# The Bronze-Loaded Acme Nut Experiment - Part 2



## graham-xrf (Mar 14, 2021)

This continues the butchering of the Acme nut on my South Bend 9. I have posted a series of extra pictures on the tail-end of Part1
--> *HERE*
This time it starts with violence from a Dremel-style tool grinder. Having decided to try and make the mold from a short piece of captured polythene water pipe, it was too tight to fit over the compound nut cylinder, "Widening" using one of those little sanders that fit tight by squeezing the rubber middle between washers seemed easy enough, though it produces a tangled string of part-melted polythene dross that obsitnately hangs onto itself. This stuff does not really "abrade", but the scheme seemed to work if the tool is spun up, and pushed in in a series of stalls.

*It all goes wrong!*
Very, very quickly - if inadvertently, the grip in the blue bit is lost. It may have been better to clamp up the tube, but I didn't want to squeeze it "out of round", and things seemed to be going well. Let go, and the blue pipe starts spinning on the sanding wheel, but unbalanced, off centre, speeding up, and then POW! It throws the tube to collide with the ceiling, and it's bullet-like flight bounces off a few things before finding the floor. Meanwhile, the grinder is vibrating like a mad thing, and it was all I could do to get to the on/off rocker switch.  The rubber tool inner was the only one I had, I "straightened it up" in a vise, but it is marked up, and will never be the same again.




Here we can see the basic setup. The threaded and untrheaded parts are the same diameter 3/8". The bit at the dial indicator end was somewhat mangled by the hard grub screw, so I filed those burrs away, and put it between V-blocks stood on a 1/2" thick glass plate I have now finally put to a good use.




I then ran into some practical issues. It did not much matter if I made the mold too high, because I could cut off the excess to have it fit with the right clearance to the casting. The very important bit to get right was to have a reasonable amount of bronze on the other side of the thread, without ending up with the nut cylinder sticking up proud of it's hole in the slide. This was going to need some nominal measurements, though not too critical.
Using the "depth gauge" measure sticking out of the end of a caliper is not as easy to get consistent results as one may think. I went through a whole series of attempts.



	

		
			
		

		
	
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In the end, the only good way was to use my recently acquired $14 bucks old Starrett depth gauge that I cleaned up and re-calibrated. It seems to read spot-on accurately, so we went with that. There are two measurements needed. One is to the top of the Acme screw, and the other is all the way to the bottom. I figured out I would need 4mm of nut sticking out of the other end, to set the thickness of the bronze around the screw.



	

		
			
		

		
	
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Much of this stuff does not have any critical measures, because they largely take care of themselves by the fact it is a mold, but we do need to make some attempt at getting the screw and the nut cylinder axis to be 90°, to prevent binding. That said, the amount of up/down flop in the screw when it is in place held by the bushing part is enough that I am sure the accuracy here is not super-critical, but that is the whole motivation for hanging it between V-Blocks.




Of course, the drilling and threading of the plastic was only done with a hand drill, though carefully. The screw can be "moved" a little in angle, and it cannot be trusted to "dangle" at a reliable 90°. This is where we "force it" a little.



	

		
			
		

		
	
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*It's like the 1950's with the car polish!*
The old stuff has been in the garage for decades. This time, I used 2 (or is it 3?) applications as per the way it must be (Ref: "_Back to the Future_" movie), but this time without the wood floor treatment wax. I don't have any promotional interest in wax from turtles, which might be deduced that the 500ml plastic bottle has lasted me about a quarter of a century!



	

		
			
		

		
	
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I buffed up the insides of the threads using a piece of knitting wool stolen from the lady of the house. So here we are, trying to complete the deed. Darn - but it's a messy business! So easy to spill, or wobble while weighing out. This time, estimating, and not wanting to end up short, I went for 24grams of powder instead of 16. That was wrong.  There was 13g of nut mix left over, so that's 10.4g of bronze too much, and 2.6g of epoxy.

The advice @Superburban  on mixing the bronze with the epoxy first, and then adding the hardener did not make things easier. The mix of powder so overwhelmed the epoxy volume that it was all just dry lumps, very difficult to get it to "coat" all the powder. When I added in the hardener, things eased up some, but I was not sure I would get the stuff fully mixed. Eventually, I got it to a consistent lump of "dough", and I just kept flattening and re-folding it until I had no loose bronze.



	

		
			
		

		
	
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Finally, with some irony in the use of thread gauges, I got it together. I put enough in on top of the nut remnant to help fill up to the threads, then I wound the screw in, and set about tamping and loading. I pushed with my thumb from the top, to force the mix down a bit "hydraulic-style".  I found by whacking the top with the flat of the spatula. I could shock the mix into giving me a somewhat shiny top, and hopefully, forced it into all the shapes below.




So here we have it. After some hours, it has "gone hard". The test nut I unwound after about 16 hours. I now have to find a non-damaging attach to be able to get some torque onto this, because it does not come with a convenient hex head. If I can get it to unwind, I will post what happened.


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## MyLilMule (Mar 14, 2021)

Interesting saga. Thanks for sharing. 

Cant you just put the handle and pin back on the screw and use that to unthread it from the new nut?


