The making of patterns for special pulleys enters largely into the [work of many pattern shops. In these patterns the rims are built up of. segments 3/8 inch to 1/2 inch in thickness. To illustrate this work fully, let us take up the successive steps in the construction of a countershaft pulley 20 inches in diameter and of 6-inch face, made to fit a shaft 1 3/4 inches in diameter. The pattern for such a pulley is shown in Fig. 177.
Fig. 177. Sectional View of Pattern for 20-Inch Pulley.
If the rim is to be finished on the face and edges only, 1/16 inch must be allowed for facing, making the outside diameter of the pattern 20 1/8 inches; and the width of the face should be 6 1/2 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 1/8 inch to 12 inches, and on the inside of the rim the draft must be 1/4 inch in 12 inches. The thickness of the rim at its thinnest edge is 1/4 inch and, with outside and inside draft added, its thickness at the arms will be about 3/8 inch. The inside diameter of the rim at the arms will be 19f inches.
This pulley should have six straight arms 3/4 inch in thickness at the hub and 5/8 inch in thickness at the rim. The width of the arms at the web should be 1 3/4 inches and at the rim 1 1/2 inches, exclusive of the connecting curves at web and rim.
Six pieces 10 1/2 inches long, 2 3/4 inches wide, and 3/4 inch in thickness, must be carefully fitted, as shown in Fig. 178. After fitting, the connecting joints are glue sized, and, when dry, carefully scraped smooth with a sharp chisel, and a saw kerf 5/16 inch deep cut in each. The tongues used for tenons in these kerfs should be a little less than 5/8 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.
The next step is to draw, from the center formed by the intersection of the six pieces, a circle 5 inches in diameter, representing the web of the arms, and, near the extremities of the pieces, the arcs of a circle 20 1/2 inches in diameter, representing 5/16 inch greater diameter than the outside diameter of the rim at the location of the arms. Carefully divide these last arcs into 6 equal spaces with the dividers, bringing the points thus obtained as nearly to the middle of the six arms as possible; and from the six points thus spaced, draw radial lines connecting them with the center or intersection of the six arms. These radial lines, shown dotted in Fig. 178, are the center lines of each arm.
Saw off the end3 of the arms on the above 20 1/2-inch arcs, and from the center again draw on the six arm pieces a third circle, whose diameter should be at least 1/8 inch less than the inside diameter of the rim - in this case 19 inches. On these arcs measure 3/4 inch on each side of the center line, and on the circle representing the web measure 7/8 inch on each side; connect these points from web to rim, and the arms will be 1 3/4 inches wide at web, and 1 1/2 inches at the rim.
Fig. 178. Layout of Arms for Fig. 177.
These lines are shown by the dotted lines in Fig. 178. The width of the ends of the arms passing through the rim should be about 2 1/2 inches, and the sides of the end sections should be drawn parallel with the center line of the arm, as shown for the hand-wheel arms in Fig. 171. The radius of the circle connecting the sides of the arms and the web must be such as to be tangent to the edge3 of the two connected arms, and also tangent to the circle marking the diameter of the web.
The smaller curve connecting the two edges of each arm with the rim must be of such radius as to be tangent to the arm and to the 19f-inch arcs which mark the inside of the rim in the plane of the arms. All these lines are shown dotted in Fig. 178.
The arms are now ready for sawing to shape on the band or scroll saw, care being taken to saw just outside of the lines so that each arm may retain its full size and width. After sawing to shape, the edges must be dressed smooth and free from all irregularities of the sawing.
Next, from the web circle, taper the arms to 5/8 inch in thickness at the extreme ends, care being taken to see that the taper of both sides of the arms is uniform from the web circle to the rim.
The shape of the arms should be elliptical or nearly so, and a cross-section at any point in an arm may be obtained in the same manner as described for the hand wheel shown in Figs. 174 and 175, and the methods used for shaping and finishing are the same.