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Possible Interview Questions/Contents

From

Manufacturing Technology II

Compiled By: Mr. B. Ramesh, B.E.; M.E.; (Ph.D.) Asso. Prof/Mechanical, St. Joseph’s Institute of Technology, Chennai-600119

What is Machining?

Machining is a process designed to change the size, shape, and surface of a material through

removal of materials that could be achieved by straining the material to fracture or by thermal

evaporation.

What are the three fundamental machining parameters?

Cutting speed (V) is the largest of the relative velocities of cutting tool or workpiece. In turning

it is the speed of the workpiece while in drilling and milling, it is the speed of the cutting tool.

Depth of cut (d) is the distance the cutting tool penetrates into the workpiece.

Feed (f) is movement of the tool per revolution. In turning, it is the distance the tool travels in

one revolution of the workpiece and is given the units of mm/rev or in./rev.

What is Material Removal Rate (MRR)?

The volume of material removed per minute. In turning, MRR= Vfd.

Chip Formation :

Chip formation affects the surface finish, cutting forces, temperature, tool life and dimensional

tolerance. A chip consists of two sides 1) the side in contact with the tool is called shiny side

(flat, uniform) due to frictional effects, 2) the other side is the free workpiece surface that has a

jagged appearance due to shear.

Types of Chips:

Continuous chips: -Occurs in ductile materials

BUE (continuous):

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Built-up-edge (BUE) forms when there is a chemical affinity between workpiece and the

tool.

- Favorable growth conditions such as high strain-hardening, low speed, large depth of

cut, low rake angle, and high temperature

- Degrades the surface finish, changes tool geometry.

Discontinuous chips: Occurs in brittle materials

Because of the discontinuous nature of the chips, forces vary continually leading to

vibrations and chatter in the machine tool with the end results of poor surface finish and loose

tolerances.

Serrated chips: Semicontinuous with zones of high and low shear strains

Occurs in metals where strength decreases sharply with temperature. Example: Titanium.

Thrust force causes deflection of the tool and reduces the depth of cut and affect tolerances. The

machine tool and tool holder must be stiff enough to withstand Ft.v

Roughness -- closely spaced , irregular deviations

Waviness -- greater spacing deviations caused by the deflections of tools, dies, thermal warping,

uneven lubrication, vibrations etc

Flaws -- scratches, holes, cracks, depressions, inclusions

Lay - direction of the predominant surface pattern

Measures of Surface Roughness

1. Arithmetic average (AA) - Widely adopted

2. Root mean square (RMS) - Used mostly prior to 1950

3. Roughness height (Peak-to-valley distance)

Variables that influence the roughness are:

BUE - more damaging effect on roughness

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Tool radius- sharper the tool, higher would be roughness

Feed - larger the feed, higher is roughness

Vibration/chatter - increase the roughness

Machinability

Machinability is a term that includes several parameters: finish, integrity, tool life, cutting

speed, force, chip formation, composition and properties of material etc. In general, tool life and

surface finish are measures of machinability.

TOOL MATERIALS AND CUTTING FLUIDS

Required Properties of Tool Materials

o Hot hardness

o Wear resistance

o Chemical inertness

o Toughness (for interrupted machining)

Cutting Fluids

Cutting fluids reduce the heat, wash away the chips, and protect the machined surface from

oxidation. It is a coolant as well as a lubricant.

The cutting fluids are applied in flood or in mist conditions. Flood cooling is applied in lathe,

milling, gun drilling, and end milling. Mist cooling is applied in grinding.

Selection of the cutting fluid depends on the workpiece (minimize chemical reactions, staining,

stress, corrosion etc), on the machine tool (slideways and bearings are to be compatible with the

fluids), and on the operator safety.

Types of lathes :

Lathes - Oldest machine tools

Engine Lathe - Simple and versatile but require a skilled machinist because all controls are

manipulated by hand. It is inefficient for large production runs.

Tracer Lathe - Machine tool with an attachment that is capable of turning parts with various

contours.

