The 'middle' rocks

I’m building a C-23.5mm, MPCNC. I’m using 5/16 hardware, not 8mm. There are no electronics yet, it is only mostly hardware assembled.
The middle section (shown in image) rocks diagonal to the X and Y axis rails. I have tightened the bolts (all of them) and it reduced the rock, tightened again and it reduced it more, but I think they’re pretty much as tight as they should go and it still rocks enough that I’m concerned it will cut inaccurately. Could this possibly be because I’m using 608 bearings (8mm hole) and 5/16 hardware? Or is there something I can do remove the rock?

Thanks guys. I’m very excited to get this beast up and running.

Add your Z axis before you do anything, it is built to work with all axis in place and nothing can be done or tested without all rails.


Do a large, 100mm on each axis, calibration print to check your printer. XYZ and all diagonals.

The build is an interference fit, no tension is required on the tension bolts to have a snug fit. Something else is wrong, Several people have missed bearings, have you used 53? Any bolts touching other bolts?