untapped balls

one of the balls i got was unthreaded. With a soft-jaw vice and a M3 tap I managed to get some thread in (don't have a bottoming tap). After grinding off some of the countersunk screw It does fit. Perhaps there should be more balls in the kit just to be safe?

I got the same problem, but I did not try to tap stainless steel, I have no appropriate tool.
Replacement ball is on the way.
As a temporary measure, I replaced one of the balls on the bed by a Nyloc nut. It seems to work well, but the calibration tests show I sometimes got bad repeatability. I am really not sure this nyloc nut is responsible of that, anyway I am waiting the ball to redo calibration tests.
Another solution could have been to grind a screw and glue the flush ball, but I am not very confident in that.

Regards, Pierre

They are not stainless! They got marred in the aluminium jaws of the vise I was using.

Mine is stainless (not magnetic) ! so there is no hope for machining.
Ordinary austenitic stainless steel is not harder (while not machined) than common steel, but shall be harder than aluminium !

Perhaps mine are stainless too, or perhaps a nickel plating? plain steel might not be the best idea for a machine that relies on the electrical contact to stainless spacers...
Anyway, I did not know and tapped a little thread in them anyway (blessed are the ignorant because they do what cannot be done?) I had more trouble with not having a bottoming tap and not wanting to grind off more of the tap I had. So I ground of some of the screw instead.