Thanks for the reply.
The part slides easily on the rod. When use my hand to clamp it down there is more resistance, but it still slides.
I think the problem is the diagonal arm length setting on my Rostock Max V2. Ahhh the pleasures of delta machines.
Off to print calibration cubes.. sigh
PS what do you think of the DC encoder version of the MPCNC in this video!? https://www.youtube.com/watch?v=uphXy3URQ6g