UNIT-I THEORY OF METAL CUTTING

Tool Wear (Failure) Tool wear occurs due to factors such as: Forces Temperatures Sliding action b/w tool & work piece They are classified as Face wear Flank wear Nose wear Face wear Flank wear Nose wear

Tool Wear (Failure) Face wear Friction caused when chip flows on face of tool Cavity is formed – crater wear If cavities are increased, tool will be broken Flank wear Caused by abrasion Occurs on the side & flanks of the cutting tool Cannot produce finished surface Nose wear Occurs at the nose of the tool If nose wear, friction b/w the tool & work piece will be more © mjk

TOOL –LIFE Tool-life is the time elapsed between two successive grinding of a cutting tool. Tool-life may be measured in the following ways: Number of pieces machined between tool sharpening. Time of actual operation, viz., the time the tool is in contact with the job. Total time of operation. Equivalent cutting speed. Volume of material removed between tool sharpening.

FACTORS AFFECTING TOOL LIFE Cutting speed Depth of cut Tool geometry Type of machining Coolant Feed Tool material Work piece material Rigidity of machine © mjk

Tool Wear & Failure - Factors Cutting speed When machining with high speed, high temp. will be developed b/w work piece & tool Hardness will be reduced Feed Life of tool depends on the amount of metal removed per minute Depth of cut Correct depth should be used to increase tool life Tool material Tool life depends on the tool material. Carbide tools have more life than high speed steel & carbon steel.

Tool Wear & Failure - Factors Tool geometry Geometry should be perfect Large rake angle reduce the tool cross section Head absorbing area will also be reduced Increase in nose radius improves tool life Rigidity of machine Strongly fitted tool on rigid m/c will have more life Coolant Coolant absorb heat produced during machining It reduces friction & temp Tool life will be increased

MACHINABILITY Machinability depends on: 1. Chemical composition of work piece material. 2. Micro-structure. 3. Mechanical properties. 4. Physical properties. 5. Cutting conditions.

Evaluation of machinability 1. Tool-life between grinds. 2. Value of cutting forces. 3. Quality of surface finish. 4. Form and size of chips. 5. Temperature of cutting. 6. Rate of cutting under a standard force. 7. Rate of metal removal.

SURFACE FINISH Cutting speed Feed Depth of cut