Hey Steve,
my machine is still unused and not assembled and I plan to measure this on the brand new ball screws. But at the moment, doing this is not in sight due to time and health issues, but it’s promised to publish it in the forum then.
Tom @TMToronto already mentioned somewhere that there are ball screws with no backlash, that simply have two ball nuts that you adjust one to the other. But the Onefinity machine is not prepared for that.
To remove backlash and obtain the optimum stiffness and wear characteristics for a given application, a controlled amount of preload is usually applied. This is accomplished in some cases by machining the components such that the balls are a “tight” fit when assembled, however this gives poor control of the preload, and cannot be adjusted to allow for wear. It is more common to design the ball nut as effectively two separate nuts which are tightly coupled mechanically, with adjustment by either rotating one nut with respect to the other, so creating a relative axial displacement, or by retaining both nuts tightly together axially and rotating one with respect to the other, so that its set of balls is displaced axially to create the preload.
– Source: Ball screw #Operation – Wikipedia
I remember also there was this mysterious issue here that was related to the seating of the ball screw.