Fig. 341.

A very effective-looking cover can be made up out of four pieces of sheet metal to the shape as shown in Fig. 341. The shape is semicircular in both directions.

The pattern is marked out as seen in Fig. 342. A semicircle is described on one side of the plan, one-half being divided into three equal parts 0, 1, 2, and 3. For the pattern, the lengths 0' 1', 1' 2', and 2' 3' are made equal to the lengths of the correspondingly numbered arcs on the section; lines drawn through each point across and, on to these, lines run down from 1 and 2; thus the points 1" and 2" are obtained. The points so found are carefully joined up with a free flowing curve; the other side of the pattern being obtained in the same manner.

Fro. 342.

It will readily be seen that all the lines required for the setting out of the pattern can be obtained by drawing only one-eighth of the plan and half of the section. The full plan has been drawn in here to better explain the method. The joints can be made either by soldering, knocking-up, or any other way as required.

Rectangular rover of Circular and Elliptical Sections.

The pattern for this is shown marked out on Fig. 343, The side pieces are quarter circle in section, and to join on to these, if the joints are to be diagonal as in plan, it will be necessary for the end pieces to be a quarter of an ellipse in shape. The pattern for the side is set out exactly as for the square cover. Before the end pattern can be struck out it is necessary to draw the shape of the half section as shown. This is done by running lines down from 3, 2, and 1 on the semicircle to the diagonal line on the plan; then drawing across and marking up M A to equal D 3, N b to equal E 2, and P a to equal F 1. Joining these points up will give a quarter of an ellipse. For the end pattern the girth line C a' b' A' must be set out to equal in length the parts with the corresponding letters on the quarter ellipse. Lines are drawn across from the points, and others to meet these from the semicircle, and thus points on the pattern curve obtained. These being carefully joined will give the pattern as required. In Fig. 344 it will be seen that the semicircular section runs lengthways of the cover; the setting out of the two patterns is, however, exactly the same as in the last case.

Fig. 343.

Fig. 344.