In addition to those dealt with in the last chapter, there are a number of hoods, hoppers, or body parts that arc formed in a somewhat different manner. Thus Fig. 15G represents an article whose top is circular and bottom oblong with semicircular ends; but in this case the centre of the top is vertically over the centre of one of the semicircular ends. On examination it will be seen that the left-hand part of surface is formed of half of a frustum of a right cone and the right-hand part of half of a frustum of oblique cone, the side parts being flat triangles. Perhaps the building up of the surface will be better understood on referring to Fig. 157, where the half-pan is shown.
To obtain the lengths of the pattern lines, the apex T (Fig. 157) of the oblique cone is obtained by joining 4 to a and producing the line to B, then running up a perpendicular to meet the line 7 d produced to T. The apex c of the right cone is found by producing the slant side to meet the centre line. Taking B as centre, the points 4, 5, and 6 are swung on to the base line, and then joined up to T.
To mark out the pattern, the first line set down is T 7, this being of the same length as the similarly-numbered line in the elevation. Then taking T as centre and radii respectively equal to T 6', T 5', and T 4' in the elevation, the three arcs are drawn. The compasses are now set to the length of one of the arcs in plan, say 4 to 5, and commencing at point 7 on the pattern, the points 6, 5, and 4 are struck off. The points on the inner curve of pattern to form top of article will be found by marking the distances along the lines from T, equal to the lengths of lines measured from T in the elevation down to where they cross the top line. Thus, T D on the pattern is equal to T d in the elevation, and so on for the remaining points. The compasses are now set to the length of line 3 4 in the plan, and with 4 on the pattern as centre, arcs are drawn, these being cut by making A 3 equal to the slant length o e in the elevation. The line 3 A is now produced, and A C set off equal in length to e c in the elevation. The point
C is now used as a centre, and the part 3 to o struck out in the usual way for a right cone development.
In laying out patterns for articles of this description, it should be noticed that the straight line and curved parts run into each other without break or unevenness. It is also as well to check the setting-out by testing if lines 3 A and 3 4 (Fig. 157) are square to each other, as they should be if the pattern be marked out correctly. For a large hood or hopper the body can be made up in as many pieces as will be suitable to the size of sheets or plates used.