Manufacturing Process - Module4.0

8/12/2019 Manufacturing Process - Module4.0

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MANUFACTURING PROCESS

BKT 1 DCP4062

4.0 MATERIAL REMOVAL PROCESSES

INTRODUCTION

Machining is a manufacturing process in which a cutting tool is used to remove excess material

from a workpart so that the remaining material is the desired part shape. The predominant

cutting action involves shear deformation of the work material to form a chip. As the chip is

removed, a new surface is exposed. Machining operations are considered to be the most

versatile manufacturing techniques for the production of highly accurate part geometries. They

can be utilized for the fabrication of one-of-a-kind products as well as for mass production.

Machining operations can be classified according to the geometry of the objects profile ;

rotational versus prismatic, as well as to the sizes of the object features. External and internal

rotational object profiles can be achieved through turning and boring operations, respectively,

carried out on lathes and/or boring machines. Prismatic profiles can be fabricated through

milling operations carried out on a variety of milling machines.

LEARNING OBJECTIVES

At the end of this course the student will be able to:

Outline the factors that influence a cutting process

Describe the different types of chip formation

State the types and functions of cutting fluids in machining

State the various tool materials

Explain several types of machining process

LEARNING OUTCOMES At the end of this course the student has the ability to:

Explain the basic theory of machining process, cutting fluids and cutting tools.

Identify suitable machining process for a specific part features


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4.1 INTRODUCTION

Final products are often obtained by machining shapes such as bar stock or plate to

size. It is important that metal cutting principles are used in turning, planning, milling, and

drilling operations as well as other processes performed by machine tools. Parts are

produced by removing metal in the form of small chips. The cutting tool that removes

these chips is the focus of many important principles.

4.2 METAL CUTTING THEORY

The simplest form of cutting tool is the single-point tool such as that found in a lathe cut-

off operation or planner or shaper work. Multiple-point cutting tools are made of two ormore single-point tools arranged together as a unit. The milling cutter and broaching

tools are examples of multiple-point cutters. But the discussion in this section deals

mostly with orthogonal tools in which the cutting edge is perpendicular to the direction of

the cut and there is no lateral flow of metal. Nor is there curvature in these idealized

forms, and all parts of the chip have the same velocity.

In analyzing the cutting process it is assumed that the chip is severed from the

workpiece by a shearing action across the plane as shown in the figure below, although

other theories exists on chip formation. Because the deformed chip is in compression

against the face of the tool, a frictional force is developed. The work of making the chip

must overcome both the shearing force and the frictional force.


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Orthogona l cu t t ing too l mo del

In the orthogonal cutting model the tool can be considered stationary and the workpiece

moving. An opposite motion pattern does not change the concept. The state of stress

before and after the shear plane is a complicated plastic flow of metal. The shear plane is determined by the rake angle of the tool and by the friction between the chip and

the tool face.

Fac tors in f luenc ing c u t t ing process

Table above outlines the factors that influence a cutting process. The major independent

variables in the cutting processes are as follows:

Tool material, coatings and tool conditions

Tool shape, surface finish and sharpness

Workpiece material, condition, and temperature

Cutting parameters, such as speed, feed and depth of cut

Cutting fluids

The characteristics of the machine tool, such as its stiffness and damping

Workholder and fixturing

Dependent variables those that are influenced by changes in the independent variables

are the following:

Type of chip produced

Force and energy dissipated in the cutting process

Temperature rise in the workpiece, the chip, and the tool


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Wear and failure of the tool

Surface finish produced on the workpiece after machining

When unacceptable conditions result from machining operations, the manufacturing

engineer must ask questions to determine the cause of the problem. The knowledge of

mechanics of chip formation; chip types; force and power requirements; temperature rise

caused by the cutting action; tool life; surface finish; and machinability will plan efficient

and economical machining operations and can select the proper equipment and tooling.

4.3 CHIP FORMATION

Tool chips have been classified into three types as shown in figure below:

Basic chip types A discontinuous, B Continuous, C Continuous with build up edge.

Type 1, Figure a, name as discontinuous or segmental chip, and represent a condition in

which the metal a head of the cutting tool is fractured into fairly small pieces. This type of

chip is obtained in machining most brittle materials such as cast iron and bronze. As the

chips are produced the cutting edge smooth over the irregularities and a fairly good finish is

obtained. Tool life is reasonably good, and failure usually occurs as a result of abrading

action on the contact surface of the tool. Discontinuous chips can also be formed on some

ductile materials if the coefficient of friction is high. However, such chips on ductile

materials are an indication of poor cutting conditions.

