical composition, hardness, microstructure and metallurgicalproperties of the workpiece material largely affects the surface finish ofworkpiece. (eg) steel having 0.1% or less carbon produce build up edge andthereby spoil surface finish.

(v) Cutting Condition

High speed cutting produces better surface finish than at low cuttingspeed. Feed also affects the surface finish. A coarse feed produces roughsurface and fine feed produces good surface finish. Also depth of cut directlyaffects the surface finish. Light depth of cut produces fine surface finish,while heavy depth of a cut produces rough surface.

(vi) Tool Material

Different tool material have different hot hardness, toughness andfrictional behavior which affects the surface finish

1.15.2 Measurement of Roughness

Following are the parameters measured in surface roughness (Fig. 1.46)

(i) Overall height hmax

Overall height is height of separation between upper and lower surfaceline occurring within sampling length (L)

hmax Lp Lv

(ii) Leveling depth hp

It is the mean height of profile above the mean line Lm. Mathematically

hp 1L

0

L

ydx


(iii) Centre Line Average hCLA

It is defined as the arithmetic average of the deviation of the profileabove and below the mean line Lm

hCLA 1L

0

L

|Y| dx

(iv) Root mean Square Value hRms

It is defined as geometrical average value of the deviation of the profileabove and below mean line.

hRms

1L

0

L

y2 dx

1/2

L p

L m

L v

yh p

h m ax

L

X

Fig. 1.46. Measures of Surface Roughness.

Y


1.15.3 Specification of Surface Roughness

ISO recommendation on surface roughness in machining specificationis given in the Fig 1.47.

Symbols are described as below:

a hCLA or centre line average value;

b Production method heat treatment and continuing

C Sampling length

d Direction of lay

e Machining allowance

The symbol d can take following symbols.

– When lay is parallel to plane of view

– The lay is perpendicular to plane of view

X – The lay is in 2 directions

M – The lay is multi-directional

C – The lay is circular

R – The lay is radial

60 o 60o

e d

a

b

c

Fig. 1.47. Drawing Symbols for Surface Roughness in Machining


1.16 CUTTING FLUIDS

In metal cutting process, heat is generated due to plastic deformationof metal, friction between chip and rake face of tool and rubbing betweenthe flank and work. This increases the temperature of both tool andworkpiece. The temperature affects the tool life causing tool failure andsurface finish of the workpiece is deteriorated. Hence cutting fluids are usedto remove the heat produced.

What is the functions of cutting fluids and its types.[AU-Nov/Dec 2010] [AU-May/June 2013]

1.16.1 Functions of cutting fluids

The main functions of cutting fluids are:

(i) To cool the cutting tool and increase the tool life.

(ii) To cool the workpiece and helps in lubrication of machine.

(iii) To reduce the friction between the chip and the tool.

(iv) To flush away the chip to keep the cutting region free.

(v) To produce the machined surface free from corrosion.

(vi) Reduce the cutting forces and energy consumption.

Describe essential properties of cutting fluid. [AU - Apr/May 2018]

1.16.2 Properties of Good Cutting fluid

A good cutting fluid should have the following characteristic properties.

(i) Good Lubricating Qualities

A cutting fluid should have good lubricating property to remove thechip from touching and adhering to the tool face and preventing formationof built up edge.

(ii) High heat absorbing capacity or cooling capacity

A good cutting fluid will remove more heat and remove the heatquickly thus reducing the temperature between tool and workpiece.


(iii) Rust resistance

Cutting fluid should prevent rusting of work, tool or machine.

(iv) Cutting fluid should have low viscosity so that chip and dirteasily settles.

(v) Cutting fluid should not be toxic in nature.

(vi) Cutting fluid should have high chemical stability such that it canbe used for longer time.

(vii) Cutting fluid should have high flash point

(viii) It should not be harmful to worker or operator

(ix) It should be non flammable

(x) It should not produce smoke or foam easily

(xi) It should not produce bad smell

(xii) It should be of low cost.

1.16.3 Types of Cutting Fluids

Cutting fluids are of the following types:

(i) Solid based cutting fluids: It may be included in the work material itselfor applied on the chip tool interface with some liquid mainly to facilitatemachining by reducing friction. Ex. graphite, molybdenum disulphide etc.

(ii) Straight cutting fluid: These are of three types

(i) Mineral oils (ii) Fatty oils (iii) Combination of mineral and fattyoils.

These oil have good lubricating properties but poor heat absorptionquality and are used for low cutting speeds.

(iii) Oil with additives: The beneficial effects of mineral oils can beimproved with the help of additives which are generally compounds ofsulphur or chlorine. Addition of sulphur compounds reduces chances of chipwelding on tool rake face.


The additives and function are given below:

Additive Function

(i) Mineral oils and other hydrocarbon Base oil

(ii) Polyglycoether (water soluble) Emulsifier

(iii) Aliphatic amines (water soluble) Neutralizing agent

(iv) Aliphatic amines in neutralized form Corrosion protection

(v) Sulfonates Corrosion protection, pressureadditive

(vi) Fatly acid amides Lubricity Improvement

(vii) Sulphur / Phosphorous additives Pressure additives

(viii) Aldehyde Derivatives Biocides

(iv) Water Soluble Cutting Fluids

These are also called water based cutting fluids. These comprise ofmineral oils, fat mixtures and emulsifiers added to water. The oil is held inthe form of microscopic droplets (colloidal) in water, which assumes a whitemilky appearance. Because of water, these have very good cooling effects.Mixture is prepared in different ratios of cutting oil and water to get thedesired heat transfer and lubricating characteristics.

1.16.4 Composition of Cutting Fluids

A cutting fluid may contain the following.

Base oil

Emulsifier

Corrosion Inhibitor

Lubricating-antiwear-extreme pressure additives

Neutralising agents

Biocides and Fungicides

Foam inhibitors

Stabilizing agent.

Table 1.1 shows the different types of coolants and lubricants usedfor different type of operations.




Describe various methods of applying cutting fluid at the cutting zone.[AU - May/June 2016] [AU - May/June 2014]

1.16.5 Method of applying cutting fluid

The method of applying a cutting fluid is very important if one wantsto use full benefit and to conserve it or reduce its wastage. The variousmethods are

(i) Nozzle-pump tank method: A pump is mounted on the tank containingfluid and outlet of pump is connected to nozzle through flexible hose. Thenozzle directs the stream of fluid at desired point.

(ii) Mist application: In this method fluid is passed through a speciallydesigned nozzle so that it forms very fine droplets of cutting fluid or produce

a mist of size 5 to 25 m directed at cutting zone.

(iii) High jet method: A narrow jet at high velocity is directed at theflank surface of the tool. It is the most recent method.

1.17 MACHINABILITY

Define the term machinability. [AU-April/May 2017]

The term machinability is used to refer to the ease with which a givenworkpiece material can be machined under a given set of cutting conditions.It is of considerable economic importance for a production engineer to knowin advance the machinability of a work material, so that its processing canbe efficiently planned.

1.17.1 Factors affecting machinability

The various factors affecting machinability are

(i) Chemical and physical properties of work material.

(ii) Microstructure of work material.

(iii) Mechanical properties of work material.

(iv) Geometry of Tool (Various angles and nose radius)

(v) Rigidity of tool and machine.

(vi) Type of tool material.

(vii) Nature of operation and cutting condition.


SOLVED PROBLEMS IN CUTTING FORCES

Problem 1.1 A dynamometer measures the following feed force 1000 N,

cutting force 3750 N, rake angle 12, Chip thickness ratio 0.3, Findthe following (i) Shear Angle (ii) Shear force (iii) Coefficient of friction

(iv) Compressive force at shear plane. (AU Apr/May 2012)

Given: Feed force (Fd 1000 N;, Cutting force Fc 3750 N,

Rake Angle 12; Chip thickness ratio r 0.3

Solution:

(i) Shear Angle

We know that tan r cos

1 r sin (from Eqn. 1.3, Pg 1.32)

Shear Angle tan 1

0.3 cos 121 0.3 sin 12

tan 1 [0.3129]

Shear Angle 17.38

(ii) Shear force Fs

Shear force Fs Fc cos Fd sin (From Eqn. 1.6, Pg 1.38)

3750cos 17.38 1000 sin 17.38

Fs 3280 N

(iii) Coefficient of friction

We know that coefficient of friction

Fc tan Fd

Fc Fd tan (From Eqn. 1.12, Pg 1.39)

3750 tan 12 10003750 1000 tan 12

0.508

0.508


(iv) Normal or Compressive force Fn

Compressive force Fn Fc sin Fd cos (From Eqn. 1.6 (a))

3750 sin 17.38 1000 cos 17.38

Fn 2074.5 N

Friction Angle tan 1 tan 10.508 26.93

Problem 1.2 The orthogonal cutting process has depth of cut 0.3 mm,

Chip thickness ratio 0.5, Width of cut 6 mm, Cutting Velocity

60 m/min, cutting force parallel to cutting velocity 1200 N, Cutting force

normal to cutting velocity 160 N, Rake angle 12. Determine the ShearAngle, Resultant cutting force, Power required for cutting, coefficient offriction, force component parallel to shear plane? (AU Apr 2013)

Given:

Depth of cut t1 0.3 mm, Chip thickness ratio r 0.5, Width of cut

b 6 mm, Cutting Velocity Vc 60 m/min, Cutting force Parallel to cutting

velocity Fc 1200 N, Cutting force normal to cutting Velocity

Fd 160 N, Rake angle 12

Solution

(i) Shear Angle


1 r sin

0.5 cos 121 0.5 sin 12 [Pg 1.32 Eqn. 1.3]

Shear Angle tan 1

0.5 cos 121 0.5 sin 12

tan 10.5458

28.62

(ii) Resultant Cutting Force F

We know that F Fc2 Fd

2 12002 1602

F 1210 N


(iii) Power required for cutting PPower P Fc Vc

P 1200 60 72,000 Nm/min

P 72000

60 1200 Nm/sec 1200 watts

Power P 1.2 kW

(iv) Coefficient of Friction (Eqn 1.12 Pg 1.39)

We know that

Fc tan Fd

Fc Fd tan

1200 tan 12 1601200 160 tan 12

0.356.

Friction Angle tan 1 tan 1 0.356 19.59

(v) Force Component Parallel to shear plane Fs

We know that Fs Fc cos Fd sin (Eqn 1.6 Pg 1.38)

1200 cos 28.62 160 sin 28.62

Fs 976.74 N

Problem 1.3 In an orthogonal cutting test with a tool of rake angle 8, thefollowing observations were made:

Chip thickness ratio : 0.2

Horizontal component of the cutting force = 1190 N

Vertical component of the cutting force = 1450 N

From Merchant’s theory, calculate the various components of the cuttingforces and the coefficient of friction at the chip tool interface. (AU May/June 2016)

Given:

Rake angle 8

Chip thickness ratio r 0.2

Horizontal component of the cutting tool Fd 1190 N


Vertical component of the cutting tool Fc 1450 N

Solution:

tan r cos

1 r sin

0.2 cos 81 0.2 sin 8

0.2037 [ Shear angle]

tan 1 0.2037 11.51

Fc tan Fd

Fc Fd tan

1450 tan 8 11901450 1190 tan 8

1.0865

(i) Shear Force

Fs Fc cos Fd sin 1450 cos 11.51 1190 sin 11.51

1183.38 N

(ii) Normal Force (or) Compressive force

Fn Fc sin Fd cos 1450 sin 11.51 1190 cos 11.51

1455.40 N

Problem 1.4 The machining of a steel with a tool having signature0-12-6-8-8-90-1 mm ORS shaped tool has the following observations. Feed

0.7 mm/rev, depth of cut 3 mm, cutting speed 60 m/min, Shear Angle

15. Power consumed while in machining 60 kW and idle power

10 kW. Calculate (i) The cutting force, (ii) Chip thickness ratio,(iii) Normal pressure on the chip (iv) Chip thickness.

