Patterns for bevel and miter gears are built as illustrated at a and b, Fig. 261. The segments are to overlap as shown, which is not only a saving of stock, but also saves time which would be required to turn the angular rim from a square construction. It will be best to make a full size layout of a radial section of hub, arm, rim, and tooth. Marking the thickness of segments on this layout will show the diameter dimensions, which can be taken directly from the layout.
The process of gluing the segments will be the same as used for the pulley rims previously considered - gluing paper between the faceplate and the first layer of segments, and also nailing through the segments into the faceplate, or placing wood screws through the faceplate into the first layer of segments. When a sufficient number of courses have been glued together, including the temporary fitting of the arms, the face f and the edge e are to be turned to correct angle and diameter. Make a template for the angle shown in Fig. 261, taking the dimensions from the full-size layout. The rib c, which will finally be a continuation of the arms, is also turned to shape and to the thickness of the ends of the arms. The rim will then present the appearance shown at b, Fig. 261.
Remove the rim from the faceplate and nail and glue six blocks to the faceplate. Turn these blocks to the inside diameter of the ring c and fasten the rim to the faceplate with six clamp pieces, shown at d, Fig. 262. In this position the edge g and the inside of rim h is turned and finished as shown. It is not necessary here to describe the method used in finding the required angles for the face and edges of the rim, but, as in the case of spur-gear teeth, the student should refer to Mechanical Drawing.
The arms, partly shown in Fig. 263, are next fitted and fastened to the rim. It is well to glue a small disk on each side of the web of the arms, as shown, which not only strengthens the arms, but serves as a fillet around the hub of the wheel.
In Fig. 264, the hub H and the ribs of the arms RR are often made loose so as to lift with the cope, which is of great advantage in molding.
The blocks for the teeth are next fitted in place, either as illustrated in Fig. 264, or in the form of alternate blocks and strips as was shown for the spur gear, Fig. 254. After all the blocks are in place, the wheel must be put in the lathe and turned to the sizes and angles required for laying out the teeth. A sharp line must be drawn on the face of the blocks, while in the lathe, to serve as a guide for the dividers while spacing the teeth.
To obtain the center lines for the tooth faces after spacing on the blocks, it will be readily seen that the ordinary try-square cannot be used as in the case of the spur gears. A temporary square or centrolinead may be made for this purpose as follows:
Take a piece of hard wood about 6 inches long, 3 1/2 inches wide, and 1/2 inch in thickness. Dress the two edges perfectly parallel and from the upper edge a, Fig. 265, with a try-square and a sharp pointed knife, draw the line c, equally distant from each end of A, and at right angles to the edge a. Lay the edge b of A against another board B, of the same thickness, and continue the line c on this board, as shown by the dotted line. With the dividers set on the extended line c on the board B, and with a radius equal to the longest distance of the outside edges of the tooth blocks from the gear center, describe the arc xy on A. Cut the edge b to this are, and see that it perfectly fits the outer rim of the tooth block. Next make a thin blade of hard wood and screw it to the head A, using the greatest care to have one edge of the blade coincide exactly with the line c. After screwing the blade to the head, its accuracy may be tested by placing a try-square against the edge a. The result will be as shown in Fig. 266, in which the edge c is radial to the arc xy. This edge will describe the center lines of the teeth radially, as required.
Fig. 203. Part of Arm Pattern for Miter Gear.
Fig. 264. Section of Miter Gear, Showing Stock Assembled for Teeth.
This temporary square can be used, within the limit of its blade, on wheels of larger diameter than that to which it has been fitted, but cannot be used for smaller wheels. For the larger gears the position will be as shown in Fig. 267, which will give the correct perpendicular if the angles at x and y are carefully made. By using in this way, only a few squares will be needed for a great number of wheels.
When the teeth are large, they must be screwed on from the inside of the rim. If small, they should be bradded from the outside or face of the tooth into the rim after the teeth have been shaped and finished.
Two templates will be necessary for laying out the ends of the teeth, the outer ends being larger than the inner. These templates are made as described for spur gears, and have the outer end bent to fit over the angles of the rim.
Fig. 265. Construction of Template.