DIY BallNut design - Could this work?

[slavkok]

Hello...

I try few things to eliminate backlash but none work's good.
Here is my work in progress:
http://www.cnczone.com/forums/showthread.php?t=84773

So Delrin nut has backlash - not good.
I try two nut's preloaded with aprox 40kg and that eliminate backlash but the motor's barely can move that.

and tomorow I got how ballnut can be designed at home.
I try to draw what I mean and make a little measure and as I know that can realy work

So how to do:
Got M14x2 thread rod and M16x2 nut. The nut has just little bigger hole to accept rod to go trought. So in "gap" ball's should be placed. I make little experiment with 1.5mm bals and should work. Just I need little smaller balls. 1.5mm works on 1'st thread and then get to stif. So I seeking now for little smaller balls to prove design.
So modified ordinary M16 nut with recirculating pipe and ordinary M14 thread rod to form ballnut system?

of course there will be some gap/backlash but I'm pointing to two nut and spring system to have zero backlash.

Slavko

[BillTodd]

It will work but, it depends on what you call work...

Ball bearings represent a point load so, even under moderate forces, they produce a large pressure loading. This means you need very hard (and smooth) surfaces to run them on, or they will rapidly wear and break-up the surface.

(Have you read the roller screw/nut thread ? - someone had a similar idea there.)

[slavkok]

... hmm I read that thread.
This rollers seem to make point contact too.
And in succesful ??? design I count 18 groves and two rollers. This make 36 points for me. In my design there is near 100 balls inside. Im little confused.

Slavko.

[BillTodd]

This rollers seem to make point contact too.

No, the contact is a sliding area (and the contact patch tends to enlarge with wear). That's why the commercial roller screw devices can withstand higher loading than ball screws.

Don't get me wrong, I'd love to see your idea going (please post pictures)

I would guess that, initially, it'll feel rough until the balls wear into the surface of the nut & thread. then you'll get a period (load & pre-load dependent) of smooth running, until the surfaces start to harden and flake.

[slavkok]

Originally Posted by BillTodd I would guess that, initially, it'll feel rough until the balls wear into the surface of the nut & thread. then you'll get a period (load & pre-load dependent) of smooth running, until the surfaces start to harden and flake.

Well I agree with that.

...and hope tha this "smooth running" time wil last long...

[BillTodd]

BTW How do you intend to recirculate the balls?


P.S. I was trying to calculate the optimum ball size for you, but my attempts to draw a realistic thread form have failed miserably. And in any case, the ball size will depend on the quality of the nut and thread you use.

[diycnc]

by the time you do all this wouldn't you be cheaper and far better off buying the inexpensive china made ball screws

i am just thinking that you buy the acme rod and nuts, balls ect then it doesn't work you buy all that stuff again to replace the stuff you ruined. you may be more expensive the china stuff and far less accurate.

i see what your trying to accomplish just don't see the point

[slavkok]

... recirculating trought brass pipe.
... The sketch I draw should be precise. All measurment except nut inner chamfer should be just right. But in reality isn't right. In that sketch the balls should be 1.57mm and I was try 1.588 and 1.5 as that is available. As I suspect 1.588 doesn't fit at all neither 1.5 mm. 1.5mm works for 1'st two revolution and get's stiffer and stiffer. As after that I measure bolt and nut I discover that both have tapper on start. Very small but big enougth to be vissible. The problem is that I can't get smaller ball. Wel I can got 1mm but that is too small.

And why I just don't buy real ballscrew? Wel I can get (here) good one without problem. But smalest one will cost me more than all other parts of machine itself. And I need 3 of them. Cheap one I can't get. If I import is from foregin country I wil have a lot of administrative problem here.
The other view is that I like to do thing.
And yes I already buy trapezoidal screw (close to ACME) and work with delrin nut. It's good enougth for wood. But I made machine out of metal to be stiffer and to machine aluminum and brass. ... and plastic of course.

For that design I decide that trapezoidal (acme) isn't good. As grove's are to deep and angles too step. Here I try regular thread rod. The angle of thread is 60 degres and as I see the balls should roll and not slide.

Slavko.

[BillTodd]

The sketch I draw should be precise. All measurment except nut inner chamfer should be just right. But in reality isn't right. In that sketch the balls should be 1.57mm and I was try 1.588 and 1.5 as that is available.

My drawing seemed to work with a 1.50mm ball, but the quality of the 'fit' will change the ball size quite a lot.

I suggest you use rolled stainless steel thread and a SS nut if you can get them, as the surface finished is much better than typical 'all-tread'.

If you have a thread cutting lathe, you may be able to relieve the nut's thread (i.e. re-thread it) to fit the available balls.

[beege]

I think this idea will be frustrating and disappointing. Regular screw threads are produced with a relatively large tolerance window. One screw's pitch may vary over its own length, same with the nut (although much less so). Measuring each pitch diameter and finding balls just the right size to have a small amount of pre-load is going to be difficult, if not impossible. You could make your own screw, and grind the threads to a total tolerance of .01mm, and measure your nut to the same window. Then you could calculate what size balls you'd need. They'd be a special size, so be prepared to pay. I'd opt for a ground, pre-assembled ball screw.

[slavkok]

... in my calculation 1.5mm should work too. But something is out of tolerance. So I think how to make nut (or modify regular one). I have chinese 12" lathe but till now I make only outer thread. I can't imagine how to make inner cut in such small diameter on lathe. Don't laught as I'm not machinist so a lot of things are strange to me. I think that grinding one drill bit onto shape of thread (60 deg) and to run that with cnc already have can be easier.
For now I have little more time to think as I'm seek and must rest.

And what beege talk about accuracy I know that with one nut I can't got good device. So I have two nut in my head with spring preload.

Slavko.

[BillTodd]

Slavko,

How about...

Using a spare piece of threaded rod and some abrasive (fine grinding paste or metal polish) to open the nut a little.

Don't laugh as I'm not machinist

Please be very careful while working on internal threads on a lathe; they can and WILL grab things (tools, fingers etc.). NEVER put your fingers into a spinning hole.