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 



2 (iff! i , rj> 



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 



619 



