Investigation of heat partition in high speed turning of high strength alloy steel

High Speed Machining (HSM) is now recognised as one of the key processes in advanced machining technology for automotive, die and mould, and aerospace industries. Machining of metals at high cutting speeds produces high temperatures in the primary shear zone, which induces plasticity in the workpiece and hence decreases the cutting forces. This investigation is concerned with the estimation of the amount of heat flowing into the cutting tool in high speed turning of BS 970-709M40EN19 (AISI/SAE-4140) high strength alloy steel. The aim is to characterise the thermal field in the cutting zone and thus understand the mechanics of HSM. Experimental results are presented of temperature measurements on the tool rake face during orthogonal cutting at cutting speeds ranging between 200 and 1200 m/min. These measured temperatures are compared with temperature fields in the cutting tool obtained from a finite element transient thermal analysis. It is shown that the tool-chip contact area, and hence the proportion of the secondary heat source conducting into the tool, changes significantly with cutting speed; it decreases with the cutting speed in the conventional and the transition regions but increases in the HSM region approaching 65% at 1200 m/min. These results are relevant to the study of thermal expansion of the cutting tools and the cutting edge wear in HSM operations.