Production Engineering-M II

8/8/2019 Production Engineering-M II

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MODULE II

MACHINABILITY OF METALS

During metal cutting, the energy dissipated gets converted in to heat.

Consequently, high temperatures are generated in the region of the tool cutting edgeand this temperature have a controlling influence on the rate of wear of the cutting tooland on the friction between the chip and the tool.

Heat is generated in three distinct regions

(i). The shear zone: here the energy needed to shear the chip is the source of heat. Inthis region about 80-85 % of the heat is generated.

(ii) The chip Tool interface region: here the energy needed to overcome friction is thesource of heat. Some plastic deformation also occurs in this region. About 15 to20 % heat is generated in this region.

(iii) The tool work interface region: here energy needed to overcome frictional rubbingbetween flank face of the tool and work piece is the source of heat. In this regiononly 1-3 % of heat generated.

Fig- 2.1: Regions of heat generation in metal cutting

Factors affecting Temperature

i)

Work piece and tool material:- Materials with higher thermal conductivity produce lower temperaturethan tools with lower conductivity.

ii) Tool Geometry- Rake angle has only slight influence on the temperature. But it increases

considerably with approach angle.iii) Cutting fluid:

- At high speeds (such as employed for carbides), cutting fluid hasnegligible effect on tool chip interface temperature. The fluid is carried
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away by the outward flowing chip more rapidly than it could be forcedbetween the tool and the chip.

Temperature Distribution in Metal Cutting

Fig shows temperature distribution in work piece and chip during metal cutting. The maximumtemperature in the cutting zone occurs not at tool tip but at some distance further up the rake

face. Material at a point such as X gets heated as it passes through the shear zone and

finally leaves as a chip.

For points such as Y, heating continues beyond the shear plane in to the frictionalheat region. These points, however, loose shear zone heat to the chip whilemoving up but gain frictional heat.

Points such as Z remain in the work piece and their temperature rises merely dueto conduction of heat in to the work piece.

GENERATION AND DISSIPATION OF HEAT IN METAL CUTTING

The Heat generated in metal cutting is due to following three reasons.

1). Deformation of Metal- during cutting process the tool exerts pressure on the work piece.This pressure transmitted to the grains of the metal adjacent to the tool face. The slipping ofsections of grains over each other produces internal friction, which generates heat ofdeformation.2). Heat due to chip distortion- the chip separated from the work piece subjected todistortion while it passes over the rake face. Distortion takes place between the grains of thecausing internal friction, which generates heat.3). Heat due to friction- during cutting process heat generated due to friction between work

piece and tool and also due to friction between tool and chip. The rubbing speed of the workpiece and chip over the tool is nearly equal to the speed.

The heat generated is dissipated in the atmosphere through the workpiece, chip and the tool. If cutting fluid is used the heat will be carried by it.

CUTTING FLUIDS

Cutting fluid is any substance (liquid, gas or solid) which is applied to a tool during a cuttingoperation to facilitate removal of chips.Function of Cutting Fluid

1. To cool the cutting tool and the work piece.2. To lubricate the chip, tool and work piece.3. To help carry away the chip.4. To lubricate some of the moving part of the machine tool.5. To improve the surface finish.6. To prevent the formation of built up edge.7. To protect the work piece against rusting.


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Requirements of a cutting Fluid

A cutting fluid should possess the following qualities in order to be of practical value:

1. It should have long life, free of excessive oxide formation that might clog circulationsystem.

2. It should be suitable for a variety of cutting tools and materials and the cuttingoperations.

3. It should have lubricating qualities, high thermal conductivity and low viscosity topermit easy flow and easy separation from impurities and chips, and should not stick towork piece or machine.

4. It should be transparent where high dimensional accuracy and fine finish are requiredin order to enable the operator to have a clear view of tool and work piece.

5. It should present no fire or accident hazards or emit obnoxious odors or vapors harmfulto operator or work piece; and should cause no skin irritation.

Types of Cutting Fluids1. Straight or neat oils

2. Water miscible cutting fluids3. Synthetics or semi chemical cutting fluids.

1) Straight or neat oils These are derived from petroleum, animal, marine or vegetable substances and

may be used straight or in combination.

The main function is lubrication and rust prevention They are chemically stable and lower in cost. They are usually restricted to light duty machining on metals of high

machinability, such as aluminum, magnesium, brass and leaded steels.2)Water miscible cutting fluids

This forms mixtures ranging from emulsions to solutions, which due to their highspecific heat, high thermal conductivity and high heat of vaporization are used onabout 90% of all metal cutting and grinding operations.