An ideal type of chip from the standpoint of tool life and finish is the simple continuous chip

Type 2 chip (Figure b), which is obtained in cutting ductile materials having a low coefficient

of friction. In this case the metal is continuously deformed and slides up the face of the tool


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without being fractured. Chips of this type are obtained at high cutting speeds are common

when cutting is done with carbide tools. Because of their simplicity they can be analyzed

easily from the standpoint of the forces involved. Continuous chips come off the bar stock

as string from a ball and can be troublesome to handle and sometimes dangerous.

The Type 3 chip, Figure c, is characteristics of those machined from ductile materials that

have a fairly high coefficient of friction. As the tools starts the cut, some of the materials,

because of the high friction coefficient, builds up ahead of the cutting edge. Some of the

workpiece may even weld onto the tool point, and thus known as a built-up edge or BUE.

As the cutting proceeds, the chips flow over this edge and up along the face of the tool.

Periodically a small amount of this BUE separates and leaves with the chip or is embedded

in the turned surface. Because of this action, the surface smoothness is not as good as with

the type 2 chip. The BUE remains fairly constant during cutting and has the effect of slightly

altering the rake angle. However, as the cutting speed is increased, the size of the BUE

decreases and the surface finish is improved. This phenomenon is also decreased by either

reducing the chip thickness or increasing the rake angle, but many on the ductile material it

cannot be eliminated entirely.

Some investigators report that 97% of the work that goes into cutting is dissipated in the

form of heat. Figure below shows the three zones in which the heat is generated. As the

shear angle is increased, the percentage heat generated in the shear plane A will

decrease because the plastic flow of the metal will take place over a shorter distance. The

shear angle can be increased by applying a coolant and reducing the friction between the

chip and the tool as by properly grinding the tool. Of the cutting variables, cutting speed has

the most effect on temperature. To increase the rate of metal removal, an increase in feedis much to be preferred over an increase in speed.


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Approximate sources of heat in three zones. A-Shear plane, B-Friction plane, C-Surface plane.

4.4 CUTTING FLUIDS

Cutting fluids are essential in most metal cutting operations. During a machining

process, considerable heat and friction are created by the plastic deformation of metal

occurring in the shear zone when the chip slides along the chip-tool interface. This heat

and friction cause metal to adhere to the cutting tools edge, causing the tool to break

down; the result is a poor finish and inaccurate work.

Cutting fluids are used extensively in machining to achieve the following results:

Reduce friction and wear, thus improving tool life and surface finish

Reduce forces and energy consumption

Cool the cutting zone, thus reducing workpiece temperature and thermal

distortion

Protect the machined surface from environmental corrosion

A cutting fluid basically may be a coolant or a lubricant. Its effectiveness in cutting

operations depends on a number of factors, such as the method of application,

temperature, cutting speed and type of machining operation. Four general types of

cutting fluids are commonly used in machining operations:

1. Oils

2. Emulsions

3. Semisynthetics

4. Synthetics


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The sources of oils can be mineral (petroleum), animal, vegetable or fish. Oil may be

compounded with any number of additives or with other oils: this process is used to

change such properties as viscosity-temperature behavior and surface tension, heat

resistance and boundary layer characteristics.

An emulsion is a mixture of two immiscible liquids, usually of oil and water in various

proportions along with additives. Milky in appearance, emulsions are also known as

water soluble oils or water-base coolants.

Synthetic solutions are chemical fluids that contain inorganic and other chemicals

dissolved in water; they do not contain any mineral oils. Various chemical agents are

added to a particular solution to impart different properties. Semisynthetic solutions are

basically synthetic solutions to which small amounts of emulsifiable oils have beenadded.

4.5 CUTTING TOOL GEOMETRY

To understand the cutting action of a single-point tool as applied to a lathe, refer to the

cutting tool figure below. The tool has been ground to a wedge shape, the included

angle being called the lip or cutting angle. The side relief angle between the side of the

tool and the work is preventing the tool from rubbing. The angle is small, usually 6 to 8for most materials. The side rake angle varies with the lip angle, which in turn depends

on the type of material being machined. If the cutting tool is supported in a horizontal

position, the back rake angle is obtained by grinding. End clearance is necessary to

prevent a rubbing action on the flank of the tool.

Nomenclature for a r ight hand cut t ingtoo l


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In grinding tools it should be noted that the lip of cutting angle varies with the kind of

material being cut. The cutting angle must be keen enough to cut well with minimum of

power consumption, yet the edge must be sufficiently strong to withstand the tool forces

involved and to carry away the heat generated. A compromise is necessary. In general it

is based on the hardness of the workpiece. Hard materials require a cutting edge of

great strength with a capacity for carrying away heat. Soft materials (for example wood)

permit the use of smaller cutting angles around 22, for soft and ductile material (copper

and aluminium) require larger angles ranging up to 47 whereas brittle material require

still larger angles.

In addition to the solid single-point tool, a carbide tip may be brazed on or inserted in a

tool holder.

