11

Machining and Machine toolsPresented By,

Alakshendra Pratap Singh

Asst. Prof.,

Mechanical Engineering Department,

Jodhpur Institute of Engineering and Technology,

jodhpur, Rajasthan

22

Cutting Fluid and Lubricants:• Before discovery of HSS, cutting was carried out at low speed and water served as coolant.

Function of the Cutting Fluid:• Cools the tool and w/p.

• Provides lubrication.

• Flush away the chips.

Advantages of using cutting fluid:• Improves tool life.

• Permits use of higher cutting speeds and larger metal removal rates.

• Lubrication helps reduce coefficient of friction and thus reduces cutting force.

• Improves surface finish by protecting newly generated surface from oxidation and corrosion.

• Reduces formation of built up edge.

33

Requirements of cutting fluid:• Should have good wetting characteristics.

• Should not gum moving parts .

• Should have good anti wear properties.

• Should not foam easily.

• Should not deteriorate in storage.

• Should have high specific heat, high thermal conductivity and high heat vaporization.

• Should be odorless and transparent.

• Should have high flash point.

• Should prevent rusting.

•Should have low viscosity.

• Should be non poisonous.

• Should have low cost, and easily available.

44

Types of cutting fluid:Gaseous Fluids:

• Applied through nozzle.

• Have poor cooling, poor lubrication poor anti seizure properties.

• Simple air or sub zero cooled air may be used.

• Other gases which can be used are CO2, Argon and oil mist.

Liquid Type:

Water:• Plain water and NaOH.

• Water containing alkali or antiseptic.

• Free availability and cheapness are its advantages.

• Has high conductivity and specific heat.

Disadvantages:

• Can cause rusting.

• Has high surface tension thus does not spread over.

55

Oil based cutting fluids:• Also referred as straight oil.

• May be mineral oil or fatty oils.

• Straight mineral oils possess lower viscosity and have good wetting and penetrating power. They are fit for light duty operations on non ferrous metals.

• Fatty oils are used for heavy duty machining work. Because of their tendency to decay and breed bacteria they are not preferred.

• Blended Fatty – Mineral oil: are obtained by mixing 10 to 30 % of fatty oil with mineral oil of different viscosity. Its advantages are

1. Cost of fatty oil reduced.

2. Improved surface finish obtained with non ferrous alloys and steel.

3. Penetrating and wetting characteristics improved.

4. Little amount of fat is sufficient to act as lubricant.

5. Blended oil do not have any undesirable effects.

• Sulphurised Blend of Fatty Mineral Oils: Sulfur increases cooling and lubricating characteristics by combining with fatty oils. These are used majorly on multi spindle automats, where they act as hydraulic fluid also along with their action of cooling an lubricating.

66

Water miscible cutting fluids:

Emulsions of soluble oils:• Soluble oil may be mineral or fatty oil containing emulsifier like soap.• Emulsions are product which when mixed with water forms milky colloidal solution.• Emulsions possess good cooling characteristics due to abundance of water in them.• For higher MRR this type of cutting fluid should be used.

Chemical Coolants:• Pure coolants: contain no lubricant and are made of water softeners and rust inhibitors.

• Coolants: have mild lubricity, contain water softeners, rust inhibitors and wetting agents

• Lubricating Coolants: contain water softeners, rust inhibitors, wetting agents and chemical lubricants and Cl, S, or P additives.

• They give high MRR, last longer, do not clog pipes

• They have inhibitors like Triethanol amine and NaNO3 for rust prevention and Sodium Molybdate for corrosion prevention.

• Phosphates and Borates for water softening.

• Soap and wetting agents for lubrication and reducing surface tension.

•Glycols for blending and germicides for controlling bacterial growth.

7

Economics of Metal Cutting:• Objective is to produce a component of required dimensions and surface finish at the minimum possible cost.

• Parameters governing cost are speed, feed, depth of cut, tool and work material, tool geometry and cutting edge.

•These constraints are pulling each other in opposite direction.

•Thus an optimized condition b/w these constraints is to be attained.

Machining Cost:1. Non Productive Cost (Cn)

2. Cutting Cost (Cc)

3. Tool Cost (Ct)

88Variation of cost and time per piece with cutting speed

99

Non Productive Time (Tn) Components:

1. Initial set up time (Ts min) (Occurring once per batch)2. Loading and unloading time (Tl min) (Occurring once per piece)3. Tool advance & withdrawal time (Ta min) (Occurring once per piece)4. Tool removing and replacing time (Tr min) (Occurring once per tool

regrind)5. Idle time (Ti min)

Let ‘Nb’ be the number of components in a batch.

