View Full Version : Anti - Backlash Nuts

08-26-2003, 10:12 AM
I was just wondering, what the exact difference is between a normal 1/2 inch leadscrew nut and an anti-backlash nut. My understanding so far, is that it will lead to in-accuracies, but how much? I can live with upto 2/16ths of an inch tolerance, so would I need to use a anti - backlash nuts?

Also, would I be able to improvise these fancy nuts?
I have seen on some sites that a slit is cut into a normal nut or something like that?!

08-26-2003, 11:35 AM
2/16? that an an eighth of an inch... That's huge... The eye can detect tolerance variations without much trouble down to a 1/100 of an inch, (particularly when inspecting a circle, a trained eye can scrutinize even better.) When you have backlash you end up with oval circles and non square squares, with gouges. your parts won't line up, 3D carvings turn out like blobs and have gouges. Backlash and rigidity are the enemies of CNC... The more you eliminate them the better.

I's not difficult at all to make an antibacklash nut from Delrin or UHMW. So why comprimise. Several of us here have done it with great success.

Regular brass or bronze nuts can be used to a fair degree by using two of them in contention or compression. But there is still some axial play in them that may or may not prove to provide some problems with vibration and or noise.

08-26-2003, 11:40 AM
Hmmm, I have read about these Delrin nuts, but, if anybody has a pic of one of these nuts I would be grateful.

When making a nut of out of Delrin, would I only need to tap it, using a tap made out of my leadscrew? Is that all?

Where can I find Delrin? I'm guessing a plastics supplier...

08-26-2003, 12:56 PM
There is some delrin going on ebay at the mo, over here (http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=2552044681&category=633), not long left on that one though. It is a fair price, if you hunt around you won't find it cheaper. As for the nut, you may not think you need anti-backlash now, but when you become profficient with your machine, you will kick yourself. There is a useful thread about the taps here (http://www.cnczone.com/showthread.php?s=&threadid=1248) and for pictures of an easy deign, check out Balsamans nuts (http://www.cnczone.com/showthread.php?s=&threadid=646&perpage=20&pagenumber=6)! Post number 119 on that page. Hope it helps.

08-26-2003, 12:59 PM
Bought some Delrin just a second ago :) . Now, I already have a 1/2 inch tap... I was wondering if that would be ok for the ol' Anti-Backlash nut?

Also what actually makes a nut made out of Delrin, an anti-backlash one, and one just made out of brass, just a backlash one?


08-26-2003, 01:09 PM
If you look at the Balsaman link I posted, you can see the slit in the nut. So if you bung in a couple of screws, the two halves get pulled together, squeezing (only very gently) the two halves of the thread together, removing the "play" from between the nut and the leadscrew. Don't mean to sound dumb, but the tap needs to be an Acme thread, you don't state what type the 1/2" tap you have is!

08-26-2003, 01:13 PM
It is an acme thread!

Hmmm, I'm not sure that I understand.
The Balsaman post doesnt look like its made out of Delrin...?

08-26-2003, 01:18 PM
No its not, it was a bronze type material I think. If you read through the thread though, he says somewhere he had some problems with that nut, due to its hardness. The delrin will provide you with less friction, will be easier to tap/cut and on a hobby/home machine, will last just as long as bronze. When you mailed me, I said I would be using bronze, this is because the new machine will be much faster than the old stepper driven one, so bronze will last longer. That machine currently has nuts made from Tufnol - a hard plastic similar to delrin, and they are running fine. Just use a nice anti-backlash design, and get cutting!

08-26-2003, 01:22 PM
So all i need to make the nut is to tap it?
Then fix this to the gantry?

08-26-2003, 01:30 PM
Doh! This will give you a regular nut. If you are happy with that, then go with it. The backlash will be someting like 0.2 - 0.4mm, but may increase with usage. If you put the slit down the middle, then add a screw across the slit, you can tighten it up to remove the backlash as the two halves come together. I hope you understand the difference between the two types of nut you can make. I guess you could just do what you said for now, then slit the nut later on as the backlash becomes critical. Personally, I would slit the nut now to create the anti-backlash nut.

08-26-2003, 01:35 PM
Ohhhh! I see now!
So you basically compress the nut to the leadscrew?

Kong don't you have msn or something like that?

08-26-2003, 01:45 PM
Like this?

