PDA

View Full Version : Having Trouble With Acetal Nuts



Jeremy_BP
08-28-2011, 10:51 PM
Hello all,
I'm trying to make some acetal leadscrew nuts as described here: http://bbs.homeshopmachinist.net/showthread.php?t=43645

I'm using a piece of 1" round delrin with a 15/32" hole drilled in it for 1/2"-10 ACME screw. I cut it in half, put it in a vise around the screw and begin heating the screw with the heat gun. I pre-heat for a few minutes, then slowly tighten the vise a tiny amount. I give it a minute to heat, tighten a tiny amount, repeat. The problem I'm running into is that the delrin nut deforms into an oval before it threads all the way. In fact, I can't even get the nut to fuse into a single piece. I'm worried with a larger hole, I'll not have deep enough threads. Any ideas would be greatly appreciated.

Evan
08-28-2011, 11:15 PM
You don't have enough heat or high enough temperature or both. Also, don't aim the heat at the delrin, only the metal. One way to prevent that is to make a shield from flashing with a hole in it.

darryl
08-28-2011, 11:34 PM
Offhand, it sounds like you need to remove more material from each half so that more material can melt before the cut sides of the nut come together. You would get this same action if the central hole wasn't bored as large- there would be more material melting and squishing out into the gaps between the halves. Without this happening, I can't see you getting the pieces 'welded' together.

Jeremy_BP
08-29-2011, 12:08 AM
Thank you for your responses.

Darryl: I tried before with smaller holes, and worse results. So far 15/32 has given me the best results.

Evan: have been aiming my heat gun at a point about 1.5" away from the delrin. The outside of the delrin is cool to the touch, the rod would burn me.
My heat gun is hot enough to change the color of the rod, so I have a hard time imagining that I have too little heat, although the more I think about it the more that makes sense. The next time I get time in the shop I will try a small torch (and god ventilation!) to see if that helps.

Another thing that may be causing trouble is the relatively wide bandsaw blade I'm using. I'm probably taking out about 1/16" in kerf. I will also try switching to a japanese handsaw with an extraordinarily fine kerf to see if that helps.

darryl
08-29-2011, 12:53 AM
Hmm- I wonder what happened with the smaller hole- as long as you took long enough for the heated rod to soften enough plastic, it should have squirted well enough to fill the threads and the two gaps. If it didn't get that far, there either wasn't enough heat soaking into the plastic, or the plastic you're using isn't right. It does take awhile for the heat to penetrate, as Evan suggested. What else could be happening- is the threaded rod losing heat to the air- maybe try not having much rod out the other side of the plastic, and if you can insulate the outer end of the side you're heating, there will be less heat loss. Don't know how much difference that will make, but you will have to proceed carefully to get the rod and the plastic up to temperature without overheating the part that you apply heat to.

As far as the thinner kerf, I think that will be going in the wrong direction. The two halves may touch before there's enough formable plastic to fill the threads, let alone flow a bit into the gaps to weld it together.

Evan
08-29-2011, 12:56 AM
Are you certain the material is acetal and not HDPE, Nylon, PVC or ABS? All look similar and are available in similar colours.

djc
08-29-2011, 02:33 AM
I'm using a piece of 1" round delrin...

Could your problem be due to the fact that your stock is _round_?

If you put two hollow semi-circles in a vice and squeeze, of course they will deform and go oval.

As I understand it, the original method used a square/rectangular split block that was then made circular _after_ the threads were formed.

You could try making a confining outer split sleeve out of metal and see if that helps.

Evan
08-29-2011, 06:03 AM
Oval isn't a problem. The lack of bonding is. It doesn't sound like it is melting correctly. After the nut is made it needs to be skimmed up to what ever shape you want it to be anyway.

loply
08-29-2011, 08:11 AM
Jeremy, I had this exact problem recently when trying to make acetal nuts, in my case I was doing a 10mm x 1mm conventional thread (not an ACME), and I think there wasn't enough contact area between the acetal and the threads to transmit the heat.

What I ended up doing, which worked really well, was to make a metal sleeve for outside my derlin rod, and I cut it in half lengthwise. I then cut the derlin in half lengthwise and put the two pieces inside the metal pieces (if you see what I mean).

I then heated the derlin directly with a paint stripping gun until it was clearly melted all along the bit I wanted to thread. I then laid my threaded rod onto one of the pieces, whereupon the derlin immediately stuck to the threads, and then picked up the other piece and plonked it on top too.

I then picked it up and popped it in the vice. The metal sleeves obviously kept the melted derlin intact.

Because they were very highly melted they stuck together really well and formed a very nice thread.

Jeremy_BP
08-29-2011, 09:48 AM
Thank you all for your replies.

The delrin I am using is this: http://www.mcmaster.com/#=dtv2p9

It is definitely possible that I am losing too much heat to the air. I have a scrap piece of the same screw I'm making the nut for that I'm using for making the nut. It it about 8" long, and the nut is about 1" long. I will try trimming it down to about 4" so less heat escapes. I do believe that I am not getting enough heat in, so I will start experimenting with that first.

Unfortunately, it could be a while until I get some time in the shop (school and all), but I will post my results so hopefully others can learn from my mistakes.

Evan
08-29-2011, 01:24 PM
One more item, buff the thread on the former to a good polish. It will make it easier to remove. I have also tried using spray on teflon coating and that works well but does present a fume hazard.