I'm converting an X3 mill to CNC and have been doing a bit of brainstorming which has thrown up the following query:

I'll be driving my ballscrews with stepper motors coupled via oldham type couplings.

I need to incorporate some thrust arrangement, preferably a pair of angular contact ball bearings configured back to back - but I don't want to increase the overhang at the drive end any more than absolutely necessary as the steppers are pretty large components.

Is mounting the thrust bearings at the non drive end of the ballscrew a bad idea?

Is mounting the thrust bearings at the non drive end of the ballscrew a bad idea?

Yes.

Thermal expansion will put axial stress on the motor and bearings. Keep them on the same side.

I know that is the conventional approach but the reason I ask is that If my calculations are right, 500mm of ballscrew raised from 20 to 60 deg C gives a total expansion of around 0.25mm in the worst case - assuming there is no rise in temperature of the ballscrew mounting i.e. the mill table / Y axis structure is held at 20 deg.

(the coefficient of expansion of mild steel at around room temperatures being 11.1 µm/m -°C, rising to 17.3 for stainless)

I can easily allow for end float in the oldham coupling of twice that without compromising the integrity of the coupling.

You are correct, there should be no problem at all with that particular coupling.

For some reason, I had a picture in my mind that an Oldham coupling was similar to a rigid coupling rather than a lovejoy coupling.