Page:Practical Treatise on Milling and Milling Machines.djvu/159

Rh Another tool, Vernier Caliper, No. 573, by use of which the bottom diameter of the teeth may be accurately measured to determine the depth of grooves, is shown in Fig. 63.

The depth of grooves may be ascertained when there are an even number of teeth by cutting two grooves opposite each other on the circumference of the blank and calipering the diameter from the bottom of the grooves, then computing the depth. When the number of teeth is uneven cut one groove and caliper the diameter from the bottom of the groove to the opposite side of the blank. In this last case be sure that the blank is of the correct diameter and runs true, otherwise the measurement will not be correct, unless allowance is made for these points.

Indexing. Indexing gear blanks is essentially the same as indexing any other work, and the instructions in Chapter IV are complete on this subject; therefore it is unnecessary to make any additional remarks here upon this point.

Fig. 63

Cutting Two or More Gears Simultaneously. If the holes in the blanks are straight, and the hubs do not project beyond the face, a number of blanks may be fastened together on a gang arbor and several gears cut at a time. Care should be taken, however, if this is done, to see that the sides of the blanks are exactly parallel, otherwise when the arbor nut is clamped, the blanks will spring the arbor, causing it to run out and making it impossible to produce accurate gears.

Cutting Bevel Gears. The teeth of bevel gears constantly change in pitch from their large to small end, and for this reason it is impossible to cut gears whose tooth curves are theoretically correct, with rotary cutters having fixed curves, such as those used for cutting these gears on a milling machine. The cutter employed must be of a curve that will make the correct form at the large end of the tooth, hence it will necessarily leave the curve too straight at the small end. It is, therefore, the practice to cut the teeth as nearly correct as possible,