I mailed Chris about this issue and he told me that it is normal for these low-cost motors to have some play in the gearbox. The question is - are there any better quality motors that I can mount on the arm without big modifications? Please see this video and tell me if it is normal to have this amount of play: https://photos.app.goo.gl/JbkGkzq95JAdzVzVA
I would like to have approx. 0,3 mm repeatability and I'm not even close to that, even with the fine calibration and keeping the arm under constant weight. I checked all connections with the shafts, belt and chain tensions, everything is really nice and tight, on every axis.