Sent from my iPhone using Tapatalk


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## graham-xrf (Mar 14, 2021)

There is a little drilling between the handle boss and the Acme shaft, to put a 1/16" little psuedo-key pin. I don't have that. Also, my time zone is such that I have to get some shut-eye. Good or bad, it will have to wait a few hours. 

[Edit: Now + some hours into the next day. The thing is stuck hard. It did not come free easily like the the first try with the M12. The little pin arrangement and shaft end looks too feeble to take the sort of torque that can be applied to a nice fat bolt head. Here we have to explore "stuckiness" Ref. "_Zen and the Art of Motorcycle Maintenance_".

Perhaps I put it into the freezer, then try heating the bronze part a bit?
Maybe pinch some copper tube around it, then into a vise or chuck?
I could try a stronger bond of Loctite 628 or 623 (not sure which), and get a grip tube around it that I know can be undone with heat?
Possibly, the J-B Weld, being epoxy, also has a temperature it cannot stand, so just heat the screw?

The advice from @mickri about getting the bolt out when the epoxy has set, but not fully cured, is probably the best way to go.
Also, finding some "Florida epoxy" that is more liquid to start with, instead of J-B Weld.

The folk who make reinforced epoxy-granite machine beds found 80% sand to 20% epoxy mix was best, which was why I tried that proportion for the bronze.

Oh Yay! I have just filed chunks out of my compound screw, then cemented the remains  into a metal-reinforced HDPE shock-protected tomb!  ]


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## Braeden P (Mar 15, 2021)

I would think that loctite in a tube then spin that sounds like a good idea to me


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## Asm109 (Mar 15, 2021)

Heat it.  Epoxy and bronze expand more than steel.


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## mickri (Mar 15, 2021)

It doesn't take much heat for the epoxy to lose its grip.  A hair dryer on high might work and a paint remover heat gun will work.  You could also try putting it in the oven at 175 to 200 degrees.

Part of the reason it is so hard to remove is that it is perfectly formed to the screw creating a mechanical bond.


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## graham-xrf (Mar 15, 2021)

Asm109 said:


> Heat it.  Epoxy and bronze expand more than steel.


You are absolutely right, and that would work better to release it. That was the basis of my thought to freeze it all first then heat the bronze from the outside.

@mickri is also right about getting epoxy to lose it's grip with heat, and the digital temperature controlled heat gun seen in Part 1 would be just the tool to do it. To some extent, we are all learning the way through this process, with my compound nut as the experiment!

*Getting it loose*
I did not have to go to the extent of using a Loctite tube to get a grip as suggested by @Braeden P , because the news is, I have managed to free it, although subsequent actions to work the screw in the thread may have hurt it. When the screw is put up a piece of 15mm copper water pipe, and the pipe crushed in a vise, it gets enough of a grip to allow biggest pipe adjustable pliers to finally make the nut part turn. It happens with a cracking sound, which may not be good, and from then on, may be turned, though it must be said, feeling pretty tight.





	

		
			
		

		
	
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I tried "running through" the screw into the nut to see if it could burnish it's way into an easier feel, but it remains "damn tight". There is zero space in there, such that it cleans off any oil. I believe I have injured it by hydraulic pressure of trapped liquid up between the threads when I cleaned it out with a squirt of IPA. Carving away the excess at the thread starts left some bits inside needing to be washed out. It is still "damn tight", but the extreme pressure exposed a hairline crack on one side at the thin place where it meets the steel. I will try to get a better picture.

The original nut had a huge crack across the brazed join, so this one is in much better condition, even as it is. I can just cut a little V and put some J-B Weld in to re-join it, but the lesson here is, we need a way to get zero backlash match on only the sloping parts of the thread, and have a little space at the peaks and troughs.




I have to file back 2.5mm to 3mm off the top, to get clearance to the compound. It cries out to be "lapped in", but this is not steel on steel. I did not want to make the nut into a permanent embedded abrasive grinding lap.

I can take care of the peaks of the threads by just taking off about 0.001", and that would stop the hydraulic entrapment, and allow some lubrication. There is not much I can do about the internal troughs.

I have thought that one layer of stretched white PTFE thread seal tape as used in plumbing and pneumatics, put over the screw before molding, may be the way. It starts out as about 0.002" thick, but stretches to lay in at about 0.0003". The lesson here is that a perfect Acme thread is not one that has zero space at the peaks and troughs.

Even so, we now have a Acme nut (still in need of some love), a set of suggested improved things to do, some idea of what not to do, and we got there with low cost ingredients and near zero machinists skills involved!


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## Braeden P (Mar 15, 2021)

maybe a wire brush used for cleaning the inside of a pipe, then cut off the handle and put it in a drill and wire brush the inside it will wear it and add oil grooves


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## Peyton Price 17 (Mar 15, 2021)

graham-xrf said:


> I have thought that one layer of stretched white PTFE thread seal tape as used in plumbing and pneumatics, put over the screw before molding, may be the way. It starts out as about 0.002" thick, but stretches to lay in at about 0.0003". The lesson here is that a perfect Acme thread is not one that has zero space at the peaks and troughs.
> 
> Even so, we now have a Acme nut (still in need of some love), a set of suggested improved things to do, some idea of what not to do, and we got there with low cost ingredients and near zero machinists skills!


