Tool Wear, Tool life & Machinability

Tool Life Useful cutting life of tool expressed in time Time period measured from start of cut to failure of the tool Time period b/w two consecutive resharpenings or replacements.

Ways of measuring tool life No. of pieces of work machined Total volume of material removed Total length of cut. Limiting value of surface finish Increase in cutting forces Dimensional accuracy Overheating and fuming Presence of chatter

Modes of tool failure Temperature failure Rupture of the tool point Plastic deformation of CE due to high temp Cracking at the CE due to thermal stresses. Rupture of the tool point Chipping of tool edge due to mechanical impact Crumbling of CE due to BUE Gradual wear at tool point Flank wear Crater wear

Tool wear Tool wear causes the tool to lose its original shape- ineffective cutting Tool needs to be resharpened

Causes of Tool Wear Attrition wear Diffusion wear Abrasive wear Electrochemical wear Chemical wear Plastic deformation Thermal cracking

Attrition wear At low cutting speeds Flow of material past cutting edge is irregular and less stream lined BUE formed and discontinuous contact with the tool Fragments of tool are torn from the tool surface intermittently High Slow and interrupted cutting Presence of vibrations Found in carbide tools at low cutting speeds

Diffusion wear Diffusion of metal & carbon atoms from the tool surface into the w/p & chip. Due to High temp High pressure Rapid flow of chip & w/p past the tool Diffusion rate depends on the metallurgical relationship Significant in carbide tools.

Abrasive wear Due to Contributes to flank wear Presence of hard materials in w/p material. Strain hardening induced in the chip & w/p due to plastic deformation. Contributes to flank wear Effect can be reduced by fine grain size of the tool & lower percentage of cobalt

Electrochemical wear When ions are passed b/w tool & w/p Oxidation of the tool surface Break down of tool material @ chip tool interface

Chemical wear Interaction b/w tool and work material Plastics with carbide tools Cutting fluid

Plastic Deformation When high compressive stresses acts on tool rake face- tool deformed downways – reduces relief angle Modifies tool geometry and accelerates other wear processes

Thermal cracking Due to cyclic thermal stresses at cutting edge Comb cracks Transverse cracks Chipping of tool

Geometry of tool wear Flank wear (edge wear) Crater wear (face wear)

Flank Wear Tool slides over the surface of the work piece and friction is developed Due to Friction and abrasion. Adhesion b/w work piece & tool- BUE Starts at CE and starts widening along the clearance face Independent of cutting conditions and tool / work piece materials Brittle and discontinuous chip Increases as speed is increased.

Primary stage rapid wear due to very high stress at tool point Wear rate is more or less linear in the secondary stage Tertiary stage wear rate increases rapidly resulting in catastrophic failure.

Crater wear Direct contact of tool and w/p Forms cavity Ductile materials – continuous chips Initiates rapid rupture near to nose Leads to weakening of the tool Increase in cutting temp Cutting forces & friction

Measurement of tool life Time for Total destruction in case of HSS or time to produce 0.75 mm wear for carbide tools Tool life expressed by Taylor’s eqn VTb = C V = cutting speed in cm/min T= tool life in min b= const= 0.1 for HSS C= 50 for HSS Cemeted carbide : b=0.125, C=100 Tool life expressed in volume of metal removed L = TVfd

Measurement of tool life Diamond indentor technique Radioactive techniques Test at elevated cutting speeds Facing tests Test with low wear criterion

Factors affecting tool life Cutting speed Physical properties of w/p Area of cut Ratio of feed to depth of cut Shape and angles of tool Tool material and its heat treatment Nature and quantity of coolants Rigidity of tool and w\p

Machinability Machinability is defined in terms of: Surface finish and surface integrity Tool life Force and power required The level of difficulty in chip control Good machinability indicates good surface finish and surface integrity, a long tool life, and low force and power requirements Machinability ratings (indexes) are available for each type of material and its condition

Factors affecting machinability of metals Material of w/p- hardness, tensile properties, strain hardenability Tool material. Size and shape of the tool. Type of machining operation. Size, shape and velocity of cut. Type and quality of machine used Quality of lubricant used in machining Friction b/w chip & tool Shearing strength of w/p material

Evaluation of machinability- factors Tool life Form and size of chip and shear angle. Cutting forces and power consumption Surface finish Cutting temperature MRR per tool grind Rate of cutting under standard force Dimensional accuracy

Evaluation of machinability Machinability decreases with increase in tensile strength and hardness Machinability of a material is assessed by any of the following. Tool life Limiting MRR at which the material can be machined for standard short tool life. Cutting force Surface finish Chip shape

Relative machinability Mg alloys Bearing bronze Al alloys Zn alloys Free cutting sheet brass Gun metal Silicon bronze, Mn bronze S.G Cast iron Malleable cast iron Gray CI Free cutting steel Sulphur bearing steel Cu-Al alloys Low carbon steels Nickel Low alloy steels Wrought iron HSS 18-8 SS Monel White CI Stellite Sintered carbides

Machinability index Machinability index= Vt/Vs x100 Vt – cutting speed of metal for 1 min tool life Vs – cutting speed of standard free cutting steel for 1 min tool life. Material MI SS 25 Low carbon steel 55-65 Cu 70 Red brass 180 Al alloys 300-1500 Mg alloys 500-2000

Machinability: Machinability of Ferrous Metals Steels If a carbon steel is too ductile, chip formation can produce built-up edge, leading to poor surface finish If too hard, it can cause abrasive wear of the tool because of the presence of carbides in the steel In leaded steels, a high percentage of lead solidifies at the tips of manganese sulfide inclusions Calcium-deoxidized steels contain oxide flakes of calcium silicates (CaSO) that reduce the strength of the secondary shear zone and decrease tool–chip interface friction and wear

Machinability: Machinability of Ferrous Metals Effects of Various Elements in Steels Presence of aluminum and silicon is harmful, as it combine with oxygen to form aluminum oxide and silicates, which are hard and abrasive Thus tool wear increases and machinability reduce Stainless Steels Austenitic (300 series) steels are difficult to machine Ferritic stainless steels (also 300 series) have good machinability Martensitic (400 series) steels are abrasive

Machinability: Machinability of Nonferrous Metals Aluminum is very easy to machine Beryllium requires machining in a controlled environment Cobalt-based alloys require sharp, abrasion-resistant tool materials and low feeds and speeds Copper can be difficult to machine because of builtup edge formation Magnesium is very easy to machine, with good surface finish and prolonged tool life Titanium and its alloys have very poor thermal conductivity Tungsten is brittle, strong, and very abrasive

Cutting fluids Decreasing power requirement Increasing heat dissipation Neat oils+ extreme pressure additives Water emulsions