Losing steps on closed loop stepper motors

@PCW: no I have not tried inverting the pulse, however the motors seem to move totally normal ig the slight drift over time.
How could that be a result of inverted step pulses?

@BeagleBrainz: it seems to be with all axis, however a bit more on the longer one.
No its not a linear error. If you move one axes and measure the distance, then the measured distance is totally fine within some few microns.
The error only occurs when milling for lets say half an hour and then measuring the position against the start position.
The coupling between the motors and the lead screw is done with those couplings that have red rubber in between.
The bearings at the end of the leadscrew are the bearing blocks that come with the leadscrew. I have tested pushing as strong as I can against the bearings and also moving the axis in between, that does not seem to have any effect on them.

The "procedure" I used for testing is. Measure the position of the tool in one axes. Then mill something. Measure it again. This results in an difference of up to 0.5 mm after half an hour of milling. Since 0.5mm is really a lot i cannot imagine how this could be from the leadscrew backlash.

The error is kind of random therefor I habe not tested from different start points.

@PCW: no I have not tried inverting the pulse, however the motors seem to move totally normal ig the slight drift over time.
How could that be a result of inverted step pulses?

Because if the step pulse is of the wrong polarity you can violate both step length and direction hold timing parameters
violating either of these parameters can cause difficult to diagnose step loss, for example if you violate the direction
hold timing you will get random step gain/loss at reversals where you cannot (by ear) determine that anything is wrong

That said, as long as you have direction hold times > (the drives hold time requirements + step length),
and step space > the drives minimum step length, inverted step pulses should not cause an issue.

An easy way bisect the issue into axis mechanics vs step motor/electrical is to mark the step motor shaft and motor frame
at some particular location, run your program, move back to the reference position and see if the marks still align

Ok, I guess i might have found the problem. The PC im using is not the newest and when I run the realtime test I get an Max Jitter Base Thread of about 90 000 ns.
In the linuxcnc documentation it is mentioned that over 100.000ns the PC is not good for running linux-cnc, however when I enter 90µs to the Max Jitter field in the GUI it is automatically reduced to 50µs.
So this seems a good candidate for me to cause the problem or what does linux-cnc do if it cannot reach the realtime requirements. Does ist "just" get slower or does is skip steps??

goaran wrote: In the linuxcnc documentation it is mentioned that over 100.000ns the PC is not good for running linux-cnc, however when I enter 90µs to the Max Jitter field in the GUI it is automatically reduced to 50µs.

That is valid only if you are using parallel port, for Mesa boards it can be worse and still work properly.
Worst case latency errors will cause lost steps.
Do you get a latency warning when running Linuxcnc, on the right lover corner with a red x ?
And stepconf usually sets the base period to 100.000 for any entry above 35.000.

I don't have a mesa board and am running directly from parallel port.
There sometimes is latency warning on startup of LinuxCNC in the right corner.
Where does StepConf set up the latency periode to 100K? When I try to enter anything above 50000 it automatically gets reduced to 50000?

In the INI file.
Open file browser, got to home/linuxcnc/configs, open the config you are using, there are several files there but only one should have .ini extension. Right click on it, open with text editor. Look fo "base period" there you should have 100.000 set.
If you are getting latency warnings with 100.000 base period you can set it to 150.000 but that will greatly limit the step rate, and the overall velocity.
I do not recall, what drives are you using ?