Edge wear and cratering are the most frequent tool breakdowns that occur. Edge wear (Figure F-29) is simply the breaking down of the tool relief Surface caused by friction and abrasion and is considered normal wear. Edge breakdowns is also caused by the tearing away of minute carbide particles by the built-up edge (Figure F-30). The cutting edge is usually chipped or broken in this case. Lack of rigidity, too much feed, or too slow a speed results in chipped or broken inserts (Figure F-31).
Tiny projections on the cutting edges of new inserts often break down and reduce tool life. Some machinists hone the edges of new inserts to increase tool life. However, if this practice is not carried out correctly, the tool can be damaged and actually have a lowered tool life. For this reason, some insert tool manufacturers provide prehoned carbide tools. Machine shop students should not hone the edges of their tools until they have gained more experience in the fied.