Making lead screw nuts

Does anyone have a source for procedure to make lead screw nuts? I need to make a few 1/4-20 size. I will just use regular threading procedure but cut the screw a little big then seat the screw in the tapped nut with lapping compound till the fit gets good. But how about larger ones with square or acme threads? Not a lot of info in the usual shop books on making the nuts. I have seen a few lead screws with grooves cut axialy at the tip. This makes the screw into a tap. I suppose the nut is cut undersize and the screw seated using the modified end as a finishing tap. Are there any other tricks or advice anyone might offer? Also how about material for the nuts?

Thanks

Rick

Can someone with copies of "Model Engineer" for 4th November and 2nd December 1988, assist you?

The late David Lammas gave a clear description of making taps etc in his " Renovating a Lathe" series.

Norman

With the quarter-twenties you could make the nut from plastic and end up with about zero backlash. Making a nut from bronze or some similar material and then doing the lapping compound routine isn't a great idea IMO, as you'd really never get all the compound back out of the nut. I made X and Y screw and nut assemblies for an XLO mill belonging to a customer a couple years ago, and was surprised to find the original nuts were plastic. The nut and the screw was worn about equally, and, based on the age of the machine and amount of use, the plastic nuts had held up about as well as you could expect bronze ones to. Sure surprised me.

Plastin Nuts. Most interesting.

Any idea what kind of plastic? There are thousands (very conservative estimate) out there.

And I wonder if they might work for larger machines?

Paul A.

Paul, I made the new ones from bronze, so I didn't have to do any research into what plastic would work. However, from fooling around with the old ones a bit, and from their appearance, my best guess would be that they were made from Nylatron GS. These nuts had lasted the better part of 25 years on a Bridgeport size Excello that was first used in production, and later in fixture work, by a company that makes jet engine parts for GE, P&W, and Rolls. They think of 6-4 Ti as a free machining material like most of us think of Ledloy, so the mill had seen plenty of tough cutting conditions. As I said before, I was amazed not only to see the plastic nuts, but to see how well they had held up.

I have seen many filled nylons hold up well to abuse. Nylon in the unfilled state tends to absorb moisture and die quickly. I have seen glass filled nylon nuts hold while the brass leadscrews stripped.

Filled acetals are much easier to machine and they are even tougher under nasty conditions. As long as temperatures are under 200 deegres F, I would go with acetals. Heck, I use unfilled Acetron GP for a lot of quickie jobs.

As I have stated before; I know little about plastic. When I need information on plastic, I call my local GE Polymers rep. They used to be called Cadillac Plastic. I give the perameters, they tell me what I need, they give the price, I write the purchase order. I like simple things.

I have made many acme lead screw nuts. 3/4" and larger I make on the lathe, using the approprate ACME threading insert. Smaller than 3/4" I have to buy the tap. Did make a 22mm-5 tpi tap for a real odd-ball lead screw. Most leadscrew nuts I make from 932-bronze or 360-brass.

A recent series of posts in the DIY CNC groups on Yahoo talked about the use of plastic lead screw nuts for small lead screws. The people who have actually done this seem to favor acetal or UHMW. They also said that to remove backlash, just compress the nut a bit.

Glenn M.

I've experimented with heat forming uhmw around a threaded shaft. I made a two sided nut by simply clamping two softened pieces of uhmw around a threaded shaft. I used some wood to make 'clamping plates' that would allow the soft uhmw to press into the threads on the shaft. The wood pieces have a shallow groove cut into each one where the shaft will lie when it's pressed in a vice or a press. The end result is a two part nut that has 'wings' which can be used for mounting the nut, and also allow you to adjust the play by skimming material off the inside surfaces of these wings. In use, one side of this two part nut can be bolted solid to whatever it needs to propel, and the other wing can either be shimmed apart some to create some play, or clamped tight to eliminate play, or have some material skimmed off then bolted together to create a tight fitting nut. Or the second wing can also be used to hold the nut to it's home in the mechanism, and both wings can be shimmed or skimmed as needed to adjust the play. Whichever would work best in the room available for the nut.
I found the initial fit to be tight without any shimming or skimming, and if anything, there needed to be a thin shim between wings to create an easier turning shaft.
It's my feeling that a nut formed this way from uhmw is superior to one turned in the lathe, in the same way that a rolled thread in steel is superior to a turned thread. Smoother threads and more accurate.
During my experiment, I determined that an accurate two part press mold would be needed, as the uhmw faithfully takes on every little offset and wrinkle in the mold. You would be looking for accuracy without a next step machining operation, as this plastic is 'springy', and will take some skill to machine to an exact size. Best to mold it to finished size right away. Having said that, it wouldn't be hard to include markers in the mold to define where to drill mounting holes in the finished nut halves.
One thing about pressing the softened plastic around a threaded shaft this way is that the shaft tends to self-center. Of course your mold can always have alignment grooves to center the shaft automatically anyway. The two sides of the mold should have some means of aligning to each other as they are pressed around the shaft and softened uhmw pieces.
One drawback to a nut made this way is that the resulting threads in each nut half don't go full depth for a full half circle, but this could be offset by making the length of the nut longer.
Something which I thought of but didn't try is to coat the threaded shaft with a graphite lube or something, prior to the clamping. The idea was that some of that lube might end up locked into the plastic, giving a self-lubricating action.
Something else that helped propel me towards this experiment was the fact that the resulting nut made this way could be as long as you wanted it to be. The longer, the less stress per thread area, and the greater the accuracy in the leadscrew since more of it's threads are engaged in the nut at one time.