Given: (ORS - Orthogonal Rake System)

From tool signature we have rake angle 12 Feed

f 0.7 mm/rev, depth of cut d 3 mm, cutting speed Vc 60 m/min,

Shear Angle 15.

Power for machining P 60 kW, Idle Power PI 10 kW.

Solution

(i) Cutting Force Fc

Net Cutting Power Pc P PI 60 kW 10 kW

Pc 50 kW


We know Power Pc Fc Vc

50 103 Fc 60

60

Vc 60 m/min

6060

m/sec

Cutting force Fc 50 103

60 60 50 kN

(ii) Chip thickness ratio r

We know tan r cos

1 r sin

tan 15 r cos 12

1 r sin 12

0.268 r 0.978

1 0.208 r

0.268 r 0.978 0.208 0.268

r 0.2593

(iii) Normal Pressure on the chip

Pressure P Force FcChip Area

Fc

w t

here w Depth of cut 3 mm , t Feed 0.7 mm

Pressure P 50 103

3 0.7 23.81 kN/mm2

(iv) Chip Thickness tc

tc Feed

Chip thickness ratio

0.70.2593

2.7 mm

Problem 1.5 A seamless tube 40 mm outside diameter is turned

orthoganally. The following data are obtained. Rake angle 40, Cutting

Speed 25 m/min, feed 0.15 mm/rev. Length of Chip (1 rev) 60 mm.

Cutting force 3000 N, feed force 1000 N. Calculate (i) Coefficient offriction (ii) Shear Angle (iii) Velocity of Chip along tool face (iv) Chipthickness.


Given:

Diameter of tube D 40 mm, Rake angle 40, Feed f 0.15 mm/rev, Cutting Speed Vc 25 m/min,

Length of Chip (rev) 60 mm, Fd 1000 N, Fc 3000 N

Solution

(i) Coefficient of friction

Fc tan Fd

Fc Fd tan

3000 tan 40 10003000 1000 tan 40

1.628

(ii) Shear Angle


1 r sin Chip thickness ratio

r t1t2

l1l2

60

D

60 D

60

40 0.4775

tan 0.4775 cos 40

1 0.4775 sin 40

0.36580.6931

0.5265

Shear Angle tan 1 0.5265 27.77

(iii) Chip Velocity Vf

Vf Vc r 25 0.4775 11.94 m/min

(iv) Chip Thickness t2

r Feed

t2 ; t2

Feedr

0.15

0.4775 0.314 mm

Problem 1.6 In orthogonal cutting of Mild steel rod of diameter 200 mmand depth of cut 1.5 mm with a cutting speed of 50 m/min and feed of 0.3

mm/rev, the following were obtained, cutting force 2000 N, Feed force

500 N, Chip thickness 0.35 mm, Contact length 1 mm, Net Power

2.5 kW and Back rake angle 15. Calculate the shear strain and strainenergy per unit volume, normal pressure. (AU Nov/Dec 2010) (AU Nov/Dec 2012)


Given:

Diameter of rod D 200 mm; Depth of cut d 1.5 mm, Cutting

Speed Vc 50 m/min, Feed f 0.3 mm/rev, Cutting force Fc 2000 N,

Feed force Fd 500 N, Chip thickness t2 0.35 mm, Contact length

L 1 mm, Net Power 2.5 kW, Back rake Angle 15

Solution:

(i) To Calculate Shear Angle


1 r sin

Chip thickness ratio r t1t2

Feed

t2

0.30.35

0.857

tan

0.857 cos 151 0.857 sin 15

0.6775

Shear Angle tan 1 [0.6775] 34.12

(ii) Shear Strain eWe know that Shear Strain

e cos

sin cos (from Eqn. 1.30) Pg 1.45

e cos 15

sin 34.12 cos 34.12 15

0.966

0.561 0.6545 2.631

Shear Strain e 2.631

(iii) Shear Stress s

Shear Force Fs Fc cos Fd sin (Eqn.1.6)

2000 cos 34.12 500 sin 34.12

1375.3 N


Shear Stress s Shear ForceShear Area

Fs sin

w.t (From Eqn. 1.27)(Pg 1.43)

1375.3 sin 34.12

1.5 0.3 1714.3 N/mm2

s 1714.3 N/mm2

[. . . t f 0.3]

Shear Velocity Vs Vc cos

cos (From Eqn. 1.2)

50 cos 15

cos 34.12 15 73.8 m/min Pg. 1.31

(iv) Shear Energy Es

Shear Energy Es s Vs

Vc sin (From Eqn. 1.34)

Shear Energy 1714.3 73.8

50 sin 34.12 4511 N/mm2 (Pg. 1.46)

(v) Normal Pressure

Fc

Area of chip

2000Feed Depth

2000

0.3 1.5 4444.4 kg/mm2

Normal Pressure 4.44 kN/mm2

Problem 1.7 In an orthogonal cutting operation on a workpiece of width2.5 mm, the uncut chip thickness was 0.25 mm and the tool rake angle waszero degree. It was observed that the chip thickness was 1.25 mm. The cuttingforce was measured to be 900 N and the thrust force was found to be810 N.(i) Find Shear Angle(ii) If the Coefficient of friction between the chip and tool was 0.5, what isthe machining constant Cm? (AU Nov 2010)


Given:

Width w 2.5 mm, Uncut chip thickness t1 0.25 mm, Rake Angle

0, Chip thickness t2 1.25 mm, Cutting force Fc 900 N, Thrust

Force Fd 810 N, Coefficient of friction 0.5

Solution

(i) To find Shear Angle

Chip Thickness ratio r t1t2

0.251.25

0.2

Shear Angle : tan r cos

1 r sin

tan 1

r cos 1 r sin

tan 1

0.2 cos 01 0.2 sin 0

tan 1 [0.2] 11.31

Shear Angle 11.31

(ii) To find Machining Constant Cm (Eqn 1.15/Pg 1.40)

Machining Constant Cm 2

Shear Force Fs Fc cos Fd sin

900 cos 11.31 810 sin 11.31Fs 723.67 N

Shear Stress s Shear force

Area

Fs sin w.t1

s 723.67 sin 11.31

2.5 0.25 227.1 N

Coefficient of friction tan

tan 1 tan 10.5 26.565

Machining Constant Cm 2

2 11.31 26.565 0

Cm 49.185


Problem 1.8 In an orthogonal machining with a tool rake angle of 10, thechip thickness was found to be 3 mm when the uncut chip thickness is setto 0.5 mm. Find the Shear Angle and friction angle [Assume Merchantformula is holding good for the machining].

Given:

Rake Angle 10, Chip thickness t2 3 mm, Uncut chip thickness

t1 0.5 mm

(i) Shear Angle

Chip Thickness ratio r t1t2

0.53

0.167

Shear Angle : tan r cos

1 sin

tan 1

r cos 1 r sin

tan 1

0.167 cos 101 0.167 sin 10

tan 1 0.16450.971

9.613

(ii) According to Merchant Theory [ Friction angle ]

2 /2 [Eqn. 1.15/Pg 1.40)

2 9.613 10 90

Friction Angle 80.77

Problem 1.9 In orthogonal machining of a tube in lathe whose outerdiameter is 80 mm and wall thickness of 4 mm to reduce its length. Thespeed of workpiece is 150 rpm and longitudinal feed is 0.4 mm/rev, cuttingratio is 0.25 with tangential force of 1000 N and axial force of 500 N. Findchip velocity and power consumed? (AU Apr 2008)


Given: Outer diameter Do 80 mm,

Wall thickness tw 4 mm, N 150 rpm;, f 0.4 mm/rev,

Cutting ratio 0.25, Fc 1000 N, Fd 500 N

Solution

1. Chip Velocity

Cutting ratio Velocity of Chip

Velocity of workpiece

Velocity of workpiece V D N1000

in m/min

D Mean diameter Do di

2

di Do 2tw 80 2 4 72 mm

D 80 72

2 76 mm

Velocity of workpiece V 76 150 35814 mm/min

V 35.814 m/min

Velocity of Chip Vc 0.25 35.814 8.954 m/min

Power Consumed P Fc V

P 1000 35.814

60 597 Watts

Problem 1.10 In an orthogonal cutting of a mild steel, following were

observed cutting force 1200 N, Feed force 500 N, Cutting velocity

100 m/min Rake Angle 12 and shear plane angle is 20. Determine thefollowing (i) Shear velocity (ii) Chip flow Velocity (iii) Work done perminute in shearing and against friction (iv) Show that work input is sum ofwork done in shearing and friction.

Given:

Cutting force Fc 1200 N, Feed force Fd 500 N, Cutting Velocity

Vc 100 mm/min. rake angle 12 Shear Angle 20


Solution

(i) Shear Velocity Vs

Vs Vc cos

cos (From eqn. 1.2)

Vs 100 cos 12

cos 20 12 98.78 m/min

(ii) Chip flow velocity Vt

Vt Vc sin

cos (From Eqn. 1.1)

Vt 100 sin 20

cos 20 12 34.54 m/min

(iii) Work done in Shearing and Friction

Work done in shear Ws Fs Vs.

Fs Fc cos Fd sin 1200 cos 20 500 sin 20

Fs 956.62 N

Ws 956.62 98.78

60 1575 W

Friction Force Ff Fd cos Fc sin

500 cos 12 1200 sin 12

Ff 738.57 N

Work done in friction WF Ff Vt 738.57 34.54

60

425.17 W

(iv) Total workdone WWs WF 1575 425.17 2000 W

Work input WI Fc V 1200 100

60 2000 W

Hence WI W


PROBLEMS ON TOOL LIFE AND WEAR

Problem 1.11 The Taylor tool-life equation for machining C-40 steel witha HSS cutting tool at a feed of 0.2 mm/rev and a depth of cut of 2 mm is

given by VTn C Where n and c are constants. The following V and Tobservations have been noted.

V, m/min 25 35T, min 90 20

Calculate (i) n and C (ii) Hence recommend the cutting speed for a desiredtool life of 60 min (AU May 2011) (AU May/June 2013)

Given: Feed f 0.2 mm/rev, d 2 mm ;

Taylors Eqn. VTn C, V1 25 m/min, V2 35 m/min,

T1 90 min, T2 20 min

Solution

(i) To find n & C

According to Taylors Eqn. VTn C 9020

n

3525 V1T1

n C V2 T2n

25 90n 35 20n 4.5n 1.4

Now V1T1n C n log 4.5 log 1.4

C 25 900.223 68.192 n log 1.4log 4.5

0.223

(ii) To find Cutting Speed at 60 min

Given: T3 60 min

Again V3 T3n C

V3 68.192

600.223 27.37 m/min

Cutting Speed V3 27.37 m/min


Problem 1.12 A HSS tool gave a tool life of 120 min at 15 m/min and25 min at 70 m/min. Calculate (i) C and n for Taylor’s equation.(ii) Cutting Speed for minimum life say 1 min?

Given: T1 120 min, V1 15 m/min, T2 25 min, V2 70 m/min

Solution

(i) To find C and n for Taylor’s Equation.

We know that VTn C 12025

n

7015V1 T1

n V2 T2n

15 120n 70 25n n log 4.8 log 4.67

Now, V1 T1n C n

log 4.67log 4.8

0.9825

C 15 1200.9825 1655.33

The Taylor Equation is VT0.9825 1655.33

(ii) Cutting Speed for Minimum Tool life say 1 min

VT0.9825 1655.33 (T 1 min)

V 10.9825 1655.33

V 1655.33 m/min

Problem 1.13 The useful tool life of a HSS tool machining mild steel 20m/min is 3 hours. Calculate the tool life when tool operates at 26 m/min.

