Acme vs Ball Screw

[Philosofy]

First, my apologies if this has been covered ad nauseam. Somewhere on CNC Zone I saw a sticky thread, which I can't find now, which I read as saying that Ball Screws are more efficient than Acme, so that's the way to go. Yet I've spoken with engineers who said that Acme was designed for zero backlash. Since zero backlash is the key for precision, shouldn't Acme be considered better? If its not as efficient, you just get a bigger motor and the problem is solved? Am I wrong? If I am, why?

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[ger21]

Backlash in both acme and ballscrews depends on the nut you use, and not the screw. Typically acme nuts do have backlash. Most people buy anti-backlash acme nuts from dumpsterCNC - anti-backlash solutions for home and industrial linear motion
Be aware that these nuts are only zero backlash when the force is lower than about 25lbs or so. At higher forces, they will have some backlash.

If you're buying chinese ballscrew, most of them have very low backlash. Probably comparable to the acme with anti backlash nuts.

[ger21]

If its not as efficient, you just get a bigger motor and the problem is solved? Am I wrong? If I am, why?

Depending on the screw and nut your using, acme can have an efficiency of less than 50%, where a ballscrew is about 95%. So you need double the motor size. With steppers, bigger motors are often slower, so doubling the motor size is not always better. Bigger motors may also require more expensive drives and power supplies.

[Philosofy]

Well, I see my answers in the "Similar Threads" which shows up in my post. Looks like Acme screws don't wear as well.

[awerby]

Ball screws (and ball nuts) are more delicate; there's more to go wrong with them. So as far as overall durability is concerned, I'd say acme screws score higher. There's more to consider than wear from abrasion, which varies depending on how well you protect your screws and what you're machining. And the precision of ball screws varies quite a bit. Only the best of them are suitable for running machine tools; the rest are better suited for opening skylights, etc. An acme screw will generally perform better than a ball screw of the same price; you have to spend a lot more to get a ball screw of equivalent precision.

[steve323]

Ball screws (and ball nuts) are more delicate; there's more to go wrong with them. So as far as overall durability is concerned, I'd say acme screws score higher. There's more to consider than wear from abrasion, which varies depending on how well you protect your screws and what you're machining. And the precision of ball screws varies quite a bit. Only the best of them are suitable for running machine tools; the rest are better suited for opening skylights, etc. An acme screw will generally perform better than a ball screw of the same price; you have to spend a lot more to get a ball screw of equivalent precision.

I always had around 5 mils of backlash with my acme screws and supposedly anti-backlash nuts. Some of that came from the crude end supports (two jam nuts) that came with my system. If I tightened the jam nuts too much, my steppers would stall.

I switched over to Chinese Ebay ballscrews and am below 1 mil of backlash and have more than twice the speed.

Steve

[awerby]

Yes, but how accurate are they? Thread "drunkeness" on cheap ball screws can be considerable.

[steve323]

Yes, but how accurate are they? Thread "drunkeness" on cheap ball screws can be considerable.

I don't know for sure. I am currently using it to cut some wooden clock gears with a diameter in the range of 3-5". The teeth are smooth without sanding and they mesh amazingly well on my depthing gauge. A typical 32 tooth gear measures 3.386" to 3.387" between any 2 pairs of teeth. I would assume that significant drunkenness would show up as differences between the X and Y measurements.

Previous gear experiments with acme screws in my system would leave many "horns" and "divots" in the teeth that would require sanding. Maybe I had bad quality acme nuts, but the quality has improved significantly with cheap ballscrews.

Steve

[LtDan]

Hope its ok to chime in. I install precision ballscrews all the time at work from steinmeyer,kuhn,star rexroth,etc. There is no substitute for a high quality ballscrew whatever the load may be with new ones I use 25mm diam 5mm pitch direct drive from servos you typically see a couple tenths (.0002) of deflection not backlash. Backlash comes from worn ones. I was patient and found awesome screws for pennys on the dollar. See photo this is a triple nut Kuhn swiss screw with hybrid ceramic balls anda INA support bearing. I see a .0001 deflection with about 50lbs of force on the table and repeats to the tenth over 18"
Picked on ebay for $160 was too long but oh well cut it down. Worth every penny
Besides who wants to deal with position error with cheap mechical drives aka chinese allthread!
Look up specs of ballscrews you find and calculate what you need.