I have an Alexander pantographic engraving machine that I want to convert to CNC. Its work table has X, Y and Z ways, so I could treat it like a vertical knee mill. But the existing leadscrews have a lot of backlash and moving the very heavy knee takes a lot of power.
So, I'm wondering about driving the pantograph itself with leadscrews to control the angle within the parallelogram and the angle between it and the rest of the machine. That would eliminate backlash and take very little power. It would be like driving a robotic arm. I'd essentially need to convert all X,Y coordinates into angular coordinates.
I'm expecting to use a PC parallel port controller and cheaply available CNC software such as Mach 3. Would this allow me to intercept the step/direction signals (in software) so that I could convert them to the actual signals needed to achieve the same dX,dY motion? Or, perhaps, to take the G codes themselves and do all my own post processing?

Has anyone used G-code type CNC with non-orthogonal axes? How are industrial robots controlled?