DIY Build of Aluminium CNC Wood Router without Workshop & Tools

[Jonathan]

How do you guys normally accurately align the bearings and ballnut?

I'm guessing I need to allow some leeway in order to be able to adjust during setup.... Is this normally done at the ballnut or at one of the bearings and if so which bearing?

Yes some leeway is exactly what you need. Ideally you have 'leeway' in the plane perpendicular to the axis of the ballscrew, not just in one direction. So to answer your question, a bit of both. If you mount the bearings in slotted holes then that gets adjustment in one axis, to get adjustment in perpendicular direction you should be able to use the small amount of clearance in the ballnut mount to adjust the ballnut centre height, but clearly it depends on the mount.

If one bearing is fixed, then spin the ballscrew so the ballnut is at that end, move it back and fourth a little to make sure it centers then tighten up the 6 ballnut bolts. Spin ballscrew so ballnut is at other end, now tighten that bearing mount so it aligns to the ballnut and the other bearing mount by default. If the ballscrew gets noticably more difficult to turn (by hand) towards either end then there is an problem with alignment, so loosen the nut/bearing depending and try again. A tiny amount of misalignment will impose a high radial force on the ballnut which well cause excessive wear since they're not designed to tolerate radial forces.