By

V. THULASIKANTHAssistant Professor

Mechanical Engineering Departmentvtkvsk@gmail.com

THEORY OF METAL CUTTING

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Orthogonal and oblique cutting– Classification of cutting tools: single, multipoint – Tool signature for single point cutting tool – Mechanics of orthogonal cutting – Shear angle and its significance – Chip formation– Cutting tool materials– Tool wear and tool life – Machinability – Cutting Fluids– Simple problems.

Syllabus

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TEXT BOOKS

1. Sharma, P.C., A textbook of Production Technology – Vol I and II, S. Chand &

Company Ltd., New Delhi, 1996.

2. Rao, P.N., Manufacturing Technology, Vol I & II, Tata McGraw Hill Publishing

Co., New Delhi, 1998.

Metal Cutting

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Metal cutting or Machining operation is to produce a desired shape, size and finish of a component by removing excess material in the form of chips.

So, primary objective of metal cutting is to produce chips which are thrown away.

Chips may constitute more than 50% of initial work piece.

Machining processes are performed on metal cutting machines, more commonly termed as machine tools using various types of cutting tools

Metal cutting process in general should be carried out at high speeds and feeds with least cutting effort at minimum cost.

Factors affecting metal cutting

1. Properties of Work material

2. Properties & geometry of cutting tool

3. Interaction between tool and work

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External

Internal

threadingforminggroovingfacingturning

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Mechanics of Metal CuttingA cutting tool exerts compressive force on the workpiece which stresses the work material beyond the yield point and therefore metal deform plastically and shears off.

Plastic flow takes place in a localized region called the shear plane.

Sheared material begins to flow along the cutting tool face in the form of chips.

Flowing chips cause tool wear.

Applied compressive force which leads to formation of chips is called cutting force.Heat produced during shearing action raises the temperature of the workpeice, cutting tool and chips.

Temperature rise in cutting tool softens and causes loss of keeness in cutting edge.

Cutting force, heat and abrasive wear are important features in metal cutting.

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Types of Metal Cutting Process

Orthogonal cutting is also known as two dimensional metal cutting in which the cutting edge is normal to the work piece. (angle = 90deg)

Oblique cutting is also known as three dimensional cutting in which the cutting action is inclined with the job by a certain angle called the inclination angle. (angle ≠ 90deg)

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Types of Cutting ToolsCutting tools performs the main machining operation.

It is a body having teeth or cutting edges on it.

They comprise of single point cutting tool or multipoint cutting tools.

Single point cutting tool : This type of tool has a effective cutting edge and removes excess material from the workpeice along the cutting edge.

These tools may be left-handed or right-handed.

Again single point cutting tools classified as solid type and the tipped tool.

Brazed tools are generally known as tool bits and are used in tool holders.

The tipped type of tool is made from a good shank steel on which is mounted a tip of cutting tool material.

Tip may be made of high speed steel or cemented carbide.

Different types of carbide tips are generally used on tipped tool.

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Geometry comprises mainly of nose, rake face of the tool, flank, heel and shank etc.

The nose is shaped as conical with different angles.

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Types of ChipsChips are separated from the workpiece to impart the required size and shape.

The chips that are formed during metal cutting operations can be classified into four types:

1. Continuous chips

2. Continuous chips with built-up edge

3. Discontinuous or segmental chips.

4. Non homogenous chips

1. Continuous chips

Chip is produced when there is low friction between the chip and tool face

This chip has the shape of long string or curls into a tight roll

Chip is produced when ductile materials such as Al, Cu, M.S, and wrought Iron are machined.

Formation of very lengthy chip is hazardous to the machining process and the machine operators.

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It may wrap up on the cutting tool, work piece and interrupt in the cutting operation.

It becomes necessary to deform or break long continuous chips into small pieces.

It is done by using chip breakers and this can be an integral part of the tool design or a separate device.

2. Continuous chips with built-up edge

When high friction exists between chip and tool, the chip material welds itself to the tool face.

Welded material increases friction further which in turn leads to the building up a layer upon layer of chip material.

Build up edge grows and breaks down when it becomes unstable.

Chips with build up edge result in higher power consumption, poor surface finish and large tool wear

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3. Discontinuous or segmental chipsChip is produced in the form of small pieces. These types of chips are obtained while machining brittle material like cast iron, brass and bronze at very low speeds and high feeds.

For brittle materials it is associated with fair surface finish, lower power consumption and reasonable tool life.

For ductile materials it is associated with poor surface finish excessive tool wear.

4. Non-homogeneous chips

It will be in the form of notches and formed due to non-uniform strain in materalduring chip formation.

Non homogenous chips are developed during machining highly hard alloys like titanium.

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Chip Control and Chip BreakersDuring machining high tensile strength materials chips has to be properly controlled.

Carbide tip tools will be used for high speeds which leads to high temperature and produce continuous chips with blue color.

