ok still have this problem , here is wath i did today
- remove mach3 with all related files, folder and driver from pc
- download and install a fresh copy of ver 3.042.020 plus config
- ran driver test at 35 kHz, everything fine here
- removed the ball screw from the lathe and clean. i tough a chip in the ball nut might give an intermitent slow down on the z axis and damage the thread
- change the electronic gearing on the z axis drive from 10 to 4. this gives me 10 000 ppr, this limits the rapids to 210 ipm but thats fine for wath i do
- ran turn diag. cut a thread in the air, everything fine.
- when everything done, i cut a thread on a piece of scrap, and got a perfect thread
I then loaded a good shaft in the lathe... guess wath same problem!!
after cuting a few more on scrap shaft, i found that the x axis moving problem dissapear. from wath i can see when the thread is cutting, the first 4 or 5 cuts are ok, then on the next one it starts fine then somewhere along the cut, it looks like mach3 stop sending the pulse to the z axis for a breif moment, the axis gets behind a few thousand for the rest of the cut, then the process repeats over the 16 toal cuts!!
i've tried suggestion posted here and still at the same spot!!!