Explod ed view of a tool with dispo sable, t r iangular, carbide cut t ing p oint inser t


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BKT 9 DCP4062

Tools angles and cut t ing s peeds for High Speed tools

4.6 CUTTING TOOL MATERIALS

The best material to use for certain job is the one that will produce the machined part at

lower cost. Desirable properties for any tool material include the ability to resist softening

at high temperature, a low coefficient of friction, good abrasive resisting qualities, and

sufficient toughness to resist fracture.

High carbon s teel :

Carbon steel were used for all cutting tools, where their carbon content ranging from

0.80% to 1.2%. These steels have good hardening ability and with proper heat

treatment, attain as great hardness as any high-speed alloys. Because these tools losehardness at temperature 300C they are not suitable for high cutting speed and heavy-

duty work.

High sp eed s teel :

Known as HSS, it high in alloy content, excellent hardenability, and will retain a good

cutting edge to temperatures of around 650C. The ability of tool to resist softening at

high temperatures is known as red hardness and is a most desirable quality. Normally it

contain of 18% tungsten , 5.5% chromium to steel as the principal alloying elements.


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BKT 10 DCP4062

Carbide :

Carbide cutting tool inserts are made only by the powder metallurgy technique. The

metal powders of tungsten carbide (94%) and cobalt (6%) are pressed to shape, semi

sintered to facilitate handling and forming to final shape, sintered in a hydrogenatmosphere furnace at 1550C and finished by grinding operation. Carbide tools permit

cutting speeds more higher but at such speeds that a much smaller feed must be used.

From an economic point of view, carbide tools should always be used if possible.

Diamond :

Diamonds used as single-point tools for light cuts and high speeds must be rigidly

supported because of their high hardness and brittleness. They are used either for hardmaterials difficult to cut with other tool materials or for light, high speed cuts on softer

materials where accuracy and surface finished are important. It is commonly used for

machining plastics, hard rubber or aluminium where the cutting speeds is high.

4.7 MACHINING PROCESSES

All machining processes remove material to form shapes. As metals are still the most

widely used materials in manufacturing, machining processes are usually used for

metals. However, machining can also be used to shape plastics and other materials

which are becoming more widespread. Basically all the different forms of machining

involve removing material from a component using a rotating cutter. The differences

between the various types arise from the relative motion between cutting tool and

workpiece and the type of cutting tool used.

Typically machining will be done using a machine tool. This tool holds the workpiece and

the rotating cutting tool and allows relative movement between the two. Usually machine

tools are dedicated to one type of machining operation, although some more flexible

tools allow more than one type of machining to be performed. The machine tool can

either be under manual or automatic (Computer Numeric Control - CNC) control.

The cutting speed of the tool is usually dictated by the type of material being machined,

in general the harder the material, the slower the machining time. Machining speed can

be increased by increasing the rotational speed of the cutter; however this will be at the

expense of the tool life.


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In order to dissipate the heat generated between the workpiece and the cutting tool,

cutting fluids are sprayed onto the tool. The cutting fluid also acts to remove cut material

away from the cutting region and lubricates the tool - workpiece interface but may

require that the component is cleaned afterwards.

Advantages

Machining processes allow high precision components to be rapidly produced.

Disadvantages

Machining processes are not suitable for removing large amounts of material.

There can be a large amount of wastage.

4.7.1 LATHE AND LATHE OPERATIONS

Lathes are generally considered to be the oldest machine tools. The most

common lathe was originally called an engine lathe because it was powered with

overhead pulleys and belts from nearby engines. Today these lathes are

equipped with individual electric motors. The lathe is one of the most versatile

machine tools used in industry. The work is held and rotated on its axis while the

cutting tool is advanced along the lines of a desired cut.

With suitable attachments, the lathe may be used for turning, tapering, form

turning, screw cutting, facing, drilling, boring, knurling, grooving and polishing

operations.

Turning

In a typical turning operation, the workpiece is clamped by one of the

workholding. Turning process is to produce straight, conical, curved, or grooved

workpieces such as shafts, spindles and pins.

Boring

The boring operation on a lathe is similar to turning. Boring is performed inside

hollow workpieces or in a hole made previously by drilling or other means. Out of

shape holes can be straightened by boring. The workpiece is held in a chuck or

in some other suitable workholding device.


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Drilling

Drilling can be performed on a lathe by mounting the drill bit in a drill chuck into

the tailstock quill (a tubular shaft). The workpiece is placed in a workholder on

the headstock, and the quill is advanced by rotating the hand wheel. Holes drilledin this manner may not be concentric because of the tendency for the drill to drift

radially. The concentricity of the hole is improved by subsequently boring the

drilled hole. Drilled holes may be reamed on lathes in a manner similar to drilling,

thus improving hole tolerances.

