Talaşlı İmalat Metal İşlemme Suranaree University of Technology Jan-Mar 2007 Machining of metals Machining of metals • Introduction/objectives • Type of machining operations • Mechanics of machining • Three dimensional machining • Temperature in metal cutting • Cutting fluids • Tool materials and tool life • Grinding processes • Non traditional machining processes • Economics of machining Chapter 7 Subjects of interest Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Objectives Objectives • This chapter aims to provide basic backgrounds of different types of machining processes and highlights on an understanding of important parameters which affects machining of metals. • Mechanics of machining is introduced for the calculation of power used in metal machining operation • Finally defects occurring in the machining processes will be discussed with its solutions. Significant factors influencing economics of machining will also be included to give the optimum machining efficiency. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Introduction Introduction • Machining is operated by selectively removing the metal from the workpiece to produce the required shape. • Removal of metal parts is accomplished by straining a local region of the workpiece to fracture by the relative motion of the tool and the workpiece. www.dragonworks.info • Conventional methods require mainly mechanical energy. • More advanced metal-removal processes involve chemical, electrical or thermal energy. Turning of metal EDM machining Tapany UdompholSuranaree University of Technology Jan-Mar 2007 • Produce shapes with high dimensional tolerance, good surface finish and often with complex geometry such as holes, slots or re- entrant angles. • A secondary processing operation (finishing process) employed after a primary process such as hot rolling, forging or casting. • Tooling must be stronger than the workpiece. Machined parts Micro machined parts Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Type of machining operations Type of machining operations Classification of machining operations is roughly divided into: • Single point cutting • Multiple point cutting • Grinding • Electro discharge machining • Electrochemical machining Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Single point cutting Single point cutting Removal of the metal from the workpiece by means of cutting tools which have one major cutting edge. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Multiple point cutting Multiple point cutting Removal of the metal from the workpiece by means of cutting tools which have more than one major cutting edge. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Grinding Grinding Removal of the metal from the workpiece using tool made from abrasive particles of irregular geometry. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Electrical discharge machining Electrical discharge machining Removal of material from the workpiece by spark discharges, which are produced by connecting both tool (electrode) and workpiece to a power supply. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Electrochemical machining Electrochemical machining Removal of material from the workpiece by electrolysis. Tool (electrode) and workpiece are immersed in an electrolyte and connected to a power supply. Tapany UdompholMechanics of machining Mechanics of machining What happens during machining of a bar on a lathe? A chip of material is removed from the surface of the workpiece. Principal parameters: • the cutting speed, v • the depth of cut, w or d • the feed, f. Geometry of single-point lathe turning Time requires to turn a cylindrical surface of length L w , w w fn L t = Where n w is the number of revolutions of the workpiece per second. …Eq. 1 Suranaree University of Technology Jan-Mar 2007 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Chip formation • The tool removes material near the surface of the workpiece by shearing it to form the chip. • Material with thickness t is sheared and travels as a chip of thickness t c along the rake face of the tool. • The chip thickness ratio (cutting ratio) r = t / t c . • Extensive deformation has taken place, as seen from the fibre texture of the polished and etched metal workpiece. Section through chip and workpiece rake angle ? ? ? ? ? ? ? ? 0.25 mm t t c Mechanism of chip formation workpiece feed t Clearance angle ? ? ? ? Machined surface t c chip tool Clearance face rake face rake angle ? ? ? ? Tapany Udomphol• The entire chip is deformed as it meets the tool, known as primary shear. Shear plane angle is ? ? ? ?. Primary shear Secondary shear Before deformation Secondary shear ? ? ? ? Shear angle • Localised region of intense shear occurring due to the friction at the rake face, known as secondary shear. Suranaree University of Technology Jan-Mar 2007 Two basic deformation zone: Well defined shear plane Tapany UdompholPrimary shear in single point cutting The relationship between rake angle, shear angle, and chip thickness ratio, r can be derived as follows ( ) ? ? ? - = = cos sin OD OD t t r c ? ? ? sin 1 cos tan r r - = and t t c ? ? ? ? ? ? ? ?