About 10 years ago when I was doing manufacturing engineering for Baker Hughes. I got together with a guy from fanuc and he gave me a formula for actually being able to calculate the lead differential (z-offset whatever you might like to call it) for changing speeds while threading with a fanuc 18T controller and a spindle that you didn't have the ability to orient (like a weiler).. I am not speaking about "in the cut" speed changing (variable lead).. I am talking about just being able to change speeds for whatever reason (usually chatter) and make the corresponding adjustment in the z offset relative to wether you were speeding up or slowing down without losing the lead... It never worked well enough to cut premium threads but it sure did a bang up job getting back in and close enough to bump down into less critical thread forms. It saved lots of time with chatter problems due to material variations or just plain running the wrong speeds. Does anybody have the slightest idea what I am talking about or seen this also? I have lost it and from time to time I would give my eye-teeth for it. Thanks for your time and any input.