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.