A novel lathe-type cutting tool in the form of a disk that may be rotated about its central axis is described and analyzed. Such a rotary tool is found to correspond to an equivalent oblique tool having an angle of inclination whose tangent is equal to the ratio of the tool and work-surface velocities. In addition to the inherent feature of the oblique tool of providing a greater effective rake angle without a corresponding decrease in the actual included angle of the tool, the rotary tool (1) provides a rest period for the cutting edge, thus enabling the edge to be cooled and the adsorbed film on the tool surface to be replenished between cuts; (2) enables the relative chip velocity to be increased to provide a lower coefficient of friction without necessitating a corresponding increase in the rate of metal removal. A rotary tool of 10-deg rake angle is capable of reducing total power required to make a given cut by about 30 per cent and at the same time to operate with a temperature about 400 F lower than that for the equivalent stationary tool. Representative test data for both rotary and stationary tools are analyzed and discussed. The shear stress on the shear plane is found to decrease with increased shear strain in the case of either a rotary tool or an oblique tool. This anomalous result is explained in terms of a size effect associated with the change in thickness of the shear plane that accompanies a change in the angle of inclination.