Some of the possible tool failure criteria are
- Tool wear
- Chipping or fine cracks developing at the cutting edge
- Wear land size
- Crater depth, width or other parameters
- Volume or weight of material worn off the tool
- Total destruction of the tool
- Failure of tool implies when the tool ceases to function satisfactorily. Among the various failures are: tool point gets blunt, gets fractured, foreign materials from work gets welded on to it, or the micro structure changes thus reducing the hardness from the optimum value. Failure could be caused due to the following reasons.
- Temperature Failure: During machining at high speeds, very high temperature exists at tool chip interface. When temperature exceeds the critical limit, the tool point gets softened. Due to this high temperature, localised phase transformation occurs. This gives rise to high residual stresses due to which cracks appear in the tool point and in such a state, it is more prone to failure in some cases tool point might even melt. This type of failure occurs quite rapidly, and is frequently accompanied by sparking and is easily recognised. Thermal cracking occurs when there is a step temp gradient due to intermitent cutting. Thermal shock combined with mechanical impact lead to failure.
- Rupture of Tool Point: At slow speeds, built up edge is formed on the tool. When it grows too much, it is unstable and breaks away with the underside of the chip, taking away? portion of tool with it. This is so with brittle tool materials, like carbides.
- Tool are generally hard. A large degree of brittleness is associated with hardness especially in small case of carbide and diamond tipped tools. Whenever the cutting forces exceed the critical limit, small portions of the cutting edge begin to chip off, or the entire tip may break away in some cases.
- The high forces, which produce this type of failure are not generally associated with steady state cutting but rather with variations in the cutting process or when cutting with excessive vibration (Chatter).