There is a write up called Threading on the Lathe Mach3 which can found in the members Docs which you should read.
If you use a VFD ( my lathe does not have one ) i would recommend setting / fixing the rpm at which you will do the thread.
During the threading cycle Mach is controlling the feedrate based on it's reading of the real time rpm. Mach will adjust the feedrate should
there be a spindle slow down , but, it will not adjust the feedrate upwards above your set rpm. Should your VFD input sense that the spindle
rpm requires adjustment, and then adjusts the rpm, that would be fighting the built in adjustment that the threading cycle is doing, thus you will have
threading problems.
-how can the synchronization of the spindle with the other axes possibly be accurate using a VFD
I would say it is more accurate based the on testing we did. The single index is very accurate and there is no advantage to multi slotted disc's
and that's why it no longer used and shouldn't be used. Remmeber that the thread cutting will only be as accurate as YOUR "lathe system".
-theres no real position control ..........meeting ANSI specs
In the write you will find a table which defines the basis that Mach threading needed to achieve.
How good your lathe system will do can be tested , just read the write up.
- if anyone is making good threads with mach and not installing a servo for the spindle let me know how
I have. Just remember that the physical end result is affected by manny things and i believe most if not all are covered in the write up.
RICH