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
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