- - - -? ? ? ? ? ? ? ? shear angle ? ? ? ? rake angle ? ? ? ? The shear strain is given by ( ) ? ? ? ? ? - = = cos sin cos ' h FF The shear angle ? ? ? ? is controlled by the cutting ratio r. Triangle ODF has been sheared to form ODF ’ , which has the same area. ? ? ? ? Wedge angle …Eq. 2 …Eq. 3 …Eq. 4 Suranaree University of Technology Jan-Mar 2007 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Rake face configuration • The amount of primary shear is related to the rake angle ? ? ? ?. (a) If ? ? ? ? is a large positive value, the material is deformed less in the chip. (b) If ? ? ? ? is a negative value, the material is forced back on itself, thus requiring higher cutting forces. (c) The tool has a negative ? ? ? ? but a small area of positive rake just behind the cutting edge. chip breaker. (a) Positive rake angle ? ? ? ? (6-30 o ) leads to low cutting forces but fragile tools (b) Negative rake angle ? ? ? ? produces higher cutting forces and more robust tools. (c) Negative rake angle tool with chip breaker – a useful compromise. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Effect of rake face contact length on chip thickness and shear plane angle • The deformed chip is flowing over a static tool, leading to frictional force similar to friction hill. • If µ µ µ µ is greater than 0.5, sticky friction will result and flow will occur only within the workpiece but not at the tool-workpiece interface. • Sticky friction is the norm in cutting due to difficulty in applying lubricant. force to move the chip chip thickness change shear angle ? ? ? ? Efficient cutting occurs when shear angle ? ? ? ? ~ 45 o . t t c = t t c > t Tapany UdompholSuranaree University of Technology Jan-Mar 2007 The cutting speed There are three velocities: 1) Cutting speed v, is the velocity of the tool relative to the workpiece. 2) Chip velocity v c , is the velocity of the chip relative to the tool face 3) Shear velocity v s , is the velocity of the chip relative to the work. Velocity relationships in orthogonal machining From continuity of mass, vt = v c t c v v t t r c c = = From kinematic relationship, the vector sum of the cutting velocity and the chip velocity = the shear velocity vector. ( ) ? ? ? ? ? - = cos cos s …Eq. 5 …Eq. 6 tool v v c O D ? ? ? ? v s workpiece t t c ? ? ? ? v v s v c ? ? ? ?- - - -? ? ? ? Kinematic relationship Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Calculation of the cutting ratio from chip length Since volume is constant during plastic deformation, and chip width b is essentially constant, ? c w W tb L = r L L t t b t L tb L w c c c c w = = = • Therefore we could also obtain r from the ratio of the chip length L c , to the length of the workpiece from which it came, L w • If L c is unknown, it can be determined by measuring the weight of chips W c and by knowing the density of the metal ? ? ? ?. …Eq. 8 …Eq. 7 tool v v c O D ? ? ? ? v s workpiece t t c L c Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Shear strain rate in cutting max ) ( s s y dt d ? ? ? = = • Where (y s ) max is the estimate of the maximum value of the thickness of the shear zone, ~ 25 mm. Example: Using realistic values of ? ? ? ? = 20, ? = 5 o , ? = 3 m.s -1 and (y s ) max ~ 25 mm. We calculate ? ? ? ? = 1.2 x 105 s -1 . This is about several orders of magnitude greater than the strain rate usually associated with high-speed metal working operation. …Eq. 9 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Forces and stresses in metal cutting ? ? ? ? ? ? ? ? Rake face chip P ’ R F s F h F v F ns P R F n F t ß ß ß ß Force component in orthogonal machining. P R - the resultant force between the tool face and the chip P ’ R - the equal resultant force between the workpiece and the chip The resultant force to the rake face of the tool can be resolved into tangential component F t and normal component F n , • The horizontal (cutting) F h and vertical (thrust) F v forces in cutting can be measured independently using a strain-gauge toolpost dynamometer. • It can be shown that ? ? sin cos v h n F F F - = ? ? ? ? P R F n F t ß ß ß ß Rake face ? ? cos sin v h t F F F + = ? ? ? ? F h ? ? ? ? F v F v cos? ? ? ? F v sin? ? ? ? F h cos? ? ? ? F h sin? ? ? ? Tapany UdompholSuranaree University of Technology Jan-Mar 2007 • If the components of the cutting force are known, then the coefficient of friction µ µ µ µ in the tool face is given by ? ? ß µ tan tan tan v h h v n t F F F F F F - + = = = P ’ R F s F h F v F ns ? ? ? ? ? ? ? ? F h sin? ? ? ? F v cos? ? ? ? F h cos? ? ? ? F v sin? ? ? ? ? ? sin cos v h s F F F - = ? ? cos sin v h ns F F F + = Finally, the resultant force may be resolved parallel F s and normal F ns to the shear plane. …Eq. 10 …Eq. 11 …Eq. 12 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 The average shear stress ? ? ? ? is F s divided by the area of the shear plane A s = bt / sin? ? ? ? bt F A F s s s ? ? sin = = And the normal stress ? ? ? ? is bt F A F ns s ns ? ? sin = = The shear stress in cutting is the main parameter affecting the energy requirement. tool O D ? ? ? ? workpiece t t c F s F s F ns ? ? ? ? O D t …Eq. 13 …Eq. 14 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 • We need to know the shear angle ? ? ? ? in order to calculate the shear stress in cutting from force measurements. • The shear angle ? ? ? ? can be measured experimentally by suddenly stopping the cutting process and using metallographic techniques to determine the shear zone. Section through chip and workpiece rake angle ? ? ? ? ? ? ? ? 