A numerical control system for providing a constant rate of metal removal in terms of surface feet per unit time. The operative relationship between the rotational speed of a rotating workpiece (RPM), the effective radius in inches of the rotating workpiece (R) and the desired rate of metal removal in surface feet per minute (SFM) is defined as: The relationship is mechanized by a plurality of digital integrators which generate pulse frequency signals proportional to each side of this relationship. These pulse frequencies are then compared with the result of this comparison affecting the speed of the rotating workpiece so as to close the control loop and fully mechanize this relationship. In one embodiment, the pulse frequencies are compared by generating phase varying signals which are then phase compared. In another, a single phase varying signal is generated and phase compared with a constant phase signal. A number of alternative embodiments are disclosed for indicating the speed of the rotating workpiece including a novel digitizer which also provides a steady state signal indicative of workpiece rotational speed.