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Thread: Anti - Backlash Nuts

  1. #21
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    You're right of course, the two nuts with a shim in between would be the better solution, but this thread is hard work, and I didn't want to confuse poor Mhiran any more than he already is!



  2. #22
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    Originally posted by anoel
    Honestly I'd go with 2 separate nuts.
    Hmm! I might go for this method, because it would mean less machining for me.

    Originally posted by anoel
    Much like the one that Mhiran posted last.
    How much like it?

    Also... I just realised that the thread on the leadscrew I am using is not the same as the ACME tap.

    Making a tap only requires tapering some of the rodding that I have doesnt it?



  3. #23
    Registered anoel's Avatar
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    This is what I have planned for the machine that I'm building. I've only got 2D apps to work with at the moment. But you'll get the idea.

    Attached Thumbnails Attached Thumbnails Anti - Backlash Nuts-abnut-gif  
    Nathan


  4. #24
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    So I am guessing that the nuts are threaded?



  5. #25
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    Ummm... yeah...

    Nathan


  6. #26
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    I like the idea of making a tap out of your acme rod. The reason is that Acme taps are expensive. Secondly, in my reading of the specs the thread fit that they cut have some clearance in it which is good for most uses of acme rod but not nececsarily for what we are trying to do.
    I have made my own with good results.

    Chris



  7. #27
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    OK! (I know that was a bit of a silly question)

    Ive read on another thread how to make the ol' taps, but, tapering an off cut of the leadscrew? Wouldn't that loose the thread? And also additional cuts are made in the tapered end, what are these?



  8. #28
    Registered anoel's Avatar
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    Ummm... yeah... Wouldn't be a nut if it wasn't. It'd be a sleeve, spacer or a bushing.

    Nathan


  9. #29
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    You taper the tap so that the cutting starts shallow and ends up deep.

    The axial cuts that you refer to create the cutting edges of the tap and they need to be deeper than the thread and wide enough to not load up with chips from the tapping. You'll want to go all the way throught the nut with the tap until there is no mor tapered part.

    Nathan


  10. #30
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    When using just one nut, the slit in the nut should be on the same axis as the screw. This will give much better thread contact and avoid the bending loads on the screw. Kerk Motion sells nuts that are split like a spindle collet and use a spring along the length of the slits to force the slits closed against the screw. The nuts are made from Acetal (Delrin).

    Steve



  11. #31
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    Tsalaf, I like that design but for mine I have on hand a bunch of 1"x2"x?" blocks of delrin if I were to use a round nut I'd go with that method. But I'm sticking with the blocks since I've got 'em.

    Nathan


  12. #32
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    Let me add a few comments.

    You can only use a home made tap if your going to tap delrin (or similar plastic)

    The nuts I made with the slit as shown in the picture are working very well. Without the slit, there was a trace of backlash but lots of side play. The slit elimanated the backlash and the axial play.

    Delrin, even without any type of slit, has very little (none) backlash when using 1/2" acme rod and a home made tap. This is because the tap is the same size as the screw, and delrin rebounds a little after being tapped. The nut ends up nice and snug.

    Eric

    Eric

    I wish it wouldn't crash.


  13. #33
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    I made my anti-backlash nut from 2" delrin rod. I milled one side flat to fit on the back of the Z-axis slide. Then I turned down one end to 3/4" dia. and slit it 4 ways. An O-ring (synthetic rubber band) sits in a groove and pulls it tight. It seems to work well. I can't measure any slop from backlash in the axis.

    Chris

    Sorry for the poor quality of the photo. It is a scanned Polaroid. My axis is shown here minus the Z-axis plate.

    Attached Thumbnails Attached Thumbnails Anti - Backlash Nuts-z-axis-jpg  


  14. #34
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    So would I make axial cuts to the rod in the green sections in the picture below? And taper it of course.