‘Ng’ be the number of components produced b/w each tool regrind; then Tn will be:

Tn = Ts + (Tl +Ta)*Nb + Tr(Nb/Ng) + Ti

Now let Cr be the cost including :

1. Labor cost (cl)2. Overhead cost rate (co)3. Depreciation cost (cd); Thus Cr becomes:

Cr = cl + co + cd

And non productive cost Cn will be:Cn = Cr*Tn

1010

Cutting Cost (Cc):It can be determined by multiplying total cutting time with cost rate Cr. Thus;

Cc = Cr*Tc*Nb

Where Tc is cutting cost per component.

Tool Cost (Ct):Includes initial tool cost Ci and tool regrinding cost Cg. Thus;

Ct = (Ci/Rg +Cg)*Nb/Ng

Here Rg is number of regrinds possible.

Concluding, machine cost per batch (Cb) will be;

Cb = Cn + Cc + Ct

And machining cost per piece (Cp) will be;

Cp = Cb/Nb

11

Machine Tools

A machine tool is a power driven device where energy is utilized to deform material to attain required shape size or to process a product to desired accuracy by removing excess material.

12

Lathe• Oldest known machine tool.

• Henry Mauldsley developed sliding carriage and screw cutting lathe in 1800 AD

• It is a general purpose machine tool used in production an repair work since it permits large variety of operations on it.

Working Principle of Lathe:• Lathe removes undesired material from a rotating w/p in from of chips with the help of a tool of harder material than the w/p, traversed either across or deep in the w/p.

• W/p should be held securely & rigidly on the machine tool.

• It is principally used to produce cylindrical surfaces and plan surfaces, at right angle to axis of rotation.

Specification of a Lathe: Lathe is generally designated by:

• Swing: Largest work dia that can be swung over the bed.

• Distance b/w head and tail stock centre.

• Sometimes by the swing and length of the bed. Sometimes by the maximum dia of bar which can be accommodated for Bar automatic Lathe..

13

14

Types of Lathe:1.Speed Lathe: • Referred so owing to its high speed of headstock spindle.

• Has headstock, tailstock and a tool post, no gear box or lead screw arrangement and carriage.

• Cone pulley used for speed variation.

• Used in wood turning, metal spinning and polishing operations.

2.Engine or Centre Lathe:• Referred so because earlier they were driven by separate engine or from

central engine with overhead belts and shafts.

• Stepped cone pulley or geared head are used for varying speed of lathe spindle.

• Tailstock is available for holding work b/w centers and accommodates tools drills, taps etc.

• Cutting tools can be controlled either by hand or power.

• A carriage and tool post for supporting tool and feeding tool in cross and longitudinal directions with reference to lathe axis is used.

• Permits use of wide range of attachments.

15

3.Turret Lathe: • Production M/c used to perform large number of operations

simultaneously.

• Usually accommodates 6 tools for different operations, which are used by indexing the turret.

4.Capstan Lathe:• Similar to Turret Lathe & incorporate side moving on auxiliary slide & can

be clamped in any position.

• Used for fast production of small parts owing to its light weight & short stroke.

5.Tool Room Lathe:

• Modern engine lathe equipped with necessary accessories for accurate tool room work.

• It is geared head driven m/c with good range in spindle speeds and feeds.

• Used for production of small tools, dies, gauges, etc.

6.Gap Bed Lathe:• A gap on bed near headstock to accommodate jobs with flanges exist.• A removable part is provided in bed, which can be removed or inserted.

7. Bench Lathe 8. Hollow spindle Lathe. 9. Vertical Turret Lathe.

16

Construction of Lathe Parts:

1.The Bed:• Its upper surface is either scraped or ground & guiding & sliding surfaces are

provided.• It has two heavy metal slides running lengthwise, with ways or V’s formed on them• It is supported on two broad box section columns and is made of cast iron.• The inner guide ways support the tailstock and external ones support saddle.• Headstock is permanently fixed to the bed• Carriage can be traversed to & b/w headstock & tailstock either manually or by the

power.• Lathe bed is made of high grade special cast iron having high vibration damping

qualities.• Top surface of bed is machined accurately.• While designing lathe bed due consideration to factors like rigidity, alignment and

accuracy should be given.

