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:

834183428343


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:

8344

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-rotary-motion/3340-rotating-ballnut-design-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 :biggrin:

[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 (http://www.mycncuk.com/forums/faqs-problems-solutions/1524-what-size-stepper-motor-do-i-need.html) 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)

8350

[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?

[GEOFFREY]

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?

Cubic acceleration - WOW!!! G.

[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:

8356

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?

[Jonathan]

I tend to put two grubscrews in each pulley, with a small brass cylinder piece under each grubscrew so that they don't mark the shaft. Do them up tight and use threadlock and the difficult bit will be getting them off, not keeping them on. Also your problem with the 12Nm motors is getting enough speed, so adding a 2.12:1 reduction is only going to reduce that speed (although it will improve acceleration). With the 4Nm motors then I'd go for 1:1 if you're mainly cutting wood since you don't need the extra resolution. If you keep the motor mount and ballnut mount as separate parts then it's easy to change the size of the motors, but more difficult to cover the belt.



I've seen this:

8356

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?

It looks like what's happened there is they've designed it with a large pulley, realised that that requires more power to drive it and just stuck a reasonably large servo motor on to compensate. Not the greatest design. They've also used one ballscew on the X-axis, which is pretty poor as it will cause the gantry to deflect/rack when the tool is near the ends. Lets look at how they justify it on their website (http://www.technocnc.com/cnc-router-systems/lc-series-cnc-routers-features.htm#Ballscrew):
'The placement of the ball screw in the center of the axis of travel eliminates the possibility of racking.'
So they've tried to justify it by saying it avoids a problem which doesn't exist with two motors done properly, and in fact they've made racking become a potential problem by using one. They've also used THK SR bearings and made out that they are excessive, when in fact they have lower load ratings in the direction they're using them and in the first pages of the datasheet is says you're better off using different bearings with equal load ratings if they're mounted upside down, as is the case here. You can do better than this company...

[paulus.v]

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.
You need to find your motor specs in order to choose the right drive. My 150V was only a guess! If you have the 68V 8A drives is better to get better steppers for them, up to 5.7 amps per phase and the inductance lower than 4.4mH. Probably will be at half the length of the 12Nm therefore near to half the holding torque and inertia.

I was searching the last two days for Nema34 stepper for my machine and found some weird things. I will post tomorrow on my build log.

[drumsticksplinter]

If you keep the motor mount and ballnut mount as separate parts then it's easy to change the size of the motors

I think that this might be the way forward, I already have 3 x 4Nm nema 23's, which I deemed not powerful enough to spin the ballscrews... Now that option is out of the window then I might look favourably on these again, especially given that I can source the drives much cheaper for them.


They've also used one ballscew on the X-axis, which is pretty poor as it will cause the gantry to deflect/rack when the tool is near the ends.

That is a major concern because even moving the gantry by hand there is some racking and I've tried to create a fair balance for rigidity vs weight! I think now I have a clearer Idea of what I need to achieve with my motors and a rotating nut design...

Would I still have problems with my ballscrews being slightly bent if going for then rotating nut? I think I can straighten them up to almost perfect with a garage press, I know its going to be a long and tedious procedure and making sure I don't make them worse along the way.

[GEOFFREY]

I think you may find the best straightening device would be something like the one we used to use to straighten motorcycle fork tubes. If you don't know what that looks like, it is a bit like a very large bearing puller, but with only two legs at 180 degrees, each with a hooked end to hook over the item to be corrected. Find the centre of each part of the bend and apply gentle pressure to that point, with the leg hooks positioned to suit. The device does not need the legs to be pivoted. I hope that makes sense. G.

[m_c]

Geoffrey, that's essentially the same as using a press with some V-blocks.

[GEOFFREY]

Geoffrey, that's essentially the same as using a press with some V-blocks.

Yes, but with far more control and the screw could be supported on centres or V-blocks to enable regular rotating to check for staightness. G.