0.25 mm t t c • Merchant predicted ? ? ? ? by assuming that the shear plane would be at the angle which minimises the work done in cutting. 2 2 4 ß ? ? ? + + = …Eq. 15 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 However, in practice, the shear plane angle ? ? ? ? is varied depending on the nature of each material (composition & heat treatment) to be machined. Based on the upper bound model of the shear zone, a criterion for predicting ? ? ? ? has been developed. The predicted shear plane angle ? ? ? ? ? ? ? ? is given by ( ) ? ? ? ? ? ? ? ? ? ? ? ? + ? ? ? ? ? ? - = - 2 45 sin 2 45 cos sin cos 1 ? ? ? ? ? k k o o o Where ? ? ? ? = rake angle k o = ? ? ? ? o /? ? ? ? 3 and ? ? ? ? o is the yield strength of the material k 1 = ? ? ? ? u /? ? ? ? 3 and ? ? ? ? u is the tensile strength of the material. …Eq. 16 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Example: Determine the shear plane angle in orthogonal machining with a 6 o positive rake angle for hot-rolled AISI 1040 steel and annealed commercially pure copper. Given Hot-rolled 1040 steel ? ? ? ? o = 415 MPa, ? ? ? ? u = 630 MPa Annealed copper ? ? ? ? o = 70 MPa, ? ? ? ? u = 207 MPa ( ) ( ) ( ) ( ) [ ] ( ) o o o o o o o o o o o o k k k k k k k k 6 1045 . 0 104 . 1 sin 2 552 . 0 6 2 sin 6 sin 2 1 552 . 0 sin 6 cos 2 6 45 sin 2 6 45 cos sin 6 cos 1 1 1 1 1 + ? ? ? ? ? ? ? ? ? ? ? ? ? ? - = = - + = - ? ? ? ? ? ? ? ? ? ? ? ? + ? ? ? ? ? ? - = - - ? ? ? ? ? ? Note that k o /k 1 is a fraction, then we can use tensile values directly in the above equations. For hot-roll 1040 steel: o o o o o o o 3 . 22 5 . 44 6 ) 6227 . 0 ( sin 2 6 1045 . 0 630 415 104 . 1 sin 2 1 1 = = + = + ? ? ? ? ? ? ? ? ? ? ? ? - × = - - ? ? ? Experimental range is 23 to 29 o o o o o o o o 8 . 10 6 . 21 6 ) 2688 . 0 ( sin 2 6 1045 . 0 207 70 104 . 1 sin 2 1 1 = = + = + ? ? ? ? ? ? ? ? ? ? ? ? - × = - - ? ? ? Experimental range is 11 to 13.5 o For annealed copper: Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Specific cutting energy • Power required for cutting is F h v • The volume of metal removed per unit time (metal removal rate) is Z w = btv bt F btv v F Z v F U h h w h = = = Where b is the width of the chip t is the undeformed chip thickness Force values of specific cutting energy for various materials and machining operations …Eq. 17 • Therefore the energy per unit volume U is given by Tapany UdompholSuranaree University of Technology Jan-Mar 2007 The specific cutting energy U depends on the material being machined and also on the cutting speed, feed, rake angle, and other machining parameters. (at cutting speed > 3 m.s -1 , U is independent of speed) 1. The total energy required to produce the gross deformation in the shear zone. 2. The frictional energy resulting from the chip sliding over the tool face. 3. Energy required to curl the chip. 4. Momentum energy associated with the momentum change as the metal crosses the shear plane. 5. The energy required to produce the new surface area. The total energy for cutting can be divided into a number of components: Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Example: In an orthogonal cutting process v = 2.5 m.s -1 , ? ? ? ? = 6 o , and the width of cut is b = 10 mm. The underformed chip thickness is 200 µm. If 13.36 g of steel chips with a total length of 500 mm are obtained, what is the slip plane angle? density = 7830 kg.m -3 . From Eq.8, thickness of chip mm t m m m kg kg bL W t c c c 341 . 0 ) 500 . 0 )( 010 . 0 ( ) . 7830 ( 01336 . 0 3 = × = = - ? From Eq.3 Chip thickness ratio 586 . 0 341 . 0 200 . 0 = = = c t t r o o o r r 32 621 . 0 6 sin 586 . 0 1 6 cos 586 . 0 sin 1 cos tan = = - = - = ? ? ? ? ß ß ß ß =?, from Eq.10 o o o v h h v n t F F F F F F 8 . 27 527 . 0 6 tan 440 1100 6 tan 1100 440 tan tan tan tan = = - + = - + = = = ß ß ? ? ß µ Tapany UdompholSuranaree University of Technology Jan-Mar 2007 If a toolpost dynamometer gives cutting and thrust forces of F h = 1100 N and F v = 440 N, determine the percentage of the total energy that goes into overcoming friction at the tool-chip interface and the percentage that is required for cutting along the shear plane. (Density ? ? ? ? = 7830 kg.m -3 .) The frictional specific energy at the tool- chip interface U f and along the shear plane U s is ? ? ? ? ? ? ? ? Rake face chip P ’ R F s F h F v F ns P R F n F t ß ß ß ß V c bt r F btv v F U t c t f = = The total specific energy is s f U U U + = btv v F U s s s = and, bt F U h = Thus h t h c t f F r F v F v F U U energy Total energy Friction = = = ( ) % 5 . 29 100 1100 ) 586 . 0 ( 553 % 553 8 . 27 sin 1185 1185 ) 1100 ( ) 440 ( , sin 2 2 2 2 ' = × = = = = + = + = = = energy friction N F N P F F P P and P F o t R h v R R R t ß From Eq. 17 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 v F v F energy Total engergy Shearing h s s = From Eq.17, N F F F F F s o o s v h s 700 32 sin 440 32 cos 1100 sin cos = - = - = ? ? From Eq.11, ( ) % 5 . 70 100 5 . 2 1100 77 . 2 700 % . 77 . 2 ) 6 32 cos( 6 cos 5 . 2 cos cos 1 = × × × = = - = - = - energy shearing s m v v o s ? ? ? 3 2 6 550 . 5 . 2 ) 10 200 ( 010 . 0 5 . 