17

2.Headstock:• Supports main spindle in the bearings and aligns it property.• It incorporates necessary transmission mechanism with speed changing

levers.• Live centre is rigidly held by taper in main hollow spindle.• Centerline of headstock is parallel to guide ways in horizontal and vertical

plane.• Headstock also have self contained clutch and brake mechanism.

3.Main Spindle:• It is a hollow cylindrical shaft such that long slender jobs can pass through it.• Spindle end facing tailstock is called spindle nose.• Spindle nose has a Morse taper hole for self locking and threads on outside.• Morse taper accommodates collet chuck and threaded portion holds faceplate.• While designing Morse taper due consideration to cutting tool thrust should be

given.• Anti friction bearing is used in headstock & spindle.• Spindle is made up of high tensile steel, suitably hardened and tempered and is

supported in roller bearing.

18

4.Tailstock:

5.Carriage: • It is located b/w headstock and tailstock and fitted on the bed.

• Can be locked on any position on the bed.

• Can be moved manually or by power.

• Consists of saddle (lower part of compound rest) & apron and slides over guide ways.

• It has form of letter ‘H’ and carries cross slide, compound rest and tool post.

• Provides three movements to the tool

1. Longitudinal Feed: Through carriage movement.

2. Cross Feed: Through cross slide movement.

3. Angular Speed: Through top slide movement.

6.Saddle:• Made of ‘H’ Shaped casting having “V’ and flat guide on one side for

mounting on lathe bed guide ways.

• Other side of saddle has male dove tail to accommodate cross slide with a jib.

19

7.Compound Rest:• Supports tool post and cutting tool. • Can be swiveled on cross slide to any right angle in horizontal plane.

9.Tool Post:• Holds various cutting tools• Holder body rests on wedge which fits into a

concave shaped ring having rocker.• This permits height of cutting edge to be

adjusted..

Tool Post

8.Cross slide:• Has a female dovetail on one side and assembled

on top of saddle.• Top surface of cross slide is provided with ‘T’

slots to enable fixing of rear tool post or coolant attachment.

20

21

Operations on Lathe:• Cylindrical and Conical Jobs.• Flat Surfaces.• Grooving• Drilling and Reaming.

• Counter sinking and counter boring.• Knurling, parting, chamfering.• Thread cutting.• Milling, slotting, grinding Etc.

22

Turret and Capstan Lathe:•The turret lathe is a form of metalworking lathe that is used for repetitive production of duplicate parts,.• These parts by the nature of their cutting process are usually interchangeable. • It evolved from earlier lathes with the addition of the turret.• Turret is an index able tool holder that allows multiple cutting operations to be performed, each with a different cutting tool, in easy, rapid succession.• With no need for the operator to perform setup tasks in between, such as installing or uninstalling tools, nor to control the tool path. •The latter is due to the tool path being controlled by the machine, either in jig-like fashion, via the mechanical limits placed on it by the turret's slide and stops, or via electronically-directed servomechanisms for computer numerical control (CNC) lathes.

23

Schematic configuration of capstan lathe.

24

Differences between capstan and turret lathes:

• Turret lathes are relatively more robust and heavy duty machines • Capstan lathes generally deal with short or long rod type blanks held in collet, whereas turret lathes mostly work on chucking type jobs held in the quick acting chucks • In capstan lathe, the turret travels with limited stroke length within a saddle type guide block, called auxiliary bed, which is clamped on the main bed as indicated in Figure, whereas in turret lathe, the heavy turret being mounted on the saddle which directly slides with larger stroke length on the main bed as indicated in Figure. • One additional guide rod or pilot bar is provided on the headstock of the turret lathes as shown in Figure, to ensure rigid axial travel of the turret head • External screw threads are cut in capstan lathe, if required, using a self opening die being mounted in one face of the turret, whereas in turret lathes external threads are generally cut, if required, by a single point or multipoint chasing tool being mounted on the front slide and moved by a short leadscrew and a swing type half nut.

25

Schematic configuration of turret lathe.

26

Tool layout:

• Schematically showing the type and configuration of cutting tools and their location and mounting.

• To draw tool layout for hexagonal headed mild steel bolt (below drawing).

• Hot rolled hexagonal mild steel bar of standard size is selected.

