Rotating Ball nut

[drumsticksplinter]

Hi Everyone,

This is my first post on here, so please be gentle...

I'm building an 8' x 4' router and have the rolling framework nearly complete. To cut a long story short I bought a 'kit' from someone who had bought the linear components, motors and spindle separately. I wish now I'd just done the spec myself and ordered everything myself....

The machine is here:




For the X axis I have 2 x 2510 ballscrews x 2800mm long with ballnuts, however, I found out that they are slightly bent as is the 1500mm long 2510 for the Y axis... I think I can get out the bends with a garage press so I'm not too concerned with that. My concern lies with the whip I'm going to experience with the long ballscrews. I'm building this machine to cut woods and plastics for prototyping and small production runs. I calculated that the maximum rapids I'll get on the X is under 150 IPM, which will do my head in waiting around all day for it to cut anything... So I'm now looking at the revolving ballnut concept!

Has anyone got any simple but tried and tested designs for a revolving bullnut. I've got large nema 34 steppers and pulleys for a 2:1 reduction.

I've already seen this:



It looks a very nifty idea, but I'm open to other designs and also looking for someone who could machine me the components please.

Many Thanks in advance

[Jonathan]

Add at least two more vertical pieces on either side of the frame to better support the rails - it will make a huge difference.

Is that a 12Nm motor on Y?!

I use rotating nuts on 2m long RM2510 ballscrews on my machine and they work very well. I posted about the design and making them here:
http://www.mycncuk.com/forums/linear...ign-ideas.html

If you're after the simplest design, then that's not it. My priorities with the latest designs in that thread were to make the moment of inertia of the rotating parts as low as possible, to get good acceleration, and to make the assembly compact. This means I get good performance with just 3 or 4NM Nema 24 stepper motors, which saves a lot of money on the motors and drivers.

I've not seen the one you have a picture of before. Where did you find it? It's nice and compact, but the problem with putting the ballnut through the bearings is you need much bigger bearings and a larger diameter shaft, both of which make the moment of inertia substantially greater since it is proportional to the diameter of the parts to the power 4.

[Fivetide]

Holey Shit thats huge !!! I WANT ONE I WANT ONE !!

[WandrinAndy]

.....This is my first post on here, so please be gentle...

Welcome to the forum Adam.......... Looks like a mother of a beast that you will have there!

Jonathan is the man for Rotating Ballnuts.......

[drumsticksplinter]

Thanks for your welcome guys, been working a little bit in the dark until now I suppose.

Add at least two more vertical pieces on either side of the frame to better support the rails - it will make a huge difference.

I have a bit of a problem doing that because my ballscrews are under the machine, therefore the gantry is driven from under the rails.

Is that a 12Nm motor on Y?!

Yep, I did have a 4nm nema 23 but I thought it would be way underpowered for the 2510 ballscrew...

Your rotating ballnut designs are impressive, but I'm slowly realising that this might be more complicated than I first thought... I don't really want to go to rack and pinion now I have the ballscrews and nuts... however its driving me nuts!!

The design I first attached was found by just searching for "rotating ballnut" in google. I first liked it because its very compact and maximises the travel of the ballscrew, which is important if I want to be able to to cut 8' x 4'

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.

[Shinobiwan]

Your rotating ballnut designs are impressive, but I'm slowly realising that this might be more complicated than I first thought...

Jonathan has that effect with most cnc related subjects

[Tenson]

Hi,

I'm curious how you have screwed into the end of the sttel box section on your gantry? I was thinking about cutting some 40mm thick end plates to go in the end of the section for my next machine, but I don't see you have done it that way?

[Jonathan]

I have a bit of a problem doing that because my ballscrews are under the machine

Move them - you don't really gain anything by keeping them there and you have a lot to gain by adding supports to the frame.

, I did have a 4nm nema 23 but I thought it would be way underpowered for the 2510 ballscrew...

If spinning the screw, then 4Nm is definitely insufficient.

I first liked it because its very compact and maximises the travel of the ballscrew, which is important if I want to be able to to cut 8' x 4'

Your screws are 2800mm, 8' is 2438mm, so surely there's plenty to spare? My design occupies 173mm of the ballscrew.

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.

If you stick with those motors then the mains voltage drivers would be much better - 68V would be the bare minimum. Try putting the numbers in irving's spreadsheet to see which is best. You'll need to enter the inertia of that assembly, which will be about the same as a 3000mm long RM2510 ballscrew, so put that in and you'll get reasonable results. The inertia of my design for RM2510 is equivalent to 1250mm of RM2510 ballscrew, hence much better acceleration is obtained than if the screw was rotated. Thinking about it, you could still use one smaller bearing with the design you found, if the bearing on the locknut end is moved off the end of the nut. Not sure how much difference it would make without drawing it.

Also it depends on what you're cutting - how fast do you really need it to go? If cutting wood most of the time, which seems likely given the dimensions, then it's important to have decent federates. The same is not true if cutting aluminium.

[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)

[Shinobiwan]

The inertia of my design for RM2510 is equivalent to 1250mm of RM2510 ballscrew, hence much better acceleration is obtained than if the screw was rotated.

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?

Surely all this talk about critical speed and higher acceleration is, in some cases, misleading given practical real world evidence that flys in the face of it?