(Take n 0.125). (AU May/June 2016) (AU April/May 2015)

Given data:

Case I

Cutting speed V 20 m/min

Tool life T 3 hours 180 min

Tool life index 0.125

We know that,

VTn C

where

C constant

20 1800.125 38.277 ~ 38.27

C 38.27


Case II: When the tool is operated at 26 m/min

V 26

T ?

n 0.125

C 38.27

VTn C

26 T0.125 38.27

T ~ 22 min

Problem 1.14 The tool life equation for HSS and carbide tool are given as

follows. Carbide: VT0.3 C1 and HSS Tool: VT0.2 C2. If the tool life is 100

min at 50 m/min, Compare the tool life of both tools at 150 m/min.

Given: For HSS tool VT0.2 C2 and

For Carbide Tool VT0.3 C1

T 100 min at V 150 m/min

Solution:

To Find T1 & T2 at 150 m/min

For Carbide Tool: VT0.3 C1

150 T10.3 199.1

T1 199.1150

1/0.3

2.57 min

HSS Tool VT0.2 C2

150 T20.2 125.6

T2 125.6150

1/0.2

0.412 min

T1

T2

2.570.412

6.24

Life of Carbide Tool is 6.24 Times life of HSS tool.

(i) To Find C1 & C2

VT0.2 C2

50 1000.2 C2

VT0.3 C1

50 1000.3 C1

C2 125.6 ; C1 199.1

T 38.27

26

10.125

T 1.4711

0.125 22.03 min


Problem 1.15 The modified Taylor equation for a Carbide tool is given as

VT0.3 f0.4 d0.2 C. It was obtained a tool life of 100 min under the following

condition. V 50 mm/min, f 0.5 mm d 1 mm. Calculate the effect of toolof life if feed is increased by 20%, Speed by 15% and depth of cut by 50%together. (AU Nov/Dec 2009)

Given:

VT0.3 f0.4 d0.2 C, T 100 min, V 50 mm/min, f 0.5 mm, d 1 mm

Solution

(i) To Find the Constant C

C VT0.3 f0.4 d0.2 C 50 1000.3 0.50.4 10.2 150.854

(ii) Effect on Tool life

Increase in Feed

20% f1 f 20100

f 1.2 f 1.2 0.5

f1 0.6 mm

Increase in Speed 15% V1 1.15V 1.15 50 57.5 mm/min

Increase in depth of cut 50% d1 1.5 d 1.5 1 1.5 mm

V1 T1 0.3 f1

0.4 d1 0.2 C

57.5 T1

0.3 0.60.4 1.50.2 150.854.

life T1

150.854

57.5 0.60.4 1.50.2

1/ 0.3

150.85450.833

1/0.3

2.9671/ 0.3

life T1 37.56 mins

The effect on tool life is T T1 100 37.56 62.44 min

Tool life is reduced by 62.44 mins.


SHORT QUESTION AND ANSWERS

Unit – I

Theory of Metal Cutting

1.1 Define MachiningMachining is a manufacturing process which involves forcing a cutting

tool through the excess material of the work piece thereby removing theunwanted material in the form of chips so as to obtain the final desired shape,size and finish on work piece.

1.2 What are the types of motion in machining?The types of motion in machining are

(i) Primary or cutting motion(ii) Feed motion

1.3 Name the types of cutting tools.Cutting tools are

(i) Single point cutting tool.

(ii) Multi point cutting tool.

1.4 Define Machine Tool.A machine tool is a power driven device in which energy is utilized

in deformation of material for shaping, sizing or processing a product byremoving the excess material in the form of a chip.

1.5 Classify machine tools.Machine Tools are classified as

(i) Tools for cylindrical work Eg, Lathes, Boring Machine,Cylindrical Grinder etc.

(ii) Tools for flat surface work (Eg Planer, Shaper, Broacher etc.

(iii) Type of production: Eg Basic machine tools, ProductionMachine Tools, special purpose m/c tools, FMS (FlexibleManufacturing System).

www.srbooks.org Short Question and Answers SQA.1

1.6 Name the parts of single point cutting tool?The various parts are shank, neck, face, base, heel, cutting edge, point,

height, width and nose.

1.7 What are the various Angles in single point cutting tool?(i) Back rake angle (ii) Side rake angle (iii) End relief angle

(iv) Side relief angle (v) Side cutting edge angle (vi) End cutting edgeangle (vii) Lip Angle

1.8 What do you understand by cutting tool signature?[AU Apr/May 2017]

Cutting tool signature is a numerical method of identification of toolstandardized by ASA, according to which the seven elements consist ofsignature of a cutting tool are always stated.

1.9 Define the term machinability and machinability index. [AU Apr/May 2017]

Machinability is used to refer the ease with which the given workpiecematerial can be machined under a given set of cutting conditions.

Machinability index is the comparison of machinability of differentmaterials to standard materials.

I cutting speed of metal investigated for 20 min tool life

cutting speed of standard steel for 20 min tool life

1.10. What is the effect of Nose radius in machining? Nose radius clears feed marks caused by previous shearing action.

It increases strength of cutting edge and hence increases tool life.

It increases heat dissipation.

1.11. Briefly explain effect of rake angle during cutting?[AU Nov/Dec 2010]

Rake angle has following functions:

Helps in flow of chip in convenient direction.

Reduces cutting force and helps to increase tool life and alsoreduces power consumption.

Improves surface finish.

SQA.2 Manufacturing Technology-II - www.airwalkbooks.com

1.12 State any two situations where positive rake angle is recommendedduring turning. [AU Nov/Dec 2011]

If the material has low yield strength.

To turn long shaft with small diameters.

To turn low strength ferrous and non ferrous materials.

Turning operation at low speed and machine with low power.

1.13 State any two situation where negative rake angle is used? To machine high strength alloys.

When machine tools are very rigid.

To machine at very high speed and feed rate.

Machining slots or keyways.

1.14 What is Back rake angle?Bake rake angle is the angle between the face of the tool and a line

parallel to the base of the shank in a plane parallel to the centre line of thepoint and at right angles to the base.

1.15 What is orthogonal rake system?[AU May/June 2014]

Orthogonal rake system is one in which the

cutting face of the tool is 90 to the line of actionor path of the tool. In other words, the edge oftool is perpendicular to the cutting velocity vectoras shown in Fig.

1.16 Classify methods of metal cutting process.(i) Orthogonal cutting process (Two dimensional)

(ii) Oblique cutting process (Three dimensional)

Feed90

o

Rake

Knife edge

Depth of cut

Orthogonal rake system


1.17 Compare orthogonal and oblique cutting.[AU May/June 2016][AU Apr/May 2015]


1. The cutting edge of the tool

remains at 90 to the direction offeed (of the tool or the work)

The cutting edge of the toolremains inclined at an acuteangle to direction of feed.

2. The chip flows in a directionnormal to the cutting edge of thetool.

The chip flow is not normal but

at an angle to the normal tothe cutting edge.

3. The cutting edge clears the widthof the workpiece on either ends.

The cutting edge may or may notclear the width of the workpiece.

4. Only two components of cuttingforce which are perpendicular toeach other are acting on tool.

Three components of cuttingforce perpendicular to each otheracts on the tool.

1.18 What is orthogonal and oblique cutting?[AU May/June 2014][AU Apr/May 2018]

Orthogonal cutting process is one in which the cutting face of the tool is

90 to the line of action or path of the tool. In other words, the edge of toolis perpendicular to the cutting velocity vector as shown in Fig. 1.21(a)

Feed

90o

Rake

Knife edge

Feed

60oRake

Roughing

Depth o f cu t


Fig. 1.21. Orthogonal and Oblique cutting


Oblique cutting process is one in which the cutting face is inclined at an

angle less than 90 to the path of the tool, the cutting action is known asoblique as shown in Fig 1.21(b).

1.19 What are the main requirements of cutting tool material?[AU Apr/May 2015]

(i) Hot Hardness

(ii) Wear resistance

(iii) Mechanical and Thermal shock resistance

(iv) Toughness

(v) Friction properties between tools & workpiece

(vi) Chemical reactivity between tool and workpiece

(vii) Ease of availability and manufacture

(viii) High thermal conductivity

(ix) Low coefficient of thermal expansion

(x) Cost of tool.

1.20 When will be the negative rake angles be used? [AU May/June 2013]

A tool has negative rake angle when the face of the tool slopes awayfrom the cutting edge and slants upwards towards the back or side of tool.It is used in turning metal with cemented carbide tipped tool in massproduction.

v

c

o

vw

ork

Chi

p

o

ab

c

di

c

(c) Oblique

Fig. 1.22. Direction of chip flow in orthogonal and oblique cutting.



1.21 Define Shear PlaneThe deformation of the material plastically beyond its yield point takes

places in a localized region called shear plane.

1.22 What are the types of chips?The types of chips are:

(i) Continuous chip

(ii) Discontinuous chips.

(iii) Continuous chip with built-up edge.

1.23 Define Chip thickness ratio?It is defined as the ratio of depth of cut t to the chip thickness

tc

Chip thickness ratio r ttc

l2l1

1.24 What are the types of forces in Metal Cutting?(i) Feed force Fd (ii) Thrust force Fr and

(iii) Main Cutting force Fc

1.25 What are the type of Chip Brakers?(i) Step type (ii) Groove type (iii) Clamp type

1.26 What is tool wear?A new tool has sharp cutting edge and smooth flanks. During

machining operation it is subjected to cutting forces, temperature, slidingaction, mechanical and thermal shocks. Under these severe condition the toolsgradually wear out and even fracture, necessitating tool change.

1.27 How do you classify tool wear?[AU Apr/May 2010][AU May/June 2013]

Tool wear is basically classified as

(i) Flank Wear (ii) Crater Wear

(iii) Nose Wear (iv) Fatigue Wear

(v) Abrasive Wear (vi) Diffusion Wear


1.28 Define Tool Life. [AU Apr/May 2010][AU Apr/May 2011]

Tool life is defined as the time elapsed between two successivegrinding of tool (or) the time for which a cutting edge or a cutting tool canbe usefully employed without grinding (in case of HSS tools) or replacement(in the case of throwaway carbide or oxide inserts) is called as tool life.

1.29 Write Taylor’s tool life equation?

Taylors tool life equation is given as VTn C

Where V Cutting speed m/min, C constant, T Tool life in

minutes, n Index depending on tool and work.

1.30. What are objectives and functions of cutting fluids?[AU Nov/Dec 2010]

The main functions of cutting fluids are:

(i) To cool the cutting tool and increase the tool life.

(ii) To cool the workpiece and helps in lubrication of machine.

(iii) To reduce the friction between the chip and the tool.

(iv) To flush away the chip to keep the cutting region free.

(v) To produce the machined surface free from corrosion.

(vi) Reduce the cutting forces and energy consumption.

1.31 Why is lubrication not required while machining cast iron?[AU May/June 2015]

The flake-line graphite structure in cast iron provides lubrication duringmachining because the flake-like Graphite structures give rise todiscontinuities in the metal matrix and subsequently reduces cutting force andalso the use of lubrication.

1.32 List the physical functions of a machine tool in machining.[AU Apr/May 2018]

(i) Firmly holding the blank and tool.

(ii) Transmit motions to the tool and blank.

(iii) Provide power to the tool-work pair for machining.


Unit – II

Turning Machines

2.1 What is lathe?

Lathe is a machine to remove metal from the job to give it the desiredshape and size producing principally a cylindrical surface. (Refer Fig 2.1)

2.2 Name the types of lathe?Speed lathe, Engine lathe, Bench lathe, Tool room lathe, Special

purpose lathe Capstan and turret lathe, Automatic lathes, NC lathes.