2 1100 - - - = × × × × = = MJm m N btv v F U h This analysis of energy distribution neglects two other energy requirements in cutting: • Surface energy required to produce new surfaces. • Momentum change as the metal crosses the shear plane (significant in high-speed machining at cutting speeds above 120 m.s -1 .) Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Type of machining chips Three general classifications of chips are formed in the machining process. (a) Continuous chip (b) Chip with a built up edge, BUE (c) Discontinuous chip Tapany UdompholContinuous chips Continuous chip is characteristic of cutting ductile materials under steady stage conditions. However, long continuous chips present handling and removal problems in practical operation. required chipbreaker. Discontinuous chips Discontinuous chip is formed in brittle materials which cannot withstand the high shear strains imposed in the machining process without fracture. Ex: cast iron and cast brass, may occur in ductile materials machined at very low speeds and high feed. Suranaree University of Technology Jan-Mar 2007 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Chip with a built-up edge (BUE) • Under conditions where the friction between the chip and the rake face of the tool is high, the chip may weld to the tool face. • The accumulation of the chip material is known as a built-up edge (BUE). • The formation of BUE is due to work hardening in the secondary shear zone at low speed (since heat is transferred to the tool). • The BUE act as a substitute cutting edge (blunt tool with a low rake angle). Chip formation with a built-up edge. Built-up edge Poor texture on the surface Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Machining force • Due to complexity of practical machining operations, the machining force F h often is related empirically to the machining parameters by equation of the type b a h f kd F = Where d is the depth of cut f is the feed k is a function of rake angle, decreasing about 1% per degree increase in rake angle. Effect of cutting speed on cutting force. Speed Force Due to low temperature at low speed work hardening …Eq. 18 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Three Three - - dimensional machining dimensional machining • Orthogonal machining such as surface broaching, lathe cutoff operations, and plain milling are two dimensional where the cutting edge is perpendicular to the cutting velocity vector. • Most practical machining operations are three dimensional. • Ex: drilling and milling. (a) Orthogonal cutting (b) Three dimensional cutting • Rotating the tool around x axis change the width of the cut. • Rotating the tool around y axis change the rake angle ? ? ? ?. • Rotating the tool around z axis ( by an inclination angle i) change the cutting process to three dimensional. x x z y z y Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Three dimensional cutting tool • has two cutting edges, which cut simultaneously. • primary cutting edge is the side-cutting edge. • secondary cutting edge is the end-cutting edge. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Multiple-edge cutting tools Drilling • Used to created round holes in a workpiece and/or for further operations. • Twist drills are usually suitable for holes which a length less than five times their diameter. Drilling machine Specialised tools for drill presses Drill-workpiece interface Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Multiple-edge cutting tools Milling • Used to produce flat surfaces, angles, gear teeth and slotting. • The tool consists of multiple cutting edges arranged around an axis. • The primary cutting action is produced by rotation of the tool and the feed by motion of the workpiece. Tool-workpiece arrangement typical for milling. Work surface Tool Primary motion Machined surface Work piece Transient surface Continuous feed motion, f Three common milling cutters. Peripheral mill End mill Face mill Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Temperature in metal cutting Temperature in metal cutting • A significant temperature rise is due to large plastic strain and very high strain rate although the process is normally carried out at ambient temperature. • Strain rate is high in cutting and almost all the plastic work is converted into heat. • Very high temperature is created in the secondary deformation zone. • At very high strain rate no time for heat dissipation temperature rise. Temperature gradient (K) in the cutting zone when machining steel. Temperature in metal cutting is therefore an important factor affecting the choice of tool materials, tool life, type of lubricant, Tapany UdompholSuranaree University of Technology Jan-Mar 2007 If all the heat generated goes into the chip, the adiabatic temperature is given by c U T ad ? = Where U = specific cutting energy ? ? ? ? = the density of the workpiece material c = specific heat of workpiece. For lower velocities, the temperature will be less than in Eq.19. The approximate chip-tool interface temperature is given by …Eq. 19 p t ad R C T T ? ? ? ? ? ? ? ? = 1 …Eq. 20 Where C ~ 0.4 and p ~ 1/3 to 1/2. R t is thermal number Note: The finite element method has been used for calculating the temperature distributions in the chip and the tool. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Cutting fluids Cutting fluids • The cutting fluids are designed to ameliorate the effects of high local temperatures and high friction at the chip-tool interface. Primary functions of cutting fluid : • To decrease friction and wear. • To reduce temperature generation in the cutting area. • To wash away the chips from the cutting area. • To protect the newly machined surface against corrosion. Also, cutting fluids help to • Increase tool life, • Improve surface finish • Reduce cutting force Cutting fluid used in machining • Reduce power consumption • Reduce thermal distortion of the workpiece. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Cutting fluids are normally liquids, but can be gases. There are two basic types of liquid cutting fluids 1) Petroleum-based nonsoluble fluids (straight cutting oils). May contain mineral oil, fatting oils, sulphur or chlorine. 2) Water-miscible fluids (soluble oils). May contain some contamination of fatty oils, fatty acids, wetting agents, emulsifiers, sulphur, chlorine, rust inhibitors and germicides. • Sulphur and chloride react with fresh metal surfaces (active sites for chemical reaction) to form compounds with lower shear strength reduce friction. • Chlorinated fluids work well at low speeds and light loads due to slower reaction of chlorine and metal whereas sulphur compounds work well at severe conditions. • Combination of both more effective. Vegetable-based cutting fluid Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Tool materials and tool life Tool materials and tool life Properties of cutting tool materials: • Hardness, particularly at high temperature • Toughness to resist failure or chipping • Chemical inertness with respect to the workpiece • Thermal shock resistance • Wear resistance, to maximise the lifetime of the tool. Tool materials: • Carbon and low alloy steels • High speed steels (HSS) • Cemented carbide • Ceramic or oxide tools • Diamond like structure www.pdbrownesouth.com Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Three main forms of wear in metal cutting 1) Adhesive wear : the tool and the chip weld together at local asperities, and wear occurs by the fracture of the welded junctions. 2) Abrasive wear : occurs as a result of hard particles on the underside of the chip abrading the tool face by mechanical action as the chip passes over the rake face. 3) Wear from solid-state diffusion from the tool materials to the workpiece at high temperature and intimate contact at the interface between the chip and the rake face. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Crater wear and flank wear Two main types of wear in cutting tool: 1) Flank wear is the development of a wear land on the tool due to abrasive rubbing between the tool flank and the newly generated surface. 2) Crater wear is the formation of a circular crater in the rake face of the tool, as a result of diffusion wear due to high temperature developed at the interface between the chip and the rake face of the tool. The predominant wear process depends on cutting speed. • Flank wear dominates at low speed. • Crater wear predominates at higher speeds Length of wear land Cutting time Typical wear curve for cutting tool Rapid wear Initial breakdown of cutting edge Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Types of wear observed in single point cutting tools The higher temperatures that occur at high cutting speeds , which results in increased tool wear. High speed steel Cemented carbide Ceramic 1) Clearance face wear 2) Crater wear 3) Oxidation wear Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Carbon and low alloy steels • High carbon tool steel is the oldest cutting tool materials, having C content ranging from 0.7 – 1.5% carbon. • Shaped easily in the annealed condition and subsequently hardened by quenching and tempering. • Due to insufficient hardenability, martensite only obtained on the surface whereas a tough interior provides the final tool very shock resistant. • H v ~ 700 after quenching and tempering. However the tool will be soften and becomes less and less wear resistance due to coarsening of fine iron carbide particles – that provide strength. • For low cutting speed due to a drop in hardness above 150 o C. Tool materials Tapany UdompholSuranaree University of Technology Jan-Mar 2007 High speed steels (HSS) • Retain their hot hardness up to 500 o C. • Cutting speed ~ 2 times higher than carbon tool steels. • Very stable secondary carbide dispersions (between 500-650 o C), giving rise to a tempering curves. Tempering curve for M2 high speed steel • Carbon content in each steel is balanced against the major alloying elements to form the appropriate stable mix of carbides with W, Mo, Cr and V. • Cobalt is added to slow down the rate of carbide coarsening material can withstand higher temperatures. • M series have higher abrasive resistance and cheaper. • Cannot stand very high speed cutting. Tapany UdompholCemented carbides • Normally made by powder processing using liquid phase sintering. • Has advantage over high speed steel in that the obtained carbides are much more stable, see Table. • They are brittle so should run without vibration or chatter. Cemented carbides fastened to the tool post. Microstructure of a K grade (WC-Co) cemented carbide. • Cobalt is used as a binder. • ? ? ? ? o ~ 1500 - 2500 MPa, depending on V f and size distribution of carbides. • Cutting temperature up to 1100 o C. • Cutting speed ~ 5x that used with high speed steel. Suranaree University of Technology Jan-Mar 2007 • Consist of heat-resistant refractory carbides (hardness) embedded in a ductile metal matrix (toughness). Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Tool coatings • Changing the tool surface properties. surface engineering • Coating can improve the performance of both high speed steel and cemented carbide tool materials. increased materials removal rates, time taken to change the tool. • Coating a very thin layer of TiC or TiN over the WC-Co tool reduces the effects of adhesion and diffusion and reduces the crater wear. • Chemical and physical vapour deposition (CVD, PVD) are two methods of depositing thin carbide layers onto materials. • TiN layer (golden colour) is hard and has low dissolution rate and friction coefficient in steel. • TiC binds well with the matrix, has good abrasion and solution wear resistance. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Ceramics or oxide tools There are three categories: 1) Alumina (Al 2 O 3 ) 2) A combination of alumina and titanium carbide 3) Silicon nitride (Si 3 N 4 ), less thermal expansion than Al 2 O 3 minimise thermal stress • For machining cast irons at high speeds. • Better wear resistance and less tendency for the tool to weld to the chip. • Cutting speed at 2-3 times > cemented carbides in uninterrupted cuts where shock and vibration are minimised (due to poor thermal shock and brittleness of ceramics). • Required rigid tool mounts and rigid machine tools. • Inherent unreliability of ceramic tooling limits its use to specialist cutting operation. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 ‘Diamond like’ structure • Diamond provides the highest hot hardness of any material. • Synthetic diamonds (1950s) and cubic boron nitride CBN (1970s), made by high pressure, high temperature pressing. The latter possesses H v = 4000. • Highest thermal conductivity ideal for cutting tool, but has two disadvantages; cost and diamond – graphite reversion at 650 o C. • Made by depositing a layer of small crystals on a carbide backing and sintering them with a binder polycrystalline diamond tooling (PCD). • Used for cutting low temperature materials, i.e., aluminium or copper alloys. • Used at very low cutting speed for very hard materials, i.e., ceramics. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Tool performance • Tool performance has been improved by the development of tool coatings. Minimum time required to surface machine a hot rolled mild steel bar Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Tool life determination Tool life can determined based on different criteria; 1) The point at which the tool no longer makes economically satisfactory parts, or 2) Defined in terms of an average or maximum allowable wear land. 3) The point at which the tool has a complete destruction when it ceases to cut, or 4) The degradation of the surface finish below some specified limit, or the increase in the cutting force above some value, or 5) When the vibrational amplitude reaches a limiting value. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 • Cutting speed is the most important operating variable influencing tool temperature, and hence, tool life. Taylor has established the empirical relationship between cutting speed v and the time t to reach a wear land of certain dimension as vt n = constant …Eq. 21 Where typical values of the exponent n are: 0.1 for high-speed steel 0.2 for cemented carbide 0.4 for ceramic tool Note: this is the machining time between regrinding the tool not the total life before it is discarded. Taylor equation Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Modified Taylor equation vt n w x f y = constant Taylor equation has been extended by including parameters such as feed f and depth of cut w as follows; …Eq. 22 Note: Taylor equation is completely empirical and as with other empirical relationships, it is dangerous to extrapolate outside of the limits over which the data extend. Length of wear land Cutting time Typical wear curve for cutting tool Rapid wear Initial breakdown of cutting edge However, tool life can be conservatively estimated by using wear curves and the replacement of the tool should be made before they have used up their economical life. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Machinability Machinability Definition: The ability of a material to be machined. Machinability depends of a number of factors: 1) Hardness – soft materials are easily sheared and require low cutting forces. 2) Surface texture – how easy it is to produce the required surface finish. Materials with high work hardening exponent n tend to form built-up edge (BUE). 3) The maximum rate of metal removal – allow low cycle times. 4) Tool life – abrasive particles can increase tool wear. 5) Chip formation – uniform discrete chips suggest good machinability. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 To improve machinability • Change the microstructure of the materials. Soft particles are often deliberately added to improve machinability. • Reducing the cutting temperature by using cutting fluid – can effectively act as coolant and lubricant. Maximum tool surface temperature remains the same but the volume of the tool that reached the high temperature is reduced. • Control surface texture – reduce the formation of built-up edge. • Increase rate of material removal – modern cutting machines, effective toolings. Effect of coolant on tool temperatures. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Grinding process Grinding process Grinding processes employ an abrasive wheel containing grains of hard material bonded in a matrix. Geometry of chip formation in grinding • Similar to multiple edge cutting but with irregularly shaped grain (tool). • Each grain removes a short chip of gradually increasing thickness. after a while sharp edges become dull. • Large negative rake angle ? ? ? ?. grains could slide over the workpiece than cut. • The depth of cut d in grinding is very small (a few µm). Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Grinding wheel • Employ aluminium oxide Al 2 O 3 or silicon carbide SiC as abrasive grain, which are often alloyed with oxides of Ti, Cr, V, Zr, etc, to impart special properties. • Since SiC is harder than Al 2 O 3 , it finds applications for the grinding of harder materials. • Diamond wheels are used for fine finishing. • Soft grade alumina wheel has a large V f of pores and low glass content surprisingly used for cutting hard materials and fast material removal, where as hard grade alumina wheel (denser) is used for soft materials and for large area grinding. Diamond grinding wheel Alumina grinding wheel Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Glass bonded grinding wheel microstructure. • Wheel performance is controlled by the strength of the bond. Binders used are depending on application, i.c., glass, rubber or organic resin. • Interconnected porosity provides the space to which the chips can go and provides a path for the coolant to be delivered to the cutting surface. • Specific cutting energy is 10 times > other cutting process since not all of particles can cut but rub on the surface, and also the rake angle is not optimised. • 70% of energy goes to the finished surface very high temp, residual stresses. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Grain depth of cut The grain depth of cut t is given by D d Crv v t g w 2 = Where C = the number of active grains on the wheel per unit area (~1 – 5 mm -2 ) D = diameter of the wheel, and r = b ’ /t v w = velocity of the workpiece v g = velocity of the grinding wheel bd v v F U w g h = v w D/2 ? ? ? ? v g d L c A B t t b ’ Geometry in surface grinding Approximate x-section of grinding chip …Eq. 23 d = wheel depth of cut and t << d Tapany UdompholSuranaree University of Technology Jan-Mar 2007 The specific cutting energy U in grinding is bd v v F U w g h = Where F h is the tangential force on the wheel v g is the velocity of grinding wheel v w is the velocity of the workpiece. …Eq. 24 Specific cutting energy And U is strongly dependent on t t U 1 ? If the grain cross section is assumed triangular, the force on a single abrasive grain F g will be D d Cv rv rt F g w g ? ? , b is the chip width , d is the wheel depth of cut …Eq. 25 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Surface temperature Large portion of energy in grinding process goes to raising the temperature. The surface temperature T w , strongly dependent on the energy per unit surface area, is given by Ud b v v F T w g g w ? ? …Eq. 25 • Ground surface temperature can be > 1600 o C, which can lead to melting or metallurgical changes, i.e., untempered martensite, grinding cracks, surface oxidation (grinding burn). • Improper grinding can also lead to residual tensile stresses in the ground surface using proper grinding fluid and softer wheel at lower wheel speeds. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Grindability Grindability is measured by using grinding ratio or G ratio, which is the volume of material removed from the work per unit volume of wheel wear. G ratio Easier to grind • The G ratio depends on the grinding process and grinding conditions (wheel, fluid, speed and feed) as well as the material. • The values of G ratio can vary from 2 to over 200. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Example: A horizontal spindle surface grinder is cutting with t = 5 mm and U = 40 GPa. Estimate the tangential force on the wheel if the wheel speed is 30 m.s -1 , the cross-feed per stroke is 1.2 mm, the work speed is 0.3 m.s -1 , and the wheel depth of cut is 0.05 mm. The rate of metal removal M = speed x feed x depth of cut 1 3 6 3 3 10 018 . 0 ) 10 05 . 0 )( 10 2 . 1 ( 3 . 0 - - - - × = × × = = s m bd v M w From Eq. 24, required power W s Nm Power s m Nm M U Power 720 . 720 ) 10 018 . 0 )( 10 40 ( 1 1 3 6 2 9 = = × × = × = - - - - But Power = F h v g N s m s Nm F h 24 . 30 . 