Water blend is usually in the ratio of one part of oil to 15 to 20 parts water forcutting and 40 to 60 parts water for grinding.

3) Synthetics( chemical) or semi chemical cutting fluids: Synthetic coolants refer to any coolant-lubricant concentrate that does not

contain petroleum oil.

Semi chemical coolants contain a small amount of mineral oils plus additives tofurther enhance lubrication properties. These incorporate best qualities of bothwater and chemical.

Advantages of Chemical Fluidsi) a very light residual film that is easy to removeii) Heat dissipation is rapid.iii) Good detergent propertiesiv) An easy concentration to control with no interference from tramp

oils.

Disadvantages :i) The lack of lubrication oiliness may cause some sticking in the moving parts

of machine tools.


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ii) The high detergency may irritate sensitive hands over a long period of time.iii) As compared to oils, there is less rust-control and lubrication and there is

some tendency to foam.

*** Pure water is by far the best cutting fluid available because of its highest heatcarrying capacity. Besides this it is cheap and easily available. Its low viscosity makesit to flow at high rates through the cutting fluid system and also penetrates the

cutting zone.But water corrodes the work material very quickly, particularly at high

temperatures prevalent in the cutting zone as well as the machine tool parts onwhich it is likely to spill. Hence other materials would be added to water to improveits wetting characteristics, rust inhibitors and any other additives to improvelubrication characteristics.

Lubrication in Metal Cutting1) Hydro dynamic lubrication2) Boundary lubricationIn hydro dynamic lubrication the surfaces under lubrication are separated by a

fluid film. The viscosity of the fluid is important in this type of lubrication in boundarylubrication a few thin layer of absorbed lubricant is present.

Selection of Cutting FluidsThe selection of a cutting fluid is dependent on the work material, tool material

and machining operation.

Table 2.1 Cutting fluids for different materials

MATERIAL CUTTING FLUIDS

Carbon steels Emulsions, low viscosity oils Alloy steels Emulsions, extreme pressure lubricants

Aluminum alloys Emulsions dilutedCopper Alloys Emulsions, fatty esters with water base solutions and chlorine.Titanium Alloys Chlorinated extreme pressure lubricantsCast iron Water base emulsions

CUTTING TOOL MATERIALS

Characteristics of an ideal tool material.

1. The material must remain harder than the work material at elevatedtemperature.(hot hardness)

2. The material must withstand excessive wear even through the relativehardness of the tool-work materials changes.(wear resistance)

3. The material must have sufficient strength and ductility to withstandshocks and vibrations and to prevent breakage.(Toughness)

4. The coefficient of friction at the chip tool interface must remain low forminimum wear and reasonable surface finish.

5. The cost and easiness of fabrication should be within reasonable limits.


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Types of Tool materials

1. Carbon steels2. Medium alloy steels3. High speed steels4. Stellites5. Cemented carbides6. Ceramics7. Diamonds8. Abrasives

1. Carbon SteelsThese are characterized by the low stability of the super cooled austenite.Therefore they have a high critical rate and low harden ability. Throughhardening can be achieved only in parts up to 12 to 15 mm in thickness ordiameter. Consequently steel may be recommended for small sized tools, whichare quenched in oil or molten salts, and for comparatively large tools (15 to 30

mm diameter) in which the cutting section is only the surface layer (files, coredrills, short reamers, etc.)

When large tools are hardened (dia over 30mm) the layer with a high hardness isso thin, even up on quenching in water, that the tools are not fit for cuttingpurposes.

Advantages

i) Cheapnessii) Low hardness( BHN 170 to 180)iii) Good machinabilityiv) Formability in the annealed state.v) Retain a tough unhardened core due to low hardenability. (This factor

improves resistance to breakage under vibration and impacts).

Disadvantagesi) Narrow range of hardening temperatureii) Necessity for rapid quenching in water or aqueous alkali solutions (salt).

** Carbon steels are applicable only for tools operating at low cutting speeds (about 12

m/min) since their hardness is substantially reduced at temperatures above 190-200o

C.

2. Medium alloy steels The high carbon medium alloy steels have carbon content akin to plain carbon

steels, but there is up to 5 % alloy content consisting of tungsten, molybdenum,chromium and vanadium. Small additions of one or more of these improve theperformance of carbon steels in respect of hot hardness, wear resistance, shock andimpact resistance and resistance to distortion during heat treatment.


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The alloy carbon steels broadly occupy a midway performance position betweenplain carbon and high speed steels.