2.3 How do you specify lathe?[AU Apr/May 2017][AU May/June 2016][AU Apr/May 2015]

1. Height of centers measured from bed.

2. Distance between the two centers i.e maximum length ofworkpiece that can be machined.

3. The maximum swing over diameter i.e maximum diameter ofthe workpiece that can be machined without touching bed.

4. Swing diameter over carriage i.e maximum diameter of the workthat can revolve over the lathe saddle and is always less thanswing over diameter.

5. Length of bed ie approximate floor space occupied by lathe.

6. Maximum bar stock that can pass through the hole of head stockspindle.

Tool

Dead cen tre

Tail S tock S leeveL ive C enter

Lathe spind le

Fig.2.1. Principle of Working of a Lathe


2.4 What are major parts of centre lathe?(i) Bed

(ii) Head stock

(iii) Tail stock

(iv) Carriage

(v) Saddle

(vi) Apron

(vii) Feed mechanism

(viii) Thread cutting mechanism

(ix) Tool post

(x) Compound rest

2.5 Explain the following parts: Lathe bed, carriage [AU Nov/Dec 2010]

Lathe bed: is base of machine consisting of two heavy metal slides runninglengthwise with ‘V’ ways and supports the different fixed and operating partswhich are mounted on it.

Carriage: A carriage is a mechanism consisting of saddle, cross slide,compound rest, tool post and apron, located between the head stock andtailstock to support, move and control the cutting tool.

2.6 What is Apron? [AU Nov/Dec 2010]

Apron consists of gears and clutches for transmitting motion from feedrod to the carriage and the split nut which engages with the lead screw duringthread cutting. It is bolted to the saddle and hangs over the front of bed. Itconverts the rotary motion of feed shaft or lead screw for a translatory motionof carriage longitudinally or of the cross slide traversely on carriage.

2.7 What are the type of Headstock mechanisms?1 Belt driven head stock - belt driven and Back gear

2. All-geared head stock

3. Variable stepped motor type.

2.8 What are advantages of all geared drive head stock? Wide range of speed with compact design.

Power at tool is almost constant for all spindle speeds.


Hand belt shift is eliminated due to lever arrangement with lessaccident.

Counter shaft arrangement is eliminated.

Smooth with reduced vibration of lathe machine operation.

2.9 What are lathe accessories? List any four lathe accessories.Lathe accessories are used for holding or supporting the work or tool.

These are

Centers, Chucks, Face plates, Collets, drill holders etc.

2.10. What are lathe attachments? List any four.Lathe attachments are additional equipments used for specific and

special purpose operations. Examples, Taper turning, milling, grinding, gearcutting attachments, ball turning rests, thread chasing dies etc.

2.11. List any four types of Chucks?3-Jaw chuck, 4-jaw chuck, collect chuck, Magnetic chuck.

2.12 What are Mandrels?Mandrels are devices used for holding and rotating hollow workpiece

that has been previously drilled or bored.

2.13. Broadly classify cutting tool?Single point cutting tools used for operations like turning, facing,

chamfering, thread cutting and general purpose operations.

Multi point cutting tools used for special operations like knurling,milling, drilling, reaming, etc.

2.14 How do you classify single point cutting tool.

(i) Method of manufacture

(a) Forged tool

(b) Brazed tool tip with carbon steel shank

(c) Fastened tool tip on to the carbon steel shank

(ii) Holding method

(a) Solid tool

(b) Tool bit inserted in tool holder


(iii) Method of operation

(a) Turning tool (b) Forming tool

(c) Chamfering tool (d) Boring tool

(e) Thread cutting tool (f) Internal thread cutting tool

(g) Facing tool (h) Parting-off tool

(i) Grooving Tool

(iv) Method of applying feed

(a) Left hand tool

(b) Right hand tool

(c) Round nose tool

2.15 What are the factors affecting tool efficiency? The accuracy with which several angles have been ground on

cutting tool.

Shape of cutting edge of tool.

Type of cutting tool material.

Heat-treatment of cutting tool.

Condition of machine.

Type and efficiency of coolants.

Correct speed and feed.

2.16 What are chip breakers? Long continuous chips are formed while machining ductile

materials which create safety hazard to machine operator and chipcoil around the tool and workpiece.

A chip breaker breaks the long curly or tangled chip so that theydo not become a safety hazard.

2.17 Name the operations performed on lathe?

1. Holding the workpiece between the Centers or by a chuck

(a) Straight turning (b) Taper turning (c) Shoulder turning (d) Stepturning (e) Chamfering (f) Polishing (g) Centering (h) Eccentric turning


(i) Thread cutting (j) Facing (k) Spinning (l) Knurling (m) Springwelding (n) Filing (o) Forming

2. Holding the work by chuck or a face plate or an angle plate

(a) Drilling (b) Reaming (c) Boring (d) Counter boring (e) Undercutting (f) Parting off (g) Taper boring

3. Operations using special attachments

(a) Grinding (b) Milling

2.18 State the different methods of taper turning.[AU Apr/May 2010][AU Apr/May 2011][AU Nov/Dec 2011]

(i) By using Form tool

(ii) By swivelling of compound rest

(iii) By offsetting the tailstock centre

(iv) By using taper turning attachment

(v) By using template and tracer method

(vi) By combining longitudinal and cross feed in special lathe

2.19 Give the change gear calculation formula?

Gearing ratio Driver teeth Stud Gear

Driven teeth Lead Screw

Pitch to be cut on work

Pitch on lead sckew

2.20 Name the type of gear train arrangement?(i) Simple gear train (ii) Compound gear train

2.21. Distinguish between capstan and Turret lathe.[AU Apr/May 2010]

S.No. Capstan Lathe Turret Lathe1. Turret is mounted on ram or

short slide which slides on thesaddle.

Turret is mounted directly on thesaddle which slides directly onthe bed.

2. Turret travel is limited by theram travel.

Turret can move on entire lengthof bed.


S.No. Capstan Lathe Turret Lathe3. This lathe does not provide

rigidity due to the over hangingof the ram beyond bed andsubjected to bending, deflectionor vibration.

Since turret is mounted onsaddle, it provides more rigidityto tool support and entire load istaken by lathe bed

4. Cannot operate at severeconditions like high cuttingspeed, feed and depth of cut.

Can operate at severe conditions.

2.22 Mention Limitations of Engine lathe/centre lathe [AU Nov/Dec 2011]

The Limitations of Centre lathe are

(i) Time required for changing and setting tool and for makingmeasurement is very large.

(ii) Only one tool can be used in normal course.

(iii) Idle time involved in setting and movement of tools betweencuts is large and cannot be used for mass production.

(iv) Skilled machinist is required.

2.23 Distinguish between capstan-turret lathe and Engine lathe?[AU Apr May/June 2016]

S.No. Capstan - Turret Lathe Engine Lathe

1. Head stock has wide range ofspeed and heavier inconstruction.

Small range of speed and light inconstruction.

2. 15 H.P motor required to drivespindle

Only 3 HP motor is required todrive spindle.

3. Four tools can be held in toolpost and additional tools can beheld in rear tool post.

Usually only one tool is held inthe tool post in case of enginelathe.


S.No. Capstan - Turret Lathe Engine Lathe

4. Tail stock is replaced by turretwhich can carry one or moretools which are indexed foroperating in sequence.

It has tail stock which canaccommodate only one tool.

2.24 Name principle parts of turret and capstan lathe?Bed, Head stock, carriage, cross slide saddle, turret.

2.25 Name the types of turret lathe.(i) Ram type turret lathe

(ii) Saddle type turret lathe

2.26 How do you specify turret lathe?(i) Maximum diameter of bar that can be passed through the

machine spindle.

(ii) Maximum swing diameter of work piece.

(iii) Spindle speeds variations (Numbers).

(iv) Feeds (Number of feeds)

2.27 Name four work holding devices in turret lathe.(i) Collet chuck

(ii) Jaw chuck

(iii) Arbors

(iv) Fixtures

2.28 Name four tool holding devices in Capston and turret lathe?(i) Straight cutter holder (ii) Multiple tool holder (iii) Slide tool

holder (iv) Form tool holder


2.29 How are automatic lathe classified? [AU Apr/May 2015]

2.30 Define Conicity?The ratio of difference in diameters of taper to its length is called

conicity

Conicity K D d

L

D : Initial diameter

d ; Machined diameter

l : Length of work

2.31 Write the formula for taper turning

tan D d

2l

Fig.2.91. Classification of Automatic Lathes.

Autom a tic la thes

H orizon ta l sp indle / Ve rtica l sp indle

Sem i-autom atics

S ing le spind le M ultip le spind le

Autom a tics

M ultip le spind leS ing le spind le

C uttingo ff

Sw iss type

Autom a ticscrew type

Specia ltype

C uttingo ff ba r

D rilling , form ing ,cutt ing off bar

Bar Specia ltype

C entretype

C entretype

C huck ingtype

C huck ingtype

Tu rret Tu rretSpecia ltype

Specia ltype


: Half taper angle

D : Bigger diameter

d ; Small diameter

l : Length of work

2.32 Name the Automate mechanisms on capstan and turret lathe.(i) Turret indexing or Geneva mechanism.

(ii) Bar feeding mechanism

(iii) Bar holding mechanism

2.33 Draw a neat sketch of Geneva mechanism used in turret lathe forautomatic indexing. [AU Apr/May 2011]

2.34 Name the major parts of swiss type automatic lathe.(i) Sliding head stock (ii) Cam shaft

(iii) Tool bracket (iv) Auxiliary attachment.


2.35 Compare parallel and progressive action of multi spindle automats.

Parallel action Progressive action

1. Same operation is done on alljobs.

Different operation are done onjobs.

2. In one cycle no.of components produced isequal to number of spindle.

3. Rate of production is very high It is moderate.

2.36 What is a centre gauge that is used in threading?[AU May/June 2014]

A centre gauge is a tool used in machining to check angle of toolbits used to cut the screw threads (i.e) threading.

Centre gauge is a small flat hand held object made of metal,Triangular notches are cut into the metal of precise dimensionsand angles and these notches are used as templates for shapingthe machine tool bit.

2.37 What are programmed automatic lathes? [AU May/June 2014]

The standard automatic lathe is programmed to produced parts bymeans of cams, stops (or) other mechanical methods.

Cam controlled Lathes are not referred as programmed lathes, butthe complete versatility in programming is provided by Numericalcontrol. In N.C lathe, the programming is provided by punchedtape and no cams are required. Latest development of NC machineis called as Computer Numerical Control (CNC) where it iscontrolled by a Central Processing Unit (CPU).

2.38 What are the advantages of automates over conventional lathe?[AU Apr/May 2018]

Minimized production time.

Large scale production of identical components.

Part accuracy is more than that obtained on other types of lathes.

One worker can look after more than one machines at a time.


2.39 What are the various mechanisms that are used for automatic feedingin lathe? [AU Apr/May 2018]

(i) Turret indexing mechanism

(ii) Bar feeding mechanism

(iii) Bar holding mechanism.


Unit – III

Shaper, Milling and Gear Cutting

Machines

3.1 What are the different types of reciprocating machine tools?(i) Shaper

(ii) Planer

(iii) Slotter

3.2 What is the use of shaper?A shaping machine (or) shaper is used to generate flat (or) plane

surfaces by means of a single point tool.

3.3 How shapers are classified according to the position and travel of ram?(ii) According to the position and travel of ram.

Horizontal type

Vertical type

Travelling head type

3.4 What is Crank shaper?It is the most common type of shaper in which a single point cutting

tool is given a reciprocating motion equal to the length of the stroke desiredwhile the work is clamped in position on an adjustable table.

3.5 What is hydraulic shaper?In a hydraulic shaper, the hydraulic power is used to give the

reciprocating motion of the ram. One of the most important advantages ofthis type of shaper is that the cutting speed and force of the ram drive areconstant from the very beginning to the end of the cut.