720 1 1 = = - - Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Non Non - - traditional machining traditional machining processes processes • The search for better ways of machining complex shapes in hard materials. • Use forms of energy other than mechanical energy. Source of energy Name of process Thermal energy processes Electrical discharge machine, EDM Laser-beam machining, LBM Plasma-arc machining, ECM Electrical energy processes Electrochemical machining, ECM Electrochemical grinding, ECG Chemical process Chemical machining process Mechanical process Ultrasonic machining, USM Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Electrical discharge machining (EDM) • Required electrically conductive materials. Workpiece – anode and tool – cathode. Independent of material hardness. • Removal of material through melting or vaporisation caused by a high- frequency spark discharge. EDM machined surface may be deleterious to fatigue properties due to the recast layer. • good selection of the proper electrode material for the workpiece • Produce deep holes, slots, cavities in hard materials without drifting or can do irregular contour. Electrical discharge machining Electrical discharge wire cutting Tapany UdompholElectrochemical machining (ECM) • Metal is removed by anodic dissolution in an electrolytic cell. Workpiece – anode, tool – cathode. • rate of metal removal depends upon the amount of current passing between the tool and the workpiece, independent of material hardness. • ECM is a cold process which results in no thermal damage to the workpiece, hence, giving a smooth burr-free surface. • Not suited for producing sharp corners or cavities with flat bottoms. Suranaree University of Technology Jan-Mar 2007 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Electrochemical grinding (ECG) • A combination of ECM and abrasive grinding in which most of the metal is removed by electrolytic action. • It is used with hard carbides or difficult-to-grind alloys where wheel wear or surface damage must be minimised. ECG equipment Schematic diagram of ECG process Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Chemical machining (CHM) • Metal is removed by controlling chemical attack with chemical reagents. Surface cleaning Masking areas not to be dissolved Attacking chemicals Cleaning Process Chemical machining of microscopic holes and grooves in glass. Photo chemical machining product Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Ultrasonic machining (USM) • The tool is excited around 20,000 Hz with a magnetostrictive transducer while a slurry of fine abrasive particles is introduced between the tool and the workpiece. • Each cycle of vibration removes minute pieces of pieces of the workpiece by fracture or erosion. • Used mostly for machining brittle hard materials such as semiconductors, ceramics, or glass. USM apparatus USM products Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Economics of machining Economics of machining Machining cost Speed , feed Tool wear tool cost Tool changing • Optimum speed which balances these opposing factors and results in minimum cost per piece. Machining cost t c n m u C C C C C + + + = Where C u = the total unit (per piece) cost C m = the machining cost C n = the cost associated with non-machining time, i.e., setup cost, preparation, time for loading & unloading, idle machine time. C c = the cost of tool changing C t = the tool cost per piece. Total cost …Eq. 26 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 1) Machining cost Machining cost C m can be expressed by ) ( m m m m O L t C + = Where t m = the machining time per piece (including the time the feed is engaged whether or not the tool is cutting. L m = the labour cost of a production operator per unit time O m = the overhead charge for the machine, including depreciation, indirect labour, maintenance, etc. …Eq. 27 2) Cost of non-machining time The cost of non-machining time C n is usually expressed as a fixed cost in dollars per piece. Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Where t g = the time required to grind and change a cutting edge t ac = the actual cutting time per piece t = the tool life for a cutting edge L g = the labour rate for a toolroom operator O g = the overhead rate for the tool room operation. 3) Cost of tool changing The cost of tool changing C c can be expressed by. ( ) g g ac g c O L t t t C + ? ? ? ? ? ? = …Eq. 28 The Taylor equation for tool life can be written n v K t 1 ? ? ? ? ? ? = fv D L t a ac ? = …Eq. 29 …Eq. 30 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 4) Tool cost per piece The tool cost per piece can be expressed by t t C C ac e t = Where C e is the cost of a cutting edge, and t ac /t is the number of tool changes required per piece. …Eq. 31 Tapany UdompholSuranaree University of Technology Jan-Mar 2007 C m C n - idle cost Cost per piece Cutting speed C t Tool cost Machining cost Tool changing C c Total unit cost C u Production rate, pieces per hour Variation of machining costs with cutting speed C u = C m + C n + C c + C t Tapany UdompholSuranaree University of Technology Jan-Mar 2007 • Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-100406-8. • Edwards, L. and Endean, M., Manufacturing with materials, 1990, Butterworth Heinemann, ISBN 0-7506-2754-9. • Beddroes, J. Bibbly M.J., Principles of metal manufacturing processes, Arnold, ISBN 0 340 731621. References References Tapany Udomphol