The face should be glue sized to prepare it for the blocks which are to form the teeth of the gear. After sizing and removing the raised grain of the wood, the periphery of the wheel must be spaced for the required number of teeth. With a try-square and very sharp awl draw lines through the points obtained by the spacing, as shown in Fig. 257. Should the teeth be of moderate size, say 3-pitch or less, the tooth blocks should be glued on bo as to meet each other on the rim of the wheel, as shown in Fig. 258, and, not being screwed on, must be nailed with brads from the face of each tooth into the rim after being shaped and finished.
Each block must be bo fitted as to reach only from line to line, Fig. 257, care being taken to have each block parallel to and coincide with its own line, reaching exactly to the line. When all the blocks are placed and glued, the wheel is returned to the lathe and the periphery turned off straight and to the required diameter for the addendum or tops of the teeth. The ends of the blocks are elso turned even with the edge of the wheel rim, and before removing from the lathe, a circular line must be drawn on the ends of the blocks, on both sides of the rim, indicating the whole depth of the teeth. The use of this line will be explained later; it is the only circular line needed for laying out, or for working out the teeth.
When the teeth are large, a tooth block is first fitted on and screwed from the inside of the rim, as shown in Fig. 254, one edge of the block touching, but not covering its line on the face of the rim. The thin strip is next fitted* glued, and bradded against the block, with the opposite edge of the strip reaching just to, but not covering the next line. A second tooth bleck is fitted and screwed in place, then a second strip, and this alternate placing of blocks and strips is continued until the surface of the rim is covered, having a block and strip for each tooth required. Care must be taken not to allow any glue to get between the blocks and the strips when gluing and nailing the strips on, as each block must be taken off, one at a time, after being laid out, to work the tooth to shape.
When all the blocks and strips are in place, the wheel must be returned to the lathe and the face of the blocks turned to the diameter required for the addendum or tops of the teeth, and the ends of the blocks also turned even with the rim. The whole depth or clearance circles are marked, one on each side, while revolving in the lathe, as explained for a wheel with smaller teeth. All parts of the rim should now be made perfectly smooth with fine sandpaper, using a holder or block to prevent rounding the corners or angles of the tooth blocks.
Beginning at the middle of a block, space the required number of teeth on the periphery of the tooth blocks, and should the first trial not result in even spaces, the trial spacing must be continued until the greatest accuracy has been obtained, that is, until all distances from point to point are exactly equal. Through each spacing point, found as above, very sharp but light lines are drawn across the face of the blocks, as was shown for the wheel rim in Fig. 257. When drawing these lines it will be found best to draw along the inside edge of the try-square blade instead of the outside as is usual. The reason for this is that on small or medium-sized wheels a much firmer base will be given for holding the square, and more accurate lines will be the result. A coat of shellac brushed over the ends and faces of the blocks, if sandpapered smooth after being allowed to dry, will greatly assist in laying out the teeth, hardening the surface, and enabling sharper lines to be drawn.
A template must next be made of the exact form of the tooth required. This will always be given full size in the detail drawings furnished to the pattern maker. Should the wheel be of small diameter, the template may be laid out and cut on the end of a long strip of zinc, but it is better to fasten the template to the end of a wooden bar, as shown in Fig. 259, a narrow slot having been cut through the back end of the zinc to allow of exact adjustment to the diameter of the wheel. This wooden bar is hung centrally on a peg or dowel which must be placed exactly in the center of the hub. For this purpose it is customary to use a block of wood as a temporary hub, the center of which may be easily found from the periphery of the blocks by the dividers. A very slight sharp notch is made in the exact center of the end of the tooth template, which must be radial to the hole in the opposite end of the bar on which the template revolves. This notch is shown in Fig. 259.
Fig. 259. Template Used to Lay Out Teeth of Spur Gear.
To use the template, place it over the center pin and bring the notch exactly in line with one of the spacing lines on the outside of a block, and with a very sharp pointed awl mark the tooth on the end of the block. Then swing the template to the next line and mark as before, continuing the process until a tooth has been laid out on the end of each block. The wheel is now turned over and the same process repeated on the other side. It will be readily seen that if the spacing lines have been squared across the face of the wheel with accuracy, the teeth laid out on the two sides will be true and perpendicular to each other, a spacing line forming the exact center of each tooth, and for this reason these lines should always be very light but sharp and clearly defined.
For convenience in cutting and paring, a second series of lines should now be drawn across the face of each block connecting the extreme ends of the lines which describe the shape of the tooth on each end of the block. Should the wheel be small and within the capacity of the band saw, all superfluous wood may easily be removed from between the teeth.
If the band saw is sharp and evenly set, and the operator skillful, the teeth may be sawed so as to need but very slight correction with the paring chisel and gouge. As the hubs usually project beyond the rim on each side of the wheel, they should be left loose and removed before placing the wheel on the saw table.
For large wheels and heavier teeth, each tooth block should be unscrewed and removed, one at a time, and planed to the lines marked on its ends and face, after which it is returned to its place before a second one is taken off. This is continued until all the teeth are shaped, when it will be necessary only to construct fillets at the base of the teeth, and also to work each space down to the whole depth or clearance circle, the circle having been drawn for this purpose and also as a guide for bringing all tooth spaces to the same depth.
Small gears, or pinions, as they are called, are usually made with a solid web instead of arms, and are glued up in solid blocks of end wood, the grain of the entire block running parallel with the face of the teeth. Such an end-wood pinion is shown in Fig. 260. It is turned and the gear laid out and cut in the same way as described for the larger wheels, except that the teeth are not glued on but are cut out in the solid disk.