Long screws and rails

[3Dsigns]

I've noticed that some of the machine manufacturers use ballscrews on the 48x96 tables. Would these long. 9' or so, screws have a tendency to sag in the middle when the gantry is at one end of the table?

Where could I get screws this long?

Also, where to find 9' rails?

Thanks

[svenakela]

They will sag (more or less) but the biggest problem is the flex (whip). One solution is to use rotating nut instead.

I'm afraid the good eBay-times when you could get big mamas for a bargain is over, either frequently scan eBay and prepare to pay more than you think or search at distributors for new ones.

Regards,
Sven

[3Dsigns]

Rotating nut? What is that?
Thanks

[svenakela]

The screw is fixed and the nut is the driving part.
Rotating ballnut?

Regards,
Sven

[3Dsigns]

So, I'm thinking the long (x?) axis motor/s would have to move with the gantry then?

[RICHARD ZASTROW]

3Dsigns, I like the idea of rotating ballnuts. Keep in mind the nut, its mounting hardware, driving motor with its mounting hardware, any belts, sprockets & couplings/adaptors will be carried on the travelling member. That additional mass (weight) will have to be accelerated / decelerated as well. That MAY create a problem, hopefully not. Good luck

[3Dsigns]

Thanks Richard.

I was playing with the idea of connectin the side columns, of the gantry, under the table like some of the industrial machines are, and driving the gantry with one motor under the center of the table. That would eliminate the possiblility of any support columns at the sides though. The long frame members would then need to be pretty stout. And I was planning to build the frame with 3.0x3.0 80/20 aluminum. The gantry will be a 5' piece of 3.0x6.0 80/20 aluminum with these two 5' rails mounted on it. Add to that the ballscrew, z-axis and motors and the gantry may be quite heavy. Couls make the frame sag.

[RICHARD ZASTROW]

With a connection across the underside of the rails, the rail support will have to be quite substantial. The area swept by the cross member will not have any structure in it. You could run a "third rail" under the traveling cross member the full length of the machine. This "third rail" would support the weight of the X-axis motor etc. It could be a simple "V" roller that wouldn't collect dust etc. or ???

[3Dsigns]

That sounds like a good idea.