I think that using the PTFE tape is the way to go.


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## graham-xrf (Mar 15, 2021)

Perhaps better is to spray-paint the screw, then use a small blade to scrape the paint from the thread sides. Then polish it up with wax, or use a spray release agent. You end up with zero backlash, but after a clean-up with something that removes the paint, you have a nut with clearance where it should be.

I should keep my head down a bit now. You can imagine how this little adventure would get roasted on some sites!


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## mickri (Mar 15, 2021)

Put anti seize on the threads.  Keep running the screw in and out and it will loosen up a bit.

I think using the liquid turtle wax was the problem.  Too thin.  A hard paste wax like mold release wax would have been a little thicker leaving more clearance.


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## RJSakowski (Mar 15, 2021)

You can run a ream through the nut to remove the crests of the nut.  That will allow oil to be carried through.


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## RJSakowski (Mar 15, 2021)

How about nickel plating the lead screw.  The plating can then be stripped chemically which should leave you with a slight clearance.








						Stripping of Plated Finishes
					

The processes, chemicals and equipment, plus control and troubleshooting.



					www.pfonline.com


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## graham-xrf (Mar 15, 2021)

RJSakowski said:


> How about nickel plating the lead screw.  The plating can then be stripped chemically which should leave you with a slight clearance.
> 
> 
> 
> ...


Nickel plating, such as has recently been done to the handle, is very thin - millionths. Yes - sure, you can plate up (say) 5 tenths or more, but much easier to apply and remove is a layer of spray paint. I know that you can remove nickel without touching the iron, because that is what platers regularly do when building up new surfaces.

If I had a spare piece of Acme screw, I would consider hacksawing some slots into it, and fashioning a tap.
If I can't jimmy this (first) try into a temporarily useful nut, I will break the bronze epoxy out, and start over, incorporating some of these suggestions.

If I put some grinding paste up there, I guess it would modify the screw before it modified the bronze epoxy much.


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## mickri (Mar 15, 2021)

Try some anti seize.  It is a very thin high pressure lubricant.

The epoxy bronze will wear before the steel screw.  You could nick up the very first thread on the screw to help open up the nut.  Then file it smooth when you are done.

I think that you are over thinking this.


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## graham-xrf (Mar 15, 2021)

mickri said:


> I think that you are over thinking this.


As I have mentioned before, "over-thinking" is not a concept I recognize. I just think as needed.
I hope to be a reasonable person meaning I always have reasons for what I think and do.

There is no seizure as in surfaces stuck against each other with high pressure friction. There is definitely high pressure, but from the nut seemingly now too small, with smooth surfaces sliding.

No other stuff, not even a film of very thin oil (tried) fits up there. It is as if, when I took it out, the bronze "relaxed slightly". What I have now is a true interference fit. The action between the screw and the nut surfaces is shiny sliding low friction, and the threads inside are smooth, burnished looking. What made the biggest difference was a tiny trace of graphite. It is just very tight.

Right now, my crack repair is hardening. Unquestionably, if I can relieve the surfaces a little, I have a vastly improved Acme nut. Perhaps I should not expect a first time try should immediately work. So far, it has been cheap ingredients, and knocked together, and we are all getting to know the recipe!

I have to be impressed with the way you make built-in threads for boat structure bolts. So far, with bolts, it kinda works. Acme threads have something else about them in contact area and design that is different. Possibly the thread half-angle is 14.5° compared to a bolt's 30°


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## nnam (Mar 15, 2021)

.....


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## mickri (Mar 15, 2021)

Have you tried anti seize yet?  With time the fit between the screw and the nut will loosen up.  The steel screw is harder than the epoxy and bronze and the imperfections in the screw will slightly wear down the epoxy and bronze.


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## Janderso (Mar 15, 2021)

graham-xrf said:


> This continues the butchering of the Acme nut on my South Bend 9. I have posted a series of extra pictures on the tail-end of Part1
> --> *HERE*
> This time it starts with violence from a Dremel-style tool grinder. Having decided to try and make the mold from a short piece of captured polythene water pipe, it was too tight to fit over the compound nut cylinder, "Widening" using one of those little sanders that fit tight by squeezing the rubber middle between washers seemed easy enough, though it produces a tangled string of part-melted polythene dross that obsitnately hangs onto itself. This stuff does not really "abrade", but the scheme seemed to work if the tool is spun up, and pushed in in a series of stalls.
> 
> ...


I started reading through this thread and thought, what the heck are you doing?
Once I realized the plan, I think it's a darn good one.
You'll get it.
Well done!!


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## nnam (Mar 15, 2021)

.....


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## graham-xrf (Mar 15, 2021)

mickri said:


> Put anti seize on the threads.  Keep running the screw in and out and it will loosen up a bit.
> 
> I think using the liquid turtle wax was the problem.  Too thin.  A hard paste wax like mold release wax would have been a little thicker leaving more clearance.


Is your release agent a type you spray on? Can you "build up" thickness?


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## DAT510 (Mar 15, 2021)

Just thinking..... What about wrapping it with Teflon Tape?  It can be drawn quite thin.  Just not sure thin enough?   Tends to conform to the shape of threads as it is drawn down.


