Despite several hours of effort and various attempts, I was unable to find a solution.
The turns of the workpiece are constant, and the pitch of the thread changes ... I am also attaching a photo.
I am also attaching  XML if anyone could help in any way.

It looks like spindle stalling under initial load.

If you run the code so it just scratches the surface of the diameter, can you see the pitch change.

Also 0.25mm depth for the first cut could be too much depending on the spindle power at 250 rpm.

Hello

The spindle is driven by a 7.5 kw motor via a gear transmission. Speed is stable, also rpm in Mach3 are stable during threading.
Thread in the pictures was made with multiple passes.
I also can't figure it out why feedrate in Mach3 showes 0,75 mm/turn if programmed pitch is 1,5mm
This feedrate is also not stable despite the fact that the spindle speed is constant.

I’m slowly getting desperate already, as I’ve invested a lot in reworking the machine, and now I can’t cut the thread.

Any advice is welcome ...
Thanks

Run a scratch pass so we can see what it looks like.

On my opinion there is something wrong in MACH3 settings.

Mach does not follow the programmed thread pitch.
If programmed pitch is 2mm, treading starts with F= 2mm/rew, but after ~ 2mm of moving this changes to F= 1mm/rew.
Spindle speed is stable and does not change. Feedrate/moving is not smooth but vibrating.
Index signal is one/turn - I tried with short time signal, longer signal, different index debounce settings... nothing change.
I have noticed under Spindle calibration menu one window for index signal "zero if one/rew". There is "1" but I can't change this value.
Could this be a reason for wrong pitch - wrong F/rew?

Thanks

OK, you know better.  Good luck.

Dear Graham

I don't know what your answer is supposed to mean
I just wrote my opinion. If the programmed step (feed rate) is 2mm / rew, and Mach works 1mm / rew, then there are probably some other settings ...
This is the same also without workpiece.

And a scratch test.... the same as pictures in my previous post .... its cut in a piece of plastic, so no (or near zero) cutting force.

MojsterMiha,
I just downloaded your xml file today and had a quick look at it nothing seemed out of place.
Scratch test is not what you did in the previous post. Get the write up I did on threading as it has lots of
 info that i am not going to repeat because that is the reason I wrote it.

From your pic of the cut thread seems that your depth of cut was too deep, slow down took place or you lost control, the controller can only slow down and not speed up, it tries to save the thread and modifies the feed rate on the following pathing.

Just do as Graham said doing scratch tests. Slow spindle speed down, try 50, 100, 150 instead of 250. When you use the Wizard it will say if you are exceeding axis capability ( not some pop up error) in the right hand corner of the screen.

What happens when you do a G1 Z*********  Fx move at the threaging feed rates you are going to use and just scribing a cut into the material. Should be constant movement , not vibrating! 

Also do you have backlash in the X axis, noticed that in the xml but also that compensation was not active. Backlash is a real PITA if you are trying to do threading.
 
RICH

Dear Graham, RICH

Thank you for your patience

In my previous post I sent the picture of thread cut in multiple passes in a plastic work piece - each pass not more than 0,1mm
I read all the thread cutting instructions, but unfortunately I didn't find the answer to the unusual doing of the machine / Mach3 in my case.

I have tried also with 50 rpm, 100,... it is the same. Spindle is driven by 7,5 kW motor and gearbox. Spindle speed is constant - I have measured it (out of Mach3) also with encoder and digital tachometer. It could not be a reason to slow down feed rate.

Z axis moves smoothly also by 5 times higher feedrate - when not threading. By all other work both axis moves with no problem at much higher feedrate.

I am attaching a picture of the thread - just one pass. Threading in this pictures was started 5mm before workpiece and thread pitch is quite constant, because it was already reduced to 1/2 (before touching workpiece)
If I start treading close to workpiece, thread looks like in my previous photo - pitch is reduced from programmed to 1/2 of programmed.

It is hard to explaine...I have  made a short video...but is too big to upload it.

From my previous post: "I have noticed under Spindle calibration menu one window for index signal "zero if one/rew". There is "1" but I can't change this value." 
Could this creates an issue?

even i have no experience in threading, had als o look to your Profile.
two questions:
-have you tryed to run Diameter mode instead of radius mode?
-witch Version of mach3 are you running?