08-26-2003, 01:52 PM
That will work, although it would be better if the slit were......jeez, difficult to explain.....image you made two nuts, then screwed them both onto the acme rod. Now image they are tight together, well thats where the slit should go. Down the middle, between the two nuts. Then, when you tighten the little connecting screws, it pulls the two nuts together tighter. Sorry for that lame attempt, but I hope you get the idea.

08-26-2003, 02:35 PM
Like this?

08-26-2003, 02:54 PM
I think I have sorted it now. Like this?

08-26-2003, 02:55 PM
How about this? I've never built one, but... ;)

08-26-2003, 02:58 PM
Damn you!
LOL, ok, my 3D was good for an MS Paint attempt.
Basically, are you telling me that is how you do it? Do you know that, that will work?

08-26-2003, 03:08 PM
Yep, woodys got it, thats what I was trying to explain! Nice one.
The slit not going all the way through keeps the two halves together, and when you put the screw in that little hole, you can pull the two halves tighter together, squeezing the threads and reducing the backlash.

08-26-2003, 03:11 PM
I get it!
So I can shove one screw down the middle through both halves and tighten it.
I guess I can't tighten it too much

08-26-2003, 03:12 PM
Honestly I'd go with 2 separate nuts. One that attaches to the axis and another that attaches to the original via two screws tighten up those two screws to pull the nuts together and eliminate the backlash. Much like the one that Mhiran posted last.

Woody, I'd suspect a chance for binding in your nut since it would bend the nut in the axial direction.

08-26-2003, 03:16 PM
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!

08-26-2003, 03:16 PM
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?

08-26-2003, 03:22 PM
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.

08-26-2003, 03:24 PM
So I am guessing that the nuts are threaded?

08-26-2003, 03:26 PM
Ummm... yeah...

08-26-2003, 03:29 PM
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.


08-26-2003, 03:30 PM
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?

08-26-2003, 03:31 PM
Ummm... yeah... Wouldn't be a nut if it wasn't. It'd be a sleeve, spacer or a bushing. ;)

08-26-2003, 03:35 PM
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.

08-26-2003, 04:35 PM
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).


08-26-2003, 05:14 PM
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.

08-26-2003, 05:51 PM
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.



08-26-2003, 09:38 PM
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.


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

08-27-2003, 05:12 AM
So would I make axial cuts to the rod in the green sections in the picture below? And taper it of course.

08-27-2003, 05:33 AM
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?

08-27-2003, 09:58 AM
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 (http://www.buildyouridea.com/cnc/cncrouter/phase_five.html), 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.

08-27-2003, 10:15 AM
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?

08-27-2003, 10:55 AM
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.

08-27-2003, 11:10 AM
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?

08-27-2003, 11:20 AM
A link to RS electronics shim stock (http://rswww.com/cgi-bin/bv/browse/Module.jsp?BV_SessionID=@@@@2057477206.1061993624@@@@&BV_EngineID=cccfadcjemglkdhcfngcfkmdgkldfhk.0&cacheID=ukie&3239446433=3239446433&stockNo=0681249&prmstocknum=0681249&prodoid=328470). 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.

08-28-2003, 05:35 AM
Today I tried to work some things out on the machine. But I am not sure if the spacer will work...

If you get 2 nuts on a bolt with a washer in between the nuts, the nuts will still lock up!

What is going on here?

08-28-2003, 06:19 AM
Change the size of the washer. As I said before, the spacing washer/shim size is critical. It may only need to be 0.05mm thick, hence the use of brass shim as it is as thin as paper. Maybe try a bit of old beer can metal in there instead of the washer?

08-28-2003, 07:15 AM
I was just testing it and the washer is 2mm? Surely thats big enough!

08-28-2003, 08:21 AM
Big enough? It's waaaaaay too big! This process is about finding a shim/washer that is small enough. You to start with something 0.1mm thick.

08-28-2003, 08:24 AM
I know it is too big, I was just testing the theory. It doesnt seem to work with that? Are you saying that if I use a thinner shim the nuts on the thread will not lock up?

08-28-2003, 08:43 AM
Yep. On the inside of the nuts, there is a thread that you have cut. This thread runs in the thread on the leadscrew, happy so far?! It is the tightness between one "tooth" in the nut, against one "tooth" on the leadscrew thread that you are trying to adjust.
So have a guess at the distance between these two parts, and that's the thickness of the shim that will be too big. I guess the distance will be something like 0.2mm, depending on how well you tapped your nuts.
Therefore, any shim THICKER than this distance will cause the nuts to lock. It may seem pointless at the moment trying to close such a small gap, but as you use your machine, the nuts will wear, and this gap will become bigger, leading to inaccuracies. So, have a beer, think about it some more, then cut up the can and try using a piece in between your nuts. You may need a couple of pieces to stop the nuts locking, you may need less, I don't know. But if you now understand the principal, you will work the rest out. Of course, you could go with one nut for now, but that would be an easy exit after all the time you have put in!