3.6 What are the four table movements for Universal shaper?

Universal shaper

Universal shaper is mostly used in tool room work. Universal shaperis having four table movements.

Vertical and Horizontal movement of the table


The table can be swivelled about an axis parallel to the ram ways

The upper portion of the table can be tilted about a secondhorizontal axis perpendicular to the first axis.

This machine is most suitable for different types of works henceis given the name “universal”.

3.7 What are the principal parts of a standard shaper?The principal parts of a standard shaper are:

(i) Base

(ii) Column (or) pillar

(iii) Cross rail

(iv) Saddle

(v) Table

(vi) Ram

(vii) Tool head

(viii) Drive mechanism

3.8 What are the work holding devices in shaper? [AU Apr/May 2018]

(i) Vises

(ii) Parallel strips

(iii) Clamps

(iv) Jack

(v) Angle plate

(vi) V-Block

3.9 How Shaper vises are classified?The shaper vises can be classified under following headings

1. Plan vise(a) Single screw (b) Double Screw

2. Swivel vise

3. Universal vise


3.10 What are the various types of shaping operation?The various types of shaping operations are:

(i) Horizontal shaping

(ii) Vertical shaping

(iii) Shaping of grooves, slots, steps and keyways

(iv) Angular shaping (or) Dove tail cutting

(v) Cutting of splines and gear teeth

(vi) Irregular cutting

3.11 What is the main fundamental difference between shaper and planer?A planer is very large and massive compared to a shaper and capable

of machining large (or) heavy workpieces. The scope of the machiningoperations on planing and shaping machines are similar, the fundamentaldifference between a shaper and a planer is that in a planer the work issupported on the table and it reciprocates past the stationary cutting tool andthe feed is supplied by the lateral movement of the tool whereas in a shaperthe cutting tool, which is mounted upon the ram reciprocates and the feed isgiven by the cross wise movement of the table.

Planers are used to produce horizontal, vertical, inclined flat surfaceson the workpiece (usually large size workpiece).

3.12 State the uses of planar. [AU May/June 2013]

It is used for producing very large/ heavy work piece.

It is used to produce horizontal, vertical, inclined flat surface.

3.13 Mention any 4 shaper specification. [AU May/June 2013]

(i) Type of drive

(ii) Floor space

(iii) Maximum travel length of ram

(iv) Weight of the shaper.

3.14 Explain milling machining process?Milling is a machining process in which metal is removed by a rotating

multiple-tooth cutter. As the multiple-tooth cutter rotates, each tooth removesa small amount of material from workpiece for each spindle revolution.


Milling is economical method of producing a wide variety of products.

3.15 What for milling machines are used?Milling machines are used for machining flat surfaces, contoured

surfaces, surfaces of revolution, external and internal gears and helicalsurfaces of various cross sections.

3.16 How column and knee type milling machines are classified?The column and knee type milling machines are classified based on

the various method of supplying power to the table, different movement ofthe table & different axis of rotation of the main spindle. i.e.

(a) Hand milling machine

(b) Plain milling machine

(c) Universal milling machine

(d) Vertical milling machine

3.17 What are the operations carried out by vertical milling machine?Vertical milling machine can perform following operations

(i) Drilling, (ii) Boring, (iii) Reaming

(iv) Facing cuts

3.18 Compare plain and universal milling machines?

Comparison between plain and universal milling machines

S.No. Plain milling machine Universal milling machine1. It is provided with three table

movementsIt is provided with four tablemovements

(i) Longitudinal (i) Longitudinal(ii) Cross (ii) Cross(iii) Vertical (iii) Vertical

(iv) Swivel2. It is more rigid and heavier in

constructionIt is not more rigid and heaviercompared to plain millingmachine

3. It is used to producenon-complicated design jobs

It is possible to produce spur,spiral, bevel gears, reamersmilling cutter and also carry outmilling & shaping operations


3.19 What are the work holding devices in milling machines?The following work holding devices are used to hold the workpieces

in the milling machine.

(i) T - bolts and clamps

(ii) Angle plate

(iii) V - Blocks

(iv) Vises

Plan vise

Swivel vise

Tool maker universal vise

(v) Special - fixtures

3.20 How the vises in milling machines are classified?Generally vises are classified into three types

Plain vise

Swivel vise

Tool maker universal vise

3.21 What are the tool (or) cutter holding devices in milling machine?

Cutter holding device (or) Tool holding device

Based on the design of the milling cutter, the tool (or) cutter holdingdevices are listed below.

(i) Arbors

(ii) Collets

(iii) Adapter

(iv) Bolted cutters

(v) Screwed on cutters

3.22 What are the different types of milling machine attachments?The different types of milling machine attachments are as follows:

(i) Vertical milling attachment

(ii) Universal milling attachment


(iii) High speed milling attachment

(iv) Slotting attachment

(v) Rack milling attachment

(vi) Universal spiral milling attachment.

3.23 How milling cutters are classified according to type of operation?

According to the type of operation

Plain milling cutter

Side milling cutter

Angle milling cutter

Form milling cutter

Metal Slitting Saw

End mill cutter

T-slot cutter

Fly cutter

Wood ruff tooth cutter

3.24 What are the different types of milling operations?The different types of milling operations are:

(i) Plain (or) slab milling

(ii) Face milling

(iii) Angular milling

(iv) Straddle milling

(v) Gang milling

(vi) End milling

(vii) Form milling

(viii) Gear cutting

(ix) T-slot milling

(x) Side milling


3.25 What do you understand by gang milling? [AU Apr/May 2017]

The gang milling operation is used for machining several surfaces ofa workpiece simultaneously by feeding the worktable against a number ofcutter having same (or) different size diameters mounted on the arbor of themachine.

Gang milling operation reduces machining time and it is widely usedin making repetitive works.

3.26 What is straddle milling? [AU Apr/May 2015]

This operation is used to produce a flat vertical surfaces on both sidesof the workpiece by using two side milling cutter mounted on the same arbor.The distance between this two cutters are adjusted by using spacing collars.Mostly straddle milling operation is used to produce square (or) hexagonalsurfaces.

3.27 Explain drilling operation?The drilling machine is one of the most important machines in

workshop. Drilling is a machining process by which an hole is produced (or)enlarged by the use of specific type of end cutting tool called drill.

A drill is a rotary end cutting tool with one (or) more cutting lips andusually one (or) more flutes for the passage of chips and cutting fluid.

3.28 How drilling machines are specified?In general, the drilling machine is specified by the following terms.

(i) Maximum diameter of the drill.

(ii) Maximum size of the workpiece and worktable.

(iii) The maximum spindle travel in mm

(iv) Number of spindle speeds and feeds available.

(v) Power input in H.P.

(vi) Floor space required in m2

(vii) Morse taper number of the drill spindle.

(viii) Net weight of the machine in kg (or) tonne.


3.29 What are the different operations done in drilling machines?Some of the important operations which can be performed on drilling

machines are as given here.

(i) Drilling (ii) Reaming (iii) Boring (iv) Counter boring(v) Counter sinking (vi) Spot facing (vii) Tapping (viii) Grinding(ix) trepanning

3.30 Explain reaming process? [AU Apr/May 2015]

Reaming

It is an accurate way of sizing and finishing a hole which has beenpreviously drilled.

Reaming operation is carried out with the help of a reamer. Thereamer is made up of multiple cutting edge tool. It may be madeup of HSS (High Speed Steel) (or) fitted with carbide cutting edgeson its shank.

The material removed by reaming process is around 0.375 mm andfor accurate work, this should not exceed 0.125 mm.

3.31 What is tapping operation?It is the operation of cutting internal threads by means of a cutting

tool called tap. A tap may be considered as bolt with accurate threads cuton it.

3.32 What is boring operation?Boring is the process of using a single point tool to enlarge previously

made hole. The boring machine is one of the most versatile machine toolsused to bore holes in large and heavy parts such as engine frames, steamengine cylinders, machine housings, etc.,

3.33 What are the operations done by horizontal boring machine?

Horizontal Boring Machine Operations

(i) Boring

(ii) Face milling operation

(iii) Drilling operation

(iv) Reaming

(v) Counter boring


(vi) Tapping

(viii) Spot facing operations

3.34 What is jig boring machine?

Jig Boring Machine

A jig boring machine is designed for locating and boring holes in jigs,fixture, dies, gauges and other precision parts. The jig boring machine is themost accurate of all other types of boring machine tools.

The accuracy of the jig boring machine is 0.0025 mm

3.35 Classify gear manufacturing methods?

(i) Non machining process (forming process)

(a) Casting:- Sand casting, Die casting, Injection moulding, Sintering

(b) Rolling

(c) Extruding

(d) Stamping

(e) Coining

(f) Powder metallurgy

3.36 Mention methods of gear cutting by form cutter? Form disc cutter in milling machine

Form end mill in milling machine

Single point cutting tool in shaping/planing machine

Formed cutter in shaper

Formed cutter in broaching machine

3.37 Mention methods of form generating of gears. [AU May/June 2013]

Gear hobbing process

Gear shaping process

Gear planing process

3.38 Mention methods of indexing. Plain or simple indexing

Direct or rapid indexing


Compound indexing

Differential indexing

3.39 Compare gear forming and gear generation method?[AU May/June 2010]

S.No. Gear forming method Gear generation method

1. Separate cutter is required toproduce the desired number ofteeth of the gear in form gearcutting.

Single cutter of any given pitchcan cut gears of any number ofteeth having the same pitch.

2. Accuracy of tooth profilegenerated is less than geargeneration.

Accuracy is high.

3. Low rate of production. High rate of production.

4. Internal and worm gears can beproduced.

Internal and worm gears cannotbe produced.

5. Methods are roll forming,extrusion, cold drawing, end mill,disc cutter.

Gear shaping, gear hobbing andgear tapping method.

3.40 What are advantages of gear hobbing? [AU Nov/Dec 2011]

1. Single hob can be used to produce any number of teeths of samemodule.

2. Any external tooth form which is uniformly spaced about thecentre so that all the teeth are identical, can be hobbed using asingle hob.

3. Spur and helical gears can be produced.

4. The indexing is continuous and there is no intermittent motionto give rise to errors. There is no loss of time due to non cuttingon the return stroke.

5. Perfect tooth profile can be hobbed.

6. Finish is dependent on the amount of feed and the accuracy ofhob and rigidity of tool.


3.41 What are advantages, limitation and application of gear shaping?

Advantages

1. A single cutter can be used for cutting spur gears of identicalpitch as that of cutter.

2. Both internal and external gears can be cut by this process.

3. Non conventional types of gears can also be cut by this process.

4. Ideally suited for mass production, batch production and singlepiece production.

5. Versatile to cut gears, cams, splines and special shapes.

6. High rate of production.

7. Machine mechanism is simple than rack type cutter process.

Limitations

1. Production rate is less than hobbing process.

2. No cutting on return stroke.

3. Worm and worm wheels cannot be generated.

Applications

1. Both internal and external gears can be generated.

2. Helical and spur gears can be generated.

3. Also can cut cams, splines and special shapes.

3.42 Give applications of gear hobbing? [AU Apr/May 2018]

Gear hobbing is used for generating spur, helical and worm gears.

3.43 Give methods of gear hobbing?Gear hobbing is done by climb hobbing, conventional hobbing.

3.44 Give the functions of flutes on taps. [AU May/June 2014]

Taps are the tools used for cutting internal thread which is similar toa threaded bolt, with one to four flutes cut parallel to its axis. The flutesperform three functions, they are

It provides cutting edges.

To conduct the cutting fluid to the cutting region and


To act as a channel to carry away the chips formed by cuttingaction.

3.45 List some of the materials of broaching tools. [AU May/June 2014]

High speed steel (H.S.S) are mostly used as material for broaches.

Brazed carbides are used for cutting edges for machining cast ironparts with close tolerances.

