Tool Failure

Tool Failure

Tool Failure -

 A tool is said to fail when it loses .its usefulness through

(i) Wear

(ii) Breakage

(iii) Chipping

(iv) Deformation, etc.

(i) Wear, which accounts for most of the tool failures, is very complex and involves chemical, physical and mechanical processes, often in combinations.

(ii) Breakage Excessive pressures acting on the cutting edge of a tool may cause immediate failure by the loss of one or two large particles or several smaller particles. Breakage is usually attributed to mechanical shock, thermal shock, thermal cracks, fatigue or excessive wear, experienced by the tool during cutting.

(iii) Chipping Chipping is a microscopic form of breakage due to the loss of many small particles from the cutting edge and is usually caused by:

(a)Un honed carbide edges. Such edges lack sufficient mechanical strength to withstand the mechanical forces encountered in cutting.

(b) Excessive vibration and chatter.

(iv) Deformation When a heavy load is applied close to the cutting edge of the tool, the surface at this point becomes indented while the adjacent face shows a corresponding bulge. The amount of deformation increases as the load is increased, until a critical strain is reached and fracture starts.

A crack forms at the periphery of the indentation, away from the cutting edge, spreads to the edge, and then goes down the adjacent face until a small flake of carbide breaks away. A tool's ability to withstand deformation is based on its hot hardness.

(v) Total tool destruction This is a point at which the tool ceases to cut or the wear rate tends toward infinity.

(vi)Formation of cracks Cracks form usually at right angles and next to the cutting edge, on the face or flank of the tool. This often results from the excessive heat generated by carbide tools operating at excessive speeds.

(vii) Temperature failure at the cutting edge As high temperatures develop, the tool becomes soft, loses its strength, stops functioning properly, and may shear off a part of the cutting edge. Such failure occurs quite rapidly at very high cutting speeds.

(viii) Inability to maintain workpiece dimensions as cutting progresses, due to increased tool wear.

(ix) Inability to impart proper surface finish on the workpiece.

(x) Increase in power requirements due to tool dulling.