Coppersmiths Joint or Backing Strap??

[ron]

Hi All
I thought I would ask this on here rather than the Simplex board as it applies to other boilers as well.
When the Simplex boiler tube is opened out to form the outer wrapper it's too short so need extensions fitted, the drawing suggests two methods, either a strap silver soldered behind or a staggered 'coppersmiths' type joint. The second type looks as if it would make a neater job and cause less problems fitting the foundation ring but doesn't look as if it would be as strong as a strap, is the 'coppersmiths' joint still an accepted method of doing this or would I be better with the strap.
Ron

[waggy]

Ron,

The strap is easier to make and solder. I would advise rivetting the strap into place after cleaning and fluxing, it can't go anywhere if you do so. I have seen the parent and strap distort as you begin to heat the job, very annoying as they never fall back into place when the heat is uniform about the whole joint. The solder will flash beautifully around the strap and rivets, nice job. You can then reduce or remove the heads if you wish, up to you.
I may be too late suggesting this but - When you cut the tube to open it out for the firebox, mark and cut off centre to give you enough material to form one side of the box in one piece. You will then have to make a larger extension piece for the other side but at least then, there is only one joint to make. Fewer joints = fewer places to leak.

Waggy.

[greasemonkey]

Hi Ron
Personally, neither, I use a seperate piece of copper rolled to make the outer and avoid the need for the joint and potential reduction in water space. As Waggy suggests cut the tube of centre you will only have one joint to worry about. If you then make the joint as high up as possible you will not reduce the water space in the leg and also avoid any potential conflict with the joint coinciding with the stay spacing. You dont want to find that your stays should go down the middle of your joint.

Andy

[John Lee]

Ron,

As you probably know, a proper coppersmiths joint is done by feathering or hammering the edges of the sheet to about half their original thickness and then cutting slits at right angles to the edge. These are bent out alternately and mated with the edge to be joined. The flaps are then hammered close to give a good friction fit that can be sealed by silver-soldering. Note.. sealed, not necessarily fully penetrated by the solder.

Final finishing is by planishing (the posh word for bashing!) the joint surface flat, so that it is the same thickness as the parent metal sheet.

All this thinning and rethickening does not seem to me (I am not a pressure vessel inspector) to be a very good idea on a hot pressure vessel, and the very root of the overlaps must be a weak point. I'd check before you try it with an inspector, even though this is (was?) the skilled craftmans preferred method and is illustrated in older model boiler making books.

Or, pragmatism, nobody will see the joint anyway, go for a strap and work around it as advised above?

Regards,

John

[ron]

John, what he suggests as an alternative to straps is not a true coppersmiths joint, it's more a series of dovetails, the more I think of it the backing strap looks the way to go.
Waggy, I cut the tube in the middle to make the two sides the same, for some reason I don't like the idea of having a joint in one side only, there's no good technical reason why not that I can think of, maybe I just like symmetry? Thanks for your tip about riveting, that's probably saved a bit of hassle, I was going to try it without.
Ron

[baggo]

I think it may be my fault for Ron calling the 'strapless' joint a coppersmiths joint as I called it that in an email to him! I think the correct name for the joint intended is a 'checked' joint where the edge of the copper is cut into alternate 'teeth which interlock with each other. There is a 'dovetail' version where the interlocking teeth are shaped as in the familiar dovetail joint used in woodwork. Both give a much longer joint surface and result in a very strong joint. The joint shown for Simplex uses a combination of the checked and dovetail joint with the outer 'teeth' dovetailed and the inner ones checked. I can imagine this joint to be a devil of a job to make so I doubt if it gets used much!

John

[John Lee]

baggo said:

I think it may be my fault for Ron calling the 'strapless' joint a coppersmiths joint as I called it that in an email to him! I think the correct name for the joint intended is a 'checked' joint where the edge of the copper is cut into alternate 'teeth which interlock with each other. There is a 'dovetail' version where the interlocking teeth are shaped as in the familiar dovetail joint used in woodwork. Both give a much longer joint surface and result in a very strong joint. The joint shown for Simplex uses a combination of the checked and dovetail joint with the outer 'teeth' dovetailed and the inner ones checked. I can imagine this joint to be a devil of a job to make so I doubt if it gets used much!

John

Hmm slightly confused here. The traditional coppersmiths joint is cut as square pieces, but I really cannot see the difference with a sawtooth pattern. The weak point is still at the root (as in the root of a thread if you see what I mean)

Here is only silver solder and a bit of hope. That will be the weak point, providing you can accurately bash the rest to even thickness.

I would go with a strap, not the ultimate example of skill, but it has to be the strongest solution. You are adding metal. And anyway, nobody will see it ;D

The assymetric thing is a good idea; never thought of that.. Slightly disturbed, as Ron is, with the "Engineer" head on, about the lack of symmetry, but hidden under cladding so what the heck.

Baggo, a real "coppersmiths joint" is something I would not even attempt on a loco size boiler. I did one under the supervision of a very experienced instructor many years ago as an apprentice. It worked... at 1" diameter after lots of coaching, and I still have the finished piece now. Apart from the above strength considerations its a lost art, at least to me!!!!

Regards,

John

[baggo]

John - the edges are cut as interlocking squares, sorry, my bad choice of word again !

One point about the 'strap' joints that had occurred to me - I understand that the straps used for joining a rolled up boiler barrel now have to be on the outside so that they can be seen by a boiler inspector. Should this now also apply to those used for extending firebox sides etc

LBSC actually suggested using a form of the coppersmiths joint on boiler barrels. However, instead of flattening the copper afterwards he suggested just filing the overlapping squares flush on the outside!

