Backlash & AB nut's

[slavkok]

Hello...

I try to understand how AB nut works. (Another begginer here)
They have spring betwen to preload and both nuts are in contact with outer face of thread. But I can't find data how much of preload should be there?
I just made new (stainles/alu) Z axis for my mill (MDF made) as I plan to make all from stainles/alu. On the old design I have UHMW block 30mm long threaded. It have smal/none backlash but endplay and machine stiffnes is bad so alltogether work as should (for light work). In new Z axis the stiffnes is much better. For now I reduce endplay with 2 bearing's. Seems good. But Backlash drive me crazy. I make same UHMW block but it's wear to fast. So back to AB nut design.

If I need (for example) 10N force to move the table then I need at least 10N preload in AB nut just to move table without backlash. So for real machining then wee need more force to overcome tool pressure. How much?

Slavko.

[slavkok]

... so nobody know what force to use for preload?

[ger21]

You probably need enough force to resist acceleration forces + cutting forces. What material are the nuts?

You can also use a design where the spring forces a "clamp" to keep the threads of the nut in tight contact with the threads on the screw. Much less force is required this way, but the load carrying ability is probably less.
http://www.dumpstercnc.com/images/1_...Start_ACME.jpg

[slavkok]

I "wish" to can cut wood, plastic, aluminum and bras.
The machine is not yet finished. I intend to post some photos how the machine is progressed.

For now I have M12 allthread and UHMW nut 30mm long. But I will replace this with some more durable.
... and already ordered trapezoidal screw.


this nut (in link) is probably good only for allthread.

[ger21]

Those nuts are used by most members here. They have them available for trapezoidal too.
http://www.dumpstercnc.com/

[slavkok]

Originally Posted by ger21 Those nuts are used by most members here. They have them available for trapezoidal too. http://www.dumpstercnc.com/

Huh... There should be something wrong. (Maybe the problem is on my head )

So As I see that nut just squeze the lead in radial direction. (As I see the spring just push bushing over tapper.) So if screew is pure trapezoid then this nut just make more friction and not remove backlash. I have little problem with translation and I'm little confused with different names. Acme, allthread, trapezoid ... etc etc...
As trapezoid thread I mean the shape of thread is square, not oval or triangle. Just pure square as is for example on minilathe for moving carriage.

[ahren]

The thread on ACME is not a pure square -- there is a 29 degree angle between teeth. Thus, with nut designs like mine and dumpster's, compressing the nut inwards onto the teeth actually does remove backlash, as the faces of the nut and the screw thread get closer together. You are correct that this technique would not work for a purely square thread. However, most threads have some form of taper -- they are difficult to cut otherwise.

Ahren
www.cncrouterparts.com

[slavkok]

Originally Posted by ahren The thread on ACME is not a pure square -- there is a 29 degree angle between teeth. Thus, with nut designs like mine and dumpster's, compressing the nut inwards onto the teeth actually does remove backlash, as the faces of the nut and the screw thread get closer together. You are correct that this technique would not work for a purely square thread. However, most threads have some form of taper -- they are difficult to cut otherwise. Ahren www.cncrouterparts.com

I discover that. In reality there is 30 degre tapper (trapezoidal screw 12x3mm) Before I get that screw I was thinking that is pure square so that is reason for my response. As my building has some progress (see http://www.cnczone.com/forums/showthread.php?t=84773 ) I now hit the backlash problem. Today I see your page and I yesterday make near same nut with little succes. As I have box rails the friction seems just to much for that nut.

Im stuck here.

Slavko.