You should now take the boiler to a practical brazier and have it properly hard-brazed in every join and round each tube, and you might cut the hole for steam-dome and have it brazed on at the same time. If this is properly done you never need be in fear when the water runs low, as the boiler might get almost red-hot without injuring it much. Of course it is not advisable, as it would blister and spoil the appearance of the paint outside. This is a good opportunity to test the boiler before fitting it up, and you should fill it with water through a hole drilled in top of dome, and then fix on the test-pump, which you could borrow from any engineering-shop. If too far away from town to do that, you must make use of the force-pump attached to your model, and work it by hand, watching the pressure-gauge in the meanwhile. Test it to 100 lb. per square inch, which will be sufficient, as 50 lb. will be a fair working pressure. Should you have to test it with your own pump, the pressure-gauge will have to be bought then, as that is an article you cannot make yourself. A small gauge of Bourdon’s make, of an inch and a half diameter, will cost about twenty-five shillings, and although it may seem a rather high price for such a small thing, it is absolutely necessary to have it, as you could not tell what dangerous pressure you had raised in the boiler without it.
Fig. 35.
This being done, proceed to make the smoke-box, which should be three inches deep, and of the same shape and dimensions shown in [Fig. 35]. This and the chimney can be made of iron, hammered up to shape and finished with a brass ring. The smoke-box can be screwed to the forward flange on boiler. The door is drawn open to show the amount of bulge it should be hammered to.
In the centre a hole should be drilled through which to pass the screw used to close it, which is attached to the loose bar A. The handle B is then screwed up tight.
The door is circular and must be large enough to overlap the opening about half an inch, and have a couple of bright iron or brass eyes (C) riveted on to form the hinge.
Fig. 36.
We can now make the back pressure-valve ([Fig. 36]). A is a front view, with plate by which it is bolted on to boiler, as at W ([Fig. 20]).
It is very simple to make, and consists of the casting A with the top and bottom covers, and the ball-valve B, which ought to be ground with a little emery-and-oil to fit perfectly. It acts in this manner. The water being forced up C from the pump, raises B and passes into the boiler. On the up-stroke of pump the pressure is removed from under B, and pressure of steam in boiler causes it to fall back and close opening entirely, preventing any water passing away from boiler. A small flange can be put on each outer side of boiler near furnace to support it on bed-plate level with smoke-box.