08-28-2003, 09:30 AM
OK, I had 2 beers LOL.
Used the can to cut some spacers, but when I shoved them onto the leadscrew, the nut moves but very roughly...
I don't see the point really I agree, hmmm. I'm not sure now!

08-28-2003, 10:06 AM
AH! But you now have movement! This is perfect, you get the idea now, and it is working, albeit roughly. So you now need to find a smaller spacer. If you are using two pieces of beer can, just use one. If not, you have two choices. You can either splash out and buy the proper brass shim from RS, from the link I provided earlier, or (better solution), you wait until your machine has been running for a week or so, which the give the nuts time to wear a little, then the spacers you have now will be perfect. The only problems you're gonna encounter is how to get to those dam nuts to adjust them once you've finnished your machine!

08-28-2003, 10:14 AM
It is sort of working. If I tighten it up more(to the max), (with the spacer) it stops working and just locks.

08-28-2003, 10:28 AM
Ok then, try adding another spacer, then tighten it.

08-28-2003, 10:30 AM
These nuts keep on locking. Ive tried 1 spacer to 8 spacers...

08-28-2003, 10:31 AM
Just a quick thought, you are using the two screws to tighten the nuts together, not turning them? If you turn the nuts, they will always lock. I'm sure you know what I mean, but check back to Anoels picture, you see the two screws? You do have those in place don't you?!

08-28-2003, 10:32 AM
Of course! :D Im not that thick! lol.

08-28-2003, 10:36 AM
Sorry, just checking! For now then, I would leave out the spacers, just screw the two nuts together, and use it. As you notice backlash developing with use of your machine, you can add the shims.

08-28-2003, 10:38 AM
Hmm, what I have been doing, is tightening the nuts together, until they are approx 1 thread away and then they will easily turn. I then have been tightening the screws, with the spacer in between the 2 nuts. This was the correct way right?

I think that I will invest in some of the brass material as I have put quite a lot of effort into these nuts now.

12-25-2003, 12:50 PM
How long do plastic nuts made this way last?

12-25-2003, 11:59 PM
Hmm, what I have been doing, is tightening the nuts together, until they are approx 1 thread away and then they will easily turn. I then have been tightening the screws, with the spacer in between the 2 nuts. This was the correct way right?

I was playing at the hardware store, the other day...found some bell washers which provided some "spring" between two nuts. I also played with a neoprene washer, for the same purpose.

Wouldn't it be possible to use some standard nuts, of whatever thread type, and just crank them down tighter, till these "squishy" spacers gave the right spacing between them? There'd be a little bit of spring, too, to help adjust for the inconsistencies in thread spacing, if you used cheaper lead screws.

It seemed to work pretty well on the allthread at the store...of course, I didn't have a dial indicator with me! ;-)

-- Chuck Knight

Steve Etter
12-26-2003, 08:50 AM
The problem you will get trying to use bell washers is that they can (and will) "give" under varying loads. The two nut technique gives you true rigidity while the bell washers don't. Your goal is to have tight positioning regardless of the load your nut/screw combination might encounter.

FYI - bell washers are often used on nuts in everyday applications to help make sure they don't back off due to vibration and such. In these applications, if the nut were to back off just a little, the bell washer would take up the slack and insure that there is still pressure between a nut and the surface it is suppose to be pressing against. If the bell washer weren't there, the system would be loose the very moment nut moved at all. This technique is really nice when used in high vibration applications where the nut has to be taken on and off on a regular basis.


12-26-2003, 01:31 PM
I wonder if Mhiran ever got the anti-backlash nut working? :)

It sounded as though he was locking the two nuts together with a spacer in between. Of course, it will always lock up if you do that. The nuts are not jammed together, but are locked against mutual rotation, at such a seperation that prevents undue binding on the screw, yet eliminates most of the backlash.

pack rat
12-26-2003, 11:06 PM
I don't know about Mhiran but tread did help me.

04-03-2008, 08:42 PM
hi, if i was to fabricate some anti-backlash nuts why is it necessary to use an ACME thread?

04-03-2008, 09:12 PM
more precise screws. long wear

02-18-2010, 09:55 AM
Kong, You deserve an award for patience. :)