    Attached Thumbnails Attached Thumbnails Anti - Backlash Nuts-3d1-gif  


  15. #35
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    Also, could I place both sides of the nut on either side of the Z axis support column, so the nuts are compressed onto this section?

    Attached Thumbnails Attached Thumbnails Anti - Backlash Nuts-abnutcopy-gif  


  16. #36
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    Well Mhiran, for someone who was willing to accept 1/16" backlash, you are really going for it with the anti-backlash designs now! Anyhow, the piccy in the last post won't necessarily work. The spacing between the nuts is critical. Too far apart and the nuts will simply lock the axis tight. Too close, and you will not gain any anti-backlash advantage. You can use the method shown, to lock each nut to the axis mounting plate, but you will need to place some shims (you can get fine brass shim from RS electronics) between one of the nuts and the mounting plate, to adjust the spacing, so the nut glides freely without locking-up. Personaly I would have the two nut sections next to each other, with only one of them mounted to the z-axis, then you can snug the two nuts together easier. You will still need the shim between the two though, in order to adjust the backlash.
    I found a neat little howto over here, obviously you will need to adapt the design to use delrin rather than steel, and your acme tap rather then the "allthread" used in his.



  17. #37
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    OK, its gone over my head again!
    My understanding is:

    If i use the 2 nuts together, I need to place a shim between say the left "half" of the nut and the Z axis mounting plate? I do not see how this will make the nut glide freely? Surely I just attach the Delrin nut to the z axis plate, so as the nut moves along the leadscrew the Z axis plate moves with it?



  18. #38
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    The easiest way to understand is to try it! Find a bolt and two nuts, anything will do. Put the nuts onto the bolt, then tighten the nuts together in the middle of the thread somewhere. The nuts cannot be turned now - they are locked on the bolt. Now, loosen them off slightly, and they will move again.
    The shim I was referring to in the last post keeps this 0.1mm or whatever gap between your nuts, so they don't lock onto the thread, allowing them to move "as one nut" along the thread of the bolt. The shim needs to be adjusted (use thicker/thinner pieces) so the bolts move with the least amount of backlash (free movement along the thread), whilst keeping the friction as low as possible.
    Please understand this before continuing!
    Now, in the example you put forward to clamp the z-axis plate in between the two nuts, you are creating the same effect - when the nuts are tightened, they will lock the whole lot onto the threads of the acme rod so you cannot move it. Zero backlash, but zero movement! So the shim will need to be used to create the correct size gap in between the nuts and z-axis carriage plate so the whole lot can move "as one unit".
    The only reason I said use the two nuts together is so it would be easier to adjust the shims between the two nuts first - to get your nice anti-backlash nut, then you can go ahead and attatch that nut-assembly to the z-axis carriage.
    Look at the link I posted in my last post. The guy has created two nuts, with a variable spacing in between them. Once the spacing is set, they are locked together with the set-screw. This is the same principle as using shims in between the nuts.
    I think you should re-read this entire thread, pick out the design you understand the most, and go with it.



  19. #39
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    I get you!
    (I tried it!)

    So that is the reason why in anoel's design he has a gap between the 2 "halves" of the nut?

    A shim / spacer would solve this I guess.

    Then so once I have the spacer, I screw 2 bolts through the entire construction to compress them together, and then attach this whole nut to the Z axis slide plate?



  20. #40
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    A link to RS electronics shim stock. If it times out, goto mechanical->engineering materials->shim stock->shim stock and it will be there. As you can see, it is ridiculously thin, but it is necessary to creat the perfect nut. Hopefully it is available elsewhere for cheaper, coz I need some too.
    So you bung some in, a small piece at a time, in between the two nuts, get as close as you can to the nuts locking onto the leadscrew, then maybe back off a tiny bit so they can move. It is up to you now, the tighter they are, the less backlash, but the more friction, ie, the nuts will wear quicker, and it will be harder for your motors to drive the machine.
    Once the nuts are done, you can then attatch the assembly to the z-axis carriage plate.



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