Page:Popular Science Monthly Volume 92.djvu/630

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��Popular Science Monihly

��ventilator, or to give sizes, as it would complicate the drawing and confuse the student. Allowance must be made on the patterns for all seams and wiring.

���Laying out a pattern for cutting a hole in the roof for insertion of the ventilator base

A very interesting and simple problem that is not generally known among sheet metal workers is shown in Fig. 2. If, when making the ventilator shown in Fig. 1, it is desired to obtain the pattern for the hole in the roof, the method is as follows: First, draw view A with the full bottom view as shown by the circle. Second, divide the circle into sixteen parts, and project the points to the base line. From where they cross the base line, run the lines up to the apex C exactly

��where on these lines, draw the center line 1-9 parallel to the roof line. Notice that now we have the exact length of the hole and the exact location of each radial line. If we can now get the exact width on each of these lines, we shall have our pattern. We can get these widths by drawing a front view. Make the drawing the same as for the side view except that the roof line is omitted. Notice particularly that the numbers on the bottom view circle have been turned one-quarter circle to the right, although on the ventilator they are, of course, in the same place. For in- stance, 1 is the highest point and 9 is the lowest point on both views. Fourth from view A, where the radial lines cross the roci line, run lines to the front view B. Make a cross where these lines intersect, the same numbered lines coming up from the bottom view circle. Connect these crosses with a curve and you will have a true front view of the joint between the ventilator and the roof. It is understood that the upper part of the oval would be back of the ventilator and therefore in- visible from the front. Sixth, to transfer these widths to the pattern, place one point of the compasses on center line, and find the width of 2-16, then lay this off on the 2-16 line of the pattern. Transfer all the other widths in the same way, connect the crosses with a curve and you will have

���as was done in the preceding problems. Third, from the points where these radial lines intersect the roof line, draw lines at exact right angles, as shown in the draw- ing. Number these lines, giving them the numbers at the bottom view points from which they started. Any-

��Fig. 3: Pattern layout for making the three parts of the ordinary scale scoop. Two of the pieces are parts of a cone while the other is a plain cylinder

��the exact pattern for the roof hole.

In Fig. 3 the "scale scoop," we have an interesting and ap- parently difficult problem, but one which is much easier than it looks. This scoop is circular in

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