They lose their required hardness at high temperature from 250 to 350 oC3. High Speed steels

High speed steels are distinguished for their high red hardness, their capacity toretain their structure (marten site), hardness and wear resistance at hightemperature generated on the cutting edges when machining at high cuttingspeeds. High speed steels are designed for the manufacture of high productiontools with high wear resistance which must retain their cutting properties attemperatures up to 600- 620 oC

High speed steels are obtained by alloying tungsten, chromium, vanadium, cobaltand molybdenum with steels.

a) 18-4-1 high speed steel: A common analysis: 18% tungsten, 4 % chromiumand 1 % vanadium, with a Carbon content of 0.6 0.7 %. This alloy istermed 18-4-1, while an increase of vanadium to 2 % provides 18-4-2 steel.

b) Cobalt high speed steel: this is sometimes called super high speed steel.Cobalt is added 2 to 15 % to increase hot hardness and wear resistance.One analysis of this steel contains 20 % tungsten, 4 % chromium, 2 %vanadium and 12 % cobalt.

c) Molybdenum high speed steels : this class contains a lower percentage oftungsten, this being compensated by the addition of molybdenum.

The steel containing 6% molybdenum, 6 % tungsten, 4 %chromium and 2 % vanadium have excellent toughness and cuttingability.

4. Stellites It is a non ferrous alloy with range of elements: cobalt 40 to 80 %; chromium

30 to 35 %; tungsten 12 to 19 %. In addition to one or more carbide formingelements, carbon is added in amounts of 1.8 to 2.5 %.

They cannot be forced to shape, but may be deposited directly on the tool shank inan oxy-acetylene flame; alternately, small tips of cast satellites can be brazed in toplace.

Satellite preserve hardness up to 1000 oC and can be operated on steel at cuttingspeed 2 times higher than for high speed steel.

These materials are not widely used for metal cutting, since they are very brittle,however, they are used extensively in some non metal cutting application, such asin rubbers, plastics where loads are gradually applied and the support is firm andwhere wear and abrasion are problems.

5. Cemented carbide These are so named because they are composed principally of carbon mixed with

other elements.

The basic ingredient of most cemented carbide is tungsten carbide.


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Abrasive particles held together by bonding material comprising the cutting edgesin grinding wheels known as abrasive wheels.

Natural abrasives- emery, corundum and diamond dust. Emery is rough and durable, containing about 70% aluminum oxide, a valuable

abrasive which does the actual cutting and is known as crystalline fused alumina. Diamond dust is slowly forging ahead as abrasive.

Carborandum is trade name for silicon carbide.- one of the most importantartificial abrasive.

9. Coated Tools

The performance of HSS tool can be improved by applying a coating of Titanium nitride(Ti N). This coating being in golden color also improves the appearance. Tool life increasednearly by ten times. Metal removal rate will be double. More number of regrind is possible. Itreduces flank wear.

Inert coating will reduce adhesive wear. Surface finish improves.

TOOL WEAR

INTRODUCTIONA new or newly ground tool has sharp cutting edges and smooth flanks. When put in to

operation, it gets subjected to cutting forces that are concentrated over a small contact areaover the rake face and flank. Also the chip slides over the rake face and machined surfacerubs over the flank surface of the cutting tool. The temperatures over the contact surface arepretty high. Each time the tool enters or exits from cut, it is subjected to mechanical as wellas thermal shock. Under such conditions the tool wear occurs.

The causes for wear can be defined to(i) The interaction between the chip and the tool and between the work and the tool.(ii) Cutting forces, and (iii) Temperature developed during cutting process.

The tool wear causes the tool to loose its original shape so that with the passage of timeit gives un satisfactory performance. This involves loss of dimensional accuracy, increasedsurface roughness and increased power consumption. After a certain degrees of wear thetool has to be replaced or re sharpened usually for further use. This leads to loss ofproduction time due to machine down time, in addition to the cost of replacing or resharpening the tool.

The tool wear depends on a number of factors such as hardness and type of toolmaterial, material and condition of work piece, dimension of work piece, feed and depth ofcut, tool geometry and cutting fluid.

TYPES OF TOOL WEAR

i). Flank Wear (Edge wear)

This wear is also called wear land. Work and tool are in contact at cutting edge only.Usually wear first appears on the clearance face of the tool in the form of wear land and ismainly due to friction and abrasion.

This wear produces wear lands on the side and end flanks of the tool on account of therubbing action of the machined surface. In the beginning the tool is sharp and the wear land


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has zero width. But very zoon it develops and grows in size on account ofabrasion, frictionand to some extent adhesion and shear.