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Turret Lathe - Several cutting tools are mounted on the turret in the cross-slide. They are

capable of performing multiple operations such as turning, boring, drilling, facing, thread

cutting.

Automatic Lathes - Also called as chucking machines, they are usually vertical and do not have

tailstock and are used for machining regular and irregular shapes.

CNC Lathe- turret lathe controlled by CNC. Automated, suitable for low to medium volumes of

production.

Operations on a Lathe

Straight turning, taper turning, grooving, threading, facing, profiling, drilling, boring, cutting off,

and knurling.

TURNING

The turning parameters include tool geometry, feed, depth of cut, and cutting speed.

Turning operations use single-point geometry cutting tools. The tool geometry affects cutting

speed, chip control, surface finish, tolerances (vibration and chatter) and cutting force.

Turning Process Capabilities

Ultraprecision machining - surface finish in nanometers, and accuracies in sub-micron range.

Examples are optical mirrors, computer memory disks, drums for copying machines. Diamond

turning is common. The workpiece materials include Cu, Al, Ag, Au, Ni, and plastics. The depth

of cut is in the nanometer range. High-stiffness machine tools, vibration-isolation tables, and

dust-free environment are needed.

Hard turning - use CBN tools for finish-machining hardened steels.

Drilling - uses standard chisel-point twist drills with diameters ranging from 0.006 in. to 3

in. Trepanning technique can be used to drill larger diameter holes about 6 in.

1. Core Drilling - Drill a larger hole on a smaller hole.

2. Step drilling - Double sized drill

3. Counterboring - stepper hole. Useful to seat bolt heads in the holes.

4. Countersinking -Hole is cone shaped for flat head screws.

5. Reaming - Enlarge the hole, provide better tolerance/finish.

6. Center Drilling - To begin the center for a hole.

7. Gun Drilling- deep holes with aspect ratios > 300

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Single point cutting tool

It is usually made from H.S.S. Beside H.S.S. machine tool is also made from High Carbon Steel,

Stellite, Ceramics, Diamond, Abrasive, etc. The main requirement of tool material is hardness. It

must be hard enough to resist cutting forces applied on work piece. Hot hardness, wear

resistance, Toughness, Thermal conductivity, & specific heat, coefficient of friction, are other

requirement of tool material. All these properties should be high.

Designation of cutting tool / Tool signature – Tool signature is the description of the cutting part of the tool. There are two system for tool

signature.

1. Machine reference system (or American Standard Association system) (ASA)

2. Tool reference system (or Orthogonal rake system) (ORS)

We discuss only reference system as it is widely used.

1. Machine reference system (or American Standard Association system) (ASA)—

In this system angles of the tool face are defined in two orthogonal planes, parallel to the axis of

the cutting tool & perpendicular to the axis of cutting tool, both planes being perpendicular to the

base of the tool.

Back rake angle

Side rake angle

End cleance angle

Side clearance angle

End cutting edge angle

Side cutting edge angle

Nose radius.

Tool wear / Tool failure – After use of some time tool is subjected to wear.

Cause of tool wear—

1. Interaction between tool & chip.

2. Cutting forces.

3. Temperature increase during cutting.

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*Effect of tool wear—

Tool wear changes tool shape, decrease efficiency. Tool wear induce loss of dimensional

accuracy, loss of surface finish. It increases power consumption.

*Classification of tool wear –

1. Flank wear

2. Crater wear on tool face

3. Chipping

4. Breakage

5. Loss of hardness at high temperature

MILLING

Three forms of milling:

o Slab Milling (Horizontal)

o Face Milling (Vertical)

o End Milling (Vertical)

Slab milling , also called as peripheral milling, the axis of cutter rotation is parallel to the

workpiece surface. The depth of cut is in the range 0.04" to 0.3". Go through Example 8.8

Face milling, (see Figure 8.65) the cutter is mounted on a spindle having axis of rotation

perpendicular to the workpiece surface. See next apages for calculations. Go through Example

8.9.

End milling, where the cutter is smaller than the face miller, can be used to produce various

profiles including dies.

