X3 mill with rails, ATC, new motor

[Jonathan]

Did you mention what size/grade ballscrews you are using?

Briefly in post #18 - I'll be getting TBI brand ballscrews with DFU nuts, C5 grade with preload. I have already put one of these on Z. For the diameters, likely 20mm on X and Y based on the stiffness calculations in the same post, but not certain yet.

Just wanted to chime in on the bearings front. I would suggest you go with angular contact bearings and ones specific for ballscrews or ones specifically made with medium preload as standard.

The ones specific for ballscrews are what I've been looking at (i.e. 60 degree contact angle, preloaded), but I'm somewhat put off by the pricing, even on eBay. This document compares the virtues of triplex angular contact bearings vs a pair of tapered roller bearings and seems to conclude that the main limitation is the additional friction. In terms of stiffness, they are pretty much the same. I don't really consider the additional torque to be an issue, as that is a secondary consideration compared to the bearing axial stiffness - in essence I can just use slightly higher rated motors, which wont be a concern if I use my own. I will use at least 15mm bore bearings, so the available sizes are not an issue.

Some time ago, I did buy some angular contact bearings from aliexpress with 12mm bore that were supposedly matched pairs. Although they have the markings of matched pairs, they don't actually seem to be so, hence I am a little wary of trying that again with more expensive bearings, e.g. BS1547TN1 or BS1747. Interestingly the load rating of ballscrew specific bearings is similar to that of similarly sized taper roller bearings - probably the stiffness requirement dictates a certain load rating.

From what I've read/calculated the stiffness of generic single row <60° angular contact bearings in matched pairs is low compared to the screw/not or simply unclear/unspecified. There's a formula floating around for calculating the bearing axial stiffness for given dimensions, which I can do from the data on the SKF site, but the resulting values don't give much confidence. I fear that if I use these the stiffness of the bearings would be noticeably less than the ballscrews, so when combined in parallel with the nut & screw stiffness, it would not be a well balanced system, hence I want to at least get 60° contact angle bearings, if not roller bearings.