27

Elementary machining operations identified as follows:Facing Centering Front Chamfering (1)Chamfering bolt head (3) Drilling Grooving (forming)Rough turning (1) – to make the bar circular from hexagon Rough turning (2) – to reduce diameter to 12 mmFinish turning – to φ10 mmThread cutting Initial parting Parting

Listed elementary operations can be combined and sequenced as follows:

1. Rough turning (1), Initial parting, Chamfering (3).2. Rough turning (2), drilling and centering for the next job.3. Finish turning.4. Spot facing and front chamfering.5. Grooving and centre chamfering.6. Thread cutting.7. Parting.

28

S. S. NoNo OperationOperation ToolTool

Tool Tool PositioPositio

nnNN SS LL CFCF

1. Stop stock & bar feed

Stop HT (1) - - - - N

2. Rough Turning (1)Initial partingChamfering

Turning ToolFormed

Parting tool

HT (2) 640 0.10

0.05

306

YY

3. Rough parting (2) Drilling (φ6),

centering

Turning ToolDrill bit

HT (3) 640 0.10

50 Y

4. Finish turning Turning tool HT (4) 640 0.05

25 Y

5. Spot facingChamfering (1)

Compound tool

HT(5) 640 0.05

5 Y

6. GroovingChamfering (2)

Forming tool FS 640 0.05

10 Y

29

7. Threading Threading Die

HT (6) 56 2 20 Y

8. Parting Parting Tool VS 640 0.05 12 Y

N – spindle speed; S – Feed; L – Tool Travel; CF – Cutting Fluid; HT – Hexagonal Turret; RS – Rear Slide; FS - Front Slide; VS – Vertical Slide;

30

Shaper:

• A shaper is a type of machine tool that uses linear relative motion between the work piece and a single-point cutting tool to machine a linear tool path

•Uses SPCT to machine flat or plane surfaces in hz, vertical and angular plane.

• Ram imparts reciprocating motion to the tool with the help of the ram on the shaper head, while w/p is fixed on the table vice.

31

Principal Parts of Shaper:1.Power Transmission:• An electric motor with V belt drive is used to transmit power to the

machine.

• A gear train is used to provide different speed.

• Speed change lever is used to shift he gears.

2. Ram:• This is the reciprocating member which carries shaper head in front.

• Cutting tool is mounted on this shaper head, which provides it vertical as well as angular movement.

• Ram slides on accurately machined guide ways on the top of the column.

• Gets drive from QRM.

3. Shaper Head:• It is clamped in front of the ram.

• Consists of tool slide, tool post and clapper box.

• Can be swiveled to any angle for getting angular cuts on the job.

• Vertical tool feed gives tool the vertical feed required.

32

4. Column:• Is rigid hollow structure made up of CI.

• Gives support to ram and gets supported by the base.

• A cross rail is mounted on column which gives drive to the table.

• On of the walls is made hollow to enable viewing and maintaining the driving QRM.

5. Base:• Supports the complete machine.

• Bolted to the floor with the help of foundation bolts.

6. Cross Rail:• Is box type structure over which saddle slides horizontally.

7. Saddle and Table:• Table is bolted on table & is capable of moving in horizontal & vertical directions.

• Table is provided with T slots for clamping fixtures.

33

Working of Shaping machine

34

The Quick Return Mechanism:

A

BB1

O

CC1

β

α

35

• Cutting operation is only in the forward stroke.

• While the return stroke is idle.

• Power is consumed in both the strokes.

•Thus it becomes necessary to reduce the non cutting or return stroke to save power and time.

• QRM enables this.

• A crank slotter mechanism is used to achieve this.

• AB is the crank rotating at the center A.

• The slotter B has both rotating (about centre A) and sliding motion (along O-C).

• Link CO, is fixed at O and can rotate about it.

• Link Co has slot in which slotter B can slide freely.

• On rotation of crank AB about A, link CO oscillates about O b/w extreme OC1 and OC2.

• Oscillation motion of CO makes ram to reciprocate through link CD.

• Ram stroke length is proportional to crank length AB.

• Forward motion is from AB to AB1 and return stroke from AB1 to AB during its anti clockwise motion.

•Ratio of forward to return stroke is ratio of angle α to angle β, usually 3:2.

36

BROACHING

37

38

BASIC PRINCIPLES OF BROACHINING…….

