Tapped Nylon Spacers as Lead Screw Nuts

[cely]

I'm not an ME and not knowing any better, I've been experimenting with lead screw nuts made from 1/2 inch long, round tapped nylon spacers (<$0.20 each). I want to use them with cheap allthread rod (<$2 per foot).

I'm able to get near zero backlash by compressing the tapped nylon spacer, but the mounting bracket is very expensive (>$10 each).

The diameter of the tapped nylon spacers is either 1/4 inch (4-40/M3, 6-32/M3.5 and 8-32/M4 threads) or 1/2 inch (10-32/M5 and 1/4-20 threads).

I'm looking for a very inexpensive way to mount the lead screw nut to the linear slide.

The mount has two important requirements. 1; it has to hold the spacer securely and 2; it has to compress the nylon spacer to achieve zero backlash. The compression has to be adjustable to account for wear, and variations in the manufacture of the spacer and the allthread rod. The amount of compression is quite small.

I am aware of the accuracy issues because the threads are not cut very precisely. So...I built an automated tool (with a DRO) that measures lead screw error and backlash in lead screws up to 36" long. Although dificult to prove, I think I get performance on the order of standard ACME threads and middle of the road lead screw nuts. Of course it is not a heavy duty solution....but it sure is cheap.

Anyway, I would appreciate some help with the bracket design.

Tapped Nylon Spacers

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[greybeard]

Hi cely.
My answer to low cost anything is to consider building the required item from a mixture of solid metal from the scrap box and filled epoxy.
could that give you a way forward ?
An alternative to the filled epoxy might be what we over on this side of the pond refer to as "body filler" - auto repair hole filler putty type stuff.
Regards
John

[AlienSteve]

FYI, nylon does tend to absorb a certain amount of moisture and swell because of it. I recall on one of the forums I am on, someone machined custom nylon pieces to make collapsible sign holders for tradeshows, he said when they first tried to use them smoewhere with high humidity, they could not get them to slide at all. My local plastics person had previously warned me off of nylon for this reason.

How about delrin or UHMW? Delrin is about $8 a pound, UHMW is $3 a pound. My local plastics person tells me that, although UHMW changes size a bit more with temperature than Delrin, that UHMW machines and wears better than Delrin.

[cely]

I have looked into drilling and tapping UHMW and acetal, but I'm trying for a very simple, off-the-shelf solution. I can purchase pre-tapped spacers for less than $0.10 each. Recall, that this project is ultra-cheap and easily adjustable.

[AlienSteve]

Originally Posted by cely I have looked into drilling and tapping UHMW and acetal, but I'm trying for a very simple, off-the-shelf solution. I can purchase pre-tapped spacers for less than $0.10 each. Recall, that this project is ultra-cheap and easily adjustable.

I'm going for ultracheap, too. UHMW is only $3.00 US per pound, which is a lot of UHMW. Of course that requires you have a friendly plastic supplier within travel distance who'll sell you scraps.

Try out the nylon, it is cheap, but watch out for the humidity problem.

[LeeWay]

The humidity issues with nylon may actually help in this case. It would tighten up some on the screw and possibly provide even better backlash characteristics.

That said, if it swells too much, it could snap. I have seen nylon bushing crack before. You can get flanged delrin bushings with 1/4" hole and 1/2" long for about $3 each from Mcmaster Carr.
They would be less if bought locally. The flange may help you mount these easier.

If you want even cheaper, then your nylon bushing could be held in another fitting or block of plastic or wood or metal by set screws that just do dig into the sides at a couple of points. That would give you a little room to drill holes for mounting.
I would however, just drill and tap a solid block and do away with the bushings. Might cost only a few cents more if you have very many to make. Heckuvalot simpler though. This is the KISS principle.

Use two short blocks like this. Mount one solid. Put a small spring in between them and just snug the other side to keep it from spinning. The whole thing couldn't cost much more than a couple bucks.

[AlienSteve]

But that means it has less backlash -only- when the humidity is high. If it were an issue of it swelling and staying that way, it isn't a problem. But it will shrink back down when the humidity goes down.

At $3 each for good delrin bushings with mounting flanges, IMHO that makes sense over futzing around with mounting potentially problematic nylon bushings.

