The deformation work absorbed by the workpiece during metal cutting is analyzed. It was shown from photomicrographs that this is appreciable and may constitute the major energy absorbed in metal cutting when the depth of cut becomes small. The so-called “size effect in metal cutting,” in part, may be explained by this workpiece deformation. Correcting cutting forces for workpiece deformation in no way detracts from Merchant’s analysis, based on the minimum-energy principle. However, the criteria of plasticity in metal cutting and the correlation of metal-cutting data with tensile data remain in doubt.