Originally Posted by
louieatienza
Automation equipment is not expensive... if you're purchasing for an industrial corporation. We're purchasing industrial equipment for personal hobby use, or one-man business - expensive. Automation equipment are tested and guaranteed to certain tolerances and performances, that engineers can factor in and trust the manufacturer's specifications so it meets their needs. These aren't consumer products; they just happen to be what a growing cottage industry wants.
Same thing for bronze leadnuts. Although, in that case, usually the leadnut is threaded outside to be received by a flange that is purchased separately. They're not cheap, but they are more compact than ballnuts, and likely far easier to maintain.
Precision ACME usually means a tolerance of around .003"-.004"/ft. Regular ACME I believe can be .004"-.009"/ft (the cheap stuff that MSC sells). A C7 ballscrew is slightly more accurate than a very good precision ACME screw. The C7 (or T7) is what is considered "transport grade," which means suitable for conveying and other transport functions that do not rely on the lead accuracy of the screw for positioning. Fortunately for most of us, .002" is more than enough for our needs, and as long as an anti-backlash mechanism is in place, the repeatability would be very good - far more important than lead accuracy.
Well, in general, the higher the precision (lower the number) of the screw, the tighter the tolerances are, meaning that even a single nut precision ground ballscrew will have anti-backlash properties simply by design. Although a lot of the machines that such screws operate in are in controlled environments, with probably (well likely) higher precision mounting surfaces than we can obtain at home. They'll also run smoother and quieter than a rolled screw, which should translate into a higher surface finish quality. I don't think any rifleman could detect a difference of .001" in the thickness of a rifle stock. If you're mounting something directly to you molded part, and expect to do so with no post processing, then maybe; but I would presume in your case those surfaces would need to be trued up after casting. The other thing is, the tolerances of the rest of your machine would have to be at the very least within the range of that of the screw, and preferably tighter. otherwise you lose much of the advantage of using the precision ground ballscrew in the first place. And that includes the radial BBs in the blocks, motor couplers, linear rails (which can come in three or more preload ratings and five accuracy ratings), drive motors, and everything they're mounted to.