Rotating Ballnut - design ideas

Thanks for the kind words...

I just mounted one ballscrew to the frame with a couple of G-clamps and some wood, so not the best but it holds. So with one ballscrew and 3nm motor I'm getting at least 12,000mm/min! It might do more but things are wobbling a bit as I have not fixed the X-axis linear rails down to the frame.

With the previous rotating nut setup (see post #1) using M20 threaded rod the best it would do reliably was about 2500mm/min.

The gantry hasn't got the spindle and Y axis screw etc mounted, so it's only about 40kg not 50kg, but still looks very promising. It's odd since for this motor and pulley (28T on motor, 30T on screw) I calculated 7300mm/min.

Will post a photo in a few mins.

Video here:

http://www.youtube.com/watch?v=mSNFD...=youtube_gdata

Whipping caused it to stall at 15m/min. That be less of a problem when everything is mounted properly.

Some pictures:

Attachment 4229

Attachment 4230

At 8m/min I couldn't stall it by pushing on the gantry. Above that I
can, but only by bracing myself against the wall and pushing with both
hands...otherwise it just pushed me along the floor.

After the video I tried adjusting the acceleration. It went up to 3m/s^2 quite happily.

Jonathan interested to know what cutter you are using to cut the teeth on your pulleys?

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Originally Posted by 2e0poz

Jonathan interested to know what cutter you are using to cut the teeth on your pulleys?

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It's just a trapezium shaped form tool. I ground it by hand on to a piece of 5/16" HSS. I mentioned it in post #37. Can post a picture if you want, but there's really nothing to it...

yes please

any more news on the ballnut jon?

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Originally Posted by AdCNC

any more news on the ballnut jon?

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Not a lot. I made one out of 4 aluminium mounts for the ballscrew from 90mm aluminium bar. Now the blade on my metal bandsaw has broken (don't really know why :sad:) ... I thought all was lost until I tried parting the bar off on the lathe. Quite unnerving having a 500mm long length of 90mm diameter bar spinning round on the lathe, but it went well and actually parted off very easily with my 2mm parting tool. I sawed through the last 10mm or so by hand for obvious reasons. Coolant made a bit of a mess ... it's actually a faster and more economical way of doing it than the bandsaw.

I used dial indicator to get the ballnut as close to on centre as I could. This completely eliminated the whip I had before. Now using 42T pulley on motor (shaft still 30T) instead of 28T I got 18m/min. There was no significant vibration or whipping at this speed, which is 50% higher than the critical speed of the screw in a fixed-fixed configuration (1180rpm)...which would require a pair of angular contact bearings at both ends (or similar). I would have tried more but I was limited by the kernel speed in mach3. That's with the motor on 1/4 step ... I'll try it on half and see what happens.

In the end I will probably stick to just (!) 12m/min since that gets me better resolution and is plenty fast enough. It probably doesn't do the ballnut any good spinning round too fast? Centripetal force may cause the oil to be flung outwards?

I'd love to try this with a 25mm lead ballscrew and really see how fast it would go...or something like this:

http://www.zappautomation.co.uk/r202...347140d1aaa921

Would probably be measuring the feedrate in meters per second with that... would need an encoder to get useful resolution though.

I've just found this quote from John S. a long time ago:

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One area where spinning ball nuts come into their own is on a CNC
lathe. ... a spin off is if you also fit a bevel gear to the nut block you can then drive this with another bevel mounted on a disengageable handwheel. This way you can
get to use a CNC lathe in manual mode without having to rely on
standing at one end turning the ball screw.

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(Source: http://groups.yahoo.com/group/CAD_CA.../message/70038)

Backdriving could still be an issue, but you could add some friction to the handle and/or a lock easily enough to combat that. That's tempted me to convert my lathe ... I could probably still use my design for the router, perhaps with better (tapered roller maybe) bearings as the forces are likely to be greater. The tricky bit will be fitting the ballnut into the cross slide.

I have now mounted both X-axis ballscrews, like this:

Attachment 4231

Interestingly with the second motor the feedrate decreased from 12m/min to 10.5m/min. I think this might be due to me not yet being able to get the second ballnut on centre which is causing the ballscrew to vibrate a bit.

