Study on the tool life analysis for free-cutting steels

Experimental study and theoretical calculation on the tool lives of free-cutting steels with different additions were carried out in terms of the friction characteristics, chip deformation, and normal stress distribution. The average friction coefficient, shear angle, and maximum normal stress were then calculated, and the influence of them on tool life was analyzed and discussed carefully. The free-cutting steel with Bi addition (MB) presented the best tool life of about 9200 and 5200 s at the speed of 150 and 200 m/min, respectively, which was mainly due to the combined effect of the lower average friction coefficient, maximum normal stress, and higher shear angle. When the cutting speed increased to 300 m/min, the tool life decreased rapidly, which was mainly caused by the serious tool wear behavior dominated by the high cutting temperature. Differing from the calculated results of shear angles based on Lee and Shaffer’s assumption, the measured shear angles decreased with the cutting speed ranging from 150 to 250 m/min, resulting from the thermal softening of workpiece material during machining. Moreover, the present findings indicated that the cutting process became unstable when the maximum normal stress on the tool nose was larger than 2 GPa, where catastrophic failures occurred, resulting in accelerated fracture of the tool. The research findings not only give more insight into the effect of friction characteristics, chip deformation, and normal stress distribution on the tool lives, but also have significance for the machining of free-cutting steels.