I'm going to machine UHMW for bearings and for the nut. To compensate for changes in size due to temperature, my plan is to saw a split in them and use a bolt and spring to keep it loaded. For both the bearings and the nuts.

Originally Posted by LeeWay The humidity issues with nylon may actually help in this case. It would tighten up some on the screw and possibly provide even better backlash characteristics. That said, if it swells too much, it could snap. I have seen nylon bushing crack before. You can get flanged delrin bushings with 1/4" hole and 1/2" long for about $3 each from Mcmaster Carr. They would be less if bought locally. The flange may help you mount these easier. If you want even cheaper, then your nylon bushing could be held in another fitting or block of plastic or wood or metal by set screws that just do dig into the sides at a couple of points. That would give you a little room to drill holes for mounting. I would however, just drill and tap a solid block and do away with the bushings. Might cost only a few cents more if you have very many to make. Heckuvalot simpler though. This is the KISS principle. Use two short blocks like this. Mount one solid. Put a small spring in between them and just snug the other side to keep it from spinning. The whole thing couldn't cost much more than a couple bucks.

[greybeard]

Hi cely,
Re-reading your original post, I'd like to ask the following -

Originally Posted by cely I'm not an ME .........

Are you able to drill and tap holes in the "bracket", even if the bracket turns out to be a short tube ?

Originally Posted by cely ........I'm able to get near zero backlash by compressing the tapped nylon spacer

Have you considered that two of your spacers, sprung together, might be a simpler solution to cope with both backlash problems and future wear, than one being compressed? (This could be one cut in half, of course, if space is tight.)

Originally Posted by cely .........I'm looking for a very inexpensive way to mount the lead screw nut to the linear slide..........I would appreciate some help with the bracket design.

Can you post a design/photo of your slide to show how much space you've got ?

Regards
John

[cely]

I can do most of those things (actually, my local machinist can), but I'm looking for a cheap, solid, off the shelf solution. I've got several ideas for brackets but they cost too much to meet my cheap goal. I'll post some digital photos soon.

I was tapping my own nylon spacers and found that I had to run the tap through the spacer 4 times, cutting a little more material each time. Literally, fitting each spacer to a particular allthread rod. The result was a nice, near zero backlash nut with very low friction. The number of passes depended upon how much I compressed the spacer in my vice. Of course, the nut didn't fit allthread rod from a different vendor.

Then I found cheap tapped nylon spacers. Not much backlash right out of the box...but I want to do better. Besides, allthread rods vary all over the place. I even asked a vendor to do a custom threaded spacer with a mild cross-thread action and some low-friction lubricant mixed in the nylon...but they refused.

I then thought about simply compressing the tapped nylon spacer. I was planning on adjusting the compression often. Others find this objectionable, so I'm willing to look for a better solution.

I don't like the idea of a spring loaded nut set, because heavy tool pressure will compress the spring. I want something cheap and solid. Depending on the size of the motors, I have max linear forces rainging from 10 to 100 pounds. I know it sounds like I shouldn't use plastic, but that's why the spacer has to be 1/2" long (or longer).

I don't know why I'm obsessing over the cost. Until I can find a $4 stepper motor, it doesn't really matter all that much.

However, I do like all the ideas.

[greybeard]

Sudden thought, are you compressing the nylon end-to-end, or sideways ?
For some reason I assumed it was e2e, but splitting the nut down one side and cramping it inside an outer sleeve would be easier.
An alternative would be to split it into two halves, keep one fixed and move the other sideways, possibly with an eccentric adjustment. Possible ?
John

[cely]

I compress the sides of the nylon spacer without modification. That is, I don't make it into a split nut. Actually, the forces required are quite small because the wall of the spacer is fairly thin. I can achieve a tight fit with just two fingers. If you think about it, the internal threads in the spacer are only 0.010" or so, away from being a tight fit.

I never thought about end-to-end compression.

Ideas like that are why I like this forum.

[greybeard]

If the side walls are thin, then my thought of e2e compression might actually be a bad move. The centre might swell out, so you'd lose contact in the middle.
Is the outside of the spacer cylindrical or hexagonal ?
John