Broaching is a machining process for removal of a layer of material of desired width and depth usually in one stroke by a slender rod or bar type cutter having a series of cutting edges with gradually increased protrusion as indicated in Fig. In shaping, attaining full depth requires a number of strokes to remove the material in thin layers step – by – step by gradually in feeding the single point tool Whereas, broaching enables remove the whole material in one stroke only by the gradually rising teeth of the cutter called broach. The amount of tooth rise between the successive teeth of the broach is equivalent to the in feed given in shaping.

SHAPING BROACHING

39

40

Construction And Operation Of Broaching

Construction of broaching tools….

•Configuration

•Material and

•Cutting edge geometry

41

Configuration of broaching tool

Both pull and push type broaches are made in the form of slender rods or bars of varying section having along its length one or more rows of cutting teeth with increasing height (and width occasionally). Push type broaches are subjected to compressive load and hence are made shorter in length to avoid buckling. •The general configuration of pull type broaches, which are widely used for enlarging and finishing preformed holes, is schematically shown in Fig.

42

43

• The essential elements of the broach (Fig.) are :

Pull end for engaging the broach in the machine. Neck of shorter diameter and length, where the broach is

allowed to fail, if at all, under overloading Front pilot for initial locating the broach in the hole Roughing and finishing teeth for metal removal Finishing and burnishing teeth Rear pilot and follower rest or retriever

44

Material of broach

•Being a cutting tool, broaches are also made of materials having the usual cutting tool material properties, i.e., high strength, hardness, toughness and good heat and wear resistance. •For ease of manufacture and re-sharpening the complex shape and cutting edges, broaches are mostly made of HSS (high speed steel). To enhance cutting speed, productivity and product quality, now-a-days cemented carbide segments (assembled) or replaceable inserts are also used specially.

45

Broaching tools

46

Broaching Machines

• Horizontal broaching machine • Horizontal broaching machines, typically shown in Fig.,

are the most versatile in application and performance and hence are most widely employed for various types of production. These are used for internal broaching but external broaching work are also possible. The horizontal broaching machines are usually hydraulically driven and occupies large floor space.

47

48

• Vertical broaching machine49

High production broaching machines

50

Unit-4

GRINDING

Grinding is the most common form of abrasive machining. It is a material cutting process which engages an abrasive tool whose cutting elements are grains of abrasive material known as grit. These grits are characterized by sharp cutting points, high hot hardness, chemical stability and wear resistance. The grits are held together by a suitable bonding material to give shape of an abrasive tool.

51

Cutting action of abrasive grains

52

Major advantages and applications of grinding

dimensional accuracy good surface finish good form and locational accuracy applicable to both hardened and unhardened material

53

Applications

• surface finishing

• slitting and parting

• stock removal (abrasive milling) finishing of flat as well as cylindrical surface

• grinding of tools and cutters and resharpening of the same.

54

Grinding wheel and work piece intraction

55

Grinding Machines

• Conventional grinding machines can be broadly classified as:

(a) Surface grinding machine

(b) Cylindrical grinding machine

(c) Internal grinding machine

(d) Tool and cutter grinding machine

56

Surface grinding machine:

• Horizontal spindle and reciprocating table

• Vertical spindle and reciprocating table

• Horizontal spindle and rotary table

• Vertical spindle and rotary table

57

Horizontal spindle reciprocating table grinder

• A: rotation of grinding wheel • B: reciprocation of worktable • C: transverse feed • D: down feed

58

A

CB

D

Vertical spindle reciprocating table grinder

59

Horizontal spindle rotary table grinder

60

Vertical spindle rotary table grinder

61

2.Cylindrical grinding machine

62

External centreless grinder

• This grinding machine is a production machine in which out side diameter of the workpiece is ground. The workpiece is not held between centres but by a work support blade. It is rotated by means of a regulating wheel and ground by the grinding wheel.

• In through-feed centreless grinding, the regulating wheel revolving at a much lower surface speed than grinding wheel controls the rotation and longitudinal motion of the workpiece. The regulating wheel is kept slightly inclined to the axis of the grinding wheel and the workpiece is fed longitudinally as shown in Fig

63

64

65

Grinding wheels

• Grinding wheel consists of hard abrasive grains called grits, which perform the cutting or material removal, held in the weak bonding matrix. A grinding wheel commonly identified by the type of the abrasive material used. The conventional wheels include aluminium oxide and silicon carbide wheels while diamond and cBN (cubic boron nitride) wheels fall in the category of superabrasive wheel.