Tool Wear

o Degrades the surface finish

o Increases the tolerance and

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o Increases the cost of machining

Mechanism of tool wear:

Adhesion: High pressure/temperature cause adhesion of asperities between the tool and the chip.

Abrasion: Hard particles in the workpiece cause abrasion of the tool-- Dominant mechanism for

flank

Plastic Flow: High temperature softens the tool and high stresses cause the plastic deformation of

the cutting edges

Diffusion: Exchange of atoms across the contact boundary between the chip and the tool. Tool

may lose "hard atoms"

Tool Life is determined by different types of wear. Flank wear is said to be the governing factor.

VB is established based on Taylor's tool life equation given by

V Tn = C (for given values of d, f)

V = cutting speed, most critical parameter

T = tool life, minutes, to develop flank wear land VB

C = constant = Tool life for 1 min

C is influenced by the type of workpiece and cutting conditions.

n is a function of the cutting tool material

Tool Wear and Tool Life

One or more of the following wear modes may occur:

i) flank

ii) notch

iii) crater

iv) edge rounding

v) edge chipping

vi) edge cracking

vii) catastrophic failure

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Conventional (Up) and Climb (Down) milling

Up Milling

- Beginning chip thickness is small

Advantages

1. Oxide scale or hard surface of work does

not matter

2. Rigidity is not critical because the cutter is

opposed by the feed of the work (machine is

even).

Drawbacks

1. Tool chatter

2. Feed marks

3. Clamp workpiece

(work moves up)

Down Milling

- Beginning chip thickness is large

Advantages

1. Low temperature (long tool life)

2. Smaller feed marks

3. Downward part of cutting force holds the

workpiece (slender parts)

Drawbacks

1. Rigid setup is needed due to the cutter

pulling the workpiece along.

2. Not suitable for oxide scale surfaces.

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Forces in Machining:

Difference between Orthogonal and Oblique Cutting

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Orthogonal cutting Oblique Cutting

The cutting edge of the tool is perpendicular

to the direction of feed motion.

The cutting edge of the tool is inclined to the

direction of feed motion.

Chip flow is expected to in a direction

perpendicular to the cutting edge.

The chip flow angle is more than zero.

There are only two components of force;

these components are mutually

perpendicular.

There are three mutually perpendicular

forces acting while cutting process.

The cutting edge is larger than cutting width. The cutting edge may or may not be larger

than cutting width.

Chips are in the form of a spiral coil. Chip flow is in a sideways direction.

High heat concentration at cutting region. Less concentration of heat at cutting region

compared to orthogonal cutting.

For a given feed and depth of cutting, the

force acts on a small area as compared with

oblique cutting, so tool life is less.

Force is acting on a large area, results in

more tool life.

Surface finish is poor. Good surface finish obtained.

Used in grooving, parting, slotting, pipe

cutting.

Used almost all industrial cutting, used in

drilling, grinding, milling.

Characteristics of a cutting tool material:

1. The material should be harder than the workpiece so that it is able to penetrate into the

workpiece and it should have hot hardness i.e. the ability of material to retain hardness at

elevated temperatures.

2. The material should have wear resistance to prevent wear and tear of the cutting tool surface.

3. It should be chemically stable so that it does not react with the workpiece and chemically

inert so that there is no oxidation and hence no scales and pits are formed on the surface.

4. The material must have sufficient strength and toughness to withstand shocks and vibrations.

5. The thermal conductivity should be high so that there is heat dissipation which is generated

during the machining process thereby increasing the life of the cutting tool.

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Common cutting tool material used:

1. Carbon steel: Carbon steels having carbon percentage as high as 1.5% are used as tool

materials however they are not able to with stand very high temperature and hence are

operational at low cutting speed.

