Rotating Ball nut

[Jonathan]

But what is much better acceleration? I use 1.5m RM2010 and happily achieve 4000mm/sec^3 at 16.5m/min with just Nema23 4Nm(video posted in my Strike thread) I don't run at that because its harsh as *$&£ but if a standard setup can achieve that I have to wonder why bother increasing it further when its already excessive?

If the inertia of the system is reduced, less torque is required to achieve a certain acceleration. To reduce the inertia you can either make the gantry lighter, or reduce the mass/size of the rotating components. Clearly optimising the rotating components is preferable since reducing the mass of the gantry will be detrimental to the overall rigidity of the machine. Since reducing the inertia of the rotating ballnut assembly to be less than the screw is not difficult, it's logical to do that as even if your motors will achieve the required acceleration you gain a greater factor of safety which is always good when it comes with no added cost. So to answer your question directly, it's to get the inertia low enough, or if it already is, then increase the factor of saftey on the torque requirement to make the system more reliable, or reduce the cost of the electrical components.

The moment of inertia of one of your RM2010 ballscrews is just over half the inertia of my RM2510 spinning nut, so if paulus used that design with the same motors and drivers he can expect to get about half the accelleration you get, assuming the mass of the gantries is similar. Similarly, if he uses the design pictured in the first post, then the acceleration would be less than 1m/s^2, which is a bit too low really. Either way if drumsticksplitter has put the numbers in the motor calculation spreadsheet, then he will have found that the 4Nm motors are ideal so long as the inertia of the rotating nut assembly is minimised.

For this size machine, then since RM2510 ballscrews are being used the only realistic option is to rotate the nut, since the critical speed would otherwise limit the feedrate to around 5m/min.

[drumsticksplinter]

To tension,

I'm curious how you have screwed into the end of the sttel box section on your gantry?

I had 6mm plate profiled out and tapped with 4 x M10 holes. These were then trued up to the ends of the box section and for the moment tacked into place. They will be fully welded once I know this layout works...

To Paulus, That indeed is very interesting and also worrying for my 12Nm motors. I was looking at the 150v drives to start with, but I got a guy in china to spec the drives and power supplies to my motors, like he did on a plasma table I recently built and he recommended a 68v 400w system. He also sold the 150v drives btw... However, I do think myself these would work best and would save having to buy power supplies for every drive.

This whole thing with acceleration and critical speed is going way over my head... I was hoping to achieve a 2:1 ish reduction to my 2510 screws to buy a bit of resolution and torque. I've sourced taperlock pulleys, because I was sick to death of damn grub screws coming loose on my last build. Therefore the smallest pulley available is a 34 tooth, which with a 72 tooth pulley gives me 2.12:1 reduction. The problem being that the 72 tooth pulley is 114mm, and going by Jonathon's recommendation to keep the moment of inertia low with small diameter components, I'm faced with another problem...

I've seen this:



Its from a Techno LC router (google search), which is a similar kind of design I was looking at. I know you can't see any detail of the ballnut and bearings, but there is a quite a big ass pulley... This particular machine is servo driven, I wonder if that's why this design works for them?

[paulus.v]

I have 2 x 12Nm nema 34's for the X (1 for each ballscrew), do you think I would get away with the rotating ballnut I found with my motors?. I know with them being bigger motors that the moment of inertia is greater than say a 4Nm, but I plan to drive them with 68V 8A drives.

From what I have read the 12Nm steppers could be a problem. Having high inductance you need to give higher voltage (maybe 150V) to run them to their specs. The torque will also drop fast with rpm. Probably you wont be able to get it moving reliably at 200ipm with decent acceleration using your 10mm screw pitch at 1:1 but I may be wrong.

Do you know the specs of your steppers?


I attach an interesting read. (1Nm=141oz-in)

[Heavyweather]

Hi friend
I have a question about rotating bull nut. whet we used this method how to fix screw to table? how install the screw?
thank's

The screw is to be fixed on one side and held under tension on the other side with a plate spring.

[JAZZCNC]

The screw is to be fixed on one side and held under tension on the other side with a plate spring.

Your a little late with that reply...

[Heavyweather]

Hi friend
I have a question about rotating bull nut. whet we used this method how to fix screw to table? how install the screw?
thank's

Your a little late with that reply...

Right, I know. But it's more complete that way I guess.

Here's more to add some to the discussion.
In German but they built their own driven nut.
http://cnc.a-ueberbach.de/tag/angetriebene-mutter/

And here you can buy a complete drive or parts for any size ballnut.
https://team-haase-shop.de/maschinenelemente.html