3.46 Mention types of gear hobbing machines?1. Horizontal work spindle

2. Vertical work spindle

3.47 What is gear finishing? Why it is done? [AU Apr/May 2017]

Surface of gear teeth produced by any of the generating process is notaccurate and of good quality.

If dimensional inaccuracies and rough surface generated, then excessivewear, lot of noises, backlash between the pair of gear occur while meshing.

In order to overcome these problems some finishing operations arerecommended for the produced gears.


Unit – IV

Abrasive Process and Broaching

4.1 What is abrasive machining?Abrasive machining is a material removal process that involves the

interaction of abrasive grits with the workpiece at high speeds and shallowpenetration depths.

4.2 Name four abrasive machining processes?1. Grinding

2. Creep feed grinding

3. Abrasive machining

4. Snagging

5. Honing

6. Lapping

7. AJM

4.3 What is Abrasive?An abrasive is a hard material that can cut or abrade (wear away)

other substances. It is extremely hard and tough and when fractured, it formssharp cutting edges and corners. Grinding wheels are made of abrasiveparticles bonded together by means of some suitable bond.

4.4 What are the types of abrasives.1. Natural abrasive: Sand stone, Emery, Corundum, Diamond, Garnet

2. Artificial abrasive: Silicon carbide, Al2O3, Boron carbide, CBN

4.5 Name the properties of abrasives.Hardness, Decomposability, Purity, Toughness, Attrition and Friability.

4.6 Give the properties of Al2O3 and its application.

Aluminium oxide is the most widely used artificial abrasive.

It is manufactured by fusing mineral bauxite (hydrated

Al2O3 SiO2 titanium oxide) and small amount of coke. The


mixture is fused in electric furnace resulting in large aluminiumoxide mass.

This mass is then crushed, washed, heated with alkalis, washedagain and finally ground and graded.

It is of reddish brown colour and is tough and sharp which hastendency to fracture easily and thus used for grinding tool steels.

4.7 Give properties and application of cubic boron nitride abrasive. CBN is newer synthetic abrasive harder than Al2O3 or SiC.

Boron nitride is the second hardest substance having 4500 ratingin knoop scale.

CBN is produced by combination of intensive heat and pressurein the presence of catalyst. CBN is a tight interlocking andalternating boron and nitrogen atom.

CBN has high thermal resistance than diamond upto 650C.

CBN changes its form from cubic to hexagonal and loose hardness

at 1400C.

4.8 What is Grinding?Grinding is a metal cutting operation performed by means of a rotating

abrasive wheel that acts as tool. Grinding is used to finish workpieces whichmust show high surface quality, accuracy, dimension and tolerance.

4.9 What are advantages of Grinding? Very suitable for cutting hardened steels.

Extremely very high smooth surfaces.

Markless surface on work piece.

Very accurate dimension with tolerance upto 25 mm.

Complex profiles can be produced by truing templates.

Grinding wheels have self sharpening properties.

Very little pressure is required to grind the workpiece, thusmagnetic chucks can be used for holding the workpiece.


4.10 Define hardness of grinding wheel. [AU Nov/Dec 2010]

Hardness is the ability to resist penetration. This is measured as knoop

hardness in kg/mm2. The knoop hardness of various abrasives are quartz

320 kg/mm2, Al2O3 (1600 - 2100), carbide (2200 - 2800), CBN (4200 -

4700) and for diamond (6000 - 9000).

4.11 What is meant by “grade” and “structure” of a grinding wheel?[AU Apr/May 2011]

Grade refers to the tenacity or hardness with which the bond holdsthe cutting points or abrasive grains in its place. It is indicated by lettersA to Z.

Structure of a grinding wheel refers to the grain spacing or the mannerin which the abrasive grains are distributed throughout the wheel. It is denotedby number from 1 to 14.

4.12 Give the characteristics of grinding wheel.(i) Material (Type of abrasive used)

(ii) Grain size

(iii) Wheel grade

(iv) Grain spacing

(v) Bond type

(vi) Size and shape

4.13 What is a bond in Grinding wheel? What are the types of bonds.A bond is a material which cements or holds the abrasive grains

together to form a grinding wheel.

The various types of bonds are vitrified bond, silicate bond, shellacbond, rubber bond, resinoid bond and oxychloride bond.

4.14 What are the advantages of vitrified bond?1. Grinding wheels produced by this process are strong and porous

which allows high metal removal with cool cutting.

2. Uniform structure of grinding wheel is obtained.

3. Wheels are chemically inert and are not affected by acid, oil,water and alkalis.


4.15 What is Grit number? Standardizing of grits or grains is accomplished by sorting or

grading the material as it passes through screens.

The number of openings per linear inch in a sieve (or screen)through which most of the particles of a particular size can passdetermine the grain size. A 80-grit would pass through a standardscreen having 80 openings per inch.

4.16 How are grinding wheels designated?Grinding wheels are designated by following six symbols.

1. Type of abrasive

2. Grain size or grit number

3. Grade of wheel

4. Structure

5. Type of bond

6. Manufacturer’s code

4.17 What are mounted wheels? Small grinding wheels of about 50 mm and less diameter are

mounted securely and permanently to steel spindle or mandrel bycementing or other means, and these are called as grinding points.

4.18 What are the factors to be considered for selecting grinding wheel?

1. Constant factors

Amount of stock removal

Material of work piece

Area of contact between wheel & work piece

Type of grinding machine used

Finish and accuracy required.

2. Variable factors

Wheel speed and workpiece speed

Condition of machine

Operator factors


3. The other factors are

(i) Severity of Grinding operation.

(ii) Type of bond.

(iii) Abrasive grain size, grade and structure.

(iv) Wet or dry grinding [i.e with or without lubrication].

4.19 What are the common faults in grinding wheels.The common faults occurring in grinding wheels are glazing, loading

and gumming.

4.20 What is loading, glazing and gumming of grinding wheels?[AU Apr/May 2017]

(i) Glazing

Condition of abrasive wheel in which the abrasive particles in thewheel become dull by wear and there is insufficient grain fracture and thegrains are not released by the bond and hence causing the cutting surface ofthe wheel to take a shiny glass like structure called Glazing of wheels.

(ii) LoadingLoading is the welding of chips to the abrasive grains (or) mechanical

trapping of chips in the pores of the grinding wheel and preventing the wheelfrom cutting freely.

(iii) GummingGumming is said to be occurred on wheel face when the grinding fluid

detoriorates and forms a sludge.

4.21 What is dressing and truing of grinding wheels?Dressing is the process of removing loading and breaks away the

glazed surface so that the sharp abrasive particles are again presented to thework.

Truing of wheels is the correction of wheels for their uneven wear onthe wheel face so as to obtain efficient cutting condition.

4.22 What is balancing of grinding wheel? Balancing can be checked by mounting the wheel on a perfectly

straight and round spindle, the assembly then being rested on levelknife edge ways on a special stand.


The wheel is then rolled a little and left.

Any out of balance will make the wheel to come to rest withheavy side underneath.

4.23 Classify grinding machines based on surface finish.

Based on Type of surface finish

(a) Rough grinders

(i) Floor stand and Bench grinders

(ii) Portable and Flexible shaft grinders

(iii) Abrasive belt grinders

(iv) Swing frame grinders

(v) Wire sawing

(b) Precision grinders

(i) Cylindrical grinders

Centre type (Plain and Universal)

Centreless type

(ii) Internal grinders

Chuck type (Plain and Universal)

Planetary

Centreless

(iii) Surface grinders

Reciprocating table (Horizontal and Vertical)

Rotating table (Horizontal and Vertical spindle)

(iv) Tool and Cutter grinder

Universal

Vertical spindle

(v) Special grinding machine

Roll grinding machines

Crank pin and Cam grinding machines


Honing and lapping machines

Super finishing thread grinders

4.24 What are the types of surface grinders? [AU Nov/Dec 2011]

1. Reciprocating table (Horizontal and Vertical)

2. Rotating table (Horizontal and Vertical)

4.25 What are the two types of grinding operations?1. Traverse grinding

2. Plunger type grinding

4.26 What are the types of centreless grinding? [AU May/June 2016]

Through feed

End feed

In feed

4.27 What are advantages of centreless grinding over cylindrical grinding.1. The rate of production is high in centreless grinding than

cylindrical grinding.

2. Job setting is not required as floating condition occurs hencetime consumed is less.

3. Work is rigidly supported and hence high stability.

4. The process is continuous and hence used for production work.

5. High skill labour/operator is not required.

4.28 What is internal grinding?Internal grinders are used for finishing internal bores and tubes which

are generally straight, tapered or stepped having more than one diameters.The finishing is generally for the purpose of bringing the hole to the correctsize and shape and to give it good surface quality. Internal grinding isfrequently used on production parts that have not been heat treated to savereaming cost.

4.29 What are types of internal grinders? Work rotating type (or) Chucking type

Planetary type

Centreless type


4.30 What is micro finishing process.Micro finishing processes are employed for final machining of

workpieces. In these processes, the peaks of surface micro irregularitiesremaining after a previous machining operations are removed.

4.31 What are types of microfinishing processes. Honing, Lapping, Superfinishing

4.32 What is Honing?Honing is an abrasive machining process of removing stock from

metallic or non metallic surfaces by means of revolving honing tool that alsoreciprocates up and down inside the work piece.

4.33 Define Lapping? [AU Nov/Dec 2010]

Lapping is an abrasive machining process for refining surface finishand geometrically accuracy of flat cylindrical and spherical surface.

4.34 State the applications of honing and lapping finishing methods.[AU Apr/May 2010]

Honing is used to remove scratches produced by grindingoperation.

Honing is done for finishing of engine cylinders, gun barrels, longtubular parts.

Effective for ferrous and non-ferrous material in hardened or softcondition.

Lapping is used for producing flat and smooth surfaces withtolerances of about 0.0125 mm and 0.005 mm.

Lapping is used in following components: piston rings, piston pins,plug gauges, valves, roller bearings, injector valves, crank shafts,slip gauges, engine pistons, valve pistons, bearing races.

4.35 What are types of Lapping operations.1. Equalising lapping 2. Form lapping

4.36 What are methods of lapping?Individual piece lapping, matched piece lapping, hand lapping, machine

lapping.


4.37 What is superfinishing? Super finishing is a microfinishing operation that is used for

surface refining of cylindrical, flat, spherical and core shaped parts.

Super finishing is fine honing operation that obtaines high surfacefinish on component.

4.38 What are applications of super finishing? Computer memory drums

Paper and textile mill rolls

Automobile cylinders, brake drums, bearing, piston, axles, clutchplates, tappet bodies, guide pins

Seal faces on housing

4.39 What is buffing and polishing? Polishing is a super finishing operation performed by a polishing

wheel for the purpose of removing appreciable metal to take outscratches, tool marks, pits and other defects from rough surfaces.

Buffing is smoothing and brightening process of a surface by therubbing action of fine abrasive in a lubricating binder appliedintermittently to a moving wheel of wood, cotton, fabric, felt orcloth.

4.40 What is principle of abrasive jet machining (AJM)? The principle of AJM is that a focussed high speed stream of

abrasive particles (size 10 to 40 microns) carried by high pressuregas or air (2 to 8 bars) at a very high velocity of about 200 to400 m/sec is made to impinge on the work surface through anozzle and the work material is removed by erosion by the highvelocity abrasive particles.

4.41 What are the parameter affecting MRR in AJM (Abrasive jetmachining)? [AU Apr/May 2011]

The factors affecting MRR (Material Removal Rate) in AJM are

Abrasive flow rate

Abrasive grain size

Stand off distance


Gas pressure

Mixing ratio

4.42 What are the advantages of AJM? Process can be used for machining of brittle, heat resistant and

fragile materials like glass, ceramic, mica, etc.

Cut intricate hole shapes in materials of any hardness andbrittleness.

