Acme thread Z axis with a zero backlash nut

discussion of design changes / improvements / suggestions
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jhllt67
Posts: 47
Joined: Fri May 04, 2012 1:57 am

Acme thread Z axis with a zero backlash nut

Post by jhllt67 » Fri Sep 07, 2012 8:44 pm

Hi all, a while ago on here someone mentioned using acme rod for the z axis to speed things up (I think it was Improbable Construct). It seemed like a good idea because the speed of the Z axis was slowing down the seek on the whole machine due grbl's single speed for all axes.

So anyway I ordered some 3/8"-8 rod from Mcmaster and a few nuts but the thread fit was much looser than the delrin nut on the original machine. It would have introduced a lot of backlash in the z. I came up with this design (turns out others have too)
IMG_20120907_142314.jpg
IMG_20120907_142314.jpg (243.39 KiB) Viewed 5015 times
The two white pieces are UHMW from a cutting board and they are pulled together by the long bolts, so one nut bears on the top of the thread while the other bears on the bottom, effectively eliminating backlash.

It bolts up to the aluminum plate like the delrin nut but I used the holes in the middle to lower it a bit.
IMG_20120907_144446.jpg
IMG_20120907_144446.jpg (185.74 KiB) Viewed 5015 times
The acme nuts are quite a bit wider than the delrin nut so I had to move z-axis slide and wheels out by 1/4" for it to fit. The slots in the z motor mount allow for this shift with no modification.
I also found a 3/8" flanged bearing with 7/8" O.D. that would fit in the original hole.

Here it is finished and back on the machine
IMG_20120907_160525.jpg
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I am really pleased with the results. I was able to increase my z speed by 8 times and haven't hit the limit yet. Before I couldn't reliably move the z above 250mm/min without occasionally losing steps. I have it set at 2000mm/min now and have had no problems. I think the increase is partly from the steeper thread pitch but also from the reduced drag from the nuts, because you can increase the tension just enough to get rid of backlash without causing binding.

Let me know what you think, if anyone wants I can post the files and a BOM.

Improbable Construct
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Re: Acme thread Z axis with a zero backlash nut

Post by Improbable Construct » Fri Sep 07, 2012 9:25 pm

That looks really nice. I like the antibacklash nut setup.
I am actually working on a acme screw solution as well.
I just got everything back together last night and cut out a long board deck this morning.
I had to leave for work before I could take any pictures but I will put some up soon.
Going off topic:
Your setup looks a lot like mine (double makerslide, DW660)
It looks like you are using one bolt through both v wheels between the x a is motor plate and the universal plate.
If you move the through bolt over one hole and put each v wheel on its own bolt and eccentric spacer you will have better adjustability.
Shapeoko #Classified some of the bolts may be original parts.
Shapeoko 1 # ???? Stainless plates, still in the box.
Shapeoko 2 # 3926 not stock
Shapeoko 3 # 0003
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http://ImprobableConstruct.com
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jhllt67
Posts: 47
Joined: Fri May 04, 2012 1:57 am

Re: Acme thread Z axis with a zero backlash nut

Post by jhllt67 » Fri Sep 07, 2012 10:48 pm

Thanks IC.

I have to admit I did draw some inspiration from your setup.
Regarding the separate bolts for the V wheels, I never noticed that the motor plate actually has holes in that location too. I'll have to get some aluminum spacers and do that mod next time I have the machine apart.

glendresser
Posts: 38
Joined: Sat Sep 08, 2012 5:45 am

Re: Acme thread Z axis with a zero backlash nut

Post by glendresser » Mon Jan 28, 2013 7:18 pm

For anyone who's done this mod, what pitch is your rod? Is there an ideal pitch?

I've got some 1/2" acme rod kicking around from a previous project and I'm thinking about using it for this. I believe it's 8 TPI. Just eyeballing yours, it looks like it's a similar TPI. (I see in IC's explanation on the wiki, he uses 12TPI). I'd really like to use it because I've already got some anti-backlash nuts and everything. The only concern I can think of is that the current motor mounting plate might not accomodate the wider nut, and I'd need to cut a new one. Is there any other reason why 1/2" 8TPI acme rod might not work?

Also, how difficult was it to modify the coupling to accomodate the larger rod?

jhllt67
Posts: 47
Joined: Fri May 04, 2012 1:57 am

Re: Acme thread Z axis with a zero backlash nut

Post by jhllt67 » Tue Jan 29, 2013 1:46 am

Hi, I chose the 3/8 - 8 thread per inch simply because it was the coarsest thread I could get in a reasonable size (theoretically this would give more speed). It does work well enough though I wouldn't go much steeper without a motor upgrade. It seems like the weight of the z axis becomes the major load as the thread count goes down, rather than the drag of the nut. Mine is noticeably harder to raise by hand than to lower.

I would guess that you will need to make a new motor plate, depending on the size of your anti-backlash nut. As you can see in the third picture mine is about maxed out with the 3/8 rod.

The coupling is a bit problematic. I was unable to find one that was perfect. The first one was slightly to loose so I added some tape. Of course that slipped so then I tried a smaller coupling and drilled it out. It works but the hole isn't perfectly concentric so it wobbles a bit. I think IC's solution here is better, where the rod is machined down to match the coupling. That gives a smooth surface for the bearing and coupling rather than having set screws try to clamp on a threaded surface. If you have access to a lathe that would be the best option I think.

glendresser
Posts: 38
Joined: Sat Sep 08, 2012 5:45 am

Re: Acme thread Z axis with a zero backlash nut

Post by glendresser » Wed Jan 30, 2013 7:47 am

Thanks, I went ahead and set this up using the spare ACME parts I had lying around, and it works great! I had a few of the dumpsterCNC delrin anti-backlash nuts on hand, and making an adapter to fit it onto the plate was the only really tricky part. Well, that and the math of trying to tweak my new settings. Looking forward to putting some old jobs through and seeing how much of a difference this makes in the actual job time. So satisfying to do something that takes less than a day in total time, doesn't require me buying anything I didn't already have lying around, and results in a massive improvement to the speed of my machine.

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