When I made the combustion chamber firebox for my Flying Scotsman I avoided using a strap on the wrapper joint by using a sunken lap joint. This was made by putting opposite 'joggles' in the two ends of the wrapper using a tool made for fitting car body panels with flush joints.



This was easy to do as the wrapper is only 1/16" but it might be a bit difficult with 1/8"!

John

[steammadman]

nice illustration baggo, that is the proper way to do a joint in any metal.
at least that is what we had bashed into us at Doncaster as apprentices
almost 60 years ago.
We were told to but two edges together and then fit a cover strap ,either
inside or out is a " lazy coppersmiths " joint.
I personally have used the" tinmans" lap on a few boilers and never had
any trouble with leakes.

[John Lee]

baggo said:

One point about the 'strap' joints that had occurred to me - I understand that the straps used for joining a rolled up boiler barrel now have to be on the outside so that they can be seen by a boiler inspector. Should this now also apply to those used for extending firebox sides etc

LBSC actually suggested using a form of the coppersmiths joint on boiler barrels. However, instead of flattening the copper afterwards he suggested just filing the overlapping squares flush on the outside!

When I made the combustion chamber firebox for my Flying Scotsman I avoided using a strap on the wrapper joint by using a sunken lap joint. This was made by putting opposite 'joggles' in the two ends of the wrapper using a tool made for fitting car body panels with flush joints.


John

That looks like the perfect compromise John, providing that when you rivet it you leave the thou or two gap for the silver solder to flash through, and work out how you will get the flux in.

Good question about the firebox sides, I suppose thereoretically now the answer is yes if you use a strap. The problem with all this, is that is seems still to be down to the individual boiler inspectors subjective opinion. I hope one will pop up on the forum.

As for forming your joggle joint in thicker material, a made up bar tool as a former and an arbor press?? On reflection access in a rolled tube might be a problem. Do the same with two lengths of material and squeeze it up with nuts and bolts?

The LBSC method.. ahh yes..hmm. Pragmatic but

Regards,

John

[spurley]

I think the 'strap on the outside' idea of rolled barrels for boiler inspection purposes applies to steel/iron boilers. I believe that copper has a far better corrosion resistance than ferrous so is probably safe enough inside, and out of view!

I remain to be corrected by any boiler inspector here

Cheers

Brian

[baggo]

Hi Brian,

I read about the straps having to be on the outside on JJ's Britannia website. I presume someone at Modelworks must have told him about it ?

John

[havoc]

The joint Baggo proposes has a lot more stress in the "joggle" than using a strap. Does not seem a good choice for a boiler to me.

A strap on the outside can continue beyond the tubeplate, while one on the inside has to stop at the tubeplate.

[ron]

After all the hammering, bashing, heating and cooling a boiler gets during making I can't see a joddled joint causing any great problem?
Ron

[havoc]

This hasn't anything to do with what you did to make the joint, but it is proper to the form of the joint. The 2 short bends to make the joggle are points that concentrate the stress when the 2 halves of the joint are pulled apart as they are when the boiler is under pressure. Simple engineering issue.

[baggo]

I think I originally came across the idea of the 'joggled' joint in one of LBSC's articles. The main reason for using it in my case was to avoid having a thick strap on the bottom of the combustion chamber. There is only about 3/16" gap between the bottom of the chamber and the boiler barrel and I was concerned about the strap interfering with the water circulation, especially as there are 4 vertical water tubes running through the chamber.

However, taking on board havoc's comments, I did a bit of a search through ME and came across an article by H F Atkinson in ME issue 3814 in which he tested to destruction several types of silver soldered joint including a plain lap joint and a joggled lap joint. In all the joints tested none of them failed at the actual joint itself or in the metal near to the joints. The writer did comment though that both the lap joints tended to straighten out under the test.

Obviously anyone with any doubts should not use this type of joint instead of a strap joint. In my case though, the joint is in compression and strengthened considerably by the 4 water tubes either side of the joint so I do not forsee any problems. The finished firebox, combustion chamber, and tube assembly has been inspected by my local boiler inspector and he is perfectly happy with it.

Interestingly in the same article the writer carried out tests on flanged joints where the plates were hammered over a sharp edge and so had no inside radius and very little outer radius. Apparently out of five such joints tested no actual joint failed but in two cases the metal fractured at the point of the bend! This highlights the need to have a nice radius on the corners of any flanged plates.

John

[havoc]

I looked in our boiler guidelines, and the lap and joggled lap joint are not forbidden. But they are "not recommended". They advise the single strap (outside or inside) and the double strap. There is a drawing advising for the dimensions of the "straight dovetail" (don't know how you call that in english), but there is no more comment about it.

[stantheman]

I understand the term used generally for the 'straight dovetailed' is in fact a 'castlleated joint' ie as in castle ramparts.

I personally would prefer to see a well dimensioned butt strap used instead of this type of joint.

One of the issues I can see is in the actual cutting of the tongues (?) so that there is just the correct fit, then making sure there is a good penetration of silver solder along the entire length of this joint.

Am I correct in thinking that the AALS do not allow this typr of joint under their own boiler requirements?

stan

[baggo]

Hi Havoc,

As mentioned, my reason for using the lap joint was purely because of my concern about restricting the circulation. After the comments raised by your good self and others I probably would not use it again and instead stick to a plain strap. I would not have considered it for a barrel joint.

Stan - I agree that the castellated joint must be very diificult to get to fit correctly and I don't think I would have a go. I am sure I would probably b****r it up and waste expensive copper! It would be an interesting exercise though ;D

John

[stantheman]

I knew I had spelt that word wrongly, but could not reason out where, MS spellchecker did not offer any help!! Surprise surprise.

Stan.