I have some BK12 bearings and 1605 ball screws and having a problem which seems to be common.

I cannot slide the ballscrews into the bearing without having to apply a potentially large amount of force.

I don't know if this is due to the end machining on the ballscrew being a little off (hence ballscrews too big to go in the bearing) or if the bearings in the housing are out of alignment with each other (hence the clearance in the BK12 is too small to accept the screw).

Either way, I don't think its a critical problem. I see people online doing one of two things:

1) Lightly sand the ballscrews with 1000 grit sandpaper until it fits.
2) Heat the BK block to cause it to expand enough to accept the screw.

1 involves metal dust being generated. 2 involves potential damage to any plastic components in the BK12 (if there are any!).

I'd love to hear what people think is best. Or maybe there is a 3rd way?