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§ inches in diameter, representing ¼ inch greater diameter than the outside diameter of the rim. Carefully divide these last arcs into six 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 con-necting them with the center or intersection of the six arms. These radial lines (shown dotted in the drawing) will be the center line of each arm.
Saw off the ends of the arms on the above 20⅜-inch arcs, and from the center again draw on the six arm-pieces a third circle, whose diameter should be at least ⅛ inch less than the inside diameter of the rim, in this case 19 inches. On these arcs measure ¾ inch on each side of the center line, and on the circle representing the web, measure ⅞ inch on each side; connect these points from web to rim, and the arms will be 1¾ inches wide at web, and 1 1/2 inches at the rim.
These lines are shown by the dotted lines in Fig. 174. The width of the ends of the arms passing through the rim should be about 2½ inches, and the sides drawn parallel with the center line of the arm, as shown for hand-wheel arms in Fig. 169. The radius of the circle connecting the sides of the arms and the web, must be such as to be tangent to the edges 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 19-inch arcs which mark the inside of the rim (less 1/8 inch). All these lines are shown dotted in Fig. 174. 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 \ 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. 170 and 171, and the methods used for shaping and finishing are the same. For building the rim, a wooden chuck 20½ inches in diameter will be necessary.
A board 7/8 inch in the thhickness and having a bar 8 inches wide and of the same thickness, well screwed to the back with wood screws will be all that is necessary for a pulley of this size. To the 8-inch bar, the iron face-plate of the lathe is screwed, and the whole turned off true in the lathe, especially the face of the chuck to which the first layer of segments is to be glued.
Strips of heavy paper are often glued between the first layer of segments and the face of the chuck, so that the rim and the chuck may be easily separated when the rim is turned and finished. The paper usually splits, allowing separation without injury to the wood. A better method, however, is to glue twelve blocks, each 2 inches long, 2 inches wide, and ¾ inch in thickness, to the face of the chuck, in the same way as that described for the small hand wheel shown in Fig. 168.
When the rim is finished it may be sawed or cut off through the blocks without injury to the chuck and its future use. The segments to form the rim should bo six in number for each layer. They should be 7/8 inch wide, and cut from 3/8-inch or ½-inch stock, lengthwise with the grain of the bond, so as to avoid end wood. The first layer is fitted and glued to the blocks (or to the face of the chuck with paper between), and securely damped with small hand-screws, three to each segment. When the glue is dry, one hour being sufficient for thin 3/8-ineh segments, place the chuck in the lathe, and carefully turn off the face of the segments true and straight to receive the second layer.
This layer, in turn, is turned off in the lathe and the third layer is glued on, hand screws being used on each layer as on the first, and the joints of the segments so broken that no two will be directly opposite each other, all joints being carried to right or left of all preceding joints, thus securing the greatest possible strength to the rim.
No nails should be used in any work of this description, Having glued on a sufficient number of layers to form half of the rim, turn it to the; required external and internal diameters, making the thin or outer edge of the half rim nest to the chuck, and carefully giving the required draft to each side.
Before removing the half rim from the chuck, turn a groove 1/16 inch to 1/8 inch in depth and of about one third of the width of its thickness in the edge of the rim, as shown in Fig. 173 at a. Re-move the half rim from the chuck (or cut from the blocks), and proceed to build up and to turn the second half b in the same way as the first. Instead, however, of turning a groove on its edge, carefully turn and fit a small projection, or tongue, to the groove in the half a, as shown in the drawing. If the work has been done with care and accuracy the groove and tongue will bring the two halves into perfect alignment.
The arms are next fitted centrally to each half of the rim, and the two halves glued together over the arms in the same way as described for the hand-wheel pattern. The projecting ends of the arms are cut off and shaped to form a part of the outside of the rim. The internal curves of the arms at the inside of the rim are also filed and shaped down so as to form true curves without cusps or irregularities. The hub is next glued up in cross-layers as described in Pig. 172, turned out, and glued centrally on each side of the arms.