Ah ok you are talking about CSS (Constant Surface Speed) Mach does it, well sort of, it doesnt work correctly unfortunately if in G95 mode which to me is really the only really useful way to use it, what happens is the axis slows or speeds when the spindle is increasing/decreasing. If in G94 then it seems to work fine but the issue then is you are not keeping a constant FPR.
It is not possible to do CSS properly in Mach3 but Mach4, I am assured, will do CSS properly.
Hood