The surfaces of many articles are what might be described as of a compound character - that is, they do not follow the surface shape of any one particular solid, but are built up of parts of surfaces of one or more solids, perhaps combined with one or several plain figures. One simple example of this has been shown in Fig. 148, and we shall now give two more typical cases of this class of pattern-marking.
In all cases it should be borne in mind that the first thing to do is to carefully analyse the surface so as to determine how it is formed. In Fig. 150 an article is shown whose top is circular, and whose bottom has straight sides, with corners formed of quarter-circles. On examination it will be seen that the .rounded parts are each a quarter of a frustum of an oblique cone, and that the flat surfaces are triangles. In setting out the pattern (Fig. 151) a quarter-plan is first drawn, as shown, and the point B obtained by joining 1 to a and 4 to c, and producing the lines until they intersect. Through B a line is drawu parallel to a o, and the points 1, 2, 3, and 4 swung on to it, as in the previous cases: these are then joined up to T. It, perhaps, should be here mentioned that the point T in this example is found by producing the back line o1 a1 until it intersects the perpendicular drawn up through B. In marking out the pattern the line 0 A is made equal in length to a1 o1 on the elevation. A line is drawn square to it through 0, and cut off on each side equal to 0 1 in the plan. The line 1 A is produced, and the point T determined by making 1 T equal in length to
T l1 from the elevation. The points 2, 3, and 4 on the pattern, and also those for the curve A C, are now obtained as in the other cases of the oblique cone. C is now taken as centre and C 4 as radius, and the arc drawn as shown, the next point 4 being fixed by making line 4 4 equal to twice the length of d 4 in the plan. The other points will be found, as previously explained, in connection with Fig. 149, each quarter of the pattern being the same shape. If the article is made up in two or more pieces, it will be an easy matter to mark out the portion of pattern required.
Hood with Hound Top and Flat Back a flat triangle for the front, a flat triangle and a quarter-frustum of an oblique cone for each side, together with a flat triangle for the back.
A pattern for a hood whose top is circular and bottom rectangular with two square and two round corners (Fig. 152) can be obtained on the same principle as in the previous articles. An inspection of the hood surface in the sketch will lead us to see that it is built up by two quarter-frustums of an oblique cone and
To get the length of the pattern lines, a half-plan and a half-elevation are set out (Fig. 153), the point B and the true lengths of lines being obtained as in the last example. The setting out of the pattern up to the lines C 2 will be exactly as in Fig. 151. Before proceeding any further with the pattern, the lengths of lines required for the remaining portion will have to be determined. To do this, set along distances l1 c1, l1 d1, and l1 01 in the elevation respectively equal to 1 c, 1 d, and 1 0 in the plan, and join up to a1. On the pattern, the line 2 1 is equal in length to the line with the same figures in the plan; and the lines C 1, D 1, O1 1 are respectively equal to lines c1 a1, dl a1, and 01 a1 in the elevation. The distance between C and D and D and O1 on the pattern will, of course, be equal to the lengths of the arcs c d and d 0 on the plan. To form the last triangles of the pattern, the lines 01 0 will be made equal in length to the back line in the elevation, or the line l1 a1 and the line 1 0 equal to the line with the same figures in the plan.
It is perhaps as well to point out that the patterns for all the objects mentioned can be struck out by the method of "triangulation." But whilst this method is general in its application, it is not so convenient for the particular cases as those shown.