A fair amount of skill is required to throw-off or stretch a flange properly. The first thing that should be done is to cut a gauge (Fig. 3) out of a bit of sheet brass, and with this mark the depth of the flange all round on the inside of the pipe. In stretching the flange on anvil, head-stake, or other tool it should be remembered that it is the outer edge of the flange that requires the greatest amount of hammering, as the length round the outside of flange will be greater than the inside by just about 6 1/4 times the width of the flange. If the pipe is made out of 1/8 in. or thicker metal the flange will have to be turned over hot, and in this case the depth of flange should be marked on the plate when flat, with centre-punch marks.
In the flanging of plate metals there is no need to exercise quite so much care to avoid the splitting of the flange as there is with sheet metals, as there is a greater volume of metal to allow for drawing. Since the introduction of mild-steel plates of uniform structure, flanging operations can be carried out with a greater degree of certainty than in the old days, when iron of an indifferent quality had to be used. All the advice in the world, however, will not make a mechanic into a good Sanger without plenty of practice.
If holes are required in the flange, no attempt should be made to put these in the sheet or plate before bending or flanging, as the flange is almost certain to break across the holes, and if, by good luck, it does not, it will be found that the holes are drawn out of shape.
In stretching, throwing-off, or flanging sheet metals, annealing plays an important part, so that as soon as an edge shows signs of becoming hard or brittle it should be at once got redhot and allowed to cool down.