If the above mentioned chips are not broken means it will adversely effect the machining in following ways,

•Spoiling cutting edge

•Raising temperature

•Poor surface finish

•Hazardous to machine operator

Two ways are employed to overcome all the above drawbacks.

First one is Proper selection of cutting conditions and second one is chip breakers are used to break the chips.

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Proper selection of cutting conditionsSince the cutting speed influences to the great extend the productivity of machining and surface finish, working at low speeds may not be desirable.

If the cutting speed is to be kept high, changing the feed and depth of cut is a reasonable solution for chip control.

Chip breakerThere are two types of chip breakers1.ΠExternal type, an inclined obstruction clamped to the tool face2. Integral type, a groove ground into the tool face or bulges formed onto the tool face

clamped

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NOMENCLATURE Of SINGLE POINT TOOL

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Feed

Back rake angle (αb)It is the angle between the face of the tool and a line parallel with base of the toolmeasured in a perpendicular plane through the side cutting edge.

This angle helps in removing the chips away from the work piece.

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Side rake angle (αs)It is the angle by which the face of tool is inclined side ways.

This angle of tool determines the thickness of the tool behind the cutting edge.

It is provided on tool to provide clearance between work piece and tool so as to prevent the rubbing of work- piece with end flank of tool.

End relief angleIt is defined as the angle between the portion of the end flank immediately belowthe cutting edge and a line perpendicular to the base of the tool, measured at right angles to the flank.

It is the angle that allows the tool to cut without rubbing on the work- piece.

Side relief angleIt is the angle that prevents the interference as the tool enters the material.

It is the angle between the portion of the side flank immediately below the side edge and a line perpendicular to the base of the tool measured at right angles to the side.

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End cutting edge angleIt is the angle between the end cutting edge and a line perpendicular to the shank of the tool.

It provides clearance between tool cutting edge and work piece.

Side cutting edge angleIt is the angle between straight cutting edge on the side of tool and the side of the shank.

It is also known as lead angle.

It is responsible for turning the chip away from the finished surface.

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Tool SignatureConvenient way to specify tool angles by use of a standardized abbreviated system is known as tool signature or tool nomenclature.

The seven elements that comprise the signature of a single point cutting tool can be stated in the following order:

Tool signature 0-7-6-8-15-16-0.81. Back rake angle (0°)2. Side rake angle (7°)3. End relief angle (6°)4. Side relief angle (8°)5. End cutting edge angle (15°)6. Side cutting edge angle (16°)7. Nose radius (0.8 mm)

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Properties of cutting tool materials

1. Red hardness or Hot Hardness: It is the ability of a material to retain its hardness at high temperature

2. Wear resistance: It enables the cutting tool to retain its shape and cutting efficiency

3. Toughness: It relates to the ability of a material to resist shock or impact loads associated with interrupted cuts

Classification tool materials1. Carbon-Tool Steels: 0.6-1.5% carbon + little amount of Mn, Si, Cr, V to increase hardness.

Low carbon varieties possess good toughness & shock resistance.

High carbon varieties possess good abrasion resistance

2. High Speed Steels (HSS): High carbon+ little amount Tungsten, Molybdenum, Cr, V & cobalt to increase

hardness, toughness and wear résistance. High operating temperatures upto 600oC.

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Two types of HSS i.e, is T-type and M-Type Vanadium increases abrasion resistance but higher percentage will decreases

grindability. Chromium increases hardenability Cobalt is added to HSS to increase red hardness.

3. Cast Cobalt Base Alloys: It is a combination of W, Cr, carbon and Cobalt which form an alloy with red

hardness, wear resistance and toughness. It is prepare by casting. Used for machining Cast iron, alloy steels, non-ferrous metals and super alloys

4. Cemented Carbides: These are carbides of W, Titanium and tantalum with small amount of cobalt

produced by means of powder metallurgy route. Two types i.e, Straight Tungsten Carbide Cobalt Grade and Alloyed Tungsten

Carbide Grade

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Straight Tungsten Carbide Cobalt Grade : Cast iron, non ferrous alloys, plastics, wood, glass etc.

Alloyed Tungsten Carbide Grade: All grades of steel at 3 to 4 times more speeds than HSS

5. Ceramic Tools: Aluminium Oxide, Silicon Carbide, Boron Carbide, Titanium Carbide, Titanium

Boride

High speed, longer tool life, superior surface finish, No coolant is required.

6. Diamond Tools: More abrasion resistance Used for turning grinding wheels Used to produce mirror surface finish. Diamond abrassive belts are used to produce TV screens Poly crystalline diamond inserts are brazed into cutting edges of circular saws for

cutting construction materials like concrete, refractories, stone etc.

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Tool Life Properly designed and ground cutting tool is expected to perform the metal cutting operation in an effective an smooth manner

If a tool is not giving satisfactory performance it is an indicative of tool failure.

