Thoughts on lining up round rails..

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My thoughts too... and yes I did, I know all about cross-tightening. The weird thing is i can tighten one bearing down fine - its only when i tighten the second.

At the moment i have just got 6mm holes through the MDF and 5mm x 30mm cap head socket screws with washers under the head so plenty of room to allow for movement as they tighten down... I expected them to run freely untightened but as I tighten them I run them back and forth a few times to align then tighten a little more... yet every time it starts to bind once I get past a certain point thats nowhere near tight :(

Any chance of a photo Irving?

Your first message described exactly how I set up my rails and I can push my 100lb+ gantry with my little finger. Sounds like one of your bearings is skewed in some way.