Rotating ball nut assembly for fast linear cnc motion

**Bongodrummer** · Dec 15, 2021, 03:52 AM

Hi all,
So I've been holding off posting the rotating ball nut setup we've been making utill I got it working well... Time will still be the test, but ATM I'm quite chuffed with it

Basically, its a way to access the precision and speed of a LONG ballscrew, without the need to spin, accelerate and decelerate the big massive screw that has comparatively huge inertia.

Here's an explanation and how the system goes together.

**Bongodrummer** · Dec 15, 2021, 03:53 AM

And here it is, actually in action after a lot of fettling and a change of ballscrew pitch!

**DIYer** · Dec 16, 2021, 05:19 AM

Thanks bongodrummer! We've added your Rotating Ball Nut Assembly to our CNC category,
as well as to your builder page: bongodrummer's Homemade Tools. Your receipt:

Rotating Ball Nut Assembly by bongodrummer

tags: CNC

**clavius** · Dec 17, 2021, 02:49 PM

I believe there were a few versions of the Bridgeport Boss series of mills that uses a similar system where the nut is spun rather than the leadscrew. I understand that the idea was that the lead screw was always in tension and would not flex and whip as it would if it were spun at high speeds for moving the table.

**nova_robotics** · Dec 20, 2021, 01:44 PM

Excellent work. I'm seriously impressed. Keep the videos coming!

**tonyfoale** · Dec 23, 2021, 02:07 PM

Originally Posted by clavius

I believe there were a few versions of the Bridgeport Boss series of mills that uses a similar system where the nut is spun rather than the leadscrew. I understand that the idea was that the lead screw was always in tension and would not flex and whip as it would if it were spun at high speeds for moving the table.

My Bridgeport has a rotating nut for the Z axis.

**Saltfever** · Dec 26, 2021, 07:20 PM

I understand your concerns about inertial effects on a long shaft and revolving the nut to compensate is a good idea. Thanks for sharing your results. I see the Y axis assembly has a lot of aluminum in it and, of course, a heavy spindle. So I am assuming the mass of the Y assembly is probably equal to or greater than the mass of the X ball screw. Even though the Y axis has a different CG location and structural shape, acceleration of similar (or greater) mass has issues as well. Are we primarily concerned with the LONG spinning X shaft and not the shorter Y assembly? Is there anything to be gained by revolving the Y nut?

**nova_robotics** · Dec 26, 2021, 08:23 PM

Originally Posted by Saltfever

I understand your concerns about inertial effects on a long shaft and revolving the nut to compensate is a good idea. Thanks for sharing your results. I see the Y axis assembly has a lot of aluminum in it and, of course, a heavy spindle. So I am assuming the mass of the Y assembly is probably equal to or greater than the mass of the X ball screw. Even though the Y axis has a different CG location and structural shape, acceleration of similar (or greater) mass has issues as well. Are we primarily concerned with the LONG spinning X shaft and not the shorter Y assembly? Is there anything to be gained by revolving the Y nut?

That's a very excellent point and quite an interesting thing to think about. The moment of inertia increases as a square of the radius. So a light 3" rotating nut might still have a greater moment of inertia than a heavy 0.75" rotating shaft. It would be interesting to do an analysis.