Form Turning

Form turning produces a concave or convex form of internal or external surfacesof a workpiece. Form tools are used to produce various shapes on round

workpieces by turning. The tool moves radially inward to machine the part.

Machining by form cutting is not suitable for deep and narrow grooves or sharp

corners because they may cause vibration and result in poor surface finish.

Knurling

Knurling is a process of impressing a diamond-shaped or straight-line pattern into

the surface of the workpiece to improve its appearance or to provide better

gripping surface. Straight knurling is often used to increase the workpiece

diameter when a press fit is required. Knurling is performed on a lathe with

hardened rolls in which the surface of the rolls is replica of the profile to be

generated. The rolls are pressed radially against the rotating workpiece, while the

tool moves axially along the part.

Grooving

Grooving, commonly called recessing, undercutting or necking, is often done at

the end of a thread to permit full travel of the nut up to a shoulder, or at the edge

of a shoulder to ensure a proper fit of mating parts. Grooves are generally

square, round or V-shaped.

Threading

Thread cutting on a lathe is a process that produces a helical ridge of uniformsection on a workpiece. This is performed by taking successive cuts with a


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threading tool bit of the same shape as the thread form required. Work to be

threaded may be held between centers or in a chuck.

4.7.2 MILLING OPERATIONS

Milling includes a number of highly versatile machining operations capable of

producing a variety of configurations with the use of a milling cutter, a multitooth

tool that produces a number of chips in one revolution.

Used for general purpose milling operations, column and knee type milling

machines are the most common milling machines. The spindle to which the

milling cutter is may be horizontal for slab milling or vertical for face and end

milling and end milling, boring and drilling operations.

Horizontal Mil l ing Machine


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Vert ical Mil l ing Machine

4.7.2.1 CLASSIFICATION OF MILLING

Peripheral Milling

In peripheral (or slab) milling, the milled surface is generated by teeth

located on the periphery of the cutter body. The axis of cutter rotation is

generally in a plane parallel to the workpiece surface to be machined.

Face Milling

In face milling, the cutter is mounted on a spindle having an axis of

rotation perpendicular to the workpiece surface. The milled surface

results from the action of cutting edges located on the periphery and face

of the cutter.

End Milling

The cutter in end milling generally rotates on an axis vertical to the

workpiece. It can be tilted to machine tapered surfaces. Cutting teeth are

located on both the end face of the cutter and the periphery of the cutter

body.


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Some of the basic type of milling cutters and milling operations; (a) slab milling

(b) face milling (c) end milling

4.7.2.2 METHODS OF MILLING

Up Milling

Up milling is also referred to as conventional milling. The direction of the cutter

rotation opposes the feed motion. For example, if the cutter rotates clockwise,

the workpiece is fed to the right in up milling.

Convention al mil l ing

Down Milling

Down milling is also referred to as climb milling. The direction of cutter rotation

is same as the feed motion. For example, if the cutter rotates

counterclockwise, the workpiece is fed to the right in down milling.


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Cl imb mi l l ing

4.7.3 PLANING

Planing is a material removal process in which the workpiece reciprocates

against a stationary single-point cutting tool producing a plane surface.

Planing is relatively simple cutting operation by which flat surfaces, as well as

various cross-sections with grooves and notches are produced along the length

of the workpiece. Planning is usually done on large workpiece.

4.7.4 SHAPING

Shaping is a material removal process in which a single-point cutting tool

reciprocates across the face of a stationary workpiece to produce a plane or

sculpted surface.

Shaping is used to machine parts; it is much like planning, except that the parts

are smaller. Cutting by shaping is basically the same as by planning. In a

horizontal shaper, the tool travels along a straight path and the workpiece is

stationary. The cutting tool is attached to the tool head, which is mounted on the

ram. The ram has a reciprocating motion and in most machines cutting is done

during the forward movement of the ram. Vertical shapers (slotter) are used to

machine notches, keyways and dies.


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4.7.5 BROACHING

The broaching operation is similar to shaping with multiple teeth and is used to

machine internal and external surfaces, such as hole of circular, square or

irregular section, keyways and the teeth of internal gears.

A broach is a long multitooth cutting tool; the total depth of material removed in

one stroke is the sum of the depths of cut of each tooth of the broach. A large

broach can remove material as deep as 38mm in one stroke.

Broaching is an important production process and can produce parts with very

good surface finish and dimensional accuracy. It competes favorably with other

processes such as boring, milling, shaping and reaming to produce similar

shapes. Although broaches can be expensive, the cost is justified with high-

quantity production runs.


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(a) Broach ing too th nom enclature and termino logy (b) I l lustrat ion of ho w a chipfi l ls the gul let during a broaching o perat ion

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Manufacturing Process - Module4.0

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