66

67

Specification of grinding wheel

• A grinding wheel requires two types of specification

• (a) Geometrical specification

• (b) Compositional specification

68

1.Geometrical specification

• This is decided by the type of grinding machine and the grinding operation to be performed in the workpiece. This specification mainly includes wheel diameter, width and depth of rim and the bore diameter. The wheel diameter, for example can be as high as 400mm in high efficiency grinding or as small as less than 1mm in internal grinding.

69

Standard wheel configuration for conventional grinding wheels

70

2.Compositional specifications

1) the type of grit material

2) the grit size

3) the bond strength of the wheel, commonly known as wheel hardness

4) the structure of the wheel denoting the porosity i.e. the amount of inter grit spacing

5) the type of bond material

6) other than these parameters, the wheel manufacturer may add their own identification code prefixing or suffixing (or both) the standard code.

71

72

2.1 Marking system for conventional grinding wheel

• The standard marking system for conventional abrasive wheel can be as follows:

• 51 A 60 K 5 V 05, where

• The number ‘51’ is manufacturer’s identification number indicating exact kind of abrasive used.

• • The letter ‘A’ denotes that the type of abrasive is aluminium oxide. In case of silicon carbide the letter ‘C’ is used.

73

51 A 60 K 5 V 05•The number ‘60’ specifies the average grit size in inch mesh. For a very large size grit this number may be as small as 6 where as for a very fine grit the designated number may be as high as 600.

•The letter ‘K’ denotes the hardness of the wheel, which means the amount of force required to pull out a single bonded abrasive grit by bond fracture. The letter symbol can range between ‘A’ and ‘Z’, ‘A’ denoting the softest grade and ‘Z’ denoting the hardest one.

•The number ‘5’ denotes the structure or porosity of the wheel. This number can assume any value between 1 to 20, ‘1’ indicating high porosity and ‘20’ indicating low porosity.

•The letter code ‘V’ means that the bond material used is vitrified. The codes for other bond materials used in conventional abrasive wheels are B (resinoid), BF (resinoid reinforced), E(shellac), O(oxychloride), R(rubber), RF (rubber reinforced), S(silicate) •The number ‘05’ is a wheel manufacturer’s identifier. 74

Selection of grinding wheels

1. Type of abrasives

•Aluminium oxide•Silicon carbide•DiamondNatural diamond Mono crystalline diamond Polycrystalline diamond •cBN (cubic boron nitride)

75

2.Grit size•The grain size affects material removal rate and the surface quality of workpiece in grinding. •Large grit- big grinding capacity, rough workpiece surface •Fine grit- small grinding capacity, smooth workpiece surface

76

3.Grade •The worn out grit must pull out from the bond and make room for fresh sharp grit in order to avoid excessive rise of grinding force and temperature. Therefore, a soft grade should be chosen for grinding hard material. On the other hand, during grinding of low strength soft material grit does not wear out so quickly. Therefore, the grit can be held with strong bond so that premature grit dislodgement (forced removal) can be avoided

77

4.Structure / concentration

•The structure should be open for grinding wheels engaged in high material removal to provide chip accommodation space. The space between the grits also serves as pocket for holding grinding fluid. On the other hand dense structured wheels are used for longer wheel life, for holding precision forms and profiles.

78

5.Bond

•vitrified bond - high stock removal •Resin bond - heavy duty grinding •Shellac bond - fine finish of rolls •Oxy chloride bond - disc grinding operation •Rubber bond - wet cut-off operation •Metal bond - large stock removal & form accuracy •Electroplated bond - making small diameter wheel , abrasive milling •Brazed bond - very high material removal either with diamond or cBN wheel

79

80

vitrified bond Vitrified bond is suitable for high stock removal even at dry condition. It can also be safely used in wet grinding. It can not be used where mechanical impact or thermal variations are like to occur.

Resin bond Conventional abrasive resin bonded wheels are widely used for heavy duty grinding because of their ability to withstand shock load. This bond is also known for its vibration absorbing characteristics and finds its use with diamond and cBN in grinding of cemented carbide and steel respectively.

• Shellac bond

• At one time this bond was used for flexible cut off wheels. At present use of shellac bond is limited to grinding wheels engaged in fine finish of rolls.

• Oxychloride bond

• It is less common type bond, but still can be used in disc grinding operation. It is used under dry condition.

81

• Rubber bond • Its principal use is in thin wheels for wet cut-off

operation. Rubber bond was once popular for finish grinding on bearings and cutting tools.

