In the making of square pipes for ventilating shafts and other purposes, it is often necessary to construct various kinds of elbows and tee-pipes. We shall, therefore, in this chapter, deal with the striking out of the patterns for a few representative cases.
A side elevation of the pipe (Fig. 26) is usually first set out, care being taken that the branches are put at the required angle. The object of this elevation is to obtain the difference between the height of the back and the throat of one of the branch pipes. This difference, which is marked A C on the figure, may be obtained by simply setting out the triangle A C B, the angle A B C having first been determined by the following rule: "To find the joint-line angle, deduct half the bend or elbow angle from 90°." Thus, in the present case the angle will be -
90 - 120 / 2 = 30°.
It will thus be seen that to get the length of line A C it is only necessary, in practice, to draw A B and the joint line B C at the proper angle. This dispenses with the elevation, which is always advisable, as in the setting out of patterns as little as possible in the way of plans and elevations should be drawn.
Those readers who have a smattering of mathematical knowledge, and can use tables, will be able to calculate the length of A C as follows: -
AC = AB x tan. ABC = 5 x tan. 30° = 5 x .58 = 2.9 in.
This height can be set directly on the pattern, and the same completely struck out without in any way using an elevation. It might be here remarked that what has been said in connection with obtaining the difference in height between the back and throat of a square pipe is also applicable to an elbow for a round pipe.
Referring again to Fig. 26, it will be noticed that the pattern is set out so that the seam will come down the centre of the throat when the sheet is bent up. This will necessitate three full widths of 5 in. each, and two half widths of 2 1/2 in., each being marked out to form the pipe girth. If the seam is to be in any other position, then the parts of the pattern must be arranged accordingly.
In Fig. 26 it will be observed that the length A C is projected by dotted lines on to the pattern. This should not be done when marking out on sheet metal, as it is most difficult to transfer lengths correctly in this way. It is done on the figure simply to better explain from where the length A C is obtained.
The pattern for the two arms may be set out in one piece, as shown in Fig. 27.
The two side gaps are cut away, and after the sheet is bent up and seamed in the form of a square straight pipe, the elbow can be made by bending along the line D E and fastening together at the throat and sides. After the elbow is formed, it will be seen that the points F F come together.
So that the elbow may have the correct offset, it is always a good plan to make a template for the required angle, and try this in the throat while the joint is being tacked.
For the special case of a square elbow it should be noted that the height of the back part of pattern above the throat portion will be equal to the diameter of the pipe.