[JFettig]

Hi, I am currently designing a mill that I plan on building over the next few summers funds willing. A consern with a few things has brought up a possibility of a spinning nut design for my x-axis. I already have the general layout figure out, although I am trying to figure out if the ballnuts are concentric.
Because funds probably wont allow, I wll probably go with precision rolled screws. The thing I am then conserned about is will the threads/body of the nut be concentric with the screw or will it be binding or trying to make the screw bend?

Basicly what I intend on doing is making a hollow shaft that screws onto the ballnut, use angular contact ball bearings and load them inside a housing with a timing pully on the very end. The whole assembly will probably be around 8" long or so which is not a problem with my design. I only now have the consern of whether it will be concentric.

I know homecnc does a spinning nut design for his z-axis, but its not a really fast setup, I intend to spin mine as fast as the nut will withstand or up to 3000rpm.

Let me know what you think.
Thanks,
Jon

[OCNC]

Attached are pic's of a spinning nut I've been working on. The bearings are R16 and are sandwiched against some MDF blocks by collars on either side. The collars hit the inner races while the blocks catch the outer ones. The bearings are from McMaster. Other parts are from Misumi and not yet optimally sized as I had to get used to their tolerances. I need to re-order the hollow shaft 10 thousanths under for an interference fit with the nut. The threaded rod and nut is Roton's Hi-Lead product (two-start 1/4" ACME ). The nuts have no backlash but are not pre-loaded. A long way from a ballscrew set up but there it is in ACME.

Chris

[Evodyne]

Jon,

Hi! I've been looking at my screws thinking the same thing. It occurred to me that the screw must be rigid axially, but could move radially to compensate for any non-concentric nut action. We are only talking enough to counter whatever runout the nut would have. Alternatively the nut mount might be slightly compliant radially (but not axially). Both accomplish the same thing.

The other concern I have is, at higher rpm, the imbalance of the nut (due to the return channels) might impart some vibration. Would a dampener like the harmonic dampeners on car engines help? I dunno. Perhaps the whole nut assembly could be balanced.

Evodyne

[JFettig]

Thanks for that, Thats my general idea, except where you have the metal cylinder sticking out the back, there will be my ballnut.

My screw will be thicker, so any offset will want to make it bind.

Jon

[jimbo]

another concern I have heard but do not know if it is true, is that the balls will undergo centrifugal forces at higher speeds and have difficulty circulating properly and/or wear faster. Can anyone verify?

[JFettig]

Evodyne, I agree, definately rigid axially, but the screw is likely to be quite rigid radially, especially at the ends.

when I was searching google, some big companies were using harmonic dampeners, I know nothing about them though. I guess it made a big difference.

Jon