As documented elsewhere I am building a small CNC mill (250mm x 250mm x 100mm working area). After much deliberation I decided to use MDF for this first attempt as its a 'cheap' trial activity. So far I have created a 500mm x 500mmx 50mm base and mounted the two 16mm rails on it. I have tried to ensure they are parallel and my plan to line them up was to:
- choose one as the reference and ensure its parallel to the edge of the base, which I have done within about .2mm then bolt the mounts down tight;
- bolt a piece of scrap mdf across the bearings and run this up and down the rails tightening the mounts on the second rail gradually until they are tight and the joined up bearings run freely end to end.

The problem I have is with step 2.. as soon as I tighten the bolts onto the bearings they start to make binding noises, even if the second rail is completely loose to move perpendicular to the rails they bind. Is it possible that the bearing block (SC16UU/SMA16U) distort and cause the balls to misalign internally? If so I can't find a point where all the bolts in the bearing are tight and its not binding :( when the two bearing are connected. Individually they run fine...

Any thoughts?