Stepper motor skipping steps

Hi all,

First of all sorry if I'm on the wrong sub forum. Since a week I can make use of a builder big builder 3d printer. It uses marlin software so it works for me the same as my own prusa i3 printer.
But I run into a problem. When the Y axis is reversing its direction the first few steps it doesn't move. If I move the Y-axis with 0.1mm steps it moves accurate, but when I go the other way around it doesn't move the first 3 steps or so until it starts moving. The results of this are non round circles of course.
What are the possible causes?
Thank you very much!

Possible causes include:

- Loose belt
- Grub screw not tight in pulley
- Too much friction
- Too little motor current

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Sounds like backlash more than skipped steps to me considering it's only on direction changes and only for the first bit. Does the motor produce a clicking noise and twitch like steppers usually do when skipping steps? A fair few of dc42's suggestions still apply, check the pulley grub screw, belt tension etc.

Thank you very much for the replies.
I tested some stuff. I removed the belt from the y-axis and did the same test and the stepper motor just worked fine. The backlash on the system is about zero. So I think it will be a combination of some friction and the stepper drivers which are set quite conservative. I already started lubricating the x and y axis' and that helped a bit. Unfortunately there is a seal which prevents me from accessing the controller board but maybe the warranty is already expired and am I allowed to open it