Update:

Can't really fault the rotating ballnut mounts. They're rigid and get a plenty high enough feedrate from quite small motors. The oil does keep flying out of the ballnuts, as predicted, so I occasionally put more in and they seem fine.
I should have incorporated dust covers as swarf does land in them. I did cut some aluminium covers on the router to protect the angular contact bearings:

Attachment 4957
Attachment 4958

More recently I have made two more mounts, this time for 20mm or 16mm screws and with polycarbonate covers, HTD pulleys, and just generally neater as I made them on the router which gets a better finish than the milling machine:

Attachment 4961
Attachment 4955
Attachment 4956
Attachment 4959
Attachment 4960
Attachment 4954

looks really good
What's the gear ratio between motor and pulley/ballnut??
Is 10mm screw pitch?

It is 1:1 and 10mm pitch, but clearly one could use it with any pitch and to change the ratio just change the pulley on the motor. The mount for the motor is long enough to accommodate different pulleys without changing the belts. It will go faster with a bigger pulley on the motor, but with a 10mm pitch screw there's not much point ... you're just loosing resolution.

So I know I may be over simplifying and havnt researched this much but...

Would it be possible, and much simpler to use a hollow shaft stepper/servo and use feed the screw through the motor and mount the motor directly to the ballnut with a flanged hub

Andrew

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Originally Posted by andrewbond

So I know I may be over simplifying and havnt researched this much but...

Would it be possible, and much simpler to use a hollow shaft stepper/servo and use feed the screw through the motor and mount the motor directly to the ballnut with a flanged hub

Andrew

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Ye that would work great.!! . . . . Now You try finding an affordable motor with a 25mm hollow shaft.!!!!!!

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Originally Posted by andrewbond

Would it be possible, and much simpler to use a hollow shaft stepper/servo and use feed the screw through the motor and mount the motor directly to the ballnut with a flanged hub

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Yes that method is used on some expensive commercial laser cutters. They use large servomotors with a hollow shaft containing the ball-nut running on a tensioned ballscrew. They get feedrates measured in meters per second.

There are few stepper or servos available with hollow shafts. The common ones have too little torque or the bore is insufficient. Anything above 16mm would be useful. It may be possible to dismantle an existing motor, machine a new shaft with the hole through the middle and use bigger, angular contact, bearings. For the size of my machine (1.7*0.74*0.4m) it's just not worth it as the 3nm steppers can already run it more than fast enough.

The advantage with using a standard motor, other than simplicity, is the timing belt allows you to change the drive ratio and reduces resonance. Currently mine is on 1:1 ratio, however it does go faster with a larger pulley on the motor. I elected to leave it at 1:1 however as 10m/min is plenty and I prefer to keep the resolution than get 15m/min+, but at least I have the option which wouldn't be the case with a hollow motor. In short I'm saying you have to be very careful with the selection of the motor, which makes it even more difficult to find a suitable one.

This is something we are developing, but it will not be for hobby use.
It will start with a servo system and the 2525 and 3232 ballnut,
We will then be moving to the 4040 and 5050 ballnut.

The only steppers that could have been used for this design were the IOS motor from IMS, but these are not made anymore.

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Originally Posted by Jonathan

Yes that method is used on some expensive commercial laser cutters. They use large servomotors with a hollow shaft containing the ball-nut running on a tensioned ballscrew. They get feedrates measured in meters per second.

There are few stepper or servos available with hollow shafts. The common ones have too little torque or the bore is insufficient. Anything above 16mm would be useful. It may be possible to dismantle an existing motor, machine a new shaft with the hole through the middle and use bigger, angular contact, bearings. For the size of my machine (1.7*0.74*0.4m) it's just not worth it as the 3nm steppers can already run it more than fast enough.

The advantage with using a standard motor, other than simplicity, is the timing belt allows you to change the drive ratio and reduces resonance. Currently mine is on 1:1 ratio, however it does go faster with a larger pulley on the motor. I elected to leave it at 1:1 however as 10m/min is plenty and I prefer to keep the resolution than get 15m/min+, but at least I have the option which wouldn't be the case with a hollow motor. In short I'm saying you have to be very careful with the selection of the motor, which makes it even more difficult to find a suitable one.