This is predominant in brittle materials having discontinues chip. Flank wear starts atthe cutting edge and then starts widening along the clearance face. It is independent of cuttingconditions and tool/ work materials.

The flank wear progress of cutting tool is shown in the figure. The curve is cutting timeVs width of wear land. It may be noted that the primary stage is one ofrapid wear due to high

stresses at the tool point. The wear rate is more or less linear in the secondary stage and is thesteady wear, but in the tertiary stage wear rate increases rapidly and resulting incatastrophic failure.

Fig-2.9: A typical Wear curve for a cutting tool

ii). Face wear (Crater wear)On the face of the tool there is a direct contact between chip and tool. Wear takes place

in the form of cavity or crater, which has its origin above the cutting edge. With time cavitygoes on widening. This is prominent in ductile materials.

The crater occurs on the rake face at the point of impingement of chip and extends endsnear the nose which cause rapid rupture. It leads o weakening the tool, increases cuttingtemperature, friction and cutting force.

These types of wear occur mainly bydiffusion.

Fig-2.10: Tool Wear Regions in Metal Cutting


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While all other types of tool failures can be effectively reduced by changing speed, feed,depth of cut, flank wear is a progressive form of deterioration. The progress of flank wear incutting tool is as shown in the graph.

iii). Nose wearIt is similar to flank wear in certain operations like finish turning. This is more prominent

than flank wear.

Limiting value of width of Wear Land at the Flank

The wear LAND ON THE flank face is not uniform width. It larger at or near the two endsof the active portion of the side cutting edge. At the nose portion the chip flow is rathercomplicated and the wearing conditions severe. At the rear portion of the flank wear land,groove or notch gets formed on account of accelerated wear. It has been suggested thataccelerated wear is caused by abrasion and metal transfer enhanced by chemical interaction

with the surrounding atmosphere. The width of the wear land is usually maximum at the rearend of the flank land.

Fig-5.13

Tool Wear Mechanisms

i) Shearing at high temperatureThe strength of hard metal decreases at high temperatures. Therefore its

shear yield stress becomes much smaller than what it is at room temperature.

Though the metal sliding over it has lower yield stress, nevertheless, the chip mayget so much work hardened as to be able to exert frictional stress sufficient tocause yielding by shear of the hard tool metal. The higher the temperature at theinterface greater is this effect.

ii) Diffusion WearWhen metal is in sliding contact with another metal and the temperature

at their interface is high, conditions may become right for the alloying atomsfrom the harder metal to diffuse in to the softer marix, thereby increasing thelatters hardness and abrasiveness ( see fig). on the other hand atoms from the

softer metal may also diffuse into the harder medium, thus, weakening thesurface layer of the latter to such an extent that particles on it are dislodged/ torn(or sheared off) and are carried away by the flowing chip material. Diffusionphenomenon is strongly dependent on temperature. For example,diffusion rate is approximately doubled for an increment of the order of 20oC inthe case of machining steel with HSS tools.


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Fig-2.2: Wear by plastic yielding and shear

Fig-2.3: Diffusion Wear Process

iii) Adhesive wear (Attrition Wear)When a softer metal slide over a harder metal it always presents a newly

formed (nascent) surface to the same portion of the hard metal. On account offriction, high temperature and pressure, particles of the softer material adhere toa few high spots of the harder metal (see fig.). As a result, flow of the softer metalover the surface of the harder metal become irregular or less laminar, and contact between the two becomes less continues. More particles join up with thosealready adhering and so called built up edge is formed. Sooner or later some ofthese fragments, which may have grown up to macroscopic size, are torn from thesurface of the hard metal. When this process continues for some time, it appearsas if the surface of the hard metal has been nibbled away and made un even.


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Fig-2.4: Adhesive Wear Mechanism

iv). Abrasive Wear

the softer metal sliding over the surface of the harder metal may containappreciable concentrations of hard particles. For example, castings may have pockets of sandin them. In these conditions, the hard particles act as small cutting edges like those of agrinding wheel on the surface of a hard metal which in due course, is worn out throughabrasion (see fig.) in addition, the particles of the hard tool metal, which intermittently get tornout from its surface are dragged along the tool surface or rolled over. These particles ploughgrooves in to the surface of the hard tool metal.

