A ventilator base may be of the form shown in
Fig. 207, which, it will be seen, amounts, geometrically, to the fitting of a cylinder on to a square pyramid concentrically. The pattern cuts, both for the pyramid and the pipe surfaces, are shown struck out in Fig. 208. A half-elevation is drawn, and a line making 45° with the base line set down, this being cut off by the line 0 2, which is drawn square to the base line. The arc 0' 2' is now described, and it will thus be seen that the figure 0' 2' 2 0 can be taken as representing one-eighth of the complete plan of the ventilator base. Line 0 0" is produced to meet the centre line in c. The line 0 2 is bisected, and lines d 1, d 2 swung on to the base line about d, and the points 1° and 2° joined up to c. For the pattern,the compasses are opened out to the length c 2°, and a circle described as shown. Five sides are now stepped around the circle, each side, 2 2, being equal in length to twice the line 0 2 from the plan. The last two sides are now bisected, so that to make up the complete pattern there will be three full sides and two half-sides. Each side is now divided into four equal parts, and from the division points lines drawn to the centre C. The compasses are next set respectively to the radii c 0", c 1", and c 2", and the three arcs drawn on the pattern to cut the radial lines. Where these arcs intersect the correspondingly numbered line will give a point on the curve. The points are then joined up, such as 0", 1", and 2", with even curves, and the net pattern is complete. (It is as well to remember that the inner curves are parts of an ellipse, as in many cases they can be marked out by a much simpler method.) Allowance is then put on the pattern for the base flange, corner laps, and side seams.
If it is desired to run the seam down a corner instead of the middle of the side, as shown, then, of course, four full sides would have to be marked out, and not three full and two half-sides as on Fig. 208.
For the cut on the bottom of pipe a girth line is first stretched out by setting along alternately the lengths of arcs 0' 1' and
1' 2'. Lines square to the girth line are then run up from each point, and these cut off respectively equal 0' 0", 0' 1", and 0' 2" from the elevation. The new-found points are then joined up to form the curve. It is as well to remember, and it will act as a test for the accuracy of the setting out, that the lengths of the curves 0" 1' and 1" 2" should be the same both on the pipe and base patterns.