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Ball screws

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Mark B13/10/2021 15:17:56
71 forum posts
36 photos

I'm planning on making my fist steps to converting a small machine to CNC and wanted some guidance on purchasing ball screws.

Eliminating backlash is a of particular importance to me as I'm planning on making small clock parts. I'm not sure that my budget will stretch to ground ball screws, so the cheaper rolled ones will be my starting point.

Lots available on ebay such as these and I wondered if anyone had any opinions on these? The kit includes the mounts which would be a good starting point potentially.

I've chosen 16mm screws here, but for a small machine (like an SX2 Mini mill for example), would 12mm be adequate?

Edited By JasonB on 13/10/2021 15:41:23

HOWARDT13/10/2021 16:44:21
800 forum posts
28 photos

With ball screws you have to take both the nut and the screw design into consideration. Obviously the best would be a ground screw with a double nut, the screw can have the greatest pitch/travel accuracy and the nut balls are selected to give the best fit with the double nudt reducing backlash to a minimum.

In machine design I used to use ground ball screws, but eventually these were replace by rolled screws and software position compensation. But this was all with proper servos and dedicated software.

No doubt there is someone here that can answer your question from experience of what you want to do. Price will be the final answer, but think about what you are trying to achieve positional tolerance wise and can you measure that. I am sure like the rest of us you don't have a temperature controlled CMM.

On a Z axis you will benefit from some sort of counterbalance, so you may well get away with a smaller screw size.. A hydraulic strut system would work, the final machines I designed had hydraulic cylinder systems.

Martin Kyte13/10/2021 16:49:29
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2608 forum posts
45 photos

As clock parts were sawn and then filed to the line rolled ball screws will do fine. My mate Chris converted an X1 to CNC without replacing the leadscrews and it worked fine crossing out wheels and hogging out clock plates. Everything was then finished by filing which just amounted to nicking out the corners and removing the machining marks. Save your money for a high speed spindle although the cheaper way round is using a hand engraving spindle with a flexi drive and mounted inside the spindle of your chosen machine. Router cutters (1mm dia) are the cutters of choice for profiling. Wheel teeth and pinions are generally cut in the normal way.

regards Martin

DC31k13/10/2021 17:12:59
586 forum posts
1 photos

If you are converting a machine, then asking about ball screw diameter is rather putting the cart before the horse.

Decide on the machine and make some drawings of the space available for the nut and screw. That will then guide you in the diameter. Ball nuts for a given screw diameter are somewhat bigger in all directions than standard nuts.

John Haine13/10/2021 19:44:50
4272 forum posts
251 photos

16mm overkill and will take up a lot of space.

Mark B13/10/2021 20:50:04
71 forum posts
36 photos

Probably worth me saying a little more about what I'm doing. I'm basically trying to replicate a rose engine for decorative dial engraving.

This is the machine so far:

20211013_203044.jpg

I'm going to attach 2 stepper motors, one for rotating the dividing head (in one direction only so backlash is not an issue), and another stepper for the left to right movement of the table which the dividing head sits on.

The idea is that you mount a dial on the dividing head which rotates as you apply an engraving tool to the surface. At the same time you move the head in a sine-wave type of pattern so add an interesting geometric shape to the dial. The sine-wave pattern could be quite small with an amplitude going down to 1 or 2mm potentially.

I've got the stepper motor for rotating the dividing head sorted although this isn't shown on the photograph.

The ball screw application is for the left to right movement of the table which the diving head sits on. This is the bit which makes the sine-wave or any other pattern I choose to make. The prototype I've made so far uses an acme thread, but the backlash is totally unacceptable for this application.

Mark B13/10/2021 20:53:21
71 forum posts
36 photos

My original post had a link to the ball screw kit from eBay, but this was removed... Is this not allowed in the forum?

JasonB13/10/2021 20:56:52
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21632 forum posts
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See last item of Code of Conduct

Mark B13/10/2021 21:10:21
71 forum posts
36 photos
Posted by JasonB on 13/10/2021 20:56:52:

See last item of Code of Conduct

Understood; I'll avoid links to eBay items. Thanks Mark

Mike Poole13/10/2021 23:11:14
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Moderator
3095 forum posts
72 photos

Although ball screws are a good solution for a low backlash precision screw there are other solutions. Nuts can be adjusted to take out backlash which can be as crude as a sawcut in the nut and a bolt to pinch it up to remove the backlash. This will wear and and as the screw will wear as well then tight spots will emerge. Plastic nuts formed on the screw also work but again don’t last forever.

Mike

Peter Greene14/10/2021 01:41:37
315 forum posts
3 photos

Be aware that, depending on pitch, ballscrews may be back-drivable if that affects you design.

Martin Connelly14/10/2021 08:07:28
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1935 forum posts
207 photos

Peter, he is going to put stepper motors on the ball screws so they will act as a brake against back driving.

Martin C

Emgee14/10/2021 09:01:00
2195 forum posts
272 photos
Posted by Martin Connelly on 14/10/2021 08:07:28:

Peter, he is going to put stepper motors on the ball screws so they will act as a brake against back driving.

Martin C

Only when powered up, the Z axis will need some kind of brake to prevent the head dropping with power removed.

Emgee

Mark B14/10/2021 10:14:03
71 forum posts
36 photos

I think I'm going to try the ball screw option which will be a SFU1204. In terms of mounting these, is there any preference to thrust bearings?

Martin Connelly14/10/2021 13:10:39
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1935 forum posts
207 photos

Emgee, no mention of putting ball screws or steppers on the head, only two, one for rotating the dividing head and one for the x axis.

Martin C

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