Possibly one of the commonest jobs an iron-plate worker is called upon to do is to make a square elbow for a round pipe. An elbow of this description may be required either for a stove pipe, a rain-water pipe, or a ventilating shaft. The pattern for it can be set out in a variety of ways, all giving the same result. One of these methods is shown in Fig. 4. This may be described as the general method, which is applicable to all kinds of pipe joints for circular non-tapering pipes. An elevation of the elbow is drawn in the usual way, and a semicircle described as shown. For the pattern the circumference of the pipe is set along the line 0 0, vertical lines are run up from each numbered point, and these cut off equal in length to the line with the same number running between base and joint lines in elevation. Before making any allowances for jointing, the method of fastening the pipes together should be decided. There are many ways in which the joint can be made, the method adopted depending upon the purposes for which the pipes are to be used. In Fig. 4 it is assumed that they are seamed together, the plan often followed in making elbows for stove pipes. A sketch of joint at back and two sketches of the joint at throat are shown. After the pipes are edged, or as it is called, paned together, it is usual to knock-up that part of the joint round the throat as shown in the bottom sketch. The four thicknesses of metal are of course hammered perfectly tight together. A knee is sometimes riveted in the throat of the elbow, which adds considerably to its strength.
For ordinary thicknesses of sheet iron, say 24 gauge, the allowance for the single throw-off may be 3-16 in., and for the double edge a little greater than § in. These allowances are shown by the dotted lines on pattern. The side seam will be grooved, and it will be sufficient to allow § in. on each side to cover for what is required for a 1/4 in. groove. The way to make allowances for the different kinds of joints will be dealt with fully in subsequent chapters. Notches at 0 0 must not be cut too large, or the result will be a hole in the joint of the elbow. The object of the notches on pattern is to avoid having to stretch or throw-off the four thicknesses of sheet which form the groove, which if attempted would, in many cases, break the grooved seam. Besides this, if the groove would stand turning over, it would result in an unsightly lump on the joint seam. It is always the safest plan to cut a long notch, as shown in the pattern at 0 0 (Fig. 4).
Without the sheet iron is of good quality, it is best to anneal around the edges for wide flange before attempting to throw it over. In fact the safest plan is to anneal twice, first before flanging, and then again after, before the edge is turned back. It might be taken as a sheet metal worker's maxim, "Never spoil a good job for the want of a little annealing."
A simpler method for marking out the pattern for a square elbow is shown in Fig. 5; but it must be distinctly borne in mind that this method applies to a square elbow only, and cannot be used for any other kind of elbow or bend. A circle equal in diameter to the pipe is described and divided into twelve equal parts, the girth line 0 0 being divided up in the same manner. Points on the curve are obtained by running construction lines up and across as shown.