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Making a Tap to Thread Plastic?

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SillyOldDuffer18/12/2018 16:54:05
3871 forum posts
779 photos

In another thread I was helped to find some PE-Xb/Al/PE barrier water pipe needed to make a loop antenna. It is a 16mm x 2.0mm plastic pipe with an internal layer of aluminium about 0.3mm thick.

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The antenna loop is about 1m in diameter with a gap at the bottom. At each end, I need to make a very good electrical connection.

loopant.jpg

One way is to hack or burn off the outer layer of plastic and put a clamping bolt through it. Messy. I would like to make a neater and weatherproof job by tapping the inside end of the loop and screwing a purpose made aluminium terminal bolt into the pipe. The terminal bolt will be drilled and tapped to take an M4 brass screw that actually terminates the cable connection:

ant_term.jpg

I made a special tap to cut the thread into the pipe. The Mark 1 version is made out of yuk steel 1/2" conduit pipe. One end is the tap, the other is tapered to clean up the plastic pipe end after hacksawing. The tap is 1.0 mm pitch.

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In theory the steel tap cuts a thread through the inner plastic pipe and penetrates the aluminium sheath. Then a matching Aluminium terminal can be screwed in to make a good corrosion resistant contact.

My problem is that the tap isn't working well enough. Although I get a working thread there is no electrical contact with the Aluminium layer. Not sure why not. Perhaps the plastic tends to deform away from the tap rather than cutting cleanly?

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Having got this far, I admit I don't know what I'm doing. To save me experimenting endlessly with home-made taps can anyone advise on how best to cut threads in plastic that will expose the aluminium layer?

Thanks,

Dave

John Reese18/12/2018 17:07:39
678 forum posts

Try cutting the inside plastic away to expose the aluminum before tapping.

Another approach would be a compression fitting on the OD of the tube. Remove the outer plastic sheath and the aluminum should pass the stop ring in the fitting. Make a tapered plug to drive inside he tubing to expand the tube so the aluminum contacts the ID of the fitting. The fitting will likely be brass so you will want to use conductive grease between the brass and the aluminum. Cap the male end of he compression fitting with a pipe cap. The cap can be drilled and tapped for a screw terminal.

Grizzly bear18/12/2018 17:15:58
195 forum posts
6 photos

Have you considered the plumbing dept.?

0.5" / 15mm olive + compression joint, or a piece of copper pipe?

Good luck, bear..

David Jupp18/12/2018 17:17:10
646 forum posts
16 photos

The tap will need to be very sharp. It might help to put the pipe in the freezer for a while to stiffen it up prior to threading (reduce the tendency to smear around the cutting points).

Are you using a tapered thread on the plugs?

Edited By David Jupp on 18/12/2018 17:17:26

Neil Wyatt18/12/2018 17:27:24
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15589 forum posts
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Soft solder a wire onto the outside.

Abrade to a shiny finish

Cover with 3 in 1 oil to stop oxide film forming

Tin with solder under the oil, scraping with the end of the bit

Solder wire to tinned patch

Les Jones 118/12/2018 17:30:20
2064 forum posts
141 photos

I would use a different approach. I would strip off the inner plastic coating to expose the aluminium and make an expanding bush. It would be a short piece of tube with some saw cuts partly along it's length that was pulled onto a tapered bush with a nut and bolt. (Or the bush could be tapped.) Another way that you could try is to make a bush with a hole through is and tap it but only with a taper tap and not all the way through. Put some saw cuts allong it's length from the end that was not fully threaded. When a bolt was screwed in it should expand it when it met the tapered part of the thread.

Les.

duncan webster18/12/2018 18:08:23
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1918 forum posts
43 photos

If this is going to be exposed to the elements you need to be careful about galvanic corrosion. Aluminium to copper is 2v difference so I suspect it would rot like mad unless you keep it bone dry

Neil Wyatt18/12/2018 20:22:31
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15589 forum posts
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For my yagi I 'potted' all the connections in silicone adhesive.

SillyOldDuffer24/12/2018 16:03:00
3871 forum posts
779 photos

Reporting back, I tried all the advice and got a result!

Deep freezing the pipe hardens the plastic and the tap does cut better as a result. Unfortunately the pipe quickly warms up and the advantage lost almost immediately.

Even with carefully sharp taps (these are essential), I found it is necessary to support the pipe with a collar. If not supported the pipe walls expand away as the tap bites. Even with support the tap still didn't cut the plastic inner cleanly and if made big enough would not enter the pipe directly. I dealt with that problem by using two taps, a small one to create a pilot thread followed by a larger to penetrate the Aluminium. Then to get a good contact, the Aluminium plug has to be made slightly larger again and forced in with a spanner. It helps considerably to remove some of the inner plastic with a Dremel.

The approach works but it is very hard work, prone to bend and doesn't solve the next-stage issue that brass and aluminium are prone to electrolytic corrosion. Also I wasn't satisfied that the Aluminium inner pipe and plug were making a really good contact. Threading left a lot of plastic smeared over the aluminium inner and electrical contact is only made over parts of the thread. As Aluminium oxidises easily I wasn't confident that the joint would last.

Reaming out the inside of the pipe to get a clean electrical contact is possible but it takes ages. I also spoilt a couple of attempts with the Dremel because it removes aluminium faster than plastic when you get to it and the Aluminium barrier is very thin. At this point I decided not to make an expanding plug. I'm certain it would work, but it's more error prone than a screw plug and it would need a permanent collar to stop the flexible pipe creeping off.

I left Neil's suggestion till last because I don't like soldering Aluminium foil and feared melting the pipe. Not a problem in practice, Neil's method is straightforward. I exposed an area of Aluminium with a sharp Stanley Knife, cleaned it to bright silver by scraping with the knife edge, applied 3-in-1 oil to stop oxidation, and tinned with a 140W solder gun. I used old-fashioned 60:40 lead electronic solder rather than the modern tin variety. Once the Aluminium is tinned it is easy to solder copper wire to the pipe, job done!

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No melting to speak of and the brown mess is only cooked 3 in 1 that cleans off. The pipe bends nicely into a perfect loop.

Unfortunately nothing is ever easy! The 16mm plastic pipe doesn't fit my standard water pipe clips and the 2.4m plank I was going to use as the support turns out to be only 2m long - too short to finish the antenna today.

Lesson learned : simple solutions are usually the best!

Cheers,

Dave

Michael Gilligan24/12/2018 16:10:05
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12701 forum posts
551 photos

Too late now, but ... a simple alternative might have been to attach a solder tag, using a screw. [either self-tapping, or into a shaped nut]

Sorry; I thought you were determined to use an internal connection, so I didn't mention it.

MichaelG.

Neil Wyatt24/12/2018 21:47:20
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15589 forum posts
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It amazes me how few people know you can solder to aluminium. I was sceptical when i was told how, and surprised how well it worked. Great for bonding a wire to a chassis.

Neil

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