X3 mill with rails, ATC, new motor

[komatias]

The stiffness you get from the bearings is also dependent on the preload class you use. Just like in the ballscrews, you can have zero backlash (P0) but you can also have P3 and P5. And of course there is the precision class and clearance to them too. The more accurate, the stiffer the bearings can be claimed to be.

You are right about the price though. On My KRV I went with heavy preload and spent around £240 for 2 sets.

You should also think about the stiffness of rotation perpendicular to the axis of the ballscrew. this will govern which way round you put the bearings. I forget which orientation is stiffer (face to face or back to back)

In the end I bought IBC bearings with 60deg angle medium preload. You can find all you need here: http://www.ibc-waelzlager.eu/dmdocum...t_Bearings.pdf

The ones I would have bought had I not bought the ones I did are: http://www.qualitybearingsonline.com...ct-20x47x15mm/

Another slightly overlook aspect is the locking nut you use should be as good as you can get it. The squareness of the thread to the face is not great in some of the chinese nuts I have come across. In my case I also wanted to lock on the front face as opposed to on the thread. Too a while to find anyone in the UK what had them.