Sheet Metal Working Simply Explained
V. — -Method of finding the miter lines and laying out the pattern for ninety-degree elbows of any number of pieces
By Arthur F. Payne
Director of Vocational Education
��SINCE the publication of the article in the September issue describing the method of laying out elbow patterns, many inquiries have been received asking for the method of finding the miter lines and laying out the patterns for elbows having any number of pieces. The author and publishers are very glad to receive this indication of interest in the series and are especially glad that it is in the scientific side of the work. They will welcome further suggestions.
Almost every pattern drafter has his own pet way of laying out the many pieced elbows, and various ingenious charts, dials, disks, tables, etc., have been worked out and placed on sale. However different these may appear, one from the other, they are all based on the same fundamental methods and principles.
In all elbows of more than two pieces, the two end sections should be one-half the size of the other sections. Note the end sections of the drawings showing the elbows in A. It is only necessary to de- velop the pattern of the bottom end section. This can be used for a pattern for all of the other sections, as will be shown later in the description. It is necessary to lay out the drawing for the complete elbow with all the miter lines to get the correct miter line of the end section. This extra work can be avoided if you will make one of the miter lines charts as shown and described later in the article.
It is always understood when speaking of elbows that we are referring to elbows of 90 deg. In the drawing A we have two elbows, one made of three pieces and the other of five, but both are 90 deg.
For an explanation of this entire problem we will take a four-piece elbow. Draw two arcs, as shown by the dotted lines C-D and E- F in drawing B. We already know that the two end sections are to be one- half size of the center sections. As there are four pieces in the elbow the two center sections will be twice the size of the two ends, as shown. If we take the size of one end section as a unit we find that
��it is repeated six times in the entire elbow. Then we must divide the arcs C-D and E- F into six equal parts. Another way of stating this is: There are four sections in this elbow; there are two end sections and two center sections. Count one for each of the end sections (1 + 1 = 2). Add two for each of the center sections (2 + 2 = 4), making a total of six units. This rule
����C % &rf~£l7 I 2 3 4 5 6 7 B 9 10 II It 15 H 15
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4-14- 9 Elbows of more than two
10 pieces and the way to lay
out a pattern for cutting
out the different sections
applies to all 90 deg. elbows of any size and any number of pieces.
The method of developing the pattern for the elbow in B is shown and has already been described in the June and September articles. It will save considerable time when laying out these elbow patterns in practical work if you will make from a piece of heavy tin one of the miter charts as shown in the drawing. This* chart will give you the miter line of the bottom end section of the elbow and all you need to do is to develop the pattern for the pipe of the diameter wanted.
To make this chart, cut out of heavy tin a triangle that is 12 in. on two sides, as shown in the drawing, then with a protractor lay off the angles as indicated