Several methods are in vogue for marking out the plate patterns, according to the practice of the particular locality. As previously stated, in work of this character that has to be hollowed or stretched, it is impossible to set out the patterns that they will work out dead true to shape. The most that can be hoped for is to get as good an approximation as possible, and at the same time take care that the pattern is slightly on the full side.
In Fig. 212 a side elevation of a bell-mouthed ventilator is shown. Its body is formed of four pieces, two cheeks and the throat and the back parts, the bell-mouth being made up in three pieces.
For the cheek pattern the simplest plan is to take the elevation of the body as the shape of the pattern; the only modification required being that shown by the dotted line, e l k. The point l can be 'obtained by making the line m n equal in length to half the mouth diameter, that is, r e. Drawing n t to touch the semicircle on a f; cutting off t p equal to t n, and then making r l equal to o p, as shown by the construction lines. An arc of a circle is then drawn through the points e l and k.
Before attempting to strike out the patterns for the back; and throat pieces the elevation of the two joint lines must first be drawn. Divide the curves a e and f k each into, say, three equal parts, and on the four lines that join the division points describe semicircles, as shown. Now mark the middle points of the semicircles, such as s and u, and set around the arc on each side a length equal to half the diameter of the respective semicircles. Thus the arc s 4' will be equal in length to r e, and the arc u 3' equal to v d, and so for each of the other two semicircles. Perpendicular lines, such as 4' 4 and 3' 3, are then drawn from the points on the semicircle to their diameters, and thus points on the joint lines obtained. These are connected with an even curve, as shown by the dotted lines, which will then give an elevation of the two side seams.
For the back pattern, a centre line A E is marked down, equal in length to the respective parts of the curve a e in the elevation. The line D 3 is set off equal in length to the arc d 3', and the line B 2 equal to arc b 2'. In the same way the other points, 1 and 4, are found. To get the lengths of the side curves on the pattern, a line, c 0, is first drawn across the two curves in the elevation, as shown; the point c being the middle of the back curve, and the line c 0 being drawn by the eye, to make as near as possible equal angles with the two curves. The parts of the joint line 0 1 and 0 4 are carefully measured along, and their lengths set above and below the point 0 on the outside line of the back pattern. In this way the points 1 and 4 are obtained. Arcs of circles, 4 E 4 and 1 A 1, are then described to form the ends' of the pattern. It should be remembered that when working up the plate for the back, that the centre will lengthen a little and the sides contract somewhat: hence the side-curves of the pattern should be made slightly longer than measured from the elevation. This is best allowed for by making the arcs 4 E 4 and 1 A 1 somewhat flatter than they would be if drawn exactly through the three points as found.
The throat pattern can be set out in identically the same manner as that for the back, and so that the reader may the more readily follow the construction, the same numbers for the outside curves have been chosen. In this the centre line, F K, on the pattern is the same length as f k on the elevation, and the lines, F 1, G 2, etc., equal in length to the arcs f 1', g 2', etc. The outside curves of the pattern will be the same length as the throat seam line; 0 1 and 0 4 on the pattern being made equal in length to 0 1 and 0 4 on the elevation. In working up the throat plate the outer edges will, of course, have to be stretched: hence they will lengthen somewhat, so that it is as well to keep F K the same length as f k on the elevation, but to draw the arcs 4 K 4 and 1 F 1 on the pattern slightly flatter, and, consequently, reduce the lengths of the side curves somewhat. The exact amounts to allow on or take off, as the case may be, are matters of experience, or of difficult calculation, the main thing being to keep on the right side, as, in any case, some small allowance must be made for trimming.
If for a large head, the bell-mouth will be made in several pieces; in the present case three have been chosen. The pattern will come out as part of the surface of a cone. The first thing, then, is to find the slant height of the cone, and thus the radius for the pattern. Join e to w, and from the middle point q draw the perpendicular q j. Make j y one-third of q j, and through the point y draw the lines z x parallel to w e, to meet the axis of the supposed cone in x. The line j z is made equal in length to the arc j w, and thus the slant height of the cone is determined. The line x z is now used as the radius for the outer curve of the bellmouth pattern; the length of this curve being made equal to one-third the circumference of the cone base circle. In working up the piece of sheet for the bell-mouth, it will be found that the draw at the ends will not be uniform; consequently it will be necessary to allow a little on the ends of the pattern, as shown by the arcs
The allowances for the joints are added to the back and throat pieces, and also to the inner side of the bell-mouth pattern.
The beading around the bell-mouth is formed either by wiring or split-tube, as shown in Fig. 213.