recently single pointed a compound nut , 1/2x10 acme ...ground an undersize bit ,(correct 29deg, but undersized abt 10%) set the compound parallel to lathe centerline ,cut to full depth + clearance w/ crosfeed...then used compound to take lateral skim cuts of .001-.002 ,checking w/ screw for desired fit ....simple & trouble free old tyme Brit method from doug king in the snow belt across the line from michigan...(works great for square thrds as well )

best wishes
docn8as

Gentlemen
I’ve come across plastic nuts in machines before. Albeit on only a few occasions. The one I recall the most was on the ram feed of an 8m Butler Elga-mill. That’s a fairly serious machine. It was some years ago, but a screw of 1 ¼ or 1 ½ 4 tpi. For a client that makes rock crushers for a living, so they know how to take a cut. A double nut arrangement with spacers in under the retainer to take up back-lash. These nuts were green, and had taken years of use. I’m assuming the green could have been Pactine. Didn’t bother to work it out, skimmed the screw and replaced them in bronze.

Regards Phil.

Rick,
This may not help you but last week I had the opportunity to visit the Schaublin factory in Switzerland and get a short tour of the facilities. I was particularly interested to see how they made their cross slide and compound leadscrews and nuts. They are the metric equivalent of 3/8" acme threads using cast iron nuts.

The cast iron nuts have about 1 1/2" of thread length. They are tapped, of course. The lead screws are turned rather than ground. The rep who escorted me said they had tried grinding them but it gave no advantage. The lead screws are turned a bit oversize and then a nut is lapped by hand to a final fit. Each lead screw and nut is a fitted assembly and they are not normally provided separately.

I wish I could have watched the whole process from beginning to end because what I saw and the explanation I got left quite a few questions in my mind. The biggest question in my mind is how they preclude the lapping process from continuing once the compound is put into service. My understanding of the lapping process leads me to think the cast iron nut accomodates the abrasive embedded in its soft structure which accomplishes the "abrasive" removal of minute amounts of material on the leadscrew. There does not seem to me to be a good way of removing the abrasive embedded in the nut to halt the lapping once the desired fit is obtained. My assumption is that the abrasive action diminishes in time but I cannot imagine it halts altogether.

I would have loved to spend the time talking to their experts and resolving those questions in my mind but that was not practical - my lack of fluency in French precluded it, for one thing. The results speak for themselves, though, with every completed compound I saw having less than .001" play.

Jim S.

Jim S
Do a Google search on "Timesaver Compounds". Their products are claimed to not embed.
I once attended a Fairbanks Morse school on large diesels and they used and recommended Greenlabel Timesaver compounds to fit crank bearnings. Perhaps that is the method.

The guys making their own acetal lead screw nuts simply drilled and tapped them.

GlennM

Thanks for all the advice. I made the 1/4-20 nut from bronze tonight. Just drilled and tapped it. Cut the screw a tad over sized and polished it using a string and lapping compound. Hopefully the threads are now so smooth, they won't hold much abrasive. It fit the nut pretty tight. Ran the screw in and out with a cordless drill and cutting oil till it got real smooth. I don't know if it would meet Schaublin specs Jim, but it looks good enough for me. Since the bronze is on hand I don't think I'll try any plastic for a while. But the information was appreciated. Same for the advice on the Engineer mag.

Thanks All

Rick

A simple method of making a plastic leadscrew nut

rjibosh said:

Does anyone have a source for procedure to make lead screw nuts? I need to make a few 1/4-20 size. I will just use regular threading procedure but cut the screw a little big then seat the screw in the tapped nut with lapping compound till the fit gets good. But how about larger ones with square or acme threads? Not a lot of info in the usual shop books on making the nuts. I have seen a few lead screws with grooves cut axialy at the tip. This makes the screw into a tap. I suppose the nut is cut undersize and the screw seated using the modified end as a finishing tap. Are there any other tricks or advice anyone might offer? Also how about material for the nuts?

Thanks

Rick

Click to expand...

I ran across a method that sounds easy. See
Making Acetal leadscrew nuts the easy way

One comment was to look for white Delrin (Acetal) since it is supposed to be less abrasive. I went to the trouble of making my own Acme tap by turning threads on a steel rod (mild steel) with a threading tool ground to 14 1/2 degrees and with the correct end width for the pitch. It took some torque to tap the Delrin, but the result was great. I did put a generous taper on the tap, but did not bother grinding relief on the cutting edges, which probably would have made it much easier to use.

Bob

Bob

Thread on HSM from years ago, with pictures.
Leadscrew removed and fettled.
Leadscrew heated in vice.
Two pieces of Delrin clamped over leadscrew, Delrin melts and fuses.
Voila - nut.

Apparently it works, how well and for how long I do not know.