copper Tube

Hi all

I have been getting drawings together for 1 1/2 inch scale Allchin and was looking at Blackgates engineering for a price for the required 3 and 3/4 inch diameter copper tube 13 gauge for the boiler but Blackgates no longer stocks this. Does anybody know where I might be able to obtain it?

M-Machine have both 3" and 3/4" listed at 10, 14 and 16 swg thickness if that helps.

Have you tried Reeves? They used to stock it but I suspect, like Blackgates, no longer do so. Rolling from sheet might be your best option.

If it were me I'd go for the stock 10swg tube - but bearing in mind that if the 3.75" is bore not OD, its extra diameter (from 0.036" extra wall thickness) could affect many other dimensions, especially on a traction-engine.

I don't know the Allchin in detail, but a slightly larger boiler barrel might need a wider firebox hence moving the hornplates and the various journals they support, outwards with possible repercussions on the shafts. It will also increase the radius of the cylinder saddle, and alter the motion centre-line height. Not to mention altering the plate-work a little.

Would it affect the scale appearance? Possibly not: scaling up that extra thickness changes the equivalent full-size width by only half an inch or so.

Even so it will far easier and arguably stronger than trying to roll the shell to the exact diameter, then joining it.

.

If you fabricate it from plate, the nearest alternatives to 13SWG (~ 2.4mm) thickness differ from it by 0.012" .

Of them, 14swg (=2mm) might still be strong enough for the pressure, but only for the pressure. It would be weaker structurally, on a boiler that is also the vehicle chassis, so needing plenty of strength.

Besides, being anything thinner might frighten the Club Boiler Admirer away.

Or 12swg (nearest 2.5mm), might be accommodated quite easily with only minor changes elsewhere, and be a trifle stronger.

(A CBA unhappy about thicker boiler walls than on the drawing, is best swapping roles with the Club Head Gardener.)

Are you sure , it is usually the OD that the size refers to

Is there some confusion here, stemming from the OP's expression of size? The need is for tube with an OD of 3.75", not the two sizes 3" and 3/4".

M-machine list 4" od which might be closed down and with a butt-strap, to save rolling.

They also seem to list 3.75" od x 2"id, but that's a lot of lathe work to make it 13g wall.

Edited By Clive Brown 1 on 10/05/2023 16:04:16

I hope not, the figures for 4" copper pipe give burst pressures of between 2215 and 3415psi, and working pressures between 419psi and 489psi.

A model boiler at 150psi is well within specification of the weakest 4" pipe, and 3¾ would be stronger. I'd expect Inspectors to be more worried about the build and joints rather than the pipe.

A seamed tube is considerably weaker than a solid drawn tube, the weak point being the seam, whatever the plate's thickness. In full size boilers seams were also prone to stress cracking and corrosion due to the boiler flexing with pressure changes. From memory, I think longitudinal seams caused most trouble, and overlap type longitudinal seams were eventually banned by the Victorians.

Is 3¾" copper pipe a standard size? It's not listed my books

Dave.

It used to be standard but generally not available now.

Standard or not, lots of Allchin builders have been able to buy it in the past, but not now it seems. Back in the day when I bought mine, stockists such as Reeves could supply a large range of odd sizes. I did wonder if tube extruders would supply them with whatever was wanted for even a modest order. Bespoke extrusion dies might have been a manageable cost.

Nigel is confusing tube(sized by OD) with pipe(ID). These are normally used for different purposes, and sized according to the relevant dimension.

I did say, sort of, that I was not sure.