Hi,
Mach4's API lists an 'override' axis, which if I understand correctly modifies the movement of the main axis, a THC function is suggested as an example.
I haven't used it so don't really know what can be achieved with it but it sounds hopeful.
Currently THC is enacted by the motion controller, the regular Z axis motion pulse stream have to be broken up and more pulses inserted or alternately pulses ignored to effect
THC variation 'on top of' regular coordinated Z axis motion. The Hicon controller of Vital Systems is the only Mach4 controller that has THC support, to my knowledge.
Warp9 have been promising THC support for the ESS for best part of a year but no dice as yet.
As for programmatically adjusting the 'steps per unit' of the fly.....Access to the data structure that includes the value is not available to us via LUA but
I would presume that the 'steps per unit' value would be stored in on of Machs pound variables. If you knew which one you could manipulate it directly. It might
not be as convenient as an API nor would it be as robust, NFS makes no representation that a pound variable will retain its precise function over later builds, but
it would work. Would it be worth asking NFS what the pound variable number is?
Craig