Prepare stock 1/2 inch thick, and saw to a length slightly greater than the chord for five segments. Stack and nail three of these pieces together, and lay out one segment, as shown in Fig. 172, as follows: outside diameter to be 12 1/2 inches; width of segment 2 inches; and the chord equaling the diameter multiplied by the sine of half of the included angle, for example, the included angle being 72 degrees, the sine of 36 degrees equals .5877, and 12.5 times .5877 equals 7.35 inches, the length of the chord. Band saw these segments and use the top segment to lay out the other segments, marking them with a pencil. Glue a sheet of paper to the faceplate on the location of the rim of the pattern, and carefully fit and glue five of these segments to the paper, fastening the segments at the same time with two 1-inch finish nails. The heads of these nails should be driven below the surface of the stock. Be sure that the segments are concentric with the center of the faceplate.
As soon as the glue is dry, turn the face and the outer and inner edges of segments true. Locate the partially completed arms on this ring and fasten temporarily with five small nails, Fig. 173. Remove from the lathe, fit and glue the five segments between the ends of the arms, clamping these segments with two hand screws each while the glue is drying. Remove the arms as soon as the segments are glued in place, to prevent them from being glued in. As soon as the glue has set firmly, remove the hand screws and turn the inside edge of these last segments to their proper diameter and form, using a small sheet-metal template of zinc to test the form while turning. This turns the rim between the arms. Glue the arms in place, and also the last layers of rim segments, using the hand screws as before. The upper half of the rim can now be turned, using the template.
Before reversing, glue and nail five blocks of wood to the faceplate between the arms, pressing these firmly against the inner edge of the rim; these will serve to center the pattern afterward. The pattern can now be removed from the faceplate with a thin chisel and mallet. The paper will split and the nails will pull themselves either out of the pattern or the faceplate, and may be removed with a pair of pliers. Refasten the pattern on the faceplate by passing slim wood screws through the arms into the faceplate, or up through the faceplate into the rim. The five blocks glued to the faceplate will keep the work concentric. These screw holes can be filled with glued plugs when finishing the pattern.
Trimming the arms to an elliptical form, as shown in the cross-section at Figs. 174 and 175, can be carried on while waiting for the glue in the rim to dry.
Fig. 175. Final Form of Arm between Rim and Hub.
The finished shape of the arm, at any point in its length, is found by drawing a cross-section of the arm at that point, as in Fig. 174. Divide the cross-section equally by the line A B; measure out 1/16 inch, as at ad and cf, and with dividers adjusted so as to be tangent to the sides of the cross-section of the arm, and to pass through a d and cf, draw the curves abc and def. After working off the sides of the arms to these curves, the angles at a, c, d, and f are carefully rounded with sandpaper, care being taken not to lessen the width of the arm at any point. The result will be as shown in Fig. 175, which gives a strong firm edge to the arm, and one which will not break or splinter off while being rammed up in the sand.
The hubs are to be turned from solid stock and with a draft or taper of | inch in 12 inches, and must have a curve of 1-inch radius at the base where they unite with the arms. If the hubs and the diameter of the cored hole are not liable to be changed, the nowel hub should be fastened firmly to the arms, and should be chucked into the arms, as shown in Fig. 176. This produces a fillet between hub and arms which is not liable to become loose. The fillets on both nowel and cope hubs should be turned on, and the cope hub should be made loose so that it will lift with the cope mold. If the pattern is to be completed as cheaply as possible, the hubs can be turned with a short 1-inch dowel, and fitted to a 1-inch hole at the center of the arms. In this case the fillet can be left off the cope hub, as a fillet this way is very fragile and easily broken. The molder will form the fillet in the green-sand mold. If the hubs are to be let into the arms, the recess in the arms can be chucked at the same time the rim is turned.
After gluing on the nubs, smooth off all connected parts of rim, arms, and hub, and finish with three coats of shellac, sandpapering smooth between each coat as already described for other patterns.