Fig-2.6: Abrasive wear mechanism


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Fig-2.7: Fatigue Wear Mechanism

v). Fatigue Wear

When two surface slide in contact with each other under pressure, asperities onthe surface interlock with those of the other. Due to the frictional stress, compressive stress isproduced on one side of each interlocking asperity and tensile stress on the other side. Afterthe asperities of a given pair have moved over or through each other, the above stresses arerelieved. New pairs of asperities are, however soon formed and the stress cycle is repeated.Thus the material of the hard metal near the surface undergoes cyclic stress. This phenomenoncauses surface cracks which ultimately combine with one another and lead to the crumbling of

the hard metal.Further, the hard metal may also be subjected variable thermal stress owing to

temperature changes brought about by cutting fluid, chip breakage and variable dimensions ofcut, again contributing to fatigue wear.

vi). Electrochemical Effect.

It has been argued that since sufficiently high temperatures exist on the chip toolinterface, a thermoelectric emf is set up in the closed circuit due to the formation of a hot junction at the chip tool interface between the dissimilar tool and the work materials. Thiscurrent may assist the wear process at the rake face in some way, for example by aiding the

diffusion of carbon ions from the carbide tool to the flowing chip.

vii). Oxidation effect

There is evidence to suggest that the formation of grooves or notches at the rake faceand the flank is on account of the sliding of portions of the chip and the machined surface which have reacted with the oxygen in the atmosphere to form abrasive oxides. For example when machining steel work piece with HSS or cemented carbide tools, groove formation isgreatly accelerated if the cutting zone is subjected to a jet of oxygen. On the other hand, it is


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retarded or even eliminated if the cutting zone is subjected to a neutral atmosphere using jetsof nitrogen or argon.

Fig-2.8 viii). Chemical Decomposition

Localized chemical reactions may occur that weaken the tool material through

formation of weak compounds or dissolution of the bond between the binder and the hardconstituents in a carbide tool. The weakened particles are easily torn away by the asperities atthe underneath of chip or on machined surface.

Effect of Cutting Speed, Feed and Depth of cut on surface finish

In most cases surface roughness decreases with increase in cutting speed, decrease infeed and depth of cut. In the machining of mild steel with cemented carbide tools, theroughness is found to decrease rapidly up to some critical value of speed after which there isvery little improvement. This is explained to be due to reduction in size of built up edge with

increase in cutting speed. Large feed is more detrimental to surface finish than a large depth ofcut. Roughness is not very much affected at low depth of cuts.

TOOL LIFE

Tool life is the useful cutting life of a tool expressed in time or some other unit. Theperiod during which the tool cuts satisfactorily is called its life. Tool life has manyinterpretation, but in general it is defined as actual cutting time between two successivegrinds.

Factors up on which tool life depends

1). Cutting speed.2). Physical properties of work piece.3). Area of cut4). Ratio of feed to depth of cut5). Shape and angles of tool6). Tool material and its heat treatment7). Nature and quantity of coolant8). Rigidity of tool, work and machine tool.


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1). Cutting Speed (Taylors Equation)Cutting speed has the maximum influence on Tool Life. Tool life decreases as the cutting

speed increases. Taylors equation defines the relation ship.VT n = CV- cutting speed in ft/ min, T- Tool life in minute, C and n are constants depends on

machine tool, work piece, tool geometry etc.

2). Physical Properties of work piece.Cutting speed depends on work piece and tool material. It is also found that Tool life can

be correlated with micro structure of work. In general hard micro constituents in the matrixresults in poor tool life. Also tool life is better with larger grain size. It is also found that similarmetallographic structures will exhibit similar machining characteristics, regardless of theirrelative properties.

The effect of properties of materials is given by the equation,

1.68

B

B

ConstantV = % reduction

H

H - Brinell hardness

3). Area of cut

The cutting speed V is inversely proportional to the area of cut and is represented by theequation given below.

kV = = C

(A+n)

V - cutting speed, A- area ofcut, k,n,C are constants

4). Feed and depth of cut

The life of the cutting tool is influenced by the amount of metal removed by the tool perminute. When we are using fine speed, the area of the chip passing over the tool face is greaterthan that of a coarse feed for a given volume of metal removal. If we offset this advantage infavor of the thick chip, the tool, forces to produce thicker chips, anyway it is possible to balancetwo opposing influences to obtain optimum feed rate.

The effect of feed and depth of cut on tool life is given by the formula,

0.19 0.36 0.08

257V =

T f a

/min

min

/min

Where V cutting speed in m

T Tool life in

f feed in mm

a depth of cut in mm

ThisWhen the feed is more, more localized action and heating of tool at chip tool interface

takes place.