• Metal bond• Metal bond is extensively used with superabrasive

wheels. Extremely high toughness of metal bonded wheels makes these very effective in those applications where form accuracy as well as large stock removal is desired.

82

• Electroplated bond• This bond allows large (30-40%) crystal exposure above

the bond without need of any truing or dressing. This bond is specially used for making small diameter wheel, form wheel and thin superabrasive wheels. Presently it is the only bond for making wheels for abrasive milling and ultra high speed grinding.

• Brazed bond • This is relatively a recent development, allows crystal

exposure as high 60-80%. In addition grit spacing can be precisely controlled. This bond is particularly suitable for very high material removal either with diamond or cBN wheel. The bond strength is much greater than provided by electroplated bond. This bond is expected to replace electroplated bond in many applications.

83

Truing and dressing of grinding wheel

• Truing • Truing is the act of regenerating the required geometry

on the grinding wheel, whether the geometry is a special form or flat profile. Therefore, truing produces the macro-geometry of the grinding wheel.

• Truing is also required on a new conventional wheel to ensure concentricity with specific mounting system. In practice the effective macro-geometry of a grinding wheel is of vital importance and accuracy of the finished workpiece is directly related to effective wheel geometry.

84

85

Dressing

• When the sharpness of grinding wheel becomes dull because of glazing and loading, dulled grains and chips are removed (crushed or fallen) with a proper dressing tool to make sharp cutting edges and simultaneously, make recesses for chips by properly extruding to grain cutting edges. Thus, these operations are for the dressing

• Dressing is the conditioning

of the wheel surface which

ensures that grit cutting edges are

exposed from the bond and

thus able to penetrate into

the work piece material.

86

• Dressing therefore produces micro-geometry. The structure of micro-geometry of grinding wheel determine its cutting ability with a wheel of given composition. Dressing can substantially influence the condition of the grinding tool.

• Truing and dressing are commonly combined into one operation for conventional abrasive grinding wheels, but are usually two distinctly separate operation for super abrasive wheel.

• Dressing of superabrasive wheel

• Dressing of the super abrasive wheel is commonly done with soft conventional abrasive vitrified stick, which relieves the bond without affecting the super abrasive grits.

87

finishing operations

88

• Honing• Honing is a finishing operation used to improve the form

tolerance of an internal cylindrical surface – in particular, it is used to improve the cylindricity. The honing tool is a metal bar holding a set of grinding stones arranged in a circular pattern. The tool brushes along the cylindrical part surface by rotating, and moving up-and-down along its axis. You can identify a honed surface by looking for the helical cross-hatched scratch marks on the part surface

• A honing stone is similar to a grinding wheel.• Any abrasive material may be used to create a honing

stone, but the most commonly used are corundum, silicon carbide, cubic boron nitride, or diamond.

89

Honing tool

90

• Honing tool

91

Honing machine

92

Lapping

• Lapping is a finishing operation. The lapping tool is made

of metal, leather, or cloth, impregnated with very fine abrasive particles. For preparing the surface of silicon wafers, lapping operations use a flat metal disc that rotates a small distance above the part. The gap is filled with a slurry containing fine abrasive grains. The rotation of the disc causes the slurry to flow relative to the part surface, resulting in very fine surface finish. This process gives dimensional tolerances of ≥ 0.5μm, and surface finish of up to 0.1 μm.

93

94

(a) Schematic illustration of the lapping process.

(b) Production lapping on flat surfaces.

(c) Production lapping on cylindrical surfaces.

95

Polishing and buffing• Polishing • Polishing is a finishing operation to improve the surface

finish by means of a polishing wheel made of fabrics or leather and rotating at high speed. The abrasive grains are glued to the outside periphery of the polishing wheel. Polishing operations are often accomplished manually

96

• Buffing

• It is a finishing operation similar to polishing, in which abrasive grains are not glued to the wheel but are contained in a buffing compound that is pressed into the outside surface of the buffing wheel while it rotates. As in polishing, the abrasive particles must be periodically replenished. As in polishing, buffing is usually done manually, although machines have been designed to perform the process automatically.

• Polishing is used to remove scratches and burrs and to smooth rough surfaces while butting is used to provide attractive surfaces with high luster.

97

98

Screw thread manufacturing

99

100

101

102

Thread cutting on lathe

103

104

105

106

107

108

109

110

111

• Tapping

• Die head

112