The depth of damage to surface is very little.

4.43 What are the applications of AJM? AJM is used in cutting slots, thin sections, contouring, drilling,

deburring and producing intricate shapes in hard & brittlematerials.

For cleaning of plastics.

For machining brittle, heat resistant and fragile materials like glass,ceramic and mica.

Etching (or) markings of glass cylinders.

4.44 What are the advantages of broaching?

Advantages of broaching

(i) Rate of production is very high

(ii) Little skilled operator is required to perform a broachingoperation.

(iii) It gives more accuracy and good surface finish

(iv) Roughing and finishing cuts are completed in one pass of thetool.

(v) It is used to produce good surface finish on both internal andexternal surfaces in work piece.

4.45 What are the limitations of broaching?(i) The tool cost is very high

(ii) Big workpiece cannot be broached

(iii) It cannot be used for removal of huge amount of stock.

(iv) Broaching machine must be rigidly supported to withstand theforces that set up during cutting.


4.46 What is duplex broaching? [AU Apr/May 2017]

Duplex broaching is an operation in which both the motions (i.e linearand rotary) can be operated simultaneously.

4.47 How do you specify the grinding wheel?[AU Apr/May 2018][AU May/June 2016][AU May/June 2013]

(i) Abrasive type (ii) Grain size

(iii) Grade (iv) Structure

(v) Bond type (vi) Manufacturer’s record

4.48 What are natural and artificial abrasive? [AU Apr/May 2015]

1. Natural abrasives

Natural abrasives are produced by uncontrolled forces of nature. Thevarious natural abrasives are

(i) Sand stone or solid quartz SiO2.

(ii) Emery (mixture of 50-60% Alumina Al2O3 and Magnetite (Iron

oxide Fe3O4)) is a natural abrasive used on coated paper and

cloth.

(iii) Corundum (75 - 90% crystalline Al2O3 and Iron oxide).

(iv) Diamonds.

(v) Garnet (off coloured stones).

2. Artificial or Manufactured Abrasives

The quality and composition of these abrasive particles can be easilycontrolled and their efficiency is far better than that of natural abrasives.These abrasives have better cutting properties. The various abrasives are

(i) Silicon carbide

(ii) Aluminium oxide

(iii) Boron carbide

(iv) Artificial diamond

(v) Cubic Boron Nitride (CBN)


Unit – V

CNC Machining

5.1 How does NC differ from CNC?CNC is a self contained NC system for a single machine tool including

a dedicated mini computer controlled by stored instructions to perform someor all the basic NC functions. CNC works on in line mode and NC in batchprocessing mode. NC is more hardware oriented. Part modification is difficultand time consuming in NC

5.2 When do you go in for incremental system? Why?.Overall error detection is easy and faster in incremental system as

compared to absolute. When we are required to cut a complicated pocket orrecess, incremental system is suitable because incremental measurements aremade from one reference point to the next one in an incremental manner.

5.3 List the main elements of NC machine tool. [AU May/June 2014]

1. Methodology of manufacture

2. The movement of machine tools

3. What tool is to be used?

4. At what speed?

5. At what feed?

6. To move from which point to which point in what path?

5.4 List out the demerits of NC machine and its applications.

Demerits:

1. Relatively high prize

2. More complicated maintenance, a special crew is desirable

3. A highly skilled and properly trained programmer is needed

4. Not suitable for long run applications

5. Machines have to be installed in A/C place


Applications

1. Aero equipments

2. Printed Circuit Board (PCB)

3. Coil winding

4. Automobile parts

5. Drawing and blueprints of complex shapes.

5.5 Define continuous path control. What is contouring system?A continuous path control system continuously monitors the positions

attained by a machine tool moving about its axes, and ensures that a desiredtool path involving two or more axis is obtained.

5.6 How will you classify the CNC machines based on Axis?CNC machines classification based on axis.

Point-to-point (positioning) system (P-Type)

Straight cutting CNC (L-Type)

Continuous path CNC system (C-Type)

5.7 Name some popular CNC systems. FANUCO - FANUC INDIA, Bangalore

HINUMERIC - HMT INDIA, Bangalore

ALLEN-BRADLEY - LAKSHMI ELECTRICCONTROLS, Coimbatore

BOSCH

CINCINNATI

SINUMERIC

TOSHIBA

5.8 What are the advantages of CNC machines? [AU Apr/May 2015]

1. Accuracy is more and it is repeatable. i.e. accuracy is kept inall ranges of speeds and feeds.

2. Production time is less.

3. Complicated part can be manufactured

4. Highly skilled and experienced operator is not necessary.


5.9 What is meant by NC? How does it differ from conventionalmachines?“Numeric control is a method of automatically operating a machine

tool by means of numbers, letters and symbols that controls movementsthrough some form of an input medium”

It is different from conventional machines by the following points:

Conventional machine tools are operated by skilled and efficientoperators using elaborate dedicated tooling.

NC machines make the same part over and over in exactly thesame way with no human intervention to introduce the error.

5.10 Illustrate the absolute mode of co-ordinate system.This system, in which

all the measurements are takenfrom a fixed origin with

co-ordinates X 0, Y 0, isknown as absolute co-ordinatesystem.

Absolute mode system‘O’ defines or igin. In absolutemode, to move the tool from

O to point P1 whoseco-ordinates are 40 and 30,the command would be move40, 30. From P1 to move topoint P2 the command would be move 70, 80.

5.11 List out the merits of NC machine

MERITS

1. Smaller batches

2. Increased flexibility

3. Production of complex parts

4. Reduced setup time

5. Elimination of special jigs & fixtures

P1(40 ,30)

O (0,0)

P2(70 ,80)

30

80

40 70


6. Machine utilization

7. Machining accuracy

8. Reduced inspection

5.12 Define circular interpolation and linear interpolation.The movement of the

tool along the circular path iscalled circular interpolation.Circular interpolation needsthe knowledge of co-ordinatesof the end points and centerof the arc or the radius andthe center of the circle.Circular interpolation may beeither clockwise (G02) or anticlockwise (G03).

Linear interpolation.

The interpolation of astraight line, which is possible by knowing the co-ordinates of its extremepoints, is known as linear interpolation. This is used for machining tapercuts, etc.

The movement of the tool in a straight line with any orientation in aplane is called linear interpolation, performed by using G01 code

5.13 What is the use of drawing tool path diagram while using manualpart programming?Drawing tool path diagram can be used to generate the co-ordinates,

which is required at the time of writing manual part programming.Determination of the tool path co-ordinates is an important part of NCprogramming and is a task, which involves numerous repetitive calculations,many of which may be quite complex.

5.14 How can one identify a CNC Machine?CNC machines can be identified by visualizing the MCU and major

elements of machines which are mounted on the machine tool or may bebuilt in the casing of the machine, feed back system, tool magazine, manualcontrol panel, etc in the machine tool.

P1

O (0 ,0)

P2

x 2x1

y 1

y2


5.15 Name at least 7 types of CNC machine. The most common CNC machine tools are:

Machining centers

Lathe machines

Drilling machines

Turning centers

Milling machines

CNC grinding machines, CNC gear hobbing & gear shaping

5.16 Define positioning systems.The machining operation is done after the tool has taken a particular

position with respect to position of the work or vice versa, i.e. movement ofslides. Hence, it is required that the tool reaches the particular fixed point inthe shortest time span and with shortest path. Due to this reason PTPs arealso called positioning type systems.

5.17 Define subroutine. [AU May/June 2013]

When a repetitive drilling (or) any machining operation has to bedone in different places, the subroutine is used to reduce the effortof writing a detailed program for each machining operation.

The subroutine program will be stored in the memory as a separateprogram so that it can be called by the main program wheneverneeded.

When the last block in the subroutine (M17) is executed, thecontrol will return automatically to the main program.

The subroutine is usually placed at the end of the main program.

5.18 Specify the limitations of Open Loop Control System?Major drawbacks of O.L.C.S are as follows:

Backlash errors may form due to wear and tear.

O.L.C.S cannot be used for high precision contouring systems.

Position and dimensional accuracies are to be maintained.

No feedback regarding the slide position.

O.L.C.S can be used with PTP drilling and laser beam cuttingmachines.


5.19 Draw a block diagram for a CNC machine.

3

2

M ach ine Tool

Feed backdevice -Z

Z

Z -axisdrive y

x

y

Leadscrewy-axis

drive

1

4

Pow er

supp ly

1 .Desired d irection, position , feed (axes velocity) and auxilia ry functions.2 .Axes feed (ve locity )and stepp ing com m ands.3 .Actual ve locity and position inform ation.4 .Sta rting and te rm ination s igna ls o f da ta reading

Computer Numerical Control System

-x

-y

-y

Z

-Z


5.20 Complex geometry parts could be machined using CNC Machineseasily - Justify.The complex geometry jobs are machined by the modern machining

centers having simultaneous control of 3 or 4 linear and 2 or 3 rotarymovements as well as positioning capabilities.

5.21 Differentiate between straight line motion control system &continuous path motion control system.

Line motion control system:

Tool works along a straight line in the direction of a majorco-ordinate axis.

PTP system is named as L-type.

Continuous path motion control system:

The NC system in which the position of the table along with thespeed (velocity) and feed of the tool is under continuous control.

Contouring or C-type

5.22 List the nature of jobs, which are suitable for NC manufacturing. Aerospace equipments

Printed Circuit Board (PCB)

Coil winding

Automobile parts

Drawing and blue prints of complex shapes

5.23 How does CNC increase the precision of a machine tool? Minimum deflection and distortion of the structural elements due

to cutting forces and thermal effects.

Least deflection and inaccuracies in the main spindle.

Accurate indexing of tools and work pieces

Low vibration

Good response to the movement given to the slides.


5.24 Define Interpolation.The calculation of successive increments in slide position to reach the

programmable point is called INTERPOLATION. Common methods ofinterpolation are linear, circular, polar and cylindrical.

5.25 Sketch axis co-ordinate system in a CNC lathe.

5.26 State the classification of CNC systems.

1. According to control system features

(a) Point to point positional control [P]

(b) Curved path or contouring system [C]

(c) Straight line [L]

2. According to feed back control system

(a) Open loop feed back control system

(b) Closed loop feed back control system

3. According to structure of controller

(a) Hardware based NC

(b) Computer based NC

+C-Z

+X

-X

+Z

+W

+U

+Z-Z

+x

-X

Coordinate System for CNC Lathe


4. According to programming method

(a) Absolute

(b) Incremental

5.27 Define: “Resolution” of a CNC system.The resolution of an NC or CNC system is a feature determined by

the designer of the control unit and is mainly dependent on the positionfeedback sensor. One has to distinguish between the programming resolutionand the control resolution. The programming resolution is the smallestallowable position increment in part programs and is referred to as the BLU,which might be of the order of 0.01 mm in typical machine tool system.

The control resolution is the smallest change in position that thefeedback device can sense.

5.28 What is the reason for using ball screw mechanism for table drive? In a ball - screw, the load between the threads of the screw and

the nut is not transmitted by direct contact, but throughintermediate rolling members (spherical balls).

Low co-efficient of friction.

Higher transmission efficiency.

Stick Slip Phenomenon is absent.

The accuracy of ball - screw is high.

5.29 Sketch the axis Co-ordinate system in a horizontal machining center.

+Y

+X

+Z

-X

-Z

-Y


5.30 Why do we go in for pneumatic clamping for CNC machines?Pneumatic clamping is done to reduce clamping / unclamping time and

to facilitate quick loading and unloading as per the component and it is alsofool-proof (i.e;) it ensures that the component cannot be loaded wrongly.

5.31 Why do we use stepper motor for axis drive? [AU May/June 2016]

Stepper motors can be used as the drive devices in open loop NCsystems. Since no feedback element is required, the system is cheaper thanits closed-loop counterpart in which NC is controlled to move very smallrotary steps in either single or multi-steps.

