Rotating Ball Nut vs Rotating Ball Screw

As I’m engineering a 1500x1500 CNC, I’m running into a lot of design decisions. One of those is linear motion, specifically selecting a ball screw.

Before I’d really done my research I ordered a 1605 x 1500mm. It is ever so slightly bent, with about 20-30 thousandths of runout at its very worst. It’s from Amazon, but was highly rated and the price was fantastic. Worth a try, and a learning experience.

As I got to doing my research I saw that for that 1500mm I was undersized and really needed something more along the lines of 25mm. I looked on Amazon and a few options exist, but wanting to avoid that experience again I looked elsewhere, too. I saw that it was going to be in the ballpark of $300-400 for a decent quality 25mm, at the least. That’s 3-4 times what this slightly bent 16mm cost.

As I weighed my options and played around with this one a bit more I noted the bend was only really an issue at high speeds. It was like a jump rope, swinging it’s mass out of balance. It was only 20-30 thousandths at worst though, and a problem only as speed increased. I thought, what if the nut was what spun, instead of the screw? The idea being that the bend isn’t an issue if it’s not spinning because the nut never catches anywhere, and if the nut is supported well it can push out such a small bend. I did some research and then found out that was a thing.

I deduced it wasn’t that the 16mm wasn’t sturdy on its own. It might sag due to its length, but the issue seemed to not be that so much. It’s just not sturdy when bent and spinning. So a potential solution is to do a rotating ball nut setup in lieu of rotating ball screw.

So, with all that prelude aside, are there any important considerations between a rotating ball screw and rotating ball nut setup? Is it reasonable that the diameter of the lead screw doesn’t matter to as great a degree with rotating nut vs rotating screw? Because if so, I’m finding that I can potentially keep costs lower, have less inertial mass to accelerate and decelerate, and potentially get muuuuuch higher speeds without actually reaching critical speed.

Any thoughts on this?


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Hi,
rotating nut designs tend to be used in much larger equipment than a 1500mm CNC router table. I would expect rotating nut on designs of 3m or more axis length.
Having said that quite a few hobbyists have made their own rotating nut designs, with the application of lots of time and some parts, and is a viable technique.

The price you are citing suggests that you are looking at rolled ballscrews, C7 grade being the most common. Given that they are rolled its unlikely they will ever be perfectly straight,
for that you'll want ground C5 or better ballscrews, but an increase in price by a factor of five. If you don't like the price of 25mm C7s you are going to weep at the price of 25mm C5s!!!

If the accuracy of C7 rolled screws is sufficient for your purpose then regular (rotating screw) 20mm or 25mm C7 are still the cheapest and most direct path to your goal. There are tens
of thousands if not hundreds of thousands of machines out there of that size using rotating screws of that size. Is there a need to reinvent the wheel?

Craig