Acme screws as linear rails?

[bb99]

I was just thinking about creating a small tabletop CNC machine and was drawing a simple picture of the table when I began to think that the linear rails were redundant if I were to replace them with 1” Acme screws. I’d either drive the two screws using a timing belt or two servos. I’d use two AB nuts in place of the bearing blocks. Does anyone know how rigid 1” Acme screws are over a 24” distance? Also, what issues/problems to look out for?

--bb99

[balsaman]

make sure they are straight.

Eric

[motomitch1]

It might have a little run out in the screw and more side clearances than Thomson rods or rails and it would take a lot bigger motors to move it and have a lot of side wear on the nut witch would lead to a lot of backlash in a short period of time.

[HuFlungDung]

Not a real good idea, from the viewpoint of how Acme thread specifications are laid out.

In pitchs finer than about 4tpi, the top of the crest, and the root of the thread, have a clearance of .010" (going from memory), which amounts to a .020" gap. So, depending on the backlash of the nuts, your gantry really has nothing to sit on. Add to that the additional wear of the nuts, and you have less bearing than ever.

This built in clearance could add a lot of wobble to your gantry, the further you get from the screw location.

Your idea might sort of work if you had zero backlash ball screws, though. However, even then, it would be best to have two ballscrew nuts per screw, in order to create some kind of a longer, stable base for your gantry.

[bb99]

Thanks!

Grate points you all make. I'll keep with the linear rails and forget the Acme screw idea.

--bb99