5.32 What is meant by encoder? Where it is used in CNC?An encoder is a rotary transducer (numerical device) that provides a

serial or parallel digital value of an angular or linear movement. The valuemay be absolute or incremental.

Types: 1. Absolute Encoders

2. Pulse Generators.

It is used in the CNC at the end of the slide screw directly or throughgearing and timer belt.

5.33 What is the role of optical grating in CNC drive?An optical grating is a strip of glass marked with a series of equally

spaced parallel lines; the lines and spaces are of equal width. They are usedas transducers on machine tools. This is used to measure the position of thetable. They are fitted on the end of lead screw, which moves the carriages.

5.34 Why do we go in for pneumatic chuck in a CNC Lathe?Reason for using pneumatic chucks on CNC lathes:

High speed clamping is possible.

Easy clamping to enable quick loading and unloading of workpiece.

Gives sufficient clamping force for use of full roughing cuts.

Swarf removal also can be done in the recess of the chuck areasduring loading and unloading duration.

Easy accessibility and ensures proper alignment.


5.35 Name the two important type of feedback devices used in closed loopsystems. Velocity feedback-tacho generators, digital operation.

Positional feedback-inductosyn, encoders.

Digital incremental, absolute measuring devices.

Analog measuring devices.

5.36 The productivity of CNC machines is relatively higher than that ofconventional machines - State the reasons for the same.The given statement is absolutely true, because CNC machines can

facilitate the following improvements in production:

Increasing the MRR (Material Removal Rate) by increasing thespindle speed, feeds and built in power and rigidity.

Reducing the time for tool change

Reducing the time for setting up a new part

Reduces the inspection time

Cycle time can be kept low with the following:

Rigid machine tools.

High spindle power at different speeds.

Rigid main spindle.

Maximum feed force, and less idle time.

5.37 List any three important measures employed in CNC machines tominimize structural deformation.Structural deformation occurs due to force acting while machining and

due to heat effect.

To avoid this structural deformation, following precautions have to beconsidered.

Providing a proper design mild steel structure having higherstiffness.

Use of ribs, braces and plates to increase the stiffness ofmachine.

Thermo-Symmetrical design of parts.


Providing large heat removing surfaces.

Use of excellent coolants.

Reduction of ambient temperature by using air conditioningunits.

5.38 Specify the advantage of slant bed design in CNC lathe?The machine bed is made of slant, generally 20 to the vertical as per

the latest trend. The main advantage of this geometry is:

1. The free flow of swarf, chip and coolant

2. Easy swarf removal.

3. More ergonomically acceptable.

4. Changing of tools is not a problem

5.39 What are the different types of machine tool bed materials?Machine tool bed materials should be selected to withstand the

damping force during machining operations. Here some materials aresuggested to match the requirements:

Cast Iron Structures.

Concrete mixed with other synthetic material Structures.

Welded Frame Works.

Epoxy Concrete.

5.40 What is stick-slip phenomenon in machine component motion?During starting of the slide, more force should be given to overcome

the frictional force. This more force will deform the lead screw elastically.The energy thus stored in the screw, together with the applied force makesthe slide to slip and move at faster rate than intended. This event may repeatitself and cause errors in positioning and consequently in a jerky motion.This phenomenon is known as stick slip phenomenon.

5.41. What is a transducer? Why is it used?A transducer is a device that converts variations of one physical

variable into another physical variable. They are used for monitoring theposition of a carriage/slide on a slide-way.

Measuring speed of rotation of spindle.


Measuring temperature of the tip of a tool.

Monitoring the power being transmitted by a shaft.

Measuring the flow of oil or cutting fluid.

5.42 What is meant by Resolver? State its purpose in a NC machine?Resolver is a feedback analog device where output is converted to a

digital form, which measures indirectly the position of the slide by measuring

the angular rotation of the screw. It posseses two windings at exactly 90 toeach other and resolves a voltage into two components at a phase difference

of 90

Purpose: The desired position of a machine tool slide is expressed as alinear dimension i.e., as a co-ordinate in digital form. This co-ordinate valueis recorded in binary code on punched tape and is used as the input data forthe machine control unit in NC systems.

5.43 How CNC reduces wear of the input devices having moving parts?CNC reduces wear by operating the devices like recirculating ball

screw, hydraulic drives for spindles, stepper motor / servomotor, feedbackdevices on closed loop systems and bearings etc.,

5.44 State the common features of CNC machines. Guide ways.

High Powered DC Spindle Drives.

Fast Response DC Axis Drives.

Tool Changer, Centralized Lubrication System, Chip Conveyers

Programming features

Readout displays

Diagnostics

5.45 List out the sequence of processing steps performed on the CNCmachine.1. Preparation of CNC co-ordinate drawing.

2. Process Planning.

3. Part Programming.

Manual Part Programing


Computer - Assisted Part Programming.

4. Tape Preparation (or) Pen drive - CD Preparation.

5. Tape Verification (or) Pen drive - CD Checking.

6. Production.

5.46 What is the concept of Cutter Compensation?Cutter diameter compensation can only be used when milling around

the outside of the work or inside a pocket in the work; it cannot be usedfor drilling operations or when milling slots with cutters of desired size. Whenwriting a part program to control path of the center of the cutter for millingoperations, it is assumed that the diameter of the milling cutter used will beparticular size. However, it is possible that owing to regrinding, the diameterof the tool will be different from assumed size and consequently the workproduced will be larger than that of required. Instead of having to reprogramthe complete path as the center of the cutter, to suit the operation, it ispossible to adjust the relative position of the cutter using cutter diametercompensation.

5.47 State the functions of G and M codes. [AU May/June 2016]

G00 Rapid transverse (Point-Point positioning)

G03 Circular interpolation - anticlockwise

M03 Start spindle rotation is clockwise.

M00 Program setup

5.48 Difference between CNC lathe and Turning Center.

CNC Lathe Turning Center

Axisymmetric geometry can beproduced

Axisymmetric as well as prismaticparts can be produced.

These are generally machined with2 axis control (Z-axis parallel tospindle & X-axis perpendicular tospindle axis.)

Addition to the X and Z-axis, CNCcontrol of the spindle rotation i.e.,C-Axis is used. Such machines areknown as turning centers.


CNC Lathe Turning Center

Tools are mounted in indexableturrets, which can hold 8, 12 or 16tools.

Two independent turrets slides aremounted for internal and external tools.

Linear tooling system. Consists of Automatic Tool Changer,Tool Clamping arrangement, Gantryloading - unloading devices, Postprocess metrology, automatic offsetcorrection and chuck changer etc.,,

5.49 Write a format of NC program?The general format is given as follows:

N G X Y Z A B C F S T M

Where

N Sequence number of the instruction.

G Preparatory function referring to a particularmachine activity.

XYZABC Co-ordinates and angular data as required.

F Feed

S Spindle Speed

T Tool code to select the tool

M Miscellaneous function as non-machiningoperations

5.50 What are the parts of a CNC program?

A CNC program consists of the following elements:

1. Type of dimensiioning system.

2. Axis designation.

3. NC Words.

4. Standard G & M Codes.

5. Tape Programming Format.

6. Machine tools zero point setting.


5.51 Define tool nose radius Compensation (TNRC).Tool moose radius compensation is only applicable to tools used on

turning centers. When movement has to take place simultaneously on bothx and z axis, such as when machining chamfers, angles or turning curves. Itis necessary to make allowance for tip-radius. The allowance is referred astool nose radius compensation.

5.52 Give the word address format and Tab sequential format for a CNClathe.Word Address Format:

N070, G81, X5764, Y04750, F475, S1000, T05, M08 <EOB>

Tab Sequential Format:

>070>81 >05764 > 04750 > 475 > 05 >08.

5.53 What are disadvantages of manual part programming?[AU Apr/May 2018]

(i) Error may occur in program

(ii) Non-optimal speed and feed

(iii) In flexible controller

5.54 What do you meant by “Canned Cycle”?[AU May/June 2014][AU Apr/May 2010]

Canned Cycle

The use of canned cycles reduces the programming length required toperform certain operations.

The cycles save the entering of at least four blocks in the programsand can form part of a loop or sub-venture.

5.55 Why do we go in for tool path graphic (TPG) mode?Tool path graphics mode helps the user to on-line checking of the part

program without running the machine. The program interprets the partprogram in question and displays the path, which would be taken by the toolduring actual operation. The path can be displayed on the CRT of theoperator’s panel or it may be drawn by a plotter interfaced with the CNCsystem.


5.56 How to codify the tools in machining centers?In part program the tools are codified by using T-wood. (ex. T01,

T02).

Each tool in machining center is identified by some form of codingdevice that can be recognized by the selector mechanism. The common codingsystems for tool identification in ATC’s are based either on the use of groupsof rings on the tool holder, or on code keys related to a definite tool.

5.57 Name atleast 5 commonly used CAM packages?1. PRO/Manufacturing

2. I-DEAS generative Manufacturing

3. UG - Manufacturing

4. Surf CAM

5. Virtual Gibbs

6. Master CAM

5.58 Differentiate the miscellaneous codes & preparatory function codes.

Miscellaneous Codes Preparatory Codes

Majority of these functions areusually operative after the motionstatements programmed in the sameblock have been executed.

Intended to be operative before anydimension instructions programmedint he same block are executed.

Referred as M Codes. Referred as G Codes.

5.59 List different types of tape format in NC systems.1. Fixed Block Format.

2. Tab Sequential Format.

3. Word Address Format.

5.60 Give four types of APT Part program statements.[AU Apr/May 2015]

1. Geometry Statements:

Also known as definition statements and are used to define geometricelements like point, line, circle, arc, plane, etc.


2. Motion Statements:

Used to define cutter path.

3. Post Processor Statements:

Applicable to specific machine tools and are used to define machiningparameters like feed, speed, coolant angles etc.

4. Auxiliary Statements

These are miscellaneous statements used to identify the part, tools,tolerance, etc.

5.61 What are the advantages of using APT Language? It is the most powerful and most extensively used programming

languages.

It has wide range of applications.

It can be used for both positioning and continuous-pathprogramming.

It is a three dimensional system that can be used to control allaxes.

APT can be used to control a variety of different machiningoperations.

5.62 What is an ATC?The Automatic Tool Changer is known as ATC. The concept of the

ATC is that a range of tool shall be available for automatic selection andpositioning relative to the job for machining to take place. It is generallyconceded that there shall be sufficient tools in all ranges on call to completeall machining operations needed for a single setting of the work piece.

Types of ATC:

1. Drum type magazine.

2. Chain type magazine.

3. Egg - box magazine.


5.63 Distinguish Mechanisation and Automation. [AU Apr/May 2017]

Mechanism

Mechanisms are rigid bodies connected by joints in order to accomplisha desired force or motion transmission.

Automation

Use of various control systems for operating equipments such asmachinery, processes in factories with minimal or reduced human interventionor with some process that is completely automated.

5.64 What is the need for micromachining? Mention the four categoriesof micromachining techniques. [AU Apr/May 2017]

Micromachining

In micromachining designer has control over the selection of materialsand the shapes of parts.

Categories of micromachining

Bulk micromachining

Surface micromachining

Micro-molding processes

Non-lithography

5.65 Define micromachining. [AU May/June 2016]

Micromachining is finishing of final work piece. In this process thesurface micro irregularities will be removed by machining.

5.66 What are the challanges in wafer machining? [AU Apr/May 2018]

Sorting silicon wafer consumes time.

Only skilled labour can operate photolithograph.

Making mask is difficult.


Documents

TECHNOLOGY - IIairwalkbooks.com/images/pdf/pdf_102_1.pdf(a) Single point cutting tools. (b) Multi point cutting tools. A single point cutting tool has a wedge like action and are used