I would roll the conical firebox section from sheet. I would roll the rings from copper bar. Brass is unacceptable. Yes all rivets must be sealed with silver solder. You could use copper elbows for the tight bends. I have used 1/4 elbows with 1/4 pipe. As for the order of assembly, I would have to stare at a bigger diagram for a while to decide.

Using the lowest temperature available throughout would be easiest. If the joints do not have too big gaps, it is not a disaster if they melt again in subsequent heats so long as they are fluxed first. If the gaps are too big, the solder might flow out of the joint and this is a drama. Close sliding fits only, no more. If you are not experienced, do lots of test joints.

Just my personal opinion but I hate to see models built to run on air only. It defeats the whole purpose of a steam engine, with the smoke, coal and smell being the main appeal, but your model, your choice.

For a start I would not make as complicated as that – for one thing you don't need those water-tubes in the firebox on a model. Nor do you need copy the original, bolted-together design (which I think is hidden under the cladding anyway.)

For the next I would strongly advise, if you have not already done this, buying books on miniature boiler-making and silver-soldering. have a look in the Tee Publishing lists, particularly the Workshop Practice series. They tend to concentrate on locomotive-type boilers but the fabrication methods are all much the same.

Then (we've not even gone into the workshop yet )…

That drawing looks as if copied from a diagram of the original, which was designed for very rapid steam-raising to suit its purpose. Simply scaling full-size boilers in detail to small model size rarely works well, so some compromises are necessary for assembly methods and indeed actually making steam. So, prepare new drawings of the boiler in miniature, to normal model-engineering boiler practice, but fairly representing the original externally. Take especial note of dimensions, for a matter I will come to…

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Why do you need those rings? Copying the full-size, with the 2-part boiler shell bolted together? That complicates things, but anyway, if you are going to build the fire-engine around the boiler, they would be hidden by the lagging.

If you do still want them, you are best making them as dummies to represent the prototype, but having no contact with the inside of the boiler. You would have to cut them from plate, and flange them by the normal methods in making miniature copper boilers. They could be soldered to the shell with a fairly-high melting-point soft solder, since they are not doing anything structural. Don't use through-fastenings to the shell, for them.

The rings were riveted on the originals, yes, but these were steel shells whose riveted joints were fullered and caulked steam-tight. Rivets are used in model boilers normally either as stays or as assembly-aids, and in both cases are silver-soldered along with the main jointing anyway.

If you fit those dummy bolting-flanges you could use brass sheet; but do not use brass on the internal surfaces of a boiler where it is exposed to high-temperature water and steam, as it is likely to decompose (the zinc dissolves out of the alloy, I don't claim to know the chemistry).

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That firebox is probably more easily cylindrical with little or no effect on performance; but it will need to be of fairly thick wall because a cylinder (and presumably, truncated cone) is much weaker under external than internal pressure.

A reasonably deep crown-plate flange inside the firebox shell will add to the hoop-strength; that central flue, fitted in an outwards-flange in the crown-plate, will act as a longitudinal stay.

You might find copper tube of appropriate diameter for it and indeed for the shell, with some compromises on scale to suit what is available. Otherwise it's rolled shell and lap-plated, silver-soldered joint. Though if you want you could roll it the firebox conical without much more difficulty – after all, some locomotive boiler have tapered boiler barrels often copied in miniature.

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That pipe inside the flue is a super-heater. In full-size they had room to form it like that, but in a model you would almost certainly have to make a return-bend to join two lengths of pipe. The usual, in model locomotives, is a "spear-point" of or bronze, with two holes at converging angles to take the ends of the tubes; held in with high-melting-point silver-solder.

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Order of assembly: the firebox and flue unit, assemble that into the outer shell which has already had its various bushes or mounting-pads for the fittings, then the foundation-ring and top tube (or flue) -plate.

Note that " then the foundation… " .

Because…

The fly in the cutting-fluid here, is the major detail you have not shown: the fire-hole. I know the, Merryweather boiler because a friend used one for a freelance narrow-gauge loco, and on that example and as far as I know all Merryweather boilers, the firehole is in the side. So it has to pass through the water-space round the firebox.

I must admit I had to think hard, but this is how I see it:

The fire-hole is a short tube of length is the width of the water-wall at the chord formed where it penetrates the wall, plus the sum of the two plate thicknesses. You would have to draw it out, because all its circumference has to penetrate fully, both walls.

However, it also constrained by the assembly; hence my point above, about drawings.

Size the innards so when the fire-hole tube has been soldered into the firebox, that assembly, with the flue, will slide into the boiler barrel with the foundation-ring not yet fitted.

Move it across the shell so the firehole tube can poke through its hole in the shell.

Then fit the foundation-ring and top-plate, ensure all aligned correctly, and silver-solder them together.

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You don't really say how big this is, but you will need different melting-point silver-solders for the different parts, and you will need put a lot of heat into the metal as rapidly as possible.

