For practical reasons, the first method of molding - for green-sand core, or, in this case, ring - the 6-inch sheave pulley shown in Fig. 159 would ordinarily not be used. The expense of molding would more than offset the alternative extra expense caused by making a dry-sand core for the groove in the outer edge of the sheave. However, consideration of this method is offered at this time solely for the study of the manner of building the pattern, as it will be found practical to use this process in making numerous other patterns. The study of the molding process is also of value. The molding of this pattern is as explained in connection with the use of the green-sand ring, under Molding Practice, Pattern Making, Part I.
The wood pattern for this casting, molded as described, is comparatively frail, so a working pattern should be made of some aluminum alloy in order that the weight will not interfere with the molding process. The metal pattern should be lathe finished all over; consequently the wood pattern shall include stock for this finish. Allow 1/16 inch on each surface for this finish, besides the allowance for the aluminum shrinkage which will be about J inch per foot. These must be added to the shrinkage allowed for the metal used in the final castings; if the final castings are to be of iron, and the metal pattern made of cast aluminum, the shrink allowance should be based on a shrinkage of 3/8 inch per foot. The wood pattern is called the master pattern.
Fig. 159. Smell Sheave Pulley.
Fig. 159. Section of Pattern tor Sheave.
A cross-section through the finished pattern for this casting is shown in Fig. 160. The groove is a semicircle 1 inch wide, and the rim containing the groove is connected with the hub by a solid web 1 inch in thickness and having four or six 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.
In all patterns of this kind, the web is first glued up in sectors, Fig. 161, six, eight, or more in number, according to the size of the sheave. 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 4 or 5 hours, it is sawed to a circle of 1/4 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. 102. 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 glucd-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.
Fig. 163. Construction Views of Sheave pattern.
In the present case, our pattern is so small that it is only necessary to use a thin board 1/4 inch in thickness for each half of the web. After sawing to 6 1/2 inches in diameter - 1/2 inch for turning - a block 1/2 inch in thickness is glued on the center of each half to form the hub, and six annular segments 1 1/4 inches wide and 1/2 inch in thickness are glued around on the outer surface of each to form the rim and groove, as shown at a and b, Fig. 163. 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, 6, of the pattern, a thin circular block 1/4 inch in thickness is glued on the inside opposite to the hub block, to form the 1/3-inch projection which will keep the two halves of the pattern in alignment, as shown in the cross-sectional drawing in Fig. 160. Having glued up the stock as described, and as shown in Fig. 163, the outside must be planed to a level surface, or so that the six segments forming the rim and the center hub block will be in the same plane.
The half pattern a is now screwed on the screw chuck of the lathe, as illustrated in Fig. 164, 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 in Fig. 159 is carefully shaped. A template, as at d, Fig. 164, will assist greatly at this stage of the work. A recess is turned at the center and in the face of a. Fig. 160, 1 1/2 inches in diameter and 1/3 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 6 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.
Fig. 164. Stock Mounted on Faceplate.
Fig. 165. SectionShowing Joint of Sheave Pattern.
A cross-section of the pattern at this stage of construction is shown in Fig. 165.