Tool wear Mechanism 2 Major wear mechanisms 1. Abrasive wear 2. Adhesion wear 3. Diusion 4. Oxidation 5. Fatigue 6. Chemical decomposing 1 1.Abrasive wear 1.Abrasive wear cont. 3 4 Soter material sliding over the ace o hard material may contain appreciable concentration o hard particles Hard particles act as small cutting edge like grinding wheel Hard particles result worn out o tool material Particles o hard material are intermittently turn out rom the surace and dragged along the surace 1
2. Adhesive Wear Adhesive Wear cont. 5 6 When soter metal slide over hard metal, parts o sot metal adhere high spots on the metal due to: Friction High temperature Pressure The spots result irregular low o chip over the ace and build up o more particles on the tool Finally the built up edges will torn rom the surace result uneven structure on the tool surace 3.Diusion Diusion cont. 7 8 When a metal is in sliding contact with another metal the temperature at the interace is high The high temperature allows the atoms o hard material to diuse into soter material matrix hence the strength and abrasiveness o the soter material Increase Atoms o the soter metal may also diuse into harder medium, thus weakening the surace o harder material medium Diusion phenomenon is strongly dependent on temperature 2
4.Oxidation 5. Fatigue Wear 9 10 Oxidation is the result o reaction between tool ace and oxygen Ex. When machining steel work piece with HSS or cemented carbide tool, groove ormation is greatly accelerated i the cutting zone is subjected to a jet oxygen. Will occur when two suraces slides in contact with other under high pressure Roughness o one surace interlocks with those o other. Due to riction, compressive orce will be produced in one side and tensile on other side,these phenomenon cause surace crack Fatigue Wear cont. Types o Wear 11 12 Flank wear(wear land) Crater wear Chipping The cracks ultimately combines with one another and lead surace crumble 3
Flank Wear Flank wear 13 14 It produces wear land on the sides and end o a lank Tool-work piece interace Predominant at low speed Primary region breakage o sharp cutting edges Secondary zone The predominant zone caused by abrasion Tertiary zone Very sensitive to temperature,caused by diusion wear Increment o MRR increases lank wear Crater wear Crater Wear 15 16 It occurs on the rake ace o the tool in the orm o pit called crater Predominant at high speed Tool-chip interace Diusion, the lose o hardened atoms at the tool chip inter ace is the major cause o creator wear 4
Chipping (premature wear ) Tool lie 17 18 It reers the breaking away o small chips rom the cutting edge o tool or an insert on account o impact or excessive plastid deormation Large chipping can be caused by interrupted cutting and sudden shock. Criteria or tool lie Change o quality o machined surace Change in magnitude o cutting orce Change in cutting temperature Tool Lie Prediction Tool Lie Prediction cont. Taylor s tool lie equation VT n =C where V is the cutting speed, T is the tool lie, n is Taylor exponent. n and C are constants depends on work material, eed, depth o cut and cutting speed. 20 Modiied Taylor s tool lie equation VT d n n n 1 2 = C where V is the cutting speed, is the eed d is depth o cut T is the tool lie, n, n 1 and n 2 are Taylor exponent. n, n 1 and n 2 and C are constants depends on work material, eed, depth o cut and cutting speed. 5
Tool lie cont. Surace Quality 21 Example A steel bar o 40mm diameter is to be turned over a length o 200mm with a depth o cut 1.2mm and eed o 0.2mm/rev at 200rpm. I the tool lie equation is given by VT 0.2 d 0.13 0.31 = 40 Ideal transverse roughness R max = 2 /8R where R is the tool nose radius and is the eed. Determine the number o components that may be turned beore regrinding the tool. a b Surace produced on steel during cutting by SEM, a) turning and b) shaping Temperature in metal cutting Heat generation in metal cutting 23 24 High temperature Reduces strength o the tool and ormation o create wear. Shortens tool lie Causes thermal distortion Caused dimensional change in work piece, making o control dimensional accuracy diicult a) Region AB is Primary shear zoneplastic deormation ( P s ) b) Region BC is Secondary shear zone riction ( P ) c) Region BD Between tool & work piece plowing orce (negligible) P m = m FV c s Pm F c Power consumed Cutting orce P = P + P P F V = 6
25 Temperature distribution in metal cutting 26 Temperature distribution in metal cutting Temperature distribution at, HSS V = 25 m s Isotherms in temperature distribution Cutting temperature control Temperature and Cutting Fluid 27 Temperature in metal cutting can be controlled by Application o cutting luid(coolant) Changing the cutting condition by reduction o cutting speed and/or eed. Selection o proper cutting Geometry ex. Positive tool orthogonal rake angle Function o cutting luids Lbi Lubricationi Cooling Chip removal 7
Cutting temperature as a unction o cutting parameters 29 Cutting temperature as a unction o cutting speed V, eed and depth o cut d 8