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Tapping a nylon hole.

My taps leave the M10 thread very tight.

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andrew lyner25/11/2019 13:56:28
178 forum posts
2 photos

I decided to make an M10 thread in a nylon block to take an aluminium screw.

The nylon is very tight, despite squeezing my M10 die in as far as it would go for the male thread.

Is there a bodge that I can do to loosen up that nylon thread? Perhaps some abrasive run between the surfaces - uo and down many times? Could I do something to the leading edges of an M10 tap? I'll try anything once.

I also (not surprisingly) have been getting stringy edges and rings of nylon which I have to tear off. I have tried various tool shapes and I have read that slow and as sharp as possible is the right way. But, at the moment, the results are all a bit random.

Edited By andrew lyner on 25/11/2019 16:43:02

Brian Wood25/11/2019 14:36:12
2188 forum posts
37 photos

Andrew,

Nylon is notoriously difficult to thread successfully with things like taps and dies. It tends to expand when a tap is run into it and shrink with a die, only to return to 'size' when the pressure is off. It is fibrous in nature which is why that is difficult. It will mould with heat but the conditions need to be well controlled

I also would avoid any kind of abrasive treatment to recover your situation, it will only become embedded and then wear whatever is threaded into it later like an everlasting abrasive lap.

I suggest you replace it with Delrin which is much more forgiving rather than flog on with a hopeless situation.

Regards Brian.

ega25/11/2019 14:52:58
1714 forum posts
150 photos

If you must use nylon you could try re-tapping with a wooden spill in one or more of the flutes, the idea being to encourage the tap to cut oversize (or screwcut the male thread to fit).

Jeff Dayman25/11/2019 15:41:19
1818 forum posts
45 photos

Just a thought - could you sawcut the nylon block parallel to the thread? This would allow the block to "open" the thread. Maybe the nylon block could be enclosed in an outer aluminum knob to hold its' tightness setting with a setscrew.

Martin Connelly25/11/2019 15:53:33
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1369 forum posts
159 photos

A helicoil tap for M10 may cut the thread to the required size if you have one. It is the same pitch but larger diameter than a normal M10 tap, test it on something first or only cut partway into the hole to ease the pressure.

Martin C

Neil Wyatt25/11/2019 16:32:20
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Moderator
17902 forum posts
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You could try boiling it in water for 20 minutes, then recut the thread, then leave in a warm place to dry for several days.

In theory the water will expand the piece, but it may make the thread swell enough to allow a bit more to be taken off.

Neil

andrew lyner25/11/2019 16:51:35
178 forum posts
2 photos

Thanks. I will try Delrin and see how I get on.

andrew lyner25/11/2019 17:56:31
178 forum posts
2 photos

@Neil I like the idea of boiling. I could try that with the existing part I'm half way through making.

Chris Evans 625/11/2019 20:05:20
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1663 forum posts

Boiling in water should stabilise the part. Nylon absorbs moisture (yes really) we used to boil injection moulded parts to speed up the post moulding inspection of sizes.

Bazyle25/11/2019 20:55:46
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5226 forum posts
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Depending on the shape you could squeeze it in a vice a bit and recut, then rotate and repeat.

Sam Stones26/11/2019 00:33:16
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746 forum posts
296 photos

Andrew,

I haven't tried this with nylon, but machining (truing) polyurethane rubber skate wheels came out better after they had been in the freezer for a while.

I could imagine that, as with most materials, nylon will shrink further, will be harder, and may tap to a marginally larger size.

I could also imagine under such cold conditions, a new (very sharp) tap could either shave a whisker off the existing threaded hole, or indeed tap a larger hole from scratch.

Watch out that you don't generate too much heat in the process.

Although I've machined nylon on a number of occasions, I too prefer machining polyacetal (Delrin).

Sam

Neil,

Perhaps the thread pitch will affected, but surely the hole will also increase in diameter at higher temperatures?

not done it yet26/11/2019 08:58:48
4651 forum posts
16 photos
Posted by Bazyle on 25/11/2019 20:55:46:

Depending on the shape you could squeeze it in a vice a bit and recut, then rotate and repeat.

Good idea. If it is round, a jubilee clip, or similar might even suffice?

andrew lyner26/11/2019 15:46:53
178 forum posts
2 photos

I had some success with this. I just squeezed the work in a vice and re-tapped, as advised. The screw, which is for adjustment, is now finger tight. I suspect things may swell up again because there will be oil around. But I can re-do the tapping so I am happy. I have also ordered a bit of Delrin to see how much easier it is to work with.

This has been yet another rewarding experience, helped by the 'brain' that is ME's knowledge and skills.

Ian Johnson 126/11/2019 19:34:22
265 forum posts
81 photos

Try a larger size drill The normal M10 tapping drill is 8.5mm, try 8.8mm, if the Nylon swells up it may swell to the correct size.

I drill a larger hole when tapping stainless steel, usually the next size up size drill, as long as you get 75% contact everything will be fine.

It works the other way round for using dies too, just make the outside diameter a few thou smaller. Less stress on the dies and taps is always a good thing.

Ian

Frances IoM26/11/2019 20:56:09
767 forum posts
26 photos
just use delrin (other trade names available) - takes threads very well just use sharp cutters, dies + taps - expect your lathe to be covered in a snake like fine swarf that wraps itself around everything; other than that very pleasant to handle.
Sam Stones26/11/2019 21:27:09
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746 forum posts
296 photos

Andrew,

I get concerned for those handling polyacetal (POM, Delrin etc.) for the first time.

While I fully agree about how easy it is to machine, do not be tempted to burn the swarf. As with other polymers (e.g. PVC), the fumes are very noxious. You'll know (and remember) if you get a whiff.

Please see also ... **LINK**

Good luck.

Sam

Nick Hulme29/11/2019 21:42:27
743 forum posts
37 photos

I machine tap Nylon with spiral flute taps, the threads have enough resistance not to shake loose but are not difficult to tighten.

Robin Graham29/11/2019 22:15:05
709 forum posts
167 photos
Posted by Sam Stones on 26/11/2019 00:33:16:

...

Neil,

Perhaps the thread pitch will affected, but surely the hole will also increase in diameter at higher temperatures?

That worried me too! Imagine a nylon part having an internal thread with a perfectly fitting nylon screw in it. Boil them up, they both swell to the same extent, and still fit together perfectly. But your unswollen tap is now smaller than the nylon screw...

Robin.

not done it yet29/11/2019 23:16:05
4651 forum posts
16 photos

There is a subtle difference between coefficient of expansion and increasing the volume by absorbing water.

Somehow I think heating the two would retain the clearance, but swelling both by adding water (fluid) would jam the threads together. Consider it with a couple of balloons and increasing the volume - inner will only swell outwards, but the outer will expand inwards as well.

SillyOldDuffer30/11/2019 09:41:16
5787 forum posts
1232 photos
Posted by Nick Hulme on 29/11/2019 21:42:27:

I machine tap Nylon with spiral flute taps, the threads have enough resistance not to shake loose but are not difficult to tighten.

Good to know it can be done, but talk of Nylon changing size when wet has left me wondering how reliable nylon to metal screw-joints are under load?

Taking stuff apart, I've seen many examples of parts screwed direct to plastic where the load is tiny. But it's more common to find metal screws engaging with brass bushes moulded into the plastic.

I don't remember seeing a load bearing joint made by screwing metal direct into any type of plastic. Where the joint needs to be strong, more likely to find a metal rod bolted through plastic (or wood) so the weight is taken by a washer and the threads are metal to metal.

Dave

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