Following are the adverse effects observed during operation;

During operation cutting tool may fail due to following;

Extremely poor surface finish on the workpieceHigher consumption of powerWork dimensions are not produced as specifiedOverheating of cutting toolAppearance of burnishing band on the work surface

Thermal cracking and softeningMechanical ChippingGradual wear

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1.Thermal Cracking and SofteningDuring cutting operation lot of heat will be generated due to this cutting tool tip and area closer to cutting edge will become hot.

Cutting tool material will be harder up to certain limit (temperature & pressure), if it crosses the limit it starts deforming plastically at tip and adjacent to the cutting edge under the action of cutting pressure and high temperature.

Tool looses its cutting ability and it is said to have failed due to softening.

Main factors responsible for this condition are;

High cutting speedHigh feed rateMore depth of cutSmall nose radiusChoice of wrong tool material

Tool life is defined as the time interval for which tool works satisfactorily between two successive grinding or re-sharpening of the tool.

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Working temperatures for common tool materials are;

Carbon tool steels 200oC - 250oCHigh speed steel 560oC - 600oCCemented Carbides 800oC - 1000oC

Tool material is subjected to local expansion and contraction due to severe temperature gradient.

Gives rise to thermal stresses further leads to thermal cracks.

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2. Mechanical Chipping

Mechanical chipping of nose an cutting edge of the tool are commonly observed causes for tool failure.

Reasons for failure are High cutting pressure, Mechanical impact, Excessive wear, too high vibrations and chatter, weak tip an cutting edge, etc.

This type of failure is pronounced in carbide tipped and diamond tools due to high brittleness of tool material.

Chipped Tip

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3. Gradual wearWhen a tool is in use for some time it is found to have lost some weight or mass implying that it has lost some material from it due to wear.

Wear locations: Crater wear location

Flank wear location

Crater wearDue to pressure of the hot chip

sliding up the face of the tool, crater

or a depression is formed on the face

of tool. (Ductile materials)

By diffusion shape of crater formed

corresponds to the shape of

underside of the chip

Crater wear

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Flank wearOccurs between tool and workpiece

interface

Due to abrasion between tool flank

and workpiece

Hard microconstituents of cut

material and broken parts of BUE.

Common in Brittle material.

The entire area subjected to flank

wear is known as WEAR LAND (VB),

occurs on tool nose, front and side

relief faces

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Mechanism of wear

Adhesion wear: Fragments of the work-piece get welded to the tool surface at high temperatures; eventually, they break off, tearing small parts of the tool with them.

• Abrasion: Hard particles, microscopic variations on the bottom surface of the chips rub against the tool surface and break away a fraction of tool with them.

• Diffusion wear: At high temperatures, atoms from tool diffuse across to the chip; the rate of diffusion increases exponentially with temperature; this reduces the fracture strength of the crystals.

•Chemical wear: Reaction of cutting fluid to material of tool

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Heat Generated During Machining

Heat finds its way into one of three placesWorkpiece, tool and chips

Heat Dissipation

Ideally most heat taken off in chipsIndicated by change in chip color as heat

causes chips to oxidizeCutting fluids assist taking away heat

Can dissipate at least 50% of heat created during machining

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Cutting Fluids—Types and Applications Cutting Fluids

Essential in metal-cutting operations to reduce heat and frictionCenturies ago, water used on grindstones100 years ago, tallow used (did not cool)Lard oils came later but turned rancidEarly 20th century saw soap added to waterSoluble oils came in 1936Chemical cutting fluids introduced in 1944

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Cutting fluid is a type of coolant and lubricant designed specifically for metalworking and machining processes.

There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols (mists), and air or other gases.

They may be made from petroleum distillates, animal fats, plant oils, water and air, or other raw ingredients.

Depending on context and on which type of cutting fluid is being considered, it may be referred to as cutting fluid, cutting oil, cutting compound, coolant, or lubricant.

What is Cutting Fluid ?

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Economic Advantages to Using Cutting Fluids

Reduction of tool costs

Reduce tool wear, tools last longer

Increased speed of production

Reduce heat and friction so higher cutting speeds

Reduction of labor costs

Tools last longer and require less regrinding, less downtime,

reducing cost per part

Reduction of power costs

Friction reduced so less power required by machining

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Characteristics of a Good Cutting Fluid

Good cooling capacityGood lubricating qualitiesResistance to rancidityRelatively low viscosityStability (long life)Rust resistanceNontoxicTransparentNonflammable

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Types of Cutting Fluids

Most commonly used cutting fluidsEither aqueous based solutions or

cutting oilsFall into three categories

Cutting oilsEmulsifiable oilsChemical (synthetic) cutting fluids

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Refrigerated Air System

Another way to cool chip-tool interfaceEffective, inexpensive and readily

availableUsed where dry machining is necessaryUses compressed air that enters vortex

generation chamberCooled 100ºF below incoming air

Air directed to interface and blow chips away

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