Everything needs drawing first, dimensioned carefully, so that it can be fitted together.

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Or just run the model on air… I think fully-working Merryweather steam fire-engines have been made, so it's feasible. You need translate full-size boiler-making to its model-engineering version.

Original ME article for those interested

As said a simpler design from 4" tube with a tube for the firebox and some simple cross tubes would make life easier and give a running engine. Add the skirt and bottom flange as dummies for looks, rivit detail can also be done as dummy ones if wanted.

Nigel, the copper boiler does not use bolted construction, the flange at the top of the "skirt" is a dummy and flue is soldered to tubeplate. The 3" STEEL boiler to Haining's design does have the bolted flange above the skirt and also around the flue that you mention as well as 43 cross tubes for good measure!

Although not a fan of Julius boiler drawings his 3D images do make it a bit easier to see what's going on and he has not departed too much from the ME design, drawing scan be downloaded from part way down this page it's the 1908 engine.

On the same page that Jason linked to above with the drawings (link here again for convenience), there is also a link to a build description, here. It is immediately to the right of the drawings' link.

The build is described in French, however the photo's are international, and even if you don't speak French an on-line translator and some imagination will assist & maybe clarify. The boiler is described and pictured on the lower part of page 2 of the build description.

Bonne chance!

Phil

Looks like that boiler has a brass foundation ring and a very narrow brass strip inside the joint where skirt meets upper boiler barrel. All best avoided Flanging of the firebox top plate looks like it will leave some excess gaps that will make for a poor solder joint too.

Fire hole tube is a tube or sleeve that you put the coal into the boiler through, it needs to pass through both skins of the lower part of the boiler but is not shown on your drawings due to the use of gas firing, dummy door could be fitted. Have a look at this one

Will post about rivits later.

Back to the Rivits, as you have not done much boiler work I would suggest avoiding drilling into the water space to fit them so.

Top row that would have gone through the smoke box tube plate or ring as it is called on your drawing. I would either make the boiler shell a little taller or lower the tubeplate so that the rivits can be fitted through the single skin of the outer tube. They don't need fully setting just cut them slightly over length, one tap to expand them enough so they don't fall out and then solder them in and file off flush internally

For the flange that would have originally joined the tapered skirt to the vertical barrel and assuming the skirt will now be a decorative add on then flange an "L" section ring, fit rivits to the vertical leg of the ring as above and then solder the ring to the outside of the barrel at an early stage. This does put the "l" on the outside but I think that the safest method but see below.

Form the tapered skirt form that with a "washer" sodered on so you have two flanges that can use bolts to joint the two together, ring of rivits as before just below the flange.

Boiler. Do away with all the bent small tubes and have 3 or 4 cross tubes through the top of the firebox which will make better use of the heat before it goes up the central flue.

Or more radical change would be to get rid of the water space around the firebox in which case the skirt can now be bolted on to act as the firebox. Then change from single flue to multi tube boiler which will increase heating area to compensate for that lost around the firebox. in which case the lower flange "L" section can fit inside as per original.

Jason –

(A few posts back) I was not assuming Bob's engine would use a bolted-together boiler, in fact I advised against it; though I did muse on the possibility as a general point.

(I'd considered it for my own project, which is copper, though with a steel ring on both external faces of the flanges to spread the bolt loads and prevent buckling.)

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Bob –

Sorry, I didn't know you'd intended spirit firing; maybe now gas.

My first reply was early on when the only information I could see was that part-sectioned GA apparently based on the full-size, but not showing how it was to be fuelled. It was somewhat natural for me to assume coal or other solid fuel, and simply omitting the grate and ash-pan on a drawing of a boiler's structure alone would not have alerted me to alternatives.

Those slot-less screws are indeed made to be pretend rivets!

Increasing the hem of the firebox (the 'inner cone' ) enough to join it directly to the outer shell, dispensing with an intervening foundation-ring, is in fact similar to the practice used by some manufacturers of full-sized boilers.

Horizontal cross-tubes in the firebox is again a common approach.

Swaging a tube to that cone is feasible, but as it thins the metal, don't start with a wall already very thin.

Looking again at your heading drawing, it seems to overcome the problem of forming the return-bend in the super-heater by making it of Omega-shape, with the advantage of not needing joints in it; but that does place the loop in the firebox. I am not sure if that would be wise as it would probably be in the flame.

Just my opinion Bob, but I don't think having the firebox inner and outer shell meet at a sharp junction with no foundation ring is a good idea. Depending on the water you use, there may be a lot of sediment buildup at the lower end of the boiler. If it is a small space tapering to a point, it will fill quickly and you may get plate overheating if there isn't water between the shell and firebox inner because the space is full of sediment. A foundation ring provides a) a water space to keep plates cool / transfer heat, b) a wide ledge for sediment to fall on, and be shifted / blown off during blowdown. Food for thought.

Edited By Jeff Dayman on 08/04/2021 19:31:49