It is sometimes necessary for sheet and plate metal workers to make what is known as tee-pipes or elbows; the patterns, therefore, of a few examples in round pipes of this kind of work will be given. The pattern for a right-angle tee-piece, both pipes being of the same diameter, is shown set out in Fig. 8. An end elevation of the top pipe is drawn first, and the quarter-circle divided into three equal parts. Lines are now drawn through each point parallel to the centre line and down to the base line. The girth of pipe is set along 0 0 lines run up and cut off equal in length to the lines in elevation drawn from base line to corresponding number. The hole is marked out by making line 33 equal to halfcircumference of pipe, lines being drawn across through each of the five intermediate points, and cut off equal in length to the lines with the same number and letter in elevation. Thus a 2 in pattern will be the same length as line a 2 in quarter-circle. Care must be taken that the hole is marked in its proper position on the sheet or plate for top pipe. The line c c should be on the longitudinal centre line of plate.
The construction lines for obtaining the pattern by a more practically useful method are set out in Fig. 9. This is a most important case, and on account of the peculiar results obtained should be carefully studied. No elevation is needed, the view shown simply being drawn to exhibit the shape of the tee-pipe. A quarter-circle of same radius as pipes is set out first, and then divided into three equal parts in the same manner as before-mentioned. Line 0 0 is drawn equal in length to the girth of the pipe, divided into twelve equal parts, and then numbered as on pattern. Through each point perpendicular lines are run up, and these are cut off the proper length by drawing lines through 1, 2, and 3 on quarter-circle parallel to line 0 0. Thus the point of intersection of line through 1 on quarter-circle with the line drawn up from 1 on girth line will be a point on curve of pattern. In the same way the other points will be obtained.
It will be noticed that the cut on pattern to form the joint is made up of four equal curves: hence in workshop practice all that is necessary to mark out is a template containing one of the curves, such as the shaded part shown at the left of pattern. This simple template can be used in a variety of ways. The pattern for the pipe can be set out by using it four times, marking the curve, and then reversing. The hole on top pipe can be drawn out in a similar manner, as will be seen by the four curves that form the hole at top of pattern. The template can also be used for setting out the patterns for a square elbow, the curve B O B showing the pattern for seam at back, and the curve 0 A 0 for seam at throat. Laps can be added on to the net patterns according to the method of joining adopted.