In the last chapter we dealt with the striking out of a pattern for a round spout fitting on to a flat surface, and pointed out a rough way in which the pattern could be altered to suit a circular body. We will now go over a method which will give an accurate shape of pattern for a round spout of any size fitting on to a circular body of any dimension.
An elevation of spout and body is shown in Fig. 115, the spout being drawn relatively large to the body, to more clearly show the method of construction. Although the appearance of the articles in Figs. 113 and 115 are not very similar, yet the same principle in marking out the pattern underlies each case. Both come under the heading of a "cone fitting on to a cylinder."
To obtain the shape of the complete cone of which the spout is a part, the side lines are extended, C 0 being made equal to C 4. On the cone-base, a semicircle is described and divided into four equal parts; lines run up from each point square to the base and then joined to C, the apex of the cone. The half-plan is now drawn, C' being the plan of the cone apex and the dotted half-ellipse representing the plan of half cone-base, the same as the half-elevation in Fig. 114. The points on the half-ellipse are joined to C', and from the points where the joining lines cross the semicircle projectors are run up to intersect the corresponding lines in the elevation of cone. In this way the joint line of spout and body is determined. Lines from the points on the joint line, and also from the lip of spout, are drawn square to the centre line of cone, and thus all the true lengths required in marking out the pattern projected on to the bottom side of spout. The pattern for the complete cone is first set out by marking along for the cone-base girth eight lengths, each equal to the length of one of the arcs on the cone-base semicircle. The required lengths to obtain points on the cut at top and bottom of the pattern are swung around from those on the bottom side of spout.
The shape of the hole in this case (Fig. 115) is shown projected on the right-hand side of figure. The lengths 0' to 1', 1', to 2', etc., for the widths of the hole at the different parts are taken from the lengths of arc with the same distinguishing numbers on the semicircle in the half-plan.
Any allowances for seam or throw-off must be put on as shown by the thick dotted line around the pattern. Whilst it may appear that for the correct marking out of the pattern for so simple an object as a spout the work is somewhat complicated, it should be borne in mind that for stock articles it certainly pays to have patterns as accurate as possible.