2. High speed steel (HSS): These are special alloy steel which are obtained by alloying

tungsten, Chromium, Vanadium, Cobalt and molybdenum with steel. HSS has high hot hardness,

wear resistance and 3 to 4 times higher cutting speed as compare to carbon steel. Most

commonly used HSS have following compositions.

a) 18-4-1 HSS i.e. 18% tungsten, 4% chromium, 1% vanadium with a carbon content of 0.6 -

0.7%. If vanadium is 2% it becomes 18-4-2 HSS.

b) Cobalt high speed steel: This is also referred to as super high speed steel. Cobalt is added 2 –

15%. The most common composition is tungsten 20%, 4% chromium, 2% vanadium and 12%

cobalt.

c) Molybdenum high speed steel: It contains 6% tungsten, 6% molybdenum, 4% chromium and

2% vanadium.

3. Cemented carbide: These are basically carbon cemented together by a binder. It is a powder

metallurgy product and the binder mostly used is cobalt. The basic ingredient is tungsten

carbide-82%, titanium carbide-10% and cobalt-8%. These materials possess high hardness and

wear resistance and it has cutting speed 6 times higher than high speed steel (HSS). Can

withstand up to 1000°C.

4. Ceramics: It mainly consists of aluminum oxide (Al2O3) and silicon nitride (Si3N4). Ceramic

cutting tools are hard with high hot hardness and do not react with the workpiece. They can be

used at elevated temperature and cutting speed 4 times that of cemented carbide. These have low

heat conductivity. Can withstand upto 1200°C.

5. Diamond: It is the hardest known material having cutting speed 15 times greater than that for

high speed tools.

6. Cubic boron nitride (CBN): It is the second hardest material after diamond and a

economical alternative to the later. It is manufactured through high temperature and pressure to

bond boron crystals in cubic form with a ceramic or metal binder to form polycrystalline

structure with nitride particles present. It is an excellent cutting tool material because it combines

extreme high hot hardness up to high temperatures of 2000°C.

7. Cermets

Cermets is the combination of ceramics and metals and produced by Powder Metallurgy

process.

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When they combine ceramics will give high refractoriness and metals will give high

toughness and thermal shock resistance.

For cutting tools usual combination as Al2O3 + W + Mo + boron + Ti etc.

Usual combination 90% ceramic, 10% metals.

Increase in % of metals reduces brittleness some extent and also reduces wear resistance.

8. UCON

UCON is developed by union carbide in USA.

It consists of Columbium 50%, Titanium 30 % and Tungsten 20%.

This is refractory metal alloy which is cast, rolled into sheets and slit into blanks. though

its hardness is only 200 BHN, it is hardened by diffusing nitrogen into surface producing

very hard surface with soft core. It is not used because of its higher costs.

9. Sialon (Si-Al-O-N)

Sialon is made by powder metallurgy with milled powders of Silicon, Nitrogen,

Aluminium and oxygen by sintering at 1800°C.

This is tougher than ceramics and so it can be successfully used in interrupted cuts.

Cutting speeds are 2 to 3 times compared to ceramics.

At present this is used for machining of aerospace alloys, nickel based gas turbine blades

with a cutting speed of 3 to 5 m/sec.

What Is Lathe?

Lathe is a machine that helps in shaping several material pieces in the desired shapes. A lathe is a

machine that rotates the work piece on the axis in order to perform various operations like

cutting, facing, knurling, deformation and more.

Types of Lathe Operation

The working of the lathe machine changes with every operation and cut desired. Some of the

common lathe operations are:

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Facing

This is usually the first step of any lathe operation on the lathe machine. The metal is cut from

the end to make it fit in the right angle of the axis and remove the marks.

Tapering

Tapering is to cut the metal to nearly a cone shape with the help of the compound slide. This is

something in between the parallel turning and facing off. If one is willing to change the angle

then they can adjust the compound slide as they like.

Parallel Turning

This operation is adopted in order to cut the metal parallel to the axis. Parallel turning is done to

decrease the diameter of the metal.

Parting

The part is removed so that it faces the ends. For this the parting tool is involved in slowly to

make perform the operation. For to make the cut deeper the parting tool is pulled out and

transferred to the side for the cut and to prevent the tool from breaking.

