Looking into a ballscrew, not sure if I should buy. Help

[irving2008]

There are so many factors here it's hard to know where to start. Building such a large industrial strength machine is a huge undertaking. Have you looked at purchasing an older second or third-hand machine?

The key issue is you have to look at the whole system, not just individual parts. The servo you have suggested has 4096bit resolution so directly driven this screw gives you 40/4096mm resolution, that's .01mm or about 4thou. Is that good enough? The rated rpm for the servo is 3000 so nominally your max feed rate is 40/1000*3000=120m/min which is huge, you'll never be able to use that and keep the machine from tearing itself to bits. But the most telling bit is that servo has only 3Nm of torque (7 instantaneous). Your gantry is going to weigh something like 120kg to get the rigidity over the width, based on 2 lengths of 8" x 4" x .25" bolted together in an L-shape + the y and z axis servos and spindle (they weigh12kg on their own!) and it's going to struggle to give you any reasonable acceleration. You need to run the numbers.

edit: just run the numbers to a first approximation and its just about doable with moderate acceleration.

however, I assume you plan to use this screw for the X-axis (though its only 8ft not the 10ft you'd originally mentioned)? If so, for a Y-axis of 4 to 5ft you'll need 2 screws to prevent racking if you plan on machining steel. That will need 2 servos, one won't cope without gearing down 3:1 and with these loads I'd not recommend belt drive.

[bikepete]

> that's .01mm or about 4thou

Pretty sure 0.01 mm is more like 0.4 thou FWIW.

The other thread where the OP describes the project is at

http://www.mycncuk.com/forums/mill-b...html#post47999

Says we're looking at 5' x 10' working area, Z travel 14".

As Irving says this sounds like a major project and which screws/rails/servos to choose is, I would think, secondary alongside making the structure sufficiently accurate and rigid to machine steel at an economically sensible speed and finish. And resolution is the least of the problems I would think - more important will be making sure everything is flat and orthogonal (even with e.g. 500 lb of steel plonked on it), that steel can be machined at an economic rate without excessive vibration and with a reasonable surface finish... etc.

Just had a quick nose around to see if there's commercial equivalents out there... best I could find:

Milltronics: BR60IL (100" x 60" x 28")

Obviously you don't need to emulate the same speed, horsepower or accuracy necessarily to have a useful home built machine but it's notable that it weighs SIX TONS (14000 lb). You could get away with much less of course but it will compromise the accuracy and finish attainable over those large travels. Rather than guessing on this, I'd probably want to get some calcs done on (as a minimum) how much your proposed structure will deflect under the machining forces you intend to put on it. This'll let you know whether it can achieve realistic tolerances for commercial work. Damping/resonance could also be an issue worth tackling at the design stage rather than later...

You'll need to be confident of its accuracy before taking on paid work, I would think - worth thinking now about how that would be proved - measuring things accurately over those sizes is not trivial...

Some good stuff here on design BTW in case you've not come across it before:

http://www.mycncuk.com/forums/faqs-p...mn-design.html

Apologies if you already know all of this - just some random thoughts after reading your plans so far. Good luck with what sounds like an ambitious build.