The pattern for a sheave pulley has been explained because it embraces so many profitable points and conditions, not only in gluing and building up, but especially in chucking and turning, all of which must be done with great care and accuracy.
The 1-inch holes in the web are bored out with a 1-inch center bit, which, when well sharpened, will not split or splinter the thin webs of the two halves of the pattern, if care is taken to reverse the bore from the opposite side when the point of the center bit comes through. The holes should be given a slight draft; as shown in Fig. 159, with a small half-round cabinet file. When very large sheave pulleys, having arms, are to be made, such as are common for power transmission by rope or cable, the patterns are not halved but are made in one piece and the groove is cored around the rim. Such a pattern is illustrated in Fig. 166a, with a wide core print ccc extending entirely around the periphery of the pattern.
A segmental core box is made for one sixth or one eighth the circumference of the wheel, as shown in Fig. 166 J, and here again only half of the core box for a full core is needed. When coring the rim as above, the core print must be made wide, at least two to three times tin-depth of the groove, so that the core may rest firmly and remain in position without tilting while the metal is being poured into the mould. The 12-inch hand wheel, Fig. 167, with five arms and a round rim finished to 1½ inches in diameter, will also serve as a good illustration of pattern construction. On the rim of the pattern, 1/16 inch over all its surface must be allowed finish, making the diameter of the rim of the pattern l ⅝ inches, and the outside diameter of ih- pattern 12½ inches, while the inside diameter of the rim will be 87 inches.
Fig. 166 b.
The rims of such patterns are usually turned in two halves. A wooden chuck, in this case a plain board 12¾ inches in diameter, and ⅞ inch to 1⅛ inches in thickness, is screwed to the iron face-plate of the lathe, and turned true on the face and on the edge to 12 ½ inches in diameter. Ten blocks 2½ inches long, 2 inches wide, and ¾ inch in thickness are glued radially at equal distances around the face of the chuck as in Fig. 168. These blocks are turned even with the edge of the chuck, and the faces are also turned off true and straight so as to form a joint with the first row of rim segments.
The segments, ten in number, five for each layer, are sawed' from a ½-inch board, and should be 2 inches wide.
Five of these are carefully fitted and glued to the face of the blocks, as shown by the dotted lines in the drawing; and when the glue is dry the chuck is returned to the lathe, and the face of the segments turned flat and true to receive the second row, which is fitted and glued to the first.
Small hand screws must be used, three on each segment, to press the first layer to the blocks, and again to press the second layer to the first. The joints of the second layer must be over the middle of the alternate blocks from the joints of the first, so as to break joints with the first. When the glue is dry, place the chuck in the lathe, and turn the half rim thus constructed to a true semicircle with an outside diameter of 12⅛ inches and an inside diameter of 8⅞ inches, using a semicircular template of sheet zinc or copper to test by while turning.
UNIVERSAL SAW TABLE.
Coburn Machine Tool Co.
When turned and Band-papered, cat from the block of the chuck by using a ⅛ inch parting or cutting-off tool, care being taken to cut close to the segments forming the half rim. Turn off the face of the blocks on the chuck true and straight a second time, and construct and turn the second half of the rim in the same way as described for the first; but great care must be taken to make the two diameters, outside and inside, of each half exactly alike, otherwise the work on one half will be lost. As it is difficult to hold these two half rims for planing and fitting together, a concave and semicircular groove turned in the face of a second board, or chuck, in which they can be laid while being planed or fitted, will be found useful.
In all rim work of this kind the circular segments should be cut lengthwise with the grain of the wood, the object being in this construction, to do away, as much as possible, with all end wood.
While waiting for the separate layers of glued segments to dry, the arms should be made so as to be ready for the two half rims when completed.
The arms in this case should be made ⅝ inch in thickness at the hub and ½ inch in thickness where they enter the rim of the wheel. The construction is as shown in Fig. 169.
Five pieces, each 5 ¾ inches long, 2⅛ inches wide, and ⅝ inch in thickness are necessary. After being carefully fitted on the trimmer, a saw kerf 5/16 inch deep is cut in each joint (a, Fig. 169), into which a thin tongue of wood is inserted and glued, the tongues serving as tenons to hold the arms together. After fitting, and before grooving with saw kerf, the joints must be glue-sized and, when dry, carefully scraped smooth with a sharp chisel.
The grain of the wood in the tongues must run at right angles to or crosswise of the joint to insure the greatest strength.
When glued together and dry, from the center or intersection of the five pieces, mark with dividers set to a radius of 5¼ inches, and cut off the ends of the arms so that they will project each halfway into the rim.