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## graham-xrf (Mar 15, 2021)

Janderso said:


> I started reading through this thread and thought, what the heck are you doing?
> Once I realized the plan, I think it's a darn good one.
> You'll get it.
> Well done!!


Thank you. I was beginning to wonder whether I had gone too radical!

One way I consider, reluctantly, is to alter the screw. The whole idea was to mold something around it, and leave the screw unmolested. It is reasonably simple to spin something up the nut to take a thousandth or so off the crests of the internal thread, and (horror), rub a thousandth or so off the metal Acme screw tops. Then only the sloping surfaces would be involved. It might still bind.

When I said "zero backlash" I had not considered an outcome that overdid it, ending up too tight 
[EDIT: The reason for this is found in post #28]


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## graham-xrf (Mar 15, 2021)

DAT510 said:


> Just thinking..... What about wrapping it with Teflon Tape?  It can be drawn quite thin.  Just not sure thin enough?   Tends to conform to the shape of threads as it is drawn down.


Indeed - that is what we considered in posts #7 and #9. PTFE stretches to about 0.0003, but it's very difficult to get a single thin uniform layer on. I have tried, and I have yet to clean up all the bits of messed up tape here. Spray painting may be the answer.


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## mattthemuppet2 (Mar 15, 2021)

with the delrin Evanuts I think the idea is to use a spare piece of threaded rod gashed at the end to act like a thread chaser. Run that through a couple of times and that's usually enough to give you sufficient clearance. Or you could mold the nut around the new screw and then use the old screw, which presumably will be a little worn.

Really neat idea that you're working on though, I think it has an awful lot of potential, especially for harder to make parts like half nuts.


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## mickri (Mar 15, 2021)

I use Meguiars Maximum Mold Release Wax.  It is a hard paste wax specifically designed to wax molds used in making fiberglass and epoxy parts.   It is not a thin spray on film.  Some of the suggestions may work.  Or may not.  Why not use a product that was designed to do what you need.

 In your application you have to be careful not to fill the threads with wax.  If you have a regular Acme nut that fits your screw I would goober up the screw with wax then run the nut on and off the screw.  This will leave you with the same thickness as the tolerance in the threads.  If you redo your nut I would not use JB Weld.  I would use a regular epoxy.  Much easier to mix and apply with a longer working time.  You want the consistency of peanut butter or just a little thicker.

 I believe Maguiars is now owned by Stoner and sold as Stoner Maximum Mold Release Wax.  I am sure that there are other similar products.  http://www.shopmaninc.com/moldrelease.html

I think that the nut you have already made will work just fine.  It just needs to be screwed on and off the screw a bunch of times.  It will loosen up in use.


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## graham-xrf (Mar 15, 2021)

mattthemuppet2 said:


> with the delrin Evanuts I think the idea is to use a spare piece of threaded rod gashed at the end to act like a thread chaser. Run that through a couple of times and that's usually enough to give you sufficient clearance. Or you could mold the nut around the new screw and then use the old screw, which presumably will be a little worn.
> 
> Really neat idea that you're working on though, I think it has an awful lot of potential, especially for harder to make parts like half nuts.


The scheme as I have done this first run does have it's imperfections. I have "got away" with a whole lot so far, but I am sure any future attempts will see this thread, and make some variations in view of what I have found. With a half-nut, at least you can cut it from a whole nut.

*Metal polish and toothpaste!*
Working it as a bit of a pastime now, using only fingers torques - no tools, and metal polish, and toothpaste, I can get it to screw through, but still stiff. Whatever abrasive are in these, they are very fine, and "wear out", unlike diamond lapping paste, which just keeps on cutting. The Acme screw is looking very shiny polished now. If I can burnish it enough this way, lapping it in, then my experiment also becomes an actual "repair".

The end result of having bronze loading particles bonded in a plastic should make a very good bearing, for something that does not have to move much. It makes me wonder how the original came to be so worn out and broken!

@mickri : My thanks. I have brought the nut to where my PC is, and I have been working it to loosen up as you suggest, When I get it running on such that I am convinced I am not going to break it, I may run it a few times back and forth using a hand drill. I am getting it to work, but ever so slowly.


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## graham-xrf (Mar 16, 2021)

OK - we make a discovery.
Much of the reason it can be so difficult to get the screw to run up the nut again, and why the force was enough to crack the mold away from metal adhesion on one side when first extracted, is because the mold was cast on a worn part of the screw - somewhere in between the V-blocks as it was set up. This wear is not on the crests of the screw. That measure was 0.372" all the way along. The wear is on the _sloping_ parts of the thread.

As I "lapped" the screw into the nut, using only metal polish and toothpaste, everything slowly becomes easier. This form of lapping is ultra slow, and relatively harmless. We start to discover that in the first inch or so, something feels "out of round", in that there are two "easier spots" 180° opposite each other. "Easier" is a relative term. It's damn tight most of the rotation. There was some feature cast, affected by the fact it was set in a U-shape, and in a part of the screw that had seen more use. I have marked up the earlier picture to show this.




If I leave it in it's current state, then I will have a repair part that should work just fine over the region of compound position most used, but it will encounter the stiffer unworn part when screwed to near it's extreme. Had I cast the nut on the unworn thread in the first inch, I might have ended up with a nut that was somewhat looser and "more sloppy" as it found the middle part of the screw.