Lathe Cutting Tools

There are several lathe cutting tools that help in cutting with the lathe machine. The commonly

used tools are mentioned below:

Carbide tip tools

Grooving tool

Cut-Off blade

Parting blades

Boring bar

Side tool

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What are the four main types of lathes?

The four main types of lathes are

Speed Lathes

Engine Lathes

Tool Room Lathes and

Turret Lathes

Speed Lathes

It is very simple is design. It only has headstock, tailstock and a very simple tool post. It can

operate in 3-4 speeds. The spindle speed is very high. It is used for light machine works like

wood turning, metal spinning and metal polishing.

Engine Lathes

Engine lathes are the most common types of lathe machine. It is designed for low power

operations as well as high power operations. Various lengths of the machine is available. The

length can be up to 60 feet. Engine lathe is commonly seen in every machine shop. Various

metals can be machines. The machine can operates at a wide range of speed ratios.

Tool room Lathes

It is a very versatile lathe machine. It can give better accuracy and finishing . It has wider range

of speeds . It can give different types of feeds. It can be a great device to manufacture die.

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Turret Lathes

It is a great machine for quick operations. It has various types of tool posts mounted on a single

structure. As a number of tools are set up on machine, the job can be completed very quickly

with the help of a single setup. A capstan wheel is used to position the next tool. A sequential

machining process can be done by using the turret lathe without moving the workpiece. It

eliminates the error that occurs due to misalignment.

Tell about Special Types of Lathe Machines

These are the machines which allows the worker to perform tasks which are not possible in

normal lathe machines. These lathes include –

bench type jeweler’s lathe

Automatic lathes

brakedrum lathes

multispindle lathes

crankshaft lathes

duplicating lathes

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What does Boring mean?

In machining, boring is the process of enlarging a hole that has already been drilled (or cast), by

means of a single-point cutting tool (or of a boring head containing several such tools), for

example as in boring a gun barrel or an engine cylinder. Boring is used to achieve greater

accuracy of the diameter of a hole, and can be used to cut a tapered hole. Boring can be viewed

as the internal-diameter counterpart to turning, which cuts external diameters.

What does Reaming mean?

Reaming is a finishing operation that is performed with multi-edge tools which provide high-

precision holes. At a high penetration rate and small depth of cuts, a superb hole quality, close

dimensional tolerance and high surface finish are achieved. The operation is performed with the

help of reamers which are round cutting tools that enlarge the size of existing holes. Reaming

should not be considered only as the correction of location and alignment of holes. The primary

purpose of the operation is fine tuning the diameter of the holes.

What does Broaching mean?

Broaching is a machining process that uses a toothed tool, called a broach, to remove material.

There are two main types of broaching:linear and rotary. In linear broaching, which is the more

common process, the broach is run linearly against a surface of the workpiece to effect the cut.

Linear broaches are used in a broaching machine, which is also sometimes shortened to broach.

In rotary broaching, the broach is rotated and pressed into the workpiece to cut an axis

symmetric shape. A rotary broach is used in a lathe or screw machine. In both processes the cut

is performed in one pass of the broach, which makes it very efficient.

Broaching is used when precision machining is required, especially for odd shapes. Commonly

machined surfaces include circular and non-circular holes, splines, keyways, and flat surfaces.

Typical workpieces include small to medium-sized castings, forgings, screw machine parts,

and stampings. Even though broaches can be expensive, broaching is usually favored over other

processes when used for high-quantity production runs.

Broaches are shaped similar to a saw, except the height of the teeth increases over the length of

the tool. Moreover, the broach contains three distinct sections: one for roughing, another for

semi-finishing, and the final one for finishing. Broaching is an unusual machining process

because it has the feed built into the tool. The profile of the machined surface is always the

inverse of the profile of the broach. The rise per tooth (RPT), also known as the step or feed per

tooth, determines the amount of material removed and the size of the chip. The broach can be

moved relative to the workpiece or vice versa. Because all of the features are built into the

broach no complex motion or skilled labor is required to use it. A broach is effectively a

collection of single-point cutting tools arrayed in sequence, cutting one after the other; its cut is

analogous to multiple passes of a shaper.

