Simple Designs for Sheet Metal Working 



XI. — Radial Line Method of Developing 

 Patterns for a Ventilator and a Scale Scoop 



By Arthur F. Payne 



Former Director of Vocational Education, Columbia University 



FOR those who have developed the 

 problems explained in the last issue, 

 Fig. 1, "pattern for ventilator on 

 roof with one-third pitch" will merely be 

 an easy review of the principles involved. 



Before starting on the pattern develop- 

 ment it is necessary to explain the mean- 

 ing of the term "one-third pitch" as ap- 

 plied to a roof. When an order comes 

 into the shop for a ventilator of any given 

 size which is to be placed on the side of a 

 roof, it is necessary to know the pitch and 

 angle of the roof. The contractors al- 

 ways state that it is a one-third, one- 

 fourth or one-sixth pitch. Before this 

 can be explained it is also necessary to be- 

 come acquainted with the terms "span," 

 "run," "rise." and "pitch." 



As shown in Fig. 1, the "span" is the 

 entire width of the roof; in this case it is 

 24 in. The "rise" is the length of a line 

 dropped straight down from the ridge until 

 it is level with the edges of the roof, in this 

 case the measurement is 8 in. The 

 "pitch" is the relation between the span 

 and the rise. In the illustration the rise 

 is 8 in., the span is 24 in., 8 in. is one-third 

 of 24 in,, therefore the pitch of this roof 

 is one-third. If the rise had been 6 in. 

 and the span 24 in., the pitch would be 

 one-fourth because the relation of 6 in. to 

 24 in. is 4 ins. Conversely, if the rise had 

 been 4 in. the pitch would be one-sixth 

 because 4 in. is one-sixth of 24. It is es- 

 sential to know the pitch of the roof be- 

 fore we can develop the ventilator pattern. 



The pattern for A only has been de- 

 veloped because jB is a simple cone, the 

 development of which was given in the 

 March issue. As the ventilator is a round 

 one, it can readily be seen that A is part 

 of a cone. First, it is necessary to draw 

 the full view of the ventilator as shown. 

 Second, draw the full cone as indicated, 

 with C for the apex and 1-9 for the base 

 line. Third, draw the one-half bottom 

 view as shown by D, and divide this into 

 eight equal spaces. Fourth, run these 



points straight up to the ba.se line, then 

 to the apex C. Fifth, where these lines 

 cross the roof line run them over to the 

 line C-9. Sixth, with the length C-9, as 

 radius strike the arc E-F, and get the 

 correct length by stepping off one of the 

 spaces on the bottom view sixteen times. 

 It must be understood that the bottom 

 view has only eight spaces because it is 

 only one-half of the bottom view, but 

 there must be sixteen spaces in the pattern 

 because it is 

 the full pat- 1 16 



tern. Seventh, 

 from the line 

 C-9, swing the 

 true lengths 

 (explained in 

 the last issue) 

 over until they 

 intersect the 

 same number- 

 ed line coming 

 up from the 



A pattern layout for making a base to a 

 ventilator to be placed on a sloping roof 



arc E-F. At the intersection make 

 a cross, connect these crosses with a 

 free curve and we have the bottom line of 

 the pattern. Eighth, to get the top line 

 of the pattern set the compasses at C and 

 G, then swing the arc G-II, which gives 

 the top hne, thus completing the full pat- 

 tern for A. The patterns for the straight 

 collars can be quickly developed by 

 methods explained in earlier articles. 



No attempt has been made in the draw- 

 ing to show the full construction of this 



613 



