From the same center describe a circle 3⅛ inches in diameter, forming the web of the arms; and from this 3⅛ -inch circle, taper the arms to ½ inch in thickness at the ends, care being taken to plane the same amount from each side, and to dress the arms evenly so that they will revolve in the same plane. This being done, from the center describe arcs on the outer ends of the arms, with a radius of 4⅜ inches (8¾ inches diameter, which is ⅛ inch less than the inside diameter of the rim), and divide the imaginary circle thus formed into five equal parts with the dividers. Draw radii from the points thus obtained, to the center. These radii will be the central lines of the arms, as shown by the dotted lines in Fig. 169. On each side of the intersection of the radii and outer circle, measure ½ inch to the right and left, and on the circle denoting the circumference of the web, mark 11/16 on each side of the radii; connect the points thus obtained, and the result will be five arms 1⅜ inches wide at the web and 1 inch wide at the rim, as shown in the drawing. The ends of the arms which enter the rim should be, in this case, 1¾ inches wide, and the sides are drawn parallel to the radius which marks the center of each arm. The curves which connect the arms at the hub must be drawn of such radius as to make the curve tangent to the circle forming the extremity of the web, and also tangent to the sides of the two connected arms as shown at d. The small circles at the intersections of the arms with the rim, must bo tangent to the edge of the arm and to the circle 8¾ inches diameter) which marks 1/16 inch less than the inside diameter of the rim as shown at c c.
Having laid out the arms as above, and as shown in the drawing by the dotted lines, saw them to shape and round them up to an elliptical form as shown in the cross-sect ion at e. Fig. 169. 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. 170.
Divide the cross-section equally by the lino A B; measure 1/16 inch; as at at a c d f ; and with dividers adjusted so as to be tangent to the sides of the cross-section of the arm, and to pass through a c and d f, draw the curves a b c and d e f.
After filing and working off the sides of the arms to these curves, the angles at a c d and f' are carefully rounded with sand paper, care being taken not to lessen the width of the arm at any point. The result will be as shown in Fig. 171, 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 arms thus shaped and finished are cut or let ¼ inch into each half of the rim, and great care must be taken to keep them central with the rim. Before marking the rim for the mortises which will receive the ends of the arms, test their positions with the dividers, spacing from the center of the arms to the outside edge of the rim, and moving the arms until a central position is obtained; after which, with the point of a knife or awl, scribe around the end of each arm. and proceed to cut, with a chisel, the mortises ¼ inch deep into each of the half rims, and so cut and fit that the two pieces of the rim may meet and form a close joint, after which they are glued and clamped together over the arms with hand screws.
The hubs are next turned, each from a solid block, or better from thin pieces ¼ inch to ⅜ inch in thickness, each thin piece being placed crosswise on the other, as shown in Fig. 172. The hubs must be turned with a draft or taper of ⅜ inch to 12 inches, and have a curve of ¼-inch radius at the base where they unite with the arms. After gluing on the hubs, smooth off all connected parts hub, and finish with three coats of shellac, smid-between each coat, as already described for other of patterns for special pulleys enters largely into ny pattern shops. In these patterns the rims are ents ⅜ inch to ½, inch in thickness.
for such a pulley is shown in Fig. 173. The diameter of the arms is 5 inches, and the diameter of the hub 3½ each end and tapering to 3¾ inches in diameter at the:
If the rim is to be finished on the face and edges only, 1/16 inch must be allowed for turning, making the outside diameter of the pattern 20⅛ inches, and the width of the face should be 6¼ inches.
In addition to 1/16 inch for finish, the draft on the outside of the rim, from each edge to the center, should be in the ratio of ⅛ inch to 12 inches, and on the inside of the rim the draft must be ⅜ inch to 12 inches.
The thickness of the rim at its edges will be 5/16 inch, and with outside and inside draft added, its thickness at the arms will be about 7/16 inch. The inside diameter of the rim at the arms will be nearly 19 3/32 inches. This pulley should have six straight arms ¾ inch in thickness at the hub and ⅜ inch in thickness at the rim. The width of the arms at the web should be 1¾ inches and at the rim 1½ inches exclusive of the connecting curves at web and rim. Six pieces 10½ inches long, 2¾| inches wide, and ¾ inch in thickness, must be carefully fitted as shown in Fig. 174.
After fitting, the connecting joints are glue sized, and when dry carefully scraped smooth with a sharp chisel, and a saw kerf 6/16 inch deep cut in each. The tongues used for tenons in these kerfs should be a little less than ⅝ inch long, the grain of the wood running always at right angles to the line of the joint to give the greatest strength to the tenons. The six pieces should be glued in two groups of three pieces each; and when dry, these two groups can easily be refitted, if necessary, and glued.