Mach4 lathe threading pitch error

Hi,
I am retrofitting a Harrison trainer lathe to Mach4 using Pokeys57cnc and stepper drives.
It is working now and I am very happy with Mach4' turn cycles wizards!
This way it is easier to program; it is faster than doing it on manual lathe, even 1 piece-project!

But.. when threading the pitch differs from what it should be.
When asking for pitch 1.5mm it is just a little off, when typing 1.475 instead 1.5 it is a nice fit for 1.5 after machining..

I searched a lot on the web but in this case I cannot find a parameter/faulty entry where to adjust it.

I checked/facts/tried:
-When moving the Zaxis 1mm, machine is moving 1mm (0.01mm precice)
-The encoder is connected to spindle using the same toothed belt sprockets. Encoder is 1024 PPR with 1 index pulse. Wired with EMC protected wire.
-The spindle speed is 430 rev/min, the screen reads around 440, fluctuating not more than 1 or two. No speed frequency module (yet) just relays start and stop.
-I tried to tell the machine the speed is 440 in Gcode but it makes no difference.,
-I tried lower rev but did not change pitch (anyway: the max speed of Zaxis is 700mm/min, 430*1.5=645mm/min so it should be ok..)
-I checked G99 is used (feed/rev)
-check box use threading is checked at Pokeys driver and encoder settings.

The question is where to find the solution; Pokeys or Mach4?

Thanks for any suggestions!!

Yesterday I tried to make more dry run space before actual cutting so that Zaxis has become stable with speed of spindle. Made no difference.
I had safety 5mm and 2 mm clearance.
It sounds very stable and the spindle does not drop in speed while cutting..

Spindle speed should not affect the pitch, as when threading, the Z axis should be geared to match the spindle, so even if the actual spindle speed isn't as requested, the Z should adjust to create the required pitch/thread.

Are you 100% sure it's a 1024 PPR encoder you have?
If it was actually 1000PPR, it would throw your pitch off by about 1%, and going by the 1.475/1.5 it's off by 1.66%.
At that big a pitch with a normal nut, being 0.6% off on pitch would probably still work without any noticeable problems.

Have you checked your Z axis calibration over a larger distance?
Checking over only 1mm isn't likely to show any minor discrepancies. I'd check it over whatever the biggest distance you can using a set of digital verniers/calipers

Thanks for the reply,
Yes i bought a new encoder, 1024 ppm was on stock. The label on the box stated 1024, but I will check on the encoder itself to make sure that is really 1024.
I thought because it has an index signal it was not possible to change the encoder PPR, it should give an error after 1 rev I thought.. but if you are right I could solve the problem by changing the PPR to correct the differences.. I will change the PPR to see any change in pitch.

I will check the Z axis calibration over a longer distance, I will let you know the outcome.
BTW when cutting metric M10 Ø10mm and pitch 1.5mm a normal nut will not fit, that is how I found out about the pitch error. A thread measuring gage does not fit either.
Thanks again and to be continued!

Update:
I checked PPR encoder and it is 1024 on encoder, so that should be ok.
I tried to change the value in Pokeys plugin to 2000 PPR, and it worked! No error at all!
The rev counter was around half rev it was before with same spindle speed.
The feedrate during threading of the Z axis was lower as well, so now I can try numbers 1.66% higher than 1024.
It solves my problem (now I can just use pitch 1.5 in the Gcode file) but this is a work around offcoarse.

The other thing to check was Z axis calibration: over 300mm the error was 0.5mm. You are right this is quite much..
But the difference I have on the pitch would be 5mm over 300mm, so I doubt that it is the reason of the error.
The stepper motors are 200 steps/rev, spindle is 6mm/rev, so 1 unit is 200/6=33.33. I just did not use the .333, so now it is better.

So first I will try with the new calibration setting if the pitch is better or not because I was not able to test it with actual material.

Hi, last update: problem solved.
First I tried to calibrate Z axis, it is now within 0.01mm on 300mm stroke.
Thread still not right pitch.

The actual solution is change the PPR to 1042 instead of the actual 1024 of the encoder.

The future projects with threading will show if this setting is suitable for other pitch threads as well..
Anyway, thanks for the help M_C.