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5). Shape and angles of tool

i) Effect of rake angle when back rake angle increases, the cutting forcedecreases, because of small shear strain. When negative rake angle is used,shear strain is more, but for practical range, the negative rake angle hashigher cutting force than positive rake angles. Large rake angles produces

chipping and smaller rake angles generates greater heat or an excessivewear and deformation in tool.

ii) Change in end cutting edge angle has little effect on tool life. Howeverlarger is the angle longer is the tool life. Similarly larger is the side cuttingedge angle longer is the tool life. But an angle higher than 15o produceschipping and the tool life decreases.

iii) Cutting angle cutting speed depends on cutting angle. As the cuttingangle increases the power required to machine increase

iv) Nose radius- increase in nose radius increase tool life. Small nose resultsin excessive stress concentration and greater heat generation. Therelationship is

0.9927 0.243V T = 331 r

6). Effect of lubricant

Lubrication decreases the cutting forces. The effect of cutting fluid is more predominantover the lower range of cutting speed, rather than at higher cutting speeds. At low speeds thecutting fliud acts as lubricant and reduces friction in tool chip interface.

7). Nature of cutting

In the case of continues cutting, the tool life is much better than in intermittent cutting.The intermittent cutting gives regular impacts on the tool leading to its failure much earlier.

MACHINABILITY

The term mach inability is used to refer to the ease with which a given work material can be machined under a given set of cutting conditions. Generally machinability is defined bydifferent criteria; say the tool life, cutting force, surface roughness etc.

Machinability Criteria

The ease of machining different materials can be compared in terms of tool life, cuttingforces or surface finish under similar cutting conditions. Other criteria such as ease of chipdisposal, cutting temperatures, operator safety etc. may also be employed.

The different factors for evaluating machinability of any metal are:

1. Tool Life2. Form and size of chip and shear angle;


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3. Cutting forces and power consumption4. Surface finish5. Cutting temperature6. Surface finish7. Rate of metal removal per tool grind8. Rate of cutting under standard force9. Uniformity in dimensional accuracy of successive parts.

Among the above tool life is the most important factor for assessing machinability. A bettermachinable metal is one which permits higher cutting speed for a given tool life. Cutting force becomes important criterion for machinability where it has to be limited considering therigidity and vibration in machine. The material which requires higher cutting forces formachining under given condition is less machinable.

For roughing operation prime consideration is for maximum metal removal rate. In thecase of finishing, the surface finish forms the criterion for machinability.

Variables affecting Machinbility

1). Machine Variables- indirectly affect mach inability

The machine variables area). rigidity of the machine,b). Power and accuracy of the machine tool,c). the machine should be rigid and have sufficient power to with stand the induced

cutting forces and to minimize deflections.

2). Tool Variables

The various tool variables affecting machinability area). geometry and tool material,b). Nature of engagement of tool with the work,c). Rigidity of tool

3). Cutting conditionThe cutting speed and dimensions of cut influence the tool life and hence machinability.

4). Work material variablesThe various work material variables affecting machinability area). Chemical composition of work piece material

b). Microstructure composition of work piece materialc). Mechanical properties like ductility, toughness, brittleness etc.d). Physical properties of work materiale). Method of production of work material.

Tool Life of machining economics. In general, if a work material produces more rapid toolwear, the tool would have to be

Effect of various machining variables on surface finish is as follows.


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1. Increase of cutting speed - improves surface finish2. Increase of feed rate - deteriorates3. Depth of cut - Deteriorates4. True rake angle - improves5. Formation of built up edge - deteriorate6. Cutter vibration and roughness of cutting edge - deteriorate7. Nose radius - improves

Evaluation of Machinability

The following criteria suggested for evaluating machinability are

1. Tool life for a given cutting speed and tool geometry2. Rate of metal removal3. Magnitude of cutting forces and power consumption4. Quality of finish of the machine surface5. Dimensional stability of the finished work6. Heat generated during cutting

7. Ease of chip disposal8. Chip hardness9. Shape and size of chips10. Power consumption per unit volume of material removed.

The main factor to be chosen for evaluating the machinability depends on type ofoperation and production requirements. However in production, tool life is generallyconsidered the most important factor for evaluating machinability. Higher the tool lifebetter is the machinability of the work material.

The machinability of a work piece tends to decrease as its hardness, tensile strength,carbon content increases. It also tends to decrease with increase in hard oxide, carbide or

silicate inclusions and also with decrease in grain size.

