Fig. 147.
Fig. 148.
Fig. 149.
The construction of a wheel, such as shown in [Fig. 145], is as follows: The rim r must be built up in segments, but when the courses of segments are high enough to reach the flat sides of the arms they should be turned in the lathe to the diameter on the inside, and the arms should be let in, as shown in the figure at o. The rest of the courses of segments should then be added. The arms are then put in, and the inside of the segments last added may then be turned up, and the outside of the rim turned. The hub should then be added, one-half on each side of the arms, as in the figure. The ribs c of the arms are then added, and the body is completed (ready to receive the teeth), by filleting in the corners. An excellent method of getting out the teeth is as follows: Shape a piece of hard wood, as in [Fig. 146], making it some five or six inches longer than the teeth, and about three inches deeper, the thickness being not less than the thickness of the required teeth at the pitch line. Parallel to the edge b c, mark the line a d, distant from b c to an amount equal to the required depth of tooth. Mark off, about midway of the piece, the lines a b and c d, distant from each other to an amount equal to the breadth of the wheel rim, and make two saw cuts to those lines. Take a piece of board an inch or two longer than the radius of the gear-wheel and insert a piece of wood (which is termed a box) tightly into the board, as shown in [Fig. 147], e representing the box. Let the point f on the board represent the centre of the wheel, and draw a radial line r from f through the centre of the box. From the centre f, with a trammel, mark the addendum line g g, pitch line h i, and line j k for the depth of the teeth (and also a line wherefrom to strike the teeth curves, as shown in [Fig. 129] if necessary). From the radial line r, as a centre, mark off on the pitch circle, points of division for several teeth, so as to be able to test the accuracy of the spacing across the several points, as well as from one point to the next, and mark the curves for the teeth on the end of the box, as shown. Turn the box end for end in the board, and mark out a tooth by the same method on the other end of the box. The box being removed from the board must now have its sides planed to the lines, when it will be ready to shape the teeth in. The teeth are got out for length, breadth, and thickness at the pitch line as follows: The lumber from which they are cut should be very straight grained, and should be first cut into strips of a width and thickness slightly greater than that of the teeth at the pitch line. These strips (which should be about two feet long) should then be planed down on the sides to very nearly the thickness of the tooth at the pitch line, and hollow on one edge to fit the curvature of the wheel rim. From these strips, pieces a trifle longer than the breadth of the wheel rim are cut, these forming the teeth. The pieces are then planed on the ends to the exact width of the wheel rim. To facilitate this planing a number of the pieces or blank teeth may be set in a frame, as in [Figs. 148] and [149], in which a is a piece having the blocks b b affixed to it. c is a clamp secured by the screws at s s, and 1, 2, 3, 4, 5, 6 are the ends of the blank teeth. The clamp need not be as wide as the teeth, as in [Fig. 148], but it is well to let the pieces a and b b equal the breadth of the wheel rim, so that they will act as a template to plane the blank teeth ends to. The ends of b b may be blackleaded, so as to show plainly if the plane blade happens to shave them, and hence to prevent planing b b with the teeth. The blank teeth may now be separately placed in the box ([Fig. 146]) and secured by a screw, as shown in that figure, in which s is the screw, and t the blank tooth. The sides of the tooth must be carefully planed down equal and level with the surface of the box. The rim of the wheel, having been divided off into as many divisions as there are to be teeth in the wheel, as shown in [Fig. 150], at a, a, a, &c., the finished teeth are glued so that the same respective side of each tooth exactly meets one of the lines a. Only a few spots of glue should be applied, and these at the middle of the root thickness, so that the glue shall not exude and hide the line a, which would make it difficult to set the teeth true to the line. When the teeth are all dry they must be additionally secured to the rim by nails. Wheels sufficiently large to incur difficulty of transportation are composed of a number of sections, each usually consisting of an arm, with an equal length of the rim arc on each side of it, so that the joint where the rim segments are bolted together will be midway between the two arms.
