Quote Originally Posted by Voicecoil View Post
What's the thinking behind the shortest possible nut please?
access/interference and balance/inertia mostly, it's no big deal and depends on the design to some degree but shorter is better in my experience.

Quote Originally Posted by Voicecoil View Post
The concentricity is an obvious thing and might rule out some of the cheaper offerings if my experience on the machine I've just completed is anything to go by. Having set everything up and got it working smoothly I thought I'd spin one ball-nut through 180deg to get better access to the lube nipple. Major fail, that side of the axis got really stiff, it turned out that the casing/flange of the ball-nut was something like 1 or 2 degrees out of alignment with the axis of the screw.
It's not always the case obvious to everyone that's why I mentioned it. However most ball-nuts, even the cheap ones, have a machined reference surface on the outer body which you can chuck upon and skim flange if needed. Thou I've not had any issues if honest.