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Milling

Milling cutters are cutting tools typically used in milling machines to perform milling operations

and occasionally in other machine tools. They remove material by their movement within the

machine or directly from the cutter's shape.

What is an End Mill ?

An endmill is a type of milling cutter, a cutting tool used in industrial milling applications. It is

distinguished from the drill bit in its application, geometry, and manufacture. While a drill bit

can only cut in the axial direction, a milling bit can generally cut in all directions, though some

cannot cut axially. End mills are used in milling applications such as profile milling, tracer

milling, face milling, and plunging.

Gear Cutting

Gear cutting is any machining process for creating a gear. The most common gear-cutting

processes include hobbing, broaching, milling, and grinding. Such cutting operations may occur

either after or instead of forming processes such as forging, extruding, investment casting,

or sand casting.

Gears are commonly made from metal, plastic, and wood. Although gear cutting is a substantial

industry, many metal and plastic gears are made without cutting, by processes such as die

casting or injection molding. Some metal gears made with powder metallurgy require subsequent

machining, whereas others are complete after sintering. Likewise, metal or plastic gears made

with additive manufacturing may or may not require finishing by cutting, depending on

application.

For very large gears or splines, a vertical broach is used. It consists of a vertical rail that carries a

single tooth cutter formed to creat the tooth shape. A rotary table and a Y axis are the cusomary

axes available. Some machines will cut to a depth on the Y axis and index the rotary table

automatically. The largest gears are produced on these machines.

Other operations such as broaching work particularly well for cutting teeth on the inside. The

downside to this is that it is expensive and different broaches are required to make different sized

gears. Therefore, it is mostly used in very high production runs.

Spur may be cut or ground on a milling machine or jig grinder utilizing a numbered gear cutter,

and any indexing head or rotary table. The number of the gear cutter is determined by the tooth

count of the gear to be cut.

To machine a helical gear on a manual machine, a true indexing fixture must be used. Indexing

fixtures can disengage the drive worm, and be attached via an external gear trainto the machine

table's handle (like a power feed). It then operates similarly to a carriage on a lathe. As the table

moves on the X axis, the fixture will rotate in a fixed ratio with the table. The indexing fixture

itself receives its name from the original purpose of the tool: moving the table in precise, fixed

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increments. If the indexing worm is not disengaged from the table, one can move the table in a

highly controlled fashion via the indexing plate to produce linear movement of great precision

(such as a vernier scale).

There are a few different types of cutters used when creating gears. One is a rack shaper. These

are straight and move in a direction tangent to the gear, while the gear is fixed. They have six to

twelve teeth and eventually have to be moved back to the starting point to begin another cut.

A popular way to build gears is by form cutting. This is done by taking a blank gear and rotating

a cutter, with the desired tooth pattern, around its periphery. This ensures that the gear will fit

when the operation is finished.

The old method of gear cutting is mounting a gear blank in a shaper and using a tool shaped in

the profile of the tooth to be cut. This method also works for cutting internal splines.

Another is a pinion-shaped cutter that is used in a gear shaper machine. It is basically when a

cutter that looks similar to a gear cuts a gear blank. The cutter and the blank must have a rotating

axis parallel to each other. This process works well for low and high production runs.

What is Hobbing?

Hobbing is a method by which a hob is used to cut teeth into a blank. The cutter and gear blank

are rotated at the same time to transfer the profile of the hob onto the gear blank. The hob must

make one revolution to create each tooth of the gear. Used very often for all sizes of production

runs, but works best for medium to high.

Finishing of gears

After being cut the gear can be finished by shaving, burnishing, grinding, honing or lapping.

A grinding wheel is a wheel composed of an abrasive compound and used for various grinding

(abrasive cutting) and abrasive machining operations. Such wheels are used in grinding

machines.

