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View Full Version : Relationship between Guide rail length and ballscrew length



CharlieRam
10-07-2014, 12:20 PM
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

Lee Roberts
10-07-2014, 01:34 PM
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
10-07-2014, 10:22 PM
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
11-07-2014, 12:59 AM
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
11-07-2014, 03:08 AM
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.

Lee Roberts
11-07-2014, 12:24 PM
Ok np glad was of help, you can always enjoy your machine once it's up and running and go on to make new parts for it if you want/need to, if you have a read up there are some good posts on here going over whip and the right rpm and so on for screw sizes.

Good luck with the build, don't forget if you need something slightly out of reach you can always post up for a quote.

.Me

Boyan Silyavski
12-07-2014, 12:37 AM
Just some examples for design considerations. Here are 2 from the machine i just build.


1.At least around 200mm shorter ball screw here / depends on the design/. I saved from both ends


12762



2. Here i also saved from both ends.


-Left side is calculated so that the spindle center/the bit/ reaches the end of the table, not the gantry carriage.
-Right side is obvious
+ the plate that is attached to ball screw housing also by being on the left and it shape / can go further right/ helps things and reduces ball screw length.

12763

One of the pleasures is not only to save on weight, but to feel the deep satisfaction when looking at it and thinking " How well thought it is" :hysterical: