There are many questions about using 8020 extrusions with thier bearings for a cnc router. I have recently completed mine that has an X axis of 53" and a Y axis of 36". I love it! There is no play in the bearings at all. They are very tight but slide easily.
Here is a link to my blog and pictures as it was assembled:
Warren's 1st cnc router log
If you have any questions, please ask.