[One of Many]

I'd expect you would need to know what type of advance the lathe was going to do, in order decide what line the sync will follow. I wouldn't think you would need to do any math in that regard.

It should be fairly simple with a good eye to get an idea where X will start cutting the existing pitch diameter. Then set the beginning or ending diameter based on the pitch height. If we are only adjusting the fixture I suggested in Z, the X should remain stable to the tool library.

I prefer cutting threads with a compound advance as in the old days. One CNC lathe I use is an EZ-Path and it does use the compound advance. If I were to set it up, I rather do it exactly as I would on a manual engine lathe and let it go from there, all be that under the lathe control. My experience has shown that making adjustments to the major diameter setting does reduce the pitch diameter at the thread advance angle. I contrived via my fixture idea, once I had it synced independent of the control, it would be a no brainer and finish the thread cycle on its own. Unfortunately, that would be a compulsory process for successive parts.

The thread sync is a function similar to electronic gearing. There is a sync pulse on the spindle and the software will move the Z in reference to the pulse feed. Commonly why it is recommended you set the thread start a few thread pitches ahead of the thread start. I have done prototype 2 lead threads in this manner. I just offset the Z 1/2 the thread pitch so the the new thread was cut in sync offset from the first. Worked like a charm at least on the EZ-Path. If I increase the spindel RPM, and recut the same part, the Z is quicker, but the sync is the same.

I have heard it suggested in other threads to move Z zero back until you achieve a resync. This takes a lot of time, but might work if others have more patience than I do.

Another method had to do with cutting a new thread the size required and then take a reference position of the spindle and Z in one of the newly created threads. Then place the part to be chased in the chuck/collet and rotated to sync its thread to the reference. Maybe this is where the floating cutter holder might pay off?

Most CNC thread reworking suggestions I have read over the years are so cumbersome, I've not tried for the sake of having other means to get it done quicker on a manual lathe.

I have racked my brains on and off the last 7+ years for a fool proof method on this. So far just a bruised brain and not much else.

DC