Page:Practical Treatise on Milling and Milling Machines.djvu/165

Rh Cutting Spiral Gears. In Chapter IV, we have gone into the subject of cutting spirals thoroughly, and, inasmuch as spiral gears are essentially cylinders having a succession of spiral grooves evenly spaced on their periphery, many of the points we have treated apply equally well to cutting them. An important point in cutting these gears is the selection of the proper cutters to use. It is impossible to give in concise form any set of rules for doing this that will be readily understood, and anyone who desires to cut spiral gears, should make a far more complete study of the subject of spiral gearing than we can possibly give in this book. It is treated upon in our "Practical Treatise on Gearing," and "Formulas in Gearing," both of which books are extremely useful to the practical workman.

One point that it is well to remember is that in calculating spirals, the angle should be figured as that at the pitch line of the teeth, and not that on the surface or periphery of a gear.

Spirals of any angle to 45 ° can be cut on all of our universal milling machines with the cutter mounted in the regular way, and the swivel table swung to the proper angle, while those of an angle up to 55° with the axis, can be cut in some of our universal machines. If, however, the required angle is greater than that to which the table can be set, a vertical spindle milling attachment is required, and the adjustment for the cutting angle is then done with the attachment.

Fig. 68

To Set Cutter Central. It is essential that the cutter be set central with the work centres, and it may be done as follows: First, set the table, or attachment, in case the latter is used, to the correct cutting angle. Take a trial piece, Fig. 68, which is simply a cylindrical piece with centre holes in the ends, and mount it on the work centres, dogging it to the spiral head spindle. Draw, or scratch the line B C on the side of the arbor at the exact height of the work centres, and then revolve the arbor one-quarter of a turn by means of the index crank; that is, bring the mark B C exactly on the top of the piece. Now, start the machine and raise the knee until a gash is cut on the top of the piece. This gash shows the position of the cutter,' and if a and