[ConKbot of Doom]

I believe ballbars are just for checking everything overall afterwards, Ie, making sure everything is working smoothly together, not just measuring the ball screws. Laser interferometers can be used for mapping ballscrews, though getting it done could be a bit pricey.

[BobWarfield]

Yep, ballbar not for mapping, though useful for other checks.

To map you need a reliable standard. It can be done by creeping gage blocks along 1" at a time, though that is painful. You can hire someone to bring their laser interferometer as has been mentioned. Or you can use a suitably accurate DRO scale.

Best,

BW