It may be possible to carry out that procedure with the ballscrew in place on the machine, something that would be impossible with a press. It could also be helpful to others who have bo access to a press. G.

[Jonathan]

That is a major concern because even moving the gantry by hand there is some racking and I've tried to create a fair balance for rigidity vs weight!

I'd be wary of compromising rigidity for weight at all. It's much more important to have a strong machine than a fast machine.


Would I still have problems with my ballscrews being slightly bent if going for then rotating nut?

The rotating nuts would still work well with a bent screw, however it will shorten the life of the ballnuts since you'll be applying a radial force to the nut which is not something they're designed for. How much it shortens the life depends on how much it's bent and the length, since that determines the force the nut has to apply to straighten it as it moves along. It's hard to quantify. How bent are they?

[drumsticksplinter]

I'd be wary of compromising rigidity for weight at all. It's much more important to have a strong machine than a fast machine.

I don't think I have compromised the rigidity really, the machine will be fairly heavy duty, but I've tried to use aluminium components where I can to help keep the weight down. I agree that the speed isn't as critical as a strong structure.

Well, I had one ballscrew that was very bent, of course when I bought them they were all packaged up, so it was hard to see that they could have been bent. I've managed to straighten the worse one almost completely, but am awaiting loan of a friends press to enable me to straighten it further. I'm wondering if I could get the ballscrews under tension whilst on the machine by tightening both ends, and hopefully pulling it straight?

[Swarfing]

You will have a problem pulling it straight. the only reliable method is to press it out. small amounts at a time, mark and check against a flat surface.

[drumsticksplinter]

You will have a problem pulling it straight. the only reliable method is to press it out. small amounts at a time, mark and check against a flat surface.

Sorry I meant once I had pressed it as straight as I can get it, then put it under tension whilst on the machine and in use.

[drumsticksplinter]

I' ve come up with an idea to house the bearings for this rotating nut idea:

8505

If I went with Jonathan's suggestion of 7207 angular contact bearings with an OD = 72mm, then I could probably machine this housing myself on my lathe. Once bored out I should be left with a 6.5mm wall thickness, do you think this will be sufficient? After all, the main force acting on this setup will be axial. I was looking at the 7008 bearings with 68mm OD, which would leave me with more wall thickness, but I can't seem to find anyone selling them?? Failing that, would I get away with deep groove bearings?? There seem to be a much better size selection.

[Jonathan]

6.5mm would be suffcient and you could use 7007 bearings, but not 7008 for the reasons given earlier. There's no problem in terms of strength from using the smaller bearings, since compared to the bearings you'd use if spinning the screw the ratings are still far greater. Don't even think about deep groove bearings. They're not designed for axial loads. When you put an axial load on one, the balls are forced to ride up on the side of the ring, which isn't ground to such a good finish, so they wear out very very quickly.

How much have you found that spindle mount for? I could probably get a block of aluminium and bore it for you for less money...

Have you thought about how to make the shaft? I'm guessing your lathe is big enough, but bear in mind it needs machining very accurately for the bearings and nut with good concentricity. It also needs thread-cutting for a locknut.

[drumsticksplinter]

Thanks again Jonathan for your good advice!


How much have you found that spindle mount for? I could probably get a block of aluminium and bore it for you for less money...

Well, I can get 2 spindle mounts for about £70 shipped, plus taxes if they stop it. I was just trying to estimate how much it was going to cost to have machined from a block and judging from recent work I've had done, ALOT! But if you could do them for less, then obviously that would be a massive bonus! AdCNC has offered to do them for me, so I plan to see him this week and have a nose at his machines while I'm there :)

Regarding the shaft, I will get this done professionally (i.e not by me) because like you said it needs to be a good fit and screw cut. My Idea is to have a flange at one end to bolt the ballnut to, then a locknut at the other to pull everything together, then the pulley after the thread on a slightly smaller diameter shaft.

[Jonathan]

I'll machine them for less than £70, PM on the way...

[ImanGH]

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

[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...:hysterical:

[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...:hysterical:

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