[jmkasunich]

Hi JMK - Good progress. Light structures and slender structures vibrate. Just because they vibrate when you strike them does not mean they will vibrate in service. Many of these vibs will not affect the performance of the machine just means the machine will sing its own song. I would not second guess these and fill with sand or anything until the machine is in service. If vibration is an issue solve it then. Light machines have many acoustic vibs that don't affect the cut. A lot of vibs come from the motors so use damp materials for the mounts.

Good advice. Should still be able to add sand later.

R&P vs rotating nut RN: RN is more complex but due to a preloaded nut no backlash. R&P will have some backlash. Have you done the numbers on std stationary nut? Since you are on the economy side of the equation I think you will find R&P is your answer. Plus its less complex. If you build your own RN it may not turn out how you expected. Peter

Looking at 2.8 or 2.9 meter long screws, so rotating screw would probably run into whip problems. I haven't done the math yet to see what critical speed would be.

I have built a rotating nut before, for a quill drive on a 3-in-1 lathe/mill/drill. In that case, the rotating nut was mounted to the machine head, and the non-rotating (but translating) screw was attached to the quill using a banjo, see below. In that case the reason was clearance and travel, not whip...

Attachment 480396

Rack & Pinion candidate: https://www.ebay.com/itm/294594387439 (two sets needed) with a 10:1 planetary gearbox between stepper and pinion
Ballscrew candidate: https://www.ebay.com/itm/143428535452 (contains two screws and nuts) with a roughly 2:1 belt reduction from stepper to nut