Uneven movement when working a file from SD card

Mostly Printed CNC - MPCNC Troubleshooting
1

Hi,
This is not quite MPCNC-related but may be useful for someone … if any solution is found…
I’m using X axis to move a wheel. So I’m trying to move the wheel little by little with distance between the stops of around 8mm (measured on the outer radius of the wheel).
When “printing” using USB & repetier - no problems, it moves equally, same distance between stops.
But when I “print” same gcode from SD card - it makes one bigger (1.5 mm more) movement each 2 or 3 times. The amount of normal movements between abnormal ones is stable.
It makes 2 normal movements. Then 1 abnormal one. Then 3 normal ones. Then 1 abnormal one. And so on. 2-1-3-1-2-1-3-2 etc.
When I repeat the job - it moves exactly where it was. Meaning that abnormal movements are where they were.

I tried this with NEMA17+DRV8825, 4498 and I even tried MKS CD + DM556 driver + nema23. The only thing changes between these setups is the pattern (how many normal movements there are between abnormal ones).

I also tried same on MKS Robin V2.4 with its corresponding old Marlin firmware. Abnormal movements remain.

Any ideas?

2

Can you share more details about the machine? What steps/mm does it have? What kind of gears/belts does it have?

3

Without more details, it is hard to troubleshoot the problem. Check your Repetier-Host app for any start g-code (Config/Printer Settings/Scripts). That is one place where the two pathways (Repetier and SD), can be different.

4

I was lucky enough to notice yesterday that I was using comma instead of dot as a delimiter of floating point. Seems that machine understood only ceil number and ditched the fractional part of a number.

You live you learn.

5

That makes sense. Thank you for sharing.