The wheels are generally made from a composite material consisting of coarse-particle aggregate

pressed and bonded together by a cementing matrix (called the bond in grinding wheel

terminology) to form a solid, circular shape. Various profiles and cross sections are available

depending on the intended usage for the wheel. They may also be made from a solid steel or

aluminium disc with particles bonded to the surface. Today most grinding wheels are artificial

composites made with artificial aggregates, but the history of grinding wheels began with natural

composite stones, such as those used for millstones.

The manufacture of these wheels is a precise and tightly controlled process, due not only to the

inherent safety risks of a spinning disc, but also the composition and uniformity required to

prevent that disc from exploding due to the high stresses produced on rotation.

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Grinding wheels are consumables, although the life span can vary widely depending on the use

case, from less than a day to many years. As the wheel cuts, it periodically releases individual

grains of abrasive, typically because they grow dull and the increased drag pulls them out of the

bond. Fresh grains are exposed in this wear process, which begin the next cycle. The rate of wear

in this process is usually very predictable for a given application, and is necessary for good

performance.

What is Dressing?

Grinding wheels are self sharpening to a small degree; for optimal use they may be dressed and

trued by the use of wheel or grinding dressers. Dressing the wheel refers to removing the current

layer of abrasive, so that a fresh and sharp surface is exposed to the work surface. Trueing the

wheel makes the grinding surface parallel to the grinding table or other reference plane, so that

the entire grinding wheel is even and produces an accurate surface.

What is surface integrity?

Surface integrity reflects the properties of a material after it has been subject to some type of

manufacturing process or modification. Engineers and product designers often plan projects

based on the known characteristics of a particular metal. For example, these designers know that

a specific steel alloy offers a set level of strength or hardness. After the material has been

modified, these original properties may no longer apply, as many manufacturing processes create

a permanent change in the material. Surface integrity helps these individuals determine how a

material will change under certain conditions, and what it's new properties are compared to its

old ones.

Changes in surface integrity can be either positive or negative. Negative changes could mean that

the material can no longer be used as intended. For example, a steel column subject to quenching

may ultimately be too brittle to support a structure. Positive changes are those that give the

material the desired finish or appearance, such as burnishing to smooth out a rough piece of

material. Positive changes in surface integrity also include those that improve properties like

hardness, strength, or moisture resistance.

What is a CNC?

A CNC, or computer numerical control machine, is a machine used mainly in manufacturing that

is controlled by programming via computer without the need for constant manual supervision. A

CNC machine can automate operations by turning hand-wheels that are impossible for a human

machinist to turn.

Name some types of CNC.

The most common types of CNC machines include milling machines, laser cutting machines,

CNC routers, drilling machines and grinders. Other types of CNC machines include swaging

machines, power presses, arbour presses and bending rollers.

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Name some applications of CNC machines.

CNC milling machines perform complicated cutting operations, like rabbeting to routing,

drilling, slot cutting and threading. CNC routers cut wood, plastics and sheet metal. CNC laser

cutting machines create precise patterns in the same materials. CNC drilling machines bore holes

into both objects and the earth. CNC grinders use a spinning wheel to grind down or shape

surfaces. Swaging machines, power presses, arbour presses and bending rollers are primarily

used for cutting and shaping sheet metal.

What is a Machining Centre?

A further development in the automation of machine tools is the ―machining centre,‖ usually a

vertical milling machine fitted with automatic tool-changing facilities and capable of several

axes of control. The tools, of which there can be more than 100, are generally housed in a rotary

magazine and may be changed by commands from the machine tool program. Thus, different

faces of a workpiece can be machined by a combination of operations without moving it to

another machine tool. Machining centres are particularly suitable for the batch production of

large and complex components requiring a high degree of accuracy.

The term ―machining center‖ describes almost any CNC milling and drilling machine that

includes an automatic toolchanger and a table that clamps the workpiece in place. On a

machining center, the tool rotates, but the work does not. The orientation of the spindle is the

most fundamental defining characteristic of a machining center. Vertical machining centers

generally favor precision while horizontal machining centers generally favor production—but

these are loose generalizations, and plenty of machining centers break out of them. Another

common machining center type is the five-axis machining center, which is able to pivot the tool

and/or the part in order to mill and drill at various orientations.

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