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Hi Jonathan,

really nice design on these- they still working a treat?

i came across these badgers http://www.drives.co.uk/fullstory.asp?id=3388- but as i say on my little build log thing- bet they're bleedy expensive-!

i'm tossing up rack and pinion vs this type affair for my long axis- (2.5m) so lurking about tying to decide to my way forward!

George

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Originally Posted by georgetheforge

really nice design on these- they still working a treat?

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Thanks... yes they're still working very well indeed. Same performance as in my post a month ago. I wouldn't hesitate to use them instead of rack and pinion on a 2.5m axis. You'll get better accuracy, longevity and it'll work out cheaper. With a RM2510 (or even a higher lead) the machine will be plenty fast enough with good acceleration.

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Originally Posted by georgetheforge

i came across these badgers http://www.drives.co.uk/fullstory.asp?id=3388- but as i say on my little build log thing- bet they're bleedy expensive-!

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They do look good, but as you say they wont be cheap.

that would indeed be interesting Jonathan- do you want to work it out and PM me- or post it's up to you-!

i'll work out the mounting of them- i was thinking of tucking them out the way under the x rails- so to minimise dust/ crap getting onto them-

thanks again

G

Hi Jonathan, any chance you could knock 2 of these units up for me?

Im running the 2020 ballscrew and need someway of using a screw of 3000mm length without whipping and this looks ideal

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Originally Posted by D-man

Hi Jonathan, any chance you could knock 2 of these units up for me?

Im running the 2020 ballscrew and need someway of using a screw of 3000mm length without whipping and this looks ideal

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Yes I can, depending on how soon you need them.

A rotating ballnut would be ideal with the a 2020, 3000mm long screw. Since (I gather) this is a woodworking machine, you would probably want to gear it 2:1 so the gantry moves 10mm for every motor revolution.

Here's one I made recently, version 3 I suppose. The significant change is using sealed angular contact bearings (turns out they do exist) and threading the shaft to enable a pair of locknuts to pre-load the bearings.

Attachment 6831Attachment 6828Attachment 6830Attachment 6829

Will post a video when I have time.

I would like it to cut Ali is there a reason you think it's just a woodworking machine? What have I done wrong lol?

If you wanted to test the R2020 screw and nut I don't have a problem sending some over as long as you send them back when your done :-)

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Originally Posted by D-man

If you wanted to test the R2020 screw and nut I don't have a problem sending some over as long as you send them back when your done :-)

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Sure he will but in 5yrs time then they'll be bent and worn out. . lol

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Originally Posted by Jonathan

. . . . .All of those figures are with a 3x safety factor on the torque, . . . . . .

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Was the 3x figure a guestimate or based on based on "real world" experience? Your reply may help me determine if some unlabled steppers I have will work reliably in my first router.

Quote:

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Originally Posted by D-man

I would like it to cut Ali is there a reason you think it's just a woodworking machine? What have I done wrong lol?

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Er, not sure really... maybe the MDF bed. That wont stop it working, but my router bed is currently MDF and it is rather limiting.

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Originally Posted by D-man

If you wanted to test the R2020 screw and nut I don't have a problem sending some over as long as you send them back when your done :-)

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Thanks for the offer, but it wouldn't really be practical/worth it .. I'll just wait and see how it goes on your machine.

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Originally Posted by boldford

Was the 3x figure a guestimate or based on based on "real world" experience? Your reply may help me determine if some unlabled steppers I have will work reliably in my first router.

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I used 3x because that's what's in irving's motor calculation spreadsheet. In reality I got a lot better than that - if you include the inertia of the pulleys in the calculation it's quite a lot more accurate.

The mdf bed is only 5mm lol it's only there to rest all my junk on lol I'm hoping to get some small extrusion to give it a nice solid bed. However this is in the future wood and plastic is its need for now.

Quote:

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Originally Posted by Jonathan

I've just found this quote from John S. a long time ago:



(Source: http://groups.yahoo.com/group/CAD_CA.../message/70038)

Backdriving could still be an issue, but you could add some friction to the handle and/or a lock easily enough to combat that. That's tempted me to convert my lathe ... I could probably still use my design for the router, perhaps with better (tapered roller maybe) bearings as the forces are likely to be greater. The tricky bit will be fitting the ballnut into the cross slide.