What I should have done when checking out the screw was to wind some copper wire into the threads, and measured across, or used "thread pins", but I don't have a set of those.

The backlash is nothing! It cannot be felt, not even "rocking". Now I know that the screw is "unique", should I come to re-make it as part of a larger dials upgrade, then it will have to have it's own new nut to match. The experiment nut only has to survive taking some 3mm off the top, to not rub on the casting, and it can become a "repair part" for the present, because it will be a huge improvement on what was there before, even if looking a bit "experimental".

Actually - it kind of has to do service now, because I sacrificed the original working nut in a manner.. well.. kinda permanent!


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## macardoso (Mar 16, 2021)

You could always replace the ACME screw with a short portion of a replacement and pin in place. Keep your awesome nut.


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## graham-xrf (Mar 16, 2021)

macardoso said:


> You could always replace the ACME screw with a short portion of a replacement and pin in place. Keep your awesome nut.


The nut is a low cost, easy and fast way to get a quite effective fitting ACME nut. "Fast" means I don't include curing time when one is off doing something else. The actual time one gives it direct attention has to be less than going up the learning curve of grinding 29° boring tools, and single point turning an internal ACME thread, all assuming you have available "another" lathe to do the job anyway!

I am not completely happy with the nut I have as a functioning thing, because even though it seems OK, and my J-B Weld crack repair looks OK from the outside, I am considering a Nut MK2, implementing some of the suggestions regarding release agents, etc. and this time, choosing to mold at the "unworn" first inch of the screw. I admit that the ease by which one can end up with exactly fitting, strong, hard wearing,  low cost, precision internal ACME thread part is just temptingly addictive. We shall see how that goes!

The solution I really fancy is shown here..
Backlash Compensated Nut Design




I know, I know! Let us be "gilding the lily"!

To do something like as in the video, to work in the confined space of that little compound may be unrealistic! The screws would have to be something like M2, or M1.6, and having to stick it all through a 5/8 round hole is a serious limitation. Milling the casting to have a squarer peg with rounded low-stress corners, or just simply have a bigger hole,  might make more room, but these are all pipe dreams. I am OK with the South Bend being what it is. Perhaps given some TLC, and getting it back to something like the rugged, yet precision thing it was originally made to be.


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## RJSakowski (Mar 16, 2021)

The third concept in the above video is the one employed in my lathe.  The concept will not compensate for uneven wear in the lead screw, however.  

What I did on my old and well used Atlas/Craftsman lathe was to add a second nut which was coupled to the original nut by a stiff compression spring.  The nut is prevented from rotating by a key and the the spring forces the nuts against opposing faces, removing all backlash.  

The nut that I used was actually an OEM nut left over from replacing it and I milled a slot in the cross slide apron to accommodate the boss on the OEM nut.  The slot length only has to be longer the the boss diameter by one thread length. The backlash nut is located to the rear of the driving nut so that it works in concert with the cutting forces when the cross slide is feeding in. The spring compression force should be strong enough to overcome cutting forces when the cross slide is feeding out but not so strong as to make moving the cross slide difficult.  A stronger spring will also result in increased wear.

The advantage of this setup is dynamic backlash compensation which results in uniform driving force. It also is possible to attain virtually zero backlash over the entire range of even a worn lead screw.   One disadvantage is that there is an increase in driving force. Another is that the range of travel of the cross slide is reduced by the distance separating the two nuts.


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## homebrewed (Mar 16, 2021)

RJSakowski said:


> The third concept in the above video is the one employed in my lathe.  The concept will not compensate for uneven wear in the lead screw, however.
> 
> What I did on my old and well used Atlas/Craftsman lathe was to add a second nut which was coupled to the original nut by a stiff compression spring.  The nut is prevented from rotating by a key and the the spring forces the nuts against opposing faces, removing all backlash.
> 
> ...


I made something a bit like that for the cross slide on my mini lathe.  The OEM adjustment for the nut is pretty lame, using three screws spaced parallel to the axis of the feed screw.  The two outer ones engage threaded holes in the nut and the middle one presses against the nut.  The idea is to tilt the nut just enough to minimize the backlash using the outer screws and then lock everything down with the middle one.  It never worked all that well.  So I cut the nut in half, ending up with two nuts.  Then I put a small compression spring between them so they are forced against the feed screw threads.  The last step is to tighten down the two outer screws to lock the nuts in place.  To accommodate wear, just loosen one of the screws so the spring again pushes the nut up firmly against the threads, retighten, done.  It's not necessary to use a real strong spring since it only is used to take out the slack.  I did have to make a shim to properly align the nuts to the axis of the feed screw.

At some point it may be necessary to elongate one of the screw holes to accommodate more wear but so far it works OK without needing to do that.

Unlike RJ's approach, unequal wear on the lead screw will result in variations in the drive force needed to move the cross slide, since the nuts are firmly locked in place.


