Unit 3 THEORY OF METAL CUTTING ME 1008

Unit – 3 THEORY OF METAL CUTTING ME 1008 Manufacturing Technology By S. SAMPATH KUMAR Assistant Professor Mechanical Engineering Department SRM Nagar, Kattankulathur – 603 203 1

Syllabus 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. 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 Mc. Graw Hill Publishing Co. , New Delhi, 1998. 2

Metal Cutting 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. Chips may constitute more than 50% of initial work piece. Machining processes are performed on metal cutting machines, 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 3

turning External Internal 4 facing grooving forming threading

Mechanics of Metal Cutting A 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. ØHeat produced during shearing action raises the temperature of the workpeice, cutting tool and chips. ØTemperature rise in cutting tool softens and causes loss of keenness in cutting edge. 5 ØCutting force, heat and abrasive wear are important features in metal cutting.

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Types of Cutting Tools ØCutting tools performs the main machining operation. ØIt is a body having teeth or cutting edges on it. ØThey comprise of single point cutting tool or multipoint cutting tools. 7

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 = 90 deg) 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 ≠ 90 deg) 8

Single point cutting tool : This type of tool has a effective cutting edge and removes excess material from the work piece along the cutting edge. ØThese tools may be left-handed or right-handed. ØAgain single point cutting tools classified as A. solid type B. tipped tool. C. Brazed tools are generally known as tool bits and are used in tool holders. ØThe tipped type of tool is made from a good shank steel on which is mounted a tip of cutting tool material. ØTip may be made of high speed steel or cemented carbide. ØDifferent types of carbide tips are generally used on tipped tool. 9

Geometry comprises mainly of nose, rake face of the tool, flank, heel and shank etc. The nose is shaped as conical with different angles. 10

Types of Chips 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 ØChip is produced when there is low friction between the chip and tool face ØThis chip has the shape of long string or curls into a tight roll ØChip is produced when ductile materials such as Al, Cu, M. S, and wrought Iron are machined. ØFormation of very lengthy chip is hazardous to the machining process and the 11 machine operators.

ØIt may wrap up on the cutting tool, work piece and interrupt in the cutting operation. ØIt becomes necessary to deform or break long continuous chips into small pieces. ØIt 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 ØWhen high friction exists between chip and tool, the chip material welds itself to the tool face. ØWelded material increases friction further which in turn leads to the building up a layer upon layer of chip material. ØBuild up edge grows and breaks down when it becomes unstable. ØChips with build up edge result in higher power consumption, poor surface finish and 12 large tool wear

3. Discontinuous or segmental chips Chip 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 materal during chip formation. Non homogenous chips are developed during machining highly hard alloys like titanium. 13

Chip Control and Chip Breakers ØDuring machining high tensile strength materials chips has to be properly controlled. ØCarbide tip tools will be used for high speeds which leads to high temperature and produce continuous chips with blue color. ØIf 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 ØTwo 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. 14

Proper selection of cutting conditions ØSince the cutting speed influences to the great extend the productivity of machining and surface finish, working at low speeds may not be desirable. Ø If the cutting speed is to be kept high, changing the feed and depth of cut is a reasonable solution for chip control. Chip breaker There are two types of chip breakers 1. � External type, an inclined obstruction clamped to the tool face 2. � Integral type, a groove ground into the tool face or bulges formed onto the tool face clamped 15

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NOMENCLATURE Of SINGLE POINT TOOL 17

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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 tool measured in a perpendicular plane through the side cutting edge. 19 This angle helps in removing the chips away from the work piece.

Side rake angle (αs) ØIt is the angle by which the face of tool is inclined side ways. ØThis angle of tool determines the thickness of the tool behind the cutting edge. ØIt 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 angle It is defined as the angle between the portion of the end flank immediately below the 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 angle It 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. 20

End cutting edge angle ØIt is the angle between the end cutting edge and a line perpendicular to the shank of the tool. ØIt provides clearance between tool cutting edge and work piece. Side cutting edge angle ØIt is the angle between straight cutting edge on the side of tool and the side of the shank. ØIt is also known as lead angle. ØIt is responsible for turning the chip away from the finished surface. 21

Tool Signature Convenient 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. 8 1. 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) 22

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 materials 1. Carbon-Tool Steels: Ø 0. 6 -1. 5% carbon + little amount of Mn, Si, Cr, V to increase hardness. ØLow carbon varieties possess good toughness & shock resistance. ØHigh carbon varieties possess good abrasion resistance 2. High Speed Steels (HSS): Ø High carbon+ little amount Tungsten, Molybdenum, Cr, V & cobalt to increase hardness, toughness and wear résistance. 23 Ø High operating temperatures upto 600 o. C.

