Tapping a nylon hole.

[andrew lynerParticipant @andrewlyner71257]

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

[andrew lynerParticipant @andrewlyner71257]

My taps leave the M10 thread very tight.

[Brian WoodParticipant @brianwood45127]

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.

[egaParticipant @ega]

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 DaymanParticipant @jeffdayman43397]

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 ConnellyParticipant @martinconnelly55370]

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 WyattModerator @neilwyatt]

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 lynerParticipant @andrewlyner71257]

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

[andrew lynerParticipant @andrewlyner71257]

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

[Chris Evans 6Participant @chrisevans6]

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.

[BazyleParticipant @bazyle]

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

[Sam StonesParticipant @samstones42903]

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 yetParticipant @notdoneityet]

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 lynerParticipant @andrewlyner71257]

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 1Participant @ianjohnson1]

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 IoMParticipant @francesiom58905]

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 StonesParticipant @samstones42903]

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 HulmeParticipant @nickhulme30114]

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

[Robin GrahamParticipant @robingraham42208]

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 yetParticipant @notdoneityet]

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.

[SillyOldDufferModerator @sillyoldduffer]

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

[andrew lynerParticipant @andrewlyner71257]

Posted by not done it yet on 29/11/2019 23:16:05:

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.

Absorption of water would only affect the surfaces as it would take years, probably, for water to permeate throughout the plastic. I'd imagine the treads would swell 'into' the hole but the body would stay much the same size.

Coopers work on the principle that everything gets bigger when you heat an iron hoop – including the hole.

(I didn't get the balloon bit. Sorry)

[Pete RimmerParticipant @peterimmer30576]

You don't need a super-hard tap to cut a thread in nylon. Turn a slightly over-sized thread in something like EN8 then mill/grind some flutes in it and use that to chase your thread slightly bigger.

[Nigel Graham 2Participant @nigelgraham2]

I find the greater problem with Nylon is not cutting the thread , as long as that is fairly coarse, but drilling the tapping (or any-purpose) hole in it.

The steel drill-bit become hot and expands, so binds. The only solution really, is peck-drilling with a short time for the tool and plastic to cool between pecks (I leave the lathe running).

Twist-drills can also dig into the plastic if you are not careful, adding to the risk of binding.

[Sam StonesParticipant @samstones42903]

Drilling (and machining) especially thermoplastics can be problematic if the drill/cutter is rubbing.

This heat can be enough to expand the plastic, and even take it above its melting point.

Because the heat is local, it would be (perhaps) the one exception where the hole reduces in diameter (as has been discussed here … "Reamer size Questions"

**LINK**

Taken slowly with very sharp cutters (sharp as for wood), and the appropriate coolant, should be the aim.

Localised heating/melting can be a source of stress and potential failure, as in stress cracking and crazing.

For comparison, the coefficients of thermal expansion of certain thermoplastics (Nylon, polyethylene, polyacetal) can be almost an order of magnitude greater than steel.

See here …

**LINK**

[Author]

Posts