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## graham-xrf (Mar 16, 2021)

homebrewed said:


> I made something a bit like that for the cross slide on my mini lathe.  The OEM adjustment for the nut is pretty lame, using three screws spaced parallel to the axis of the feed screw.  The two outer ones engage threaded holes in the nut and the middle one presses against the nut.  The idea is to tilt the nut just enough to minimize the backlash using the outer screws and then lock everything down with the middle one.  It never worked all that well.  So I cut the nut in half, ending up with two nuts.  Then I put a small compression spring between them so they are forced against the feed screw threads.  The last step is to tighten down the two outer screws to lock the nuts in place.  To accommodate wear, just loosen one of the screws so the spring again pushes the nut up firmly against the threads, retighten, done.  It's not necessary to use a real strong spring since it only is used to take out the slack.  I did have to make a shim to properly align the nuts to the axis of the feed screw.
> 
> At some point it may be necessary to elongate one of the screw holes to accommodate more wear but so far it works OK without needing to do that.
> 
> Unlike RJ's approach, unequal wear on the lead screw will result in variations in the drive force needed to move the cross slide, since the nuts are firmly locked in place.


OK - as I understand it, you use the spring as a convenience to take up slack when you are deciding to remove the built-up wear, you secure it in the new position with the locknut, so you have "equal" wear on both sides of the screw.

Reading @RJSakowski 's approach, as I understand it, the screw is pushing on the nut, taking all the cutting forces when feeding in, or turning along X. If withdrawing the tool, as in (say) facing outward from the middle, or boring a hole with the tool on the operator side, then the stoutness of the spring is in play. Maybe I misunderstand some.

That same idea is behind using two (thin , flat) spur gears up against each other, one fixed to the shaft with a boss, and the other free to rotate. There is a peg or slot to fix a tension spring between them, to keep the teeth of the free gear trying to rotate "the other way", so taking up backlash. This scheme was common on the tuning dial arrangements of old radios.

I have also seen the huge pinions that rotate the 25m dish at Chilbolton. There they have a complete servo drive system, doubled! One rotates one way, and the other to drives against it. One drive gets to point, and the other has to follow the first, with enough force to be certain it can move things if it has to, but with a force just short of overcoming the other drive.

This whole repair thing started out because of inability to turn up a mounting for XRF radioactive sources!


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## nnam (Mar 16, 2021)

......


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## RJSakowski (Mar 16, 2021)

I recall the split gears used on radio dials. It's been a long time!  

In feeding out as in boring or internal threading, the spring provides the cutting force so aggressive cuts would cause a deflection.  A final "spring" pass should take care of that though.  If desired, it could be avoided by flipping the tool and cutting from the back side.


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## graham-xrf (Mar 16, 2021)

RJSakowski said:


> I recall the split gears used on radio dials. It's been a long time!
> 
> In feeding out as in boring or internal threading, the spring provides the cutting force so aggressive cuts would cause a deflection.  A final "spring" pass should take care of that though.  If desired, it could be avoided by flipping the tool and cutting from the back side.


 The Joe Pieczynski method!

My South Bends have 8TPI threaded spindle mounts for the chucks. Thus running in reverse risks the whole lot unwinding and dropping onto the bed at speed. Unless I can find a way to stop them unscrewing themselves, I can only use that method with a collet chuck. As it happens, the MT3 collet chuck set for my mill-drill (when it finds it's home), would fit. I need to work up a draw-bar for it. Doing all threading this way is attractive, because the tool travel is away from trouble. You can let the carriage keep moving to the right, and disengage at leisure (nearly).

Recalling the infinitely incremental automatic brake wear compensation in the front wheels of a French car I once owned, it seems to me that if the nut was split, and one half permanently encouraged to rotate so as to take up the slack whenever this became possible, with some arrangement to make the rotation "one way" only, one could know the nut was always without backlash, and yet able to transfer the full forces, whether shoving or tugging.

Also, it would be great if the dial end of the compound were a stepped bolt-on bulkhead, so that nut arrangements could be assembled without constraints from the dovetail slide. Another way might be to have as much of the nut as possible on the dovetail side to be made a female threaded blind hole, so that nut arrangements need not be confined to be within a circle perpendicular to the nut axis. When it is moved into place, the slide can be attached by adding the engagement cylinder, which has the mating male thread, looking like a bolt with a very tall cheese head! Duh - I know that many of us with 20-20 hindsight could probably design a better compound, and I suppose some do.


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## RJSakowski (Mar 16, 2021)

graham-xrf said:


> ........which has the mating male thread, looking like a bolt with a very tall cheese head!


Them's fightin' words to a Wisconsin boy!
https://onmilwaukee.com/articles/cheeseheads10things


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## graham-xrf (Mar 16, 2021)

RJSakowski said:


> Them's fightin' words to a Wisconsin boy!
> https://onmilwaukee.com/articles/cheeseheads10things


Eh? Sorry!
In UK, the catalogues (catalogs?) list round head, and instrument head, and hex cap-screw - and "cheese head", because that is the shape of a cheese (before they cut it up). Cylindrical, unrelated to footballs. We have other insults for cricket


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## RJSakowski (Mar 16, 2021)

I know what a cheesehead screw is.  They are uncommon here for some reason.  I first encountered them on a trip to the UK in 1997.  We have round head screws  as well.  I wasn't sure what the instrument head was.  We have a raised head countersunk screw that is referred to as an oval head.  Another type is a fillister head which would be similar to the cheese head but with a shorter head and slightly oval head with rounded edges.  They are common in electrical connectors and sometimes have slight ridge on the underside for a better bite when making electrical connections.