Ø Two types of HSS i. e, is T-type and M-Type Ø Vanadium increases abrasion resistance but higher percentage will decreases grindability. Ø Chromium increases hardenability Ø Cobalt is added to HSS to increase red hardness. 3. Cast Cobalt Base Alloys: Ø 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. Ø Used for machining Cast iron, alloy steels, non-ferrous metals and super alloys 4. Cemented Carbides: Ø These are carbides of W, Titanium and tantalum with small amount of cobalt produced by means of powder metallurgy route. Ø Two types i. e, Straight Tungsten Carbide Cobalt Grade and Alloyed Tungsten Carbide Grade 24

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: Ø Aluminium Oxide, Silicon Carbide, Boron Carbide, Titanium Boride Ø High speed, longer tool life, superior surface finish, No coolant is required. 6. Diamond Tools: Ø More abrasion resistance Ø Used for turning grinding wheels Ø Used to produce mirror surface finish. Ø Diamond abrassive belts are used to produce TV screens Ø Poly crystalline diamond inserts are brazed into cutting edges of circular saws for cutting construction materials like concrete, refractories, stone etc. 25

Tool Life q. Properly designed cutting tool is expected to perform the metal cutting operation in an effective an smooth manner q. If a tool is not giving satisfactory performance it is an indicative of tool failure. q. Following are the adverse effects observed during operation; ØExtremely poor surface finish on the workpiece ØHigher consumption of power ØWork dimensions are not produced as specified ØOverheating of cutting tool ØAppearance of burnishing band on the work surface q. During operation cutting tool may fail due to following; ØThermal cracking and softening ØMechanical Chipping 26 ØGradual wear

Tool life is defined as the time interval for which tool works satisfactorily between two successive grinding or re-sharpening of the tool. 1. Thermal Cracking and Softening ØDuring cutting operation lot of heat will be generated due to this cutting tool tip and area closer to cutting edge will become hot. ØCutting 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. ØTool looses its cutting ability and it is said to have failed due to softening. Main factors responsible for this condition are; §High cutting speed §High feed rate §More depth of cut §Small nose radius 27 §Choice of wrong tool material

ØWorking temperatures for common tool materials are; Carbon tool steels 200 o. C - 250 o. C High speed steel 560 o. C - 600 o. C Cemented Carbides 800 o. C - 1000 o. C ØTool material is subjected to local expansion and contraction due to severe temperature gradient. ØGives rise to thermal stresses further leads to thermal cracks. 28

2. Mechanical Chipping ØMechanical chipping of nose an cutting edge of the tool are commonly observed causes for tool failure. ØReasons for failure are High cutting pressure, Mechanical impact, Excessive wear, too high vibrations and weak tip an cutting edge, etc. ØThis type of failure is pronounced in carbide tipped and diamond tools due to high brittleness of tool material. Chipped Tip 29

3. Gradual wear When 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 wear Due 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 30 the chip Crater wear

Flank wear Occurs between tool and workpiece interface Due to abrasion between tool flank and workpiece The entire area subjected to flank wear is known as WEAR LAND (VB), occurs on tool nose, front and side relief faces 31

Machinability The major factor involved in metal cutting are, Forces and power absorbed Tool wear and tool life Surface finish Dimensional accuracy Machining cost This factor depend upon a large variables, such as Property of work material Tool geometry Cutting condition Machine tool rigidity 32

Qualitative measure of machinability The easy with which it could be machined, 2. The life of tool before tool failure or re sharpening 3. The quality of machined surface. 4. The power consumption per unit volume of material removed. 1. 33

The machinability may be evaluated as given below Long tool life at a given cutting speed Low power consumption per unit volume of material removed. Maximum metal removal per tool re sharpening High quality of surface finish Good and uniform dimensional accuracy of successive parts Easy disposable chips. 34

Cutting Fluids—Types and Applications Cutting Fluids ØEssential in metal-cutting operations to reduce heat and friction ØCenturies ago, water used on grindstones Ø 100 years ago, tallow used (did not cool) ØLard oils came later but turned rancid ØEarly 20 th century saw soap added to water ØSoluble oils came in 1936 ØChemical cutting fluids introduced in 1944 35

What is Cutting Fluid ? ØCutting fluid is a type of coolant and lubricant designed specifically for metalworking and machining processes. ØThere are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels and other gases. Ø They may be made from petroleum distillates, animal fats, plant oils, water and other raw ingredients. ØDepending on context, type of cutting fluid is being considered, it may be referred to as cutting fluid, cutting oil, cutting compound, coolant, or lubricant. 36

Economic Advantages to Using Cutting Fluids Reduction of tool costs Reduce tool wear, tools life longer Increased speed of production Reduce heat and friction so higher cutting speeds Reduction of labor costs Tools life longer and require less regrinding, less downtime, reducing cost per part Reduction of power costs Friction reduced so less power required by machining 37

Characteristics of a Good Cutting Fluid ØGood cooling capacity ØGood lubricating qualities ØRelatively low viscosity ØStability (long life) ØRust resistance ØNontoxic ØTransparent ØNonflammable 38

Types of Cutting Fluids Most commonly used cutting fluids Either aqueous based solutions or cutting oils Three categories Cutting oils Emulsifiable oils Chemical (synthetic) cutting fluids 39