Threading Copper Tube

[SteveW]

Guys,

Yet another set back with the club Sweet Pea: I had to remake one of the steam feed pipes from under the smoke box to the cylinder. Domestic 8mm HC pipe was ideal but nowhere thick enough once threaded at 5/16"x32. It did turn out to be the easiest to put a good thread on which set me up badly for my next attempts.

I got hold of some 1/4" internal thick walled tube but it was too fat. I found that I could spin it down to 5/16" dia in the lathe using a 8-9mm ER collette to hold it and a bar lever off the bed. It seemed to require little pressure to bring the size down (it was soften first). I used copper grease to lubricate it.

The first two attempts at threading failed. On the first the die rippled the thread and the second cut very under size. I tried various lubricants (except sour milk) but nothing really worked.

I finally tried a squirt of spray PTFE lubricant from Screwfix (UK) and it worked like a dream.

WARNING: I also made the mistake of annealing the final bit without first cleaning all the residue and ended up this the mum and dad of brain aches very shortly after. Whether the two events are related I don't know but on reflection: PTFE + heat....

[Shawki Shlemon]

Hi
Threading a copper pipe is hard at the best of times.If it need to be done then there is no way out . If it is possible I build the end with silver solder to give me more meat to work with and is better to cut thread. If not I use thick wall tube and as fine thread as possible , use kero for cutting medium ,open the die in the holder as much as possible,and when cutting the thread only back up the die to break the chips but as little as one need ,continue to the finish ,blow away the chips and very carefully back the die out .

[paul]

As a total noob I tried to thread some 4mm copper pipe recently and totally messed it up. It's so good to know it wasn't just me!! ;D

[ron]

Hi Steve
Two things I found out threading copper, the die needs to be sharp, ie brand new and the best lubricant is Fairy Liquid, I would also agree with Shawki about opening the die in the holder as far as possible.
Ron

[lancelot]

Very true , on the occasions I had to thread copper pipe, I used P.T.F.E.thread lub., I also used the thickest wall pipe possible to enable a good thread, The tailstock die holder needed to be opened out a little on diam. to allow for the Die. spreading out a bit more. take the cut slowly, clearing swarf as you go, at end of cut release Die. from holder and remove Die. by hand.
All the best for now,
John.

[SteveW]

Guys,

Re replies above... Yep did all that. New die, lubes (various), tail stock holder, max opening and it was still took three goes to get a good result.

The most successful bit was spinning the tube diameter down to 5/16". I also tried threading on the stress hardened (best result) and annealed, it ripped.

Chances are the best results would be by rolling the thread into the metal although maybe a bit tricky on tube.

[jackrae]

how about driving a plug into the end of the tube for the length of the threaded part. This will stop the tube " relaxing" under the die and may give better results.

[lancelot]

Hand rolling...Plug in end of tube...this rings a bell and brings back memories of a major repair I was involved with on a large stationary steam engine owned by the railway, at the time I was an apprentice, if I can remember the job was to thread a copper pipe of somewhere around 21/2'' diam. with a relatively fine thread,this was done with the use of ''Hand Chasers'' a multi point tool, used to cut the thread by hand , ...now some what of a lost art...control of lathe speed and skill in the use of the tool, produced a very reasonable thread, the threaded part was coated with sealant, screwed into fitting and if thread was in any way 'loose'' could be expanded using a set of rollers or taper drift.
Now I realise there's a big difference between ...2 1/2'' and 1/4'' but I believe hand chaser tools of 32 and 40 t/p/i are still available, the thing is, is it worth a try?
All the best for now,
John.

[richcarlstedt]

Instead of turning down the copper from 8mm to 5/16, cut a piece of copper tube longer than you need, and place it on a hard surface (bench top) and then place a clean flat piece of wood on top of it and start rolling the wood back and forth putting pressure on it. The tube literally becomes a 'bearing" between the flat surfaces.
Three things will occur, the tube will become VERY straight, and will get smaller as it converts diameter to length. It also will work harden to make threading easier.

If the tube slips because the surfaces are to slick, put a VERY fine emery paper(600)or polishing paper on one surface to engage the copper.

Rich

[SteveW]

Rich,

I didn't turn the tube down I 'spun' it down. I put the tube in an ER32 Collete with enough stuck out, put a small pile of wood on the far side of the lathe bed for a fulcrum and used a bit of 5/8" rod under the tube and over the fulcrum. Then with the lathe running and a bit of copper grease on on the tube I persuaded the OD down to the required 5/16" by simply lifting my end of the rod and moving it around. I was quite surprised how much longer it got.

[jackrae]

using the forming technique and a half nut you might be able to form, rather than cut, the thread. If so then it should be stronger, and harder, than a cut thread
Jack

[Phil Sutton]

Very interesting,but what about cutting an internal thread,ie with a tap-will the same apply ?or is this a whole new ball game?

Phil

[jackrae]

Hi Phil, now that's another interesting problem
You could try laying a length of lubricated screwed rod into the tube then "shrinking" the tube into the threads by swaging it down. I would imagine tube would have to be fully softened and you'd need some means of ensuring the rod remained central until the copper started to bite it - unless the tube started as a snug fit on the rod OD.
Then "simply" unscrew the rod from the finished threaded copper tube !!!
Just remember - nothing's impossible - just either difficult or impractical.

[SteveW]

Guys,

It occurs to me that this thread has evolved to rolling threads into the metal. In ME the other week was a picture of a machine rolling lathe lead screws and somewhere I've got a couple of thread dies with three threaded rollers spaced 120 degrees around the die body.

How these three rollers sync up is another problem but it must be successful enough for someone to make them. It's likely that if one was desperate enough to thread copper tube making a thread rolling die looks quite do-able

[Phil Sutton]

jackrae said:

Hi Phil, now that's another interesting problem
You could try laying a length of lubricated screwed rod into the tube then "shrinking" the tube into the threads by swaging it down. I would imagine tube would have to be fully softened and you'd need some means of ensuring the rod remained central until the copper started to bite it - unless the tube started as a snug fit on the rod OD.
Then "simply" unscrew the rod from the finished threaded copper tube !!!
Just remember - nothing's impossible - just either difficult or impractical.

Your not joking Jack....was it ever thus
Time for a rethink then.

Phil

[Pierre]

Hi All,
I've decided to build for my granchildren (?, who knows....this will be my first grand scle loco) one of the Sweet Pea. I need your advices and may be what to do an not to do.....Tender, saddle etc.......
WHO CAN HELP......
Any good advice and picture will be of the upmost help ...
Pierre