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## mickri (Mar 16, 2021)

To keep the chuck from unscrewing you need what I call a chuck keeper.  Why do I call it a chuck keeper.  Because it keeps the chuck from unscrewing when running in reverse.  Simple to make.  The headstock end is a very close fit to the ID of the chuck.  Then there is a long bar that goes through the spindle with a nut and washer that holds it in place.  As long as the threads on the nut are different from the 8 tpi on the spindle the two different threads work against each other preventing either from unscrewing.  Both ends are machined to fit the spindle to keep it centered in the spindle.   I did not come up with this.  Another forum member told me about it.


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## homebrewed (Mar 17, 2021)

Sherline has a slightly different approach to an anti backlash nut design for their mills that might be useful for some other applications.  They have a nut in the "usual" place, i.e., underneath the table.  On the same axis, they have another nut on the exterior of the table.  It is knurled on the OD.  A thick washer, knurled on its OD is mounted on the exterior of the table such that the knurls are engaged, pretty much like gears.  To remove the backlash, the screw holding the washer is slightly loosened and rotated to tighten the exterior nut, removing the backlash.  Then the screw is retightened, locking the adjustment in place.  There is no fundamental reason that the two nuts in an anti-backlash scheme have to be immediately adjacent to each other!

I used a similar scheme on the X axis of my mini mill:  and now I can perform climb milling in steel, at least on that axis.  Backlash is roughly .001 inch and has has been easy to keep that way, since the adjustment is point is easily accessible.  The table design makes it relatively easy to mod the X axis in this fashion, not so much on the Y.


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## AGCB97 (Mar 17, 2021)

mickri said:


> I think using the liquid turtle wax was the problem.  Too thin.  A hard paste wax like mold release wax would have been a little thicker leaving more clearance.


 The problem is the melting point of the wax. The curing epoxy creates heat and if the wax melts below that temperature, the epoxy seeps through it.  This is the case with car wax.

I had the same problem with mold release when building a fiberglass cowl for one of my experimental airplanes. I asked an epoxy expert/salesman at Oshkosh flyin about it. He said to use Johnsons Paste Wax. Cured the problem on later molds. By the way, that same wax is excellent for preventing rust on iron surfaces in high humidity conditions. I coat all the surfaces in my wood shop in the spring before those conditions arrive.
Aaron


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## graham-xrf (Mar 17, 2021)

mickri said:


> To keep the chuck from unscrewing you need what I call a chuck keeper.  Why do I call it a chuck keeper.  Because it keeps the chuck from unscrewing when running in reverse.  Simple to make.  The headstock end is a very close fit to the ID of the chuck.  Then there is a long bar that goes through the spindle with a nut and washer that holds it in place.  As long as the threads on the nut are different from the 8 tpi on the spindle the two different threads work against each other preventing either from unscrewing.  Both ends are machined to fit the spindle to keep it centered in the spindle.   I did not come up with this.  Another forum member told me about it.
> 
> View attachment 359486


This I do very much like, and one of these surely must join the list of essential tools to have.


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## graham-xrf (Mar 17, 2021)

homebrewed said:


> Sherline has a slightly different approach to an anti backlash nut design for their mills that might be useful for some other applications.  They have a nut in the "usual" place, i.e., underneath the table.  On the same axis, they have another nut on the exterior of the table.  It is knurled on the OD.  A thick washer, knurled on its OD is mounted on the exterior of the table such that the knurls are engaged, pretty much like gears.  To remove the backlash, the screw holding the washer is slightly loosened and rotated to tighten the exterior nut, removing the backlash.  Then the screw is retightened, locking the adjustment in place.  There is no fundamental reason that the two nuts in an anti-backlash scheme have to be immediately adjacent to each other!
> 
> I used a similar scheme on the X axis of my mini mill:  and now I can perform climb milling in steel, at least on that axis.  Backlash is roughly .001 inch and has has been easy to keep that way, since the adjustment is point is easily accessible.  The table design makes it relatively easy to mod the X axis in this fashion, not so much on the Y.


This is nice and simple in concept, and compact. You get me thinking. If you happen to have a Sherline, or can get get a picture of your mill mod knurled part, when you hqave opportunity, it would be much appreciated.

I do hate backlash, though for many, at least a small amount has that "feel", so you know from where to read the graduations. Backlash or not, if ever the tool has to move back, I always back up a tad further, then adjust forward to the place I want. A knurled nut "take-up" arrangement need only be adjusted occasionally.

Of course, for CNC work, especially with servo motors, any backlash at all is big trouble! If the motor moves at at all, without the feedback showing any response (through backlash), it will cause the electronics to drive up the currents to max in microseconds, and then have to drive the other way to stop the overshoot, which is similarly being incorrectly reported.  If you want to see stuff doing a blinding loud high speed vibrating instability oscillation jiggle either to overcurrent trip, or snapping stuff, then allow backlash. That is why one of the most popular mods to mill drills for CNC is to replace the Acme nut stuff with ballscrews.

Fortunately, here we are only talking of what is in a little compound, not normally in the works of a CNC drive train, and hardly gets moved much.


