Comment on 'The cutting of metals by plastic buckling' by Udupa et al.

In a recent paper on machining annealed copper at a low cutting speed, and at an uncut chip thickness one tenth of the mean grain size of the copper, Udupa et al. (Proc. R. Soc A473:20160863, doi:10.1098/rspa.2016.0863) report chip thicknesses larger than 10 times the uncut thickness and then a new mode of chip formation. Plastic bulging occurs in the surface of the copper ahead of the tool, leading to chip formation by a series of folds. The strain in the chip is less than that expected in a chip formed by shear according to long-standing classical theory. The authors suggest that the foundations of that theory need to be re-examined. In response, continuum mechanics numerical simulations presented here show a continuous transition from the classical condition towards that observed by Udupa et al. as the ratio of chip thickness to uncut thickness increases above ≈ 7. Bulging is obtained by introducing (approximately) material heterogeneity to the simulations at a grain size scale but whether such heterogeneity is essential for the bulging flows remains an open question.