The Evolution of Cutting Tool Materials

It is evident from this discussion concerning optimisation, that the parameters of: tool life, feedrate and cutting speed form a complex relationship, which is illustrated in Fig. 2a. Consequently, if you change one parameter, it will affect the others, so a compromise has to be reached to obtain the optimum performance from a cutting tool. Preferably, the ideal cutting tool should have superior performance if five distinct areas (see Fig. 2b):

• Hot hardness – is necessary in order to maintain sharp and consistent cutting edge at the elevated temperatures that are present when machining.

NB If the hot hardness of the tooling is not sufficient for the temperature generated at the tool’s tip, then it will degrade quickly and be useless.

• Resistance to thermal shock – this is necessary in oder to overcome the effects of the ontinuous
cycle of heating and cooling that is typical in a milling oeration, or when an intermittent cutting operation occurs on a lathe (e.g. an eccentric turning operation).

NB If this thermal shock resistance is too low, then rapid wear rates can be expected, typified in the past, by ‘comb cracks’ on High-speed steel (HSS) milling cutters.

• Lack of affinity – this condition should be present between the tool and the workpiece, since any degree of affinity will lead to the formation of a builtup edge (BUE) – see the chapter on Machinability and Surface Integrity.

NB This BUE will modify the tool geometry, leading to poorer chip-breaking ability, with higher forces generated, leading to degraded workpiece surface finish. Ideally, the cutting edge should be inert to any reaction  with the workpiece.

• Resistance to oxidation – a cutting edge should have the desirable condition of having a high resistance to oxidation.

NB This oxidation resistance of the cutting tool is necessary, in order to reduce the debilitating wear that oxidation can produce when machining at elevated temperatures.

• Toughness – allows the cutting edge of the insert to absorb the cutting forces and shock loads produced whilst machining, particularly relevant when intermittent cutting operations occur.

NB If an insert is not sufficiently tough, then when unwanted vibrations are induced, this can result in either premature failure, or worse, a shattered cutting edge.