The groove is a semicircle 1 inch wide, and the rim containing the groove is connected with the hub by a solid web ¼ inch in thickness, and having four or Biz holes, each 1 inch in diameter, this web taking the place of arms. If there is to be no finish, on the sheave, as is usual, the only allowance to be made on the pattern, which must be parted, will be for shrinkage and for draft.
A cross-section through the finished pattern for this casting is shown in Fig. 159.
In all large patterns of this kind, the web is first glued up in sectors, six, eight or more in number, according to the size of the sheave (see Fig. 160). The sectors are fitted by hand or on the trimmer, the ends are glue-sized, and when the sizing is dry the joints are carefully scraped smooth, and the whole glued together. After, drying for four or five hours, it is sawed to a circle of ½-inch greater diameter than the finished pattern, and the block for the hub is glued over the center. Six segments to form the outer rim are glued around on the outer edge, care being taken to break joints as shown in Fig. 161. If the groove is to be large, the six segments should be of half the thickness only, and a second set of segments of like thickness glued over the first, breaking joints not only with the first set, but also with sectors of the web.
In other words, in all glued-up rims, no two joints should be directly over each other. All joints must be so broken and so distributed as to give the greatest possible strength to the rim. In the present cage, our pattern is so small that it is only necessary to use a thin board, ¼ inch in thickness for each half of the web. After sawing to 60½ inches in diameter, ½ inch for turning, a block \ inch in thickness is glued on the center of each to form the hub; and six segments 1¼ inches wide and ½ inch in thickness, are glued around on the outer surface of each to form the rim and groove, as shown in Fig. 162.
Care must be taken to place the segments so that the grain of the web will be crossed by two of the segments as shown in the drawings.
On the second half b of the pattern, a thin circular block ¼ inch in thickness is glued on the inside opposite to the hub block, to form the projection (⅛ inch) which will keep the two halves of the pattern in alignment, as shown in the cross-sectional drawing in Pig. 159. Having glued up the stock as described, and as shown in Fig. 162, the outside must be planed to a level surface, or bo that the sis segments forming the rim and the center hub block will be in the same plane.
The half pattern is now screwed on the screw chuck of the lathe as illustrated in Fig. 163, and the inside, or the parting face c, is turned perfectly straight and true. The edge is turned down to 6 inches in diameter, and the quartered circle shown by the dotted lines is carefully shaped. A template, made as shown at d, will assist greatly at this stage of the work. A recess is turned at the center, and in the face of a, Fig. 159, 1½ inches in diameter and ⅛ inch deep, to receive the corresponding projection on the half pattern b which is to keep the two halves in alignment.
The half pattern a, is now removed from the screw chuck, and the second half b is screwed on and turned in the same manner except that the central projection is carefully turned to fit in the recess in a.
Before removing b from the chuck, test by trying the second half a, and change b until a perfect fit is obtained between the two halves, not only in the central recess and projection, but also in the two curves which form the semicircular groove of the rim. A cross-section of the pattern at this stage of construction is shown in Fig. 164.
A disc or chuck of wood 5½ inches in diameter is now screwed to the iron face-plate, or the screw chuck, and turned off true on the face with a projection ⅛ inch high which will fit into the recess in the middle of the parting face of a. This projection will center the half pattern a on the face plate, and it can be held in position by two or four short wood screws driven through the web into the wooden chuck as shown in Fig. 165.
Care must be taken to place the screws in such a position that the screw holes will be cut or bored out when making the four or six openings 1 inch in diameter in the finished web of the pulley. The screws must be small and slender and the heads well countersunk out of reach of the turn, ing tools. The face of the half pattern is now turned to the required shape, the template shown at e in Fig. 165, being used for the purpose. Having finished with fine sandpaper, remove the half pattern, and turn off the projection on the center of the wooden chuck; turn a recess instead to receive the projection on b, and proceed with this second half as with the first. If the wood has been well seasoned, and the work carefully done a perfect 6-inch sheave pulley pattern will be obtained, such as is shown in Fig. 159.