Advantages of high machinability

1. Good surface finish2. Higher cutting speed can be used3. Less power consumption4. Metal removal rate is high5. Less tool wear

Machinability index

It is a quantitative measure of machinability. It is used to compare the machinability ofdifferent metals and acts as a quick and reliable checking method. The rated machinabiluityof two or more metals may vary for different process of cutting such as heavy turning, lightturning, forming, milling etc. the machinability index of different materials is taken relativeto the index which is standardized.

The machinability index of free cutting steel is arbiterly fixed at 100%. For othermaterials the index is found as below.


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Cutting speed of materialfor20min toollifeMachinability index, I =

cuttingspeedof standardsteelfor 20min toollife

. .Vi

i e IVs

=

The free cutting steel which is considered as a standard has carbon content of 0.13,Maximum manganese of 0.06 to 1.10 and sulphur of 0.008 to 0.03 %

Table: 2.2 ; the machinability Index for some common materials are

MaterialMachinability index

Low carbon steel55.6

Stain less steel25

Red brass180 %

Alluminium alloy390 1500

Magnesium alloy500 2000

TOOL LIFE

A tool cannot be used for un limited time. When it has been used for a considerabletime, it ceases to cut satisfactorily unless it is re sharpened. It has definite life.

Tool life is the useful cutting life of a tool expressed in time or some other unit. Theperiod during which the tool cuts satisfactorily is called its life. Tool life has manyinterpretations, but in general it is defined as actual cutting time between twosuccessive grinds.

Tool life is an important factor in a cutting tool performance since considerable time islost whenever tool is ground and reset. Therefore, the cutting tool should have longer life.The tool life between reconditioning and replacement can be measured in a number ofways.

1. Volume of material removed between two successive grindings.2. Number of pieces machined between two successive grindings.3. Total time of operation.

4. Equivalent cutting speed.Tool life (T) based on volume of metal removed between two successive grindings for a

definite depth of cut,; feed and cutting speed can be represented by the relation.


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3Vvol = 1000 Vt mm /min

Where, depth of cut in mm

feedin mm/rev

V-cuttingspeed in m/min

t - time totoolfailurein min

a f

a

f

Factors up on which tool life depends

1). Cutting speed.2). Physical properties of work piece.3). Area of cut4). Ratio of feed to depth of cut5). Shape and angles of tool6). Tool material and its heat treatment7). Nature and quantity of coolant8). Rigidity of tool, work and machine tool.

1). Cutting SpeedCutting speed has the maximum influence on Tool Life. When cutting speed increases,

the cutting temperature increases. Due to this hardness of the tool decreases. Hence the toolflank wears and crater failure occurs. Hence it is obvious that the Tool life decreases as thecutting speed increases.

Taylors Equation

Taylor developed an empirical relationship between cutting speed and tool life. It describes the

relation ship.VT n = C,V- cutting speed in ft/ min, T- Tool life in minute, C and n are constants depends on

machine tool, work piece, tool geometry etc.

n = 0.1 to 0.5 for HSStools

= 0.2 to 0.4 for tungstencarbide tools

= 0.4 to0.6 for ceramictools

C = constant,it is numerically equal to cutting speed that gives a tool life of 1 min.

2). Physical Properties of work piece.Cutting speed depends on work piece and tool material. It is also found that Tool life can

be correlated with micro structure of work. In general hard micro constituents in the matrixresults in poor tool life. Also tool life is better with larger grain size. It is also found that similarmetallographic structures will exhibit similar machining characteristics, regardless of theirrelative properties.

The effect of properties of materials is given by the equation,


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1.68

B

B

ConstantV = % reduction

H

H - Brinell hardness

3). Area of cut

The cutting speed V is inversely proportional to the area of cut and is represented by theequation given below.

kV = = C

(A+n)

V - cutting speed, A- area ofcut, k,n,C are constants

4). Ratio of feed to depth of cut

When the feed is more, more localized action and heating of tool at chip tool interfacetakes place.

5). Shape and angles of tool

v) Effect of rake angle when back rake angle increases, the cutting forcedecreases, because of small shear strain. When negative rake angle is used,shear strain is more, but for practical range, the negative rake angle hashigher cutting force than positive rake angles. Large rake angles produceschipping and smaller rake angles generates greater heat or an excessive

wear and deformation in tool.vi) Change in end cutting edge angle has little effect on tool life. Howeverlarger is the angle longer is the tool life. Similarly larger is the side cuttingedge angle longer is the tool life. But an angle higher than 15o produceschipping and the tool life decreases.

vii) Cutting angle cutting speed depends on cutting angle. As the cuttingangle increases the power required to machine increase

viii) Nose radius- increase in nose radius increase tool life. Small nose resultsin excessive stress concentration and greater heat generation. Therelationship is

0.9927 0.243

V T = 331 r

6). Effect of lubricant

Lubrication decreases the cutting forces. The effect of cutting fluid is more predominantover the lower range of cutting speed, rather than at higher cutting speeds. At low speeds thecutting fliud acts as lubricant and reduces friction in tool chip interface.

