Building a Boiler for a 3 1/2" Gauge Locomotive

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sbwhart:
An update:-

I've now tried two more blower tubes using tube from different suppliers, John and maccmodels, Johns tube was 5/32" I don't know what the wall thickness was, the macc tube was 5/32 * 22g (0.028 wall thickness) they both went ping when I tried tightening them down, now you're probably thinking I over tightened, but this is far from the truth, as a rule I'm what you would call a gentle engineer, some times I would admit I'm too gentle, and having one tube break I was extremely careful. The drawing calls for 5/32" tube it doesn't say what gauge. To put some numbers around the problem in the UK the standard gauge for 5/32 pipe seems to be 22g (0.028" or 0.71mm) this has to be threaded 5/32*40 ME that will leave a wall thickness less than 0.015" or 0.38mm no wonder it's breaking.

I'm left with a number of options:-

Find some 3/32 tube with a thicker gauge, I've located a supplier who has some 20g (0.036" or 0.92mm) tube but I think he will only be interested in supplying a couple of tons which is out of the question I'm going to call him on Monday and see if he will supply me a small sample (don't ask don't get)

I can get 3/16 * 18g (0.048" or 1.2mm) tube, I would have to increase the thread in the fitting to 3/16 * 40 ME this would leave me a wall thickness of 0.032" or 0.81mm or more than twice what I've got know.

Fit a solid stay and rout the blower around the outside of the boiler.

I think I'm going to go for the 3/16 tube unless any of you Guys can suggest anything else.

Stew

lordedmond:
Save yourself the bother run the blower feed out side the boiler and fit solid stays

Stuart

madjackghengis:
This idea might be more bother than it's worth Stew, but you could carefully peen the end of the 3/16ths tube down to 5/32nds the length of the thread and thus upset and thicken the tube right where you are threading, and actually have it stronger right there, making up for the cutting of the threads weakening the tube. That keeps you with the original threads in the fitting, no loss of strength in it opening it up for the 3/16ths, and still using the larger and thicker tubing. It's not too hard to keep a soft tube like copper, round, while reducing its diameter thirty thousandths, but I'd anneal the peened end of it before threading, just to be sure a crack doesn't show up from work hardening. I've had to do it to get a piece of straight tube into a tapered hole so it could be well soldered in, and had success. I like to buy studs rather than make them because threaded studs as bought are roll threaded and much stronger than cutting threads for your own studs and I'd recommend rolling the thread instead of cutting it, but I don't know anyone with thread rolling equipment other than for bicycle spokes, and I think they are for fixed sizes and not merely adjustable for different spoke thicknesses. There's always the possibility of motorcycle spokes happening to be the right gauge and a shop having a rolling machine. For what it's worth, :poke: mad jack

NickG:
Stew,

I will whip mine out of the boiler tonight and see how it's been done just for info. Suppose you could route it outside the boiler but where? It could look messy unless you went through cab floor underneath running board. You'd have to go in through the side of the smoke box though as the cylinders are underneath it.

Nick

lordedmond:
NickG

most of the British standard class loco's have the blower feed outside not though the boiler. my class 4 tank has it outside and fits just under the ejector elbow, just a part of the smoke box furniture so to speak with the ejector itself bolted on there it look fine

I am not evan sure if any 12 inch to the foot loco's had the blower feed integral with the boiler

if the sight of the pipe offends then disguise it as a hand rail

then normal model why was to fit a concentric bush 40 tpi outside and 40 tpi internal and screw that in to the boiler shell and pipe together therefor no stress on the pipe but a good seal if you use copper slip it will seal

Stuart

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