Lathe thread cutting issues at specific RPM

In this video you can observe the overshooting (velocity skips over 900) at the thread start.

Threading video

Sure looks like a commanded position issue
so something in linuxcnc or gcode not hardware

Hi,

I made some HAL scope screenshots.

The first one was taken during the normal threading cycle.
The second one is with spindle full stopped between each passes.

You can seen the velocity overshot on Z axis.
Unfortunately I couldn't find any signal of acceleration of Z axis to include.

looks to me as the axis needs tuning , chances are your running the axis faster than the actual motor can accomplish .
reduce your settings in the ini , alternatlvely you could increase the supplied voltage and current from your power supply

can you give rating of psu used .

I dont think this is related to the 7I43 or stepper motion at all
(other than perhaps max acceleration and velocity settings)

If you have reasonable ferror limits set and you dont get following errors,
then the Z motion is exactly what linuxcnc's motion module commanded.

That is, nothing the the stepgen or Z driver/mechanics does affects the commanded Z position
and the fact that there are no following errors means that your overshoot was commanded
by the motion module, so thats why it looks like something funny in motion or the
G code thats causing this problem.

EDIT

Do you have an issue with non tapered threads?

The fact that spring passes went wrong suggest trouble at X close to 0