Thank you for your reply, I should have said that I'm now using a 1hp 3 phase motor to drive the spindle, it doesn't seem to slow down while cutting.
Anyway, what is bugging me is that the threading program won't start if the "Use Spindle Feedback In Sync Mode" is checked. And it needs to be checked according to some sources.
Also, some people report that they have good results in threading using the parallel port :
https://www.machsupport.com/forum/in...p?topic=9861.0
For this reason I'm thinking it's a matter of getting the settings right... But you guys would know better.
Does anyone have other ideas?