Thoughts on lining up round rails..

[irving2008]

I'd very much doubt that the housings are deforming and causing the bearings to bind.

I would think that when you're tightening the bearings onto the MDF, they are twisting some way causing the problems.
How are you tightening the bearings onto the MDF?
Have you tried tightening the bolts up in an alternating pattern (ie. top right, bottom left, bottom right, top left), and only tightening each bolt up a fraction (say an eight of a turn) at a time?

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 :(