Thanks Rich,
I had never heard of G68/G69. It doesn't appear on the old list of G Commands that I printed out from Mach3 years ago. I looked it up in Peter Smid's CNC Programming Handbook and I now understand.
Presumably Mach3 recognizes and responds to G68/G69 commands?
I'll give it a try as soon as I can get an opportunity. (busy right now having been sucked temporarily back from retirement into the thrust and cut of the real world)
Regards and once again, thanks for the tip.
Chrisjh