Fig. 2785.
Fig. 2786.
Fig. 2787.
Let it be required to make a pattern for a flanged pulley, such as shown in section in [Fig. 2784]. It would be constructed in two halves composed of a number of courses as from 1 to 8, and each course would be composed of segments of the form shown in [Fig. 2785]. The length of the arc of these segments must be such that it will require a certain number of these to complete the circle of that part of the cylinder which the segment is to form; and the manner of accomplishing this is shown in [Fig. 2786], in which the circle c is of the diameter of the outside, while circle d is that of the outside of the pulley proper, circle e is of the diameter of the inside of the pulley rim. These circles are divided into as many equal divisions as there are to be segments in the circumference; hence the number of divisions determines the length of arc of the segments. Thus a would be a segment for the body of the pulley, and f a segment for the rim. A template is then made of each one of these segments, as at a and f. This template must be made slightly larger in every direction than the respective divisions, to allow for the stuff that will be turned off in truing the pattern in the lathe and in jointing the segments to one another during the building. The templates are employed to mark out on the board which should first be planed to the required thickness. This will be a trifle thicker than the course so as to allow for truing the surface of each finished course in the lathe. The courses are best built up on the chuck of the lathe on which they are to be turned, and a saving in time will be effected if there are two chucks, so that a course on one half of the pattern may be built up while the glue of another course on the other half is drying. On the lathe chuck, and directly beneath, where the joints of the segments will come, pieces of paper as at a, c, e, g, [Fig. 2787], and if the segments are long ones, intermediate pieces of paper, as b, d, f, h, will be necessary. The radial edges of the segments are trimmed on what is termed a shooting board, which is a device such as shown in [Fig. 2788], in which a is a piece of board on which is fastened the piece b. s is a piece projecting above b, and is provided to rest the segment s′ against, the flat surface of the latter lying on the board b. It is thus held in a fixed position, ready to have its edges e planed, the whole being laid upon the bench against the bench stop g. If, however, it is more convenient to rest the shooting board across the bench, a piece c may be fastened beneath a, so as to come against the edge of the bench as in [Fig. 2789], in which t is the bench. The plane is laid with its side on a as in [Fig. 2790], so that the surface of a acts as a guide, keeping the edge of the plane vertical, and thus planing the edges of the segment square. The plane is operated by hand in the usual manner (save that it lies on its side), taking its cut most off the outside or inside of the edge of the segment s′, according as the position of the latter is varied. In some of the shooting boards manufactured by tool makers, the height of b from a is adjustable, so that all parts of the plane blade edge may be used, which saves grinding, since only that part of the edge that is used dulls. Also there is provided means whereby the required lateral position of the segment may be adjusted; such a device is shown at p, [Fig. 2788], which is a plate having a slot through it, through which passes the thumb screw v, which screws into s. Hence the plate may be adjusted so that when one end of the segment rests against the end of s, and the other against the end of p, its edge e will be in the proper position to be planed to correct angle by the plane, whose line of action is in this case rendered positive by means of a slide on the plane, acting in a groove in the base a.