Page:Practical Treatise on Milling and Milling Machines.djvu/160

154 and then finish the gears by hand, filing the small ends of the teeth to get the correct curve.

Pitch of Bevel Gear. The pitch of a bevel gear is always considered as that at the largest end of the teeth.

Data Required to Cut Bevel Gears with Rotary Cutter. Pitch and number of teeth in each gear. The whole depth of tooth spaces at both large and small ends of teeth.

The thickness of teeth at both ends.

The height of teeth above the pitch line at both ends. The cutting angle; the angle to set spiral head on milling machine, and the proper cutter or cutters.

Scratch Depth Line on Blank. Before placing the blank on machine, measure the length of face, angles and outside diameter of blank, and, if all dimensions are correct, place the blank on the arbor and fasten it securely in place ; then scratch the whole depth of space at large end with a depth of gear tooth gauge similar to that shown in Fig. 64.

Selection of Cutter for Bevel Gears. The length of teeth or face on bevel gears is not ordinarily more than one-third the apex distance, Ab, Fig. 65, and cutters usually carried in stock are suitable for this face. If the face is longer than one-third the apex distance, special thin cutters must be made.

Rule for Selecting Cutter. Measure the back cone radius a b for the gear, or b c for the pinion. This is equal to the radius of a spur gear, the number of teeth in which would determine the cutter to use. Hence twice a b times the diametral pitch equals the number of teeth for which the cutter should be selected for the gear. Looking in the list given on page 149, the proper number for the cutter can be found.

Thus, let the back cone radius a b be 4" and the diametral pitch be 8.