How to mold a Nylon nut?
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  1. #1
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    Default How to mold a Nylon nut?

    I have a project to do in my spare time(if that ever happens).We have a bunch of linear actuators on our presses that for the most part are trouble free.They cost about $3,600 ea. So every one I salvage can add up quickly.

    Our newest press has decided to break the screw end off several of them.Since this has never happened on any or our older presses I have been unable to find a definite cause.The only difference in them is the new press has a $14,000 compound air cylinder with a built in valve body in stead of a $500. standard cylinder.

    I quizzed some of the factory personnel and they have never heard of any else having the same problem.The press is out of warranty.I told the factory rep I can't fix bad engineering and consider it a defect related to the new cylinders.

    Funny although nobody else has this problem they have a fix for it!They sent a couple boxes of parts in and are waiting on down time to install.
    The broken part is available @1,100 ea. I think I can make them for a couple hundred. ea.https://connect.xfinity.com/appsuite...&callback=yell
    Attached Thumbnails Attached Thumbnails 20190812_111708.jpg  

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    Are those annular groove or a thread on the ID of the aluminum (?) cylinder. If they're threads, why not just machine the nut out of nylon or acetal and thread it in with some primer and red loctite.

    If they're annular grooves, it might be worth looking at Moglice. I know some people have used it to cast new nuts.

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    As you can see the material is either hot injected or cold mixed and poured in. The material smells like Nylon (burned) but the color is not familiar,may be a friction modifier.I can make the hex tube and increase the strength or get hex tube and modify.I can buy Nylon6/6 two part make the mold cap for the end.I'm not sure what to use for a mold release on the screw. Since it is a thermo plastic I have a lot of Nyloil and Nylon with moly/s added.What about melting and pouring it in?

    Doing it cold would obviously be simpler.The load is 10,000N.

    This is a homebrew job not a production one so although all the pitfalls may apply I only have to make 3 or 4. Has any body done something like this ?

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    The grooves are actually the same pitch thread as the other end that the rod end screws into,It is locked by cross drilled holes at the capped end and the thread.The load is bi-directional.

    I suppose I could machine the nut as you suggest and heat the Aluminum tube and screw it in.Gluing Aluminum and plastic is tricky. But by machining I could get my choice of material.

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    You can't hack mold nylon. It has to be perfectly dry before melting. Unfortunately, your choices of grades and modifiers are much smaller if you buy an extruded bar to machine your own. I think a nylon may be your best choice for this application.

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    It might be easier to just fill the end, cold or hot, and drill and tap it.It's a 16mx4 Trapezoid single start.Maybe single point it,it's not that long.

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    Quote Originally Posted by ratbldr427 View Post
    I quizzed some of the factory personnel and they have never heard of any else having the same problem.
    I swear every OEM must have all new hires repeat that phrase constantly for their first week of employment. I've been in a green equipment dealership and been given that line by the service manager. He didn't know I came through the shop and saw they were working on another piece of equipment with the same problem.

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    What breaks? Aluminum? Maybe they are clueless about so-so and actually strong aluminum - or equally clueless about over a dozen stress riser grooves needing to be just so in design and execution

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    I think delrin would be easier than nylon.

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    John, The hex aluminum with nut is very weak in sheer.The crest of the rod end threads have only .040" material to the hex flats.However the actuator should only ever see tension or compression loads in operation.Rated at 220lbs.

    I thought that it(the actuator) failed first, however there is more to the story.The 35mm pin that the air cyl mounts to broke also.Cyl(two mounted back to back with electronic valve body in between) is100mm dia with 120psi and completes its cycle in under 40ms.I now am pretty sure the pin broke first and the actuator was collatoral damage not the other way around.The pin is cantilever mount to the frame.It has no radius at the flange instead an undercut.The replacements I made have a generous radius.

    The pins may have been stressed near failure on the older presses(never a failure in 20years) and the new cylinder may have pushed them over the edge. I didn't rummage through the boxes of parts to see what the fix is,be interesting.
    Like I told the rep"To paraphrase Forrest Gump, I may not know why it broke but I know bad engineering when I stumble on it"

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    Quote Originally Posted by gbent View Post
    I swear every OEM must have all new hires repeat that phrase constantly for their first week of employment. I've been in a green equipment dealership and been given that line by the service manager. He didn't know I came through the shop and saw they were working on another piece of equipment with the same problem.
    Usually this statement is accompanied with a two-foot-high stack of bibles so the person uttering the magic incantation (we've NEVER seen that before!) can place their
    right hand atop the stack for verification.

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    So that's why the rep asked if I had a bible handy!

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    In then old days cast in place babbit. I have read of making the nut in acetal with small hole. Cut in half the long way and squeeze it tight to mold the threads in. I think there may be heat involved.
    Why not buy a bronze nut and machine the outside to match?
    Bill D

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    Like Bill D mentioned in the prior message - forming the nut out of acetal.

    Not trying to upset anyone by linking to a hobby machining website.

    First link is the original discussion but without images - the original poster deleted them. There are **30** pages in the thread - lots of conversation. The second link is is a compact one page discussion of how to form your own nuts, with images. Making Acetal leadscrew nuts the easy way Making Acetal leadscrew nuts... DIY - Denford Software & Machines

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    I did some more head scratching & investigating and came up with the most likely cause of failure.I found one of the actuators that had some plastic deformation on the flats on one side.It was ready to be pulled in two.So when the rod end pulled the hex apart at the end of the rod end threads(all broke there) the rod end flipped over and jammed against the frame and then the air cyl had enough pressure plus the added leverage to break the steel mount pin.

    The factory rep came in and installed the fix.The piston end of the cylinders have adjustable metered out exhaust silencers.The rod ends have some crappy foam silencers with no control.That apparently effected the speed in that direction enough to fatigue the hex tube.The fix was some standard screw in non adjustable silencers(don't know if they have a fixed orifice in them or not).

    As for the actuator repair; the hex tube doesn't have to be aluminum,there are no dynamic loads to contend with so weight is no concern.Although I would like to use a nonmetal nut I think I am going bore some hex 12L14 steel and insert a phosphor-bronze nut in one end and reduce the thread size of the rod end on the other.The strength will be many times the currant set up.I found TR16x4 taps for under $50.00 and metric hex steel 4' for under $100.00.So I can dick around and make 4 for less than $200.ea.Beats the $1,100.00 plus freight ea the manufacturer wants

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    I downloaded that thread when the photos were still available. If anyone wants the photos, I can make the pages available for download on my Dropbox account.



    Quote Originally Posted by aribert View Post
    Like Bill D mentioned in the prior message - forming the nut out of acetal.

    Not trying to upset anyone by linking to a hobby machining website.

    First link is the original discussion but without images - the original poster deleted them. There are **30** pages in the thread - lots of conversation. The second link is is a compact one page discussion of how to form your own nuts, with images. Making Acetal leadscrew nuts the easy way Making Acetal leadscrew nuts... DIY - Denford Software & Machines

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    The best is when they drop the "nobody else" line followed by a recall months later.

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    That's what this tech told me.At first I thought he was just playing dumb,however after further conversations I realized that he wasn't playing !


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