It should be borne in mind that the most important point in the making of patterns is accuracy in determining the lines that are required for the pattern. It is always better to spend a little extra time in finding the correct length of these lines than to have an ill-fitting article, or to waste time in cutting or chiselling it into shape. If the pattern is for a stock article, then the greatest possible care should be exercised, so as to obtain a pattern as near perfection as possible; but, on the other hand, if it is required to set out a pattern for an odd job, the workman who has an ounce of common sense will know it is foolish to spend as much time in the setting out as will eat up the cost of the job.
Fig. 1 shows a sketch of the pipe, with flange fitted on the slant end. Generally, for those who have had but little practice at setting out, it is the best plan to draw an elevation of the pipe or pipe-joint for which the patterns are required. First draw the centre line (Fig. 2), and then, as it were, "clothe" this with the pipe by marking half its diameter on each side, and draw lines parallel to the centre line, cutting them off to the required length. Now draw "base line," as shown, and on this describe a semicircle, and divide it into six equal parts by using the compasses at the same radius with 0, 3, and 6 as centres. Draw lines from "baseline" to "joint line," passing through the points 1, 2, 3, etc., and parallel to the centre line, or square with the base line.
The pattern can now be developed by drawing a line, 0 0, equal in length to the "circumference or girth of the pipe. This length can be obtained by carefully measuring along one of the six arcs 0 to 1 or 2 to 3, etc., into which the semicircle is divided, and setting it along the straight line twelve times. The arc can be measured by bending along it a strip of sheet metal or stiff paper, or a bit of thin wire; or it can be more accurately found by using the wellknown rule for calculating the circumference of a circle: - "Multiply the diameter by 22, and divide by 7." Thus, in the present case, if the diameter of the pipe is 10 1/2 in., its circumference will be 33 in., and, dividing this by 12, the length of one of the arcs will be 2¾ in.
The simplest plan, however, and the one most often adopted in ordinary practice, is to take the lengths directly from the drawn semicircle. Lines perpendicular to 0 0 should be run up from each point, and numbered as shown, and their lengths cut off equal to the corresponding lines between "base" and "joint" lines in the elevation. In workshop practice, it is most convenient to take these lengths off with the compasses, and set them up the proper lines; but in developing a pattern on paper, the heights can be projected from elevation on the pattern, as shown with line cutting off point 2. The points marked can now be joined up with a free-flowing curve, and thus the net pattern is completed. To add the proper allowances for thickness of metal - laps, seams, joints, and wiring - is the most important part of the making of patterns, and this will be dealt with fully in subsequent chapters. In the present case, whatever is allowed for the side-riveted seam, half must be put on to each end of pattern. Thus, suppose the lap is 1 1/4 in., then § in. will be the allowance for each end of pattern. It will be noticed that the centre lines for the rivet-holes are the end lines of the net pattern.
The thick dotted line at top represents the allowance for small flange for fastening ring to pipe, either by riveting, brazing, or soldering. The thin dotted line shows the allowance to be made if the whole flange is to be thrown off the pipe. Care must be exercised so as to get the allowance for flange the same width all along the pattern. This can be best done by setting the compasses at the required width, and drawing them along the curve at top of net pattern.
Attention is called to the method of numbering adopted. The figure 0 will in all similar cases be placed against the seam of pipe, and it will thus always come on the outside lines of net pattern.
The ring to form the flange can be set out from the elevation of pipe. The long diameter 0 6 will be equal in length to the joint line, and the intermediate points can also be taken from the same line. The widths at the different parts of the ring can be taken from the lines with the corresponding numbers on the semicircle in elevation. These points will now all be joined with a curve and the width of flange marked around. As the hole in the flange-ring is an ellipse there are many other ways that might be employed for marking it out - some shorter, some longer - and the best of these methods shall be shown as occasion demands.