Joe90's BIG RAT Build.

[Andrewg]

Hi Joe,
Thanks for asking many of the questions I need answers to as well!. I am currently trying to start my own design from the Z axis out. You would think this would be the simplest axis with least design options, but still the choices come. I had also picked up the tit-bit about rails on Z plate for stiffness, so grateful that Jazz has spelled out the trade offs.

It seems to come down to how to sandwich rails/bearings and ballscrew/ballnut between two plates neatly and compactly.
A 16mmm ballscrew nut and holder is 40mm high, so that is the minimum distance between plates,
However BK12 are 25mm from base to ballscrew centre, and ballscrew holder is 20mm, So using these simple components, plates must be 45mm+ apart
If you are going to have to pack up rails or bearings to span this gap, then 20mm rails are far from too big, as at 30mm combined height, you will need a 15+mm space. FK bearings and clever design might make it possible to pack things a little tighter, but I am still looking around for a reference design that solves all this with minimal parts and machining.

Good luck with your design