Relationship between Guide rail length and ballscrew length

[Lee Roberts]

Hi, Just a quick question. If I was to build a machine with 1500mm guide rails on X and the bearing blocks were 250mm apart (is that enough or should I spread them further) then the actual travel distance will be only 1250mm so should I order a ballscrew with an approximate length of 1300mm or would you have the ballscrew span the whole length of the MC? same goes for the Y gantry and Z too.

Cheers, Charlie

I always say 200mm minimum on the spacing but Jonathan has said in the past 250, the stiffness is proportional to the spacing squared so the more the better.

Its hard to say with the lead screw as I don’t know the machine design, if the nut is never going to travel the full length of the machine then you may only need it as long as necessary to secure it firmly while maintaining the amount of travel you need, if it makes sense to do that at each end of the said axis then do that, if you can get a good fixing point with-in the said axis and this reduces the overall length of the screw (cost as well) then do that instead.

.Me

[Boyan Silyavski]

If i were you, I would definitely design it so that no unnecessary extra ball screw length remains. No need to spin something which is doing nothing.

I agree that the reasonable minimum is 250mm for a strong machine

[Lee Roberts]

If i were you, I would definitely design it so that no unnecessary extra ball screw length remains. No need to spin something which is doing nothing.

I agree that the reasonable minimum is 250mm for a strong machine

Yea exactly, reducing the length will go towards eliminating any "whip" in the screw as well, or was you thinking of a rotating nut design Charlie?

.Me

[CharlieRam]

Cheers guys, No I am not going for a rotating ballnut, this build is going to be hard enough without access to any real machines to make the parts I need! It just got me thinking while I was looking for rails and ballnuts that all the 'sets' I saw have ballscrews as long as or slightly longer than the rails in the kits and I wondered if I was missing something.
Thanks again, the shorter the better for me.