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## MyLilMule (Mar 17, 2021)

mickri said:


> To keep the chuck from unscrewing you need what I call a chuck keeper.  Why do I call it a chuck keeper.  Because it keeps the chuck from unscrewing when running in reverse.  Simple to make.  The headstock end is a very close fit to the ID of the chuck.  Then there is a long bar that goes through the spindle with a nut and washer that holds it in place.  As long as the threads on the nut are different from the 8 tpi on the spindle the two different threads work against each other preventing either from unscrewing.  Both ends are machined to fit the spindle to keep it centered in the spindle.   I did not come up with this.  Another forum member told me about it.
> 
> View attachment 359486


Why are the simplest ideas such genius? LOL.


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## RJSakowski (Mar 17, 2021)

homebrewed said:


> Sherline has a slightly different approach to an anti backlash nut design for their mills that might be useful for some other applications.  They have a nut in the "usual" place, i.e., underneath the table.  On the same axis, they have another nut on the exterior of the table.  It is knurled on the OD.  A thick washer, knurled on its OD is mounted on the exterior of the table such that the knurls are engaged, pretty much like gears.  To remove the backlash, the screw holding the washer is slightly loosened and rotated to tighten the exterior nut, removing the backlash.  Then the screw is retightened, locking the adjustment in place.  There is no fundamental reason that the two nuts in an anti-backlash scheme have to be immediately adjacent to each other!
> 
> I used a similar scheme on the X axis of my mini mill:  and now I can perform climb milling in steel, at least on that axis.  Backlash is roughly .001 inch and has has been easy to keep that way, since the adjustment is point is easily accessible.  The table design makes it relatively easy to mod the X axis in this fashion, not so much on the Y.


The fundamental reason for having the two nuts in close proximity  is that  it maximizes the effective travel of the lead screw. If the lead screw is sufficiently long enough, not a problem but if your dealing with a prior design, any additional separation decreases your effective travel.  In my particular setup, the nuts are separated by 1 inch and my effective travel is reduced from 5.1" to 4.1".


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## Larry$ (Mar 17, 2021)

The backlash compensating nut design or something similar is common on acme screws. I think it is on my lathe cross slide.


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## homebrewed (Mar 17, 2021)

RJSakowski said:


> The fundamental reason for having the two nuts in close proximity  is that  it maximizes the effective travel of the lead screw. If the lead screw is sufficiently long enough, not a problem but if your dealing with a prior design, any additional separation decreases your effective travel.  In my particular setup, the nuts are separated by 1 inch and my effective travel is reduced from 5.1" to 4.1".


The added nut reduced my effective travel by about the same amount.  As you said, it depends on the particular machine's design.


graham-xrf said:


> This is nice and simple in concept, and compact. You get me thinking. If you happen to have a Sherline, or can get get a picture of your mill mod knurled part, when you hqave opportunity, it would be much appreciated.
> 
> I do hate backlash, though for many, at least a small amount has that "feel", so you know from where to read the graduations. Backlash or not, if ever the tool has to move back, I always back up a tad further, then adjust forward to the place I want. A knurled nut "take-up" arrangement need only be adjusted occasionally.
> 
> ...


I don't own a Sherline mill (it was in the lab where I worked).  Also, the Sherline's table design is substantially different compared to my X2 mini-mill, so my rendition is different, but uses the same principle.  Here's a photo of my version:




The table on my mill doesn't have a flat surface for the external nut to press against so I added one, attached with two screws.  The nut passes through a hole in the plate, and was drilled with a 1/16" hole to accommodate a piano wire adjusting finger that was bent at 90 degrees.  It isn't glued in place, it must be free to rotate in order to allow adjustment.  It was inserted in a slotted piece that's used to adjust the backlash.

Before drilling the hole in the nut I threaded it onto the feed screw to mark the location of the hole.  I also faced the end of the nut that presses against the plate so it would bear evenly against the plate.

This and the Sherline scheme avoid the potential issue of applying radial force to the nut, which could cause binding or excess wear.  That's why I didn't use a grub screw to hold the nut in place.


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## RJSakowski (Mar 17, 2021)

I wouldn't recommend a dynamic backlash compensation system like the one on my lathe for use on a mill.  It works on the lathe because for the most part, cutting is done with infeed so the force exerted by the work on the cross slide and the force exerted by the spring  are in the same direction.

A mill must be able to exert substantial cutting force in both directions and a spring loaded compensation system would require a fairly stiff spring which would increase wear on the lead screw and nuts.


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## homebrewed (Mar 18, 2021)

Well I messed up in my description of my mill mod a bit.  I said that the nut passes through a hole in the plate I added, but it doesn't -- only the feed screw does.  The hole is a fairly close fit so the nut has a decent contact area on the plate.

Before I made the plate, I cut a template out of paper to double-check the clearances.


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## graham-xrf (Mar 18, 2021)

homebrewed said:


> Well I messed up in my description of my mill mod a bit.  I said that the nut passes through a hole in the plate I added, but it doesn't -- only the feed screw does.  The hole is a fairly close fit so the nut has a decent contact area on the plate.
> 
> Before I made the plate, I cut a template out of paper to double-check the clearances.


Thanks very much for the picture. That was a good mod.


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