7). Nature of cutting


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in the case of continues cutting, the tool life is much better than in intermittent cutting.The intermittent cutting gives regular impacts on the tool leading to its failure much earlier.

Optimum Machining PracticesIdeally the work piece must be turned to the size with one roughening and finish cut.

The great part of the excess material should be removed in the roughing cut as fast as

possible without leaving a surface too torn and rough and without warping the work piece.For optimum machining, proper consideration should be given to1. The size and shape of work piece2. Material to be removed,3. Kind of tool used4. Nature of cut to be made5. Speed, depth and feed of cut to be employed, so that the best results are achieved withminimum cost.

It is important to bear in mind that the tool forces are markedly higher at slow cuttingspeeds and as such the tools with low tensile strength (ceramics) are likely to chip or crack.At higher cutting speeds or with harder work pieces, the feed and the ratio of feed to depth

of cut become more important; it become more advisable to use high depth to feed ratiothan the high feed to depth ratio.

Fig- (a) Cutting speed more or less constant

with increase in cutting speed.

0

5

10

15

20

25

30

35

40

0 20 40 60

cutting speed

tool

cuttingforce

Fig- (b) Power requirements increase directly in

proportion to the cuting speed

0

5

10

15

20

25

30

35

40

0 10 20 30 40

cutting speed

Powerrequirement


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Fig- (c) Power requirements increase directly

in proportion to the cuting speed

0

5

10

15

20

25

30

35

40

-1 1 3 5 7 9 11 13 15

Depth of cut, rate of feed

Toolcutti

ngforce

Fig- (d) Power requirements increase as the

rate of feed is increased

0

5

10

15

20

25

30

35

40

-1 1 3 5 7 9 11 13 15

rate of feed

Powerreq

uirement

Fig- (e) chip/ tool interface temperature

increases as the cutting speed is increased

and also the tool force increases as the tool

temperature increases

0

10

20

30

40

-1 1 3 5 7 9 11 13 15

cutting speed

chiptoolinterface

temperature

Ideally work piece must be turned in to the size by one roughing cut and onefinishing cut. The greater part of metal will be removed in the roughing cut as fast aspossible.

Economics of Machining

It is not sufficient to device a feasible procedure for a desired component orcommodity. The variables affecting the economics of a machining operation are numerous which include the tool material, machine tool capacity, cutting conditions. Ecoomicselection of a cutting condition involves technical and cost data which are not readlyavailable to the operator, so that an optimum selection can seldom be achieved by thisapproach.

Taylor stressed this point some years ago and suggested that an optimum can only beapproached, if the selection is made by a planning engineer with access to all relevantinformation.

In selecting economic operating conditions, machine tool capabilities must be taken into account. Often the desired conditions may not be attainable on the machine tool


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proposed for particular operation. It is then necessary to either change the operatingcondition or review the machine tool selection by a cost comparison, to see if a change inmachine tool is effectively justified. The change may involve purchase of new machine orpossibly modifying the existing machine. The capacity limits of a machine tool limiting theselection of machining conditions may be listed as follows.(1). Machine tool maximum limit.(2). Machine tool maximum speed.

(3). Machine tool maximum power.(4). Maximum available cutting or thrust force.(5). Speed and feed limits for the desired surface finish.(6). Machine tool feed and speed steps.

Cost per component, production rate and profit rate- the machining cost per component ismade up of a number of different costs. For simplicity, single pass case will be considered.

i) Non productive cost per component (C1). It includes the cost of loading and unloadingcomponent, the ideal time cost and other non cutting time costs not included in the totalcost per component. This cost is determined by adding all non productive time T1 andmultiplying it by the cost rate x. the cost rate includes the labor and overhead cost rates.

Thus C1 = xT1.ii). The cost of machining time (C2) it is found by multiplying the cost x, by the

machining time per component Tc. The machining time is the time required for the tool totraverse the component (feed engaged), whether the tool is continuesly in contact with thework or not. The cost C2 is, therefore, given by C2 = x Tc.

.

Production Engineering-M II

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