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Perhaps interestingly, the George Taylor milling machines used a similar arrangement. The lead-screw being turned by the hand-wheel and the nut being turned by the self-act. http://www.lathes.co.uk/taylormiller/

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Originally Posted by Jonathan

I used 3x because that's what's in irving's motor calculation spreadsheet. In reality I got a lot better than that - if you include the inertia of the pulleys in the calculation it's quite a lot more accurate.

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Many thanks for the pointer. That's really useful information which may save me some grief.

I'm just wondering, that timing pulley you used? was it around 26 teeth 5mm pitch?

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Originally Posted by jcb121

I'm just wondering, that timing pulley you used? was it around 26 teeth 5mm pitch?

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For the one in post #61 I used a 30T HTD 5m pulley. Clearly it depends on the ratio you require, but if you can keep the pulleys small that helps a lot with reducing the moment of inertia of the rotating assembly.

yeh, i was thinking of using a 24 tooth one as that would give me a lot of factors to change the gearing.

but it has the same amount of factors as 30 so either will be good in my design :)

But factors aren't the only factor...
Ideally you don't want to be using less than a 15T pulley as that bends the belt a lot and gets lower tooth engagement, so if you need a 1:2 ratio then you've got to go with at least 30T. However if you're happy with ratios from 15:24 and you can fit the ballscrew shaft through the 24T, then use it as it's inertia will be about 60% less than the 30T pulley. That will help get high acceleration, although you need to consider the system as a whole not just the pulley.

Hi Jonathan,
have you seen these bearings, which are meant for the purpose?

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Originally Posted by silyavski

Hi Jonathan,
have you seen these bearings, which are meant for the purpose?

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No, I hadn't spotted that. I see they've used bearings with a higher contact angle than most angular contact bearings, as that allows the bearing spacing to be lower for the same stiffness. I don't think this is too big a concern with the rotating ballnut bearings, as they are oversized compared to the bearings one would use if spinning the screw.

I've just finished making three more rotating nut mounts for someone who spotted the video on YouTube who's making a large machine with two SFE2525 ballscrews and one SFU2010 ballscrew. I simplified the design by changing the housing for a block machined from 4" square aluminium bar, and a separate piece cut on my router for the motor mount.


Some pictures:

Attachment 10994Attachment 10995Attachment 10996Attachment 10997Attachment 10998Attachment 10999Attachment 11000Attachment 11001Attachment 11002Attachment 11003


A video, feedrate displayed assumes connection to 25mm pitch ballscrew:

http://www.youtube.com/watch?v=kmIqQ...ature=youtu.be

Drawings:
Attachment 11004Attachment 11005
Attachment 11006


Feel free to copy or modify the drawings and use them to make your own or get the parts machined by your favorite company... I don't mind.

Very nice indeed Jonathan, I can see a lot of work has gone into those. ..Clive

Hi Johnathan,

nice work! I have a question: How did you fasten the pulleys on the rotating ballnut shaft and motors?

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Originally Posted by Clive S

Very nice indeed Jonathan, I can see a lot of work has gone into those. ..Clive

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Thanks, yes they take forever, or at least it feels like forever when you're only able to machine things at weekends. It's been three months since I agreed to make them.

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Originally Posted by paulus.v

How did you fasten the pulleys on the rotating ballnut shaft and motors?

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M5 and M4/M5 grubscrews respectively. So long as you threadlock them it works. The reason I didn't use a clamping method is that tends to require a larger diameter pulley, particularly on the ballnut shaft, which would increase the moment of inertia.

Basically from what i understand 24 tooth pulley / OD 37.08 at the lower point at the tooth/ can not be fit, had to be machined on the shaft directly? So the next in line is the 26 tooth pulley which is OD 40.24 at the lower tooth point, so that will give 40-35=5/2=2.5mm wall, which seems quite thin. Then 28 tooth pulley is 43.42-35=8.42/2=4.21.

So it seems if not machined directly, the minimum pulley will be 28 tooth? Or am i missing something?

The minimum pulley size that I could fit on the shaft was 30T, so that's what I used. By machining the pulley teeth directly into the shaft, as with my first version, a smaller pulley could indeed be used, but it adds a lot of time to the machining.

The shafts are 35mm diameter, so it's a bit thin even with 30T as you can see from the previous one I made, here:

Attachment 11008

Thanks.

Now wouldn't this exact RM2510 ball nut simplify things and lead to another design, even possibly with 2 of them?