Page:Practical Treatise on Milling and Milling Machines.djvu/166

160 b are equal, the cutter is centred with the trial piece, which will, of course, bring it central with the work.

The same method is employed when using a vertical spindle milling attachment, except the scratched line is left at the side of the piece where it is at the exact height of the centres. The gash is then cut and examined as described above.

Test Settings and Index Gears. Before cutting a blank, it is well to raise the knee until the cutter will just make a slight trace on the work to see if the lead obtained by the change gears is correct. If the material in the gear blank is expensive, it is sometimes advisable to make a cast iron blank to experiment with before cutting into the expensive material.

Fastening Blanks. Spiral gears are more liable to slip in cutting than spur gears. Small blanks may be dogged to the spindle, but the dog must be far enough from the blank so that it will not interfere with the cutter. For blanks that are more than three or four inches in diameter, it is better to use a taper shank arbor held directly in the spindle; and for still heavier work, the arbor may be drawn into the spindle with a threaded rod.

Cutting Teeth. In cutting the teeth, either the cutter should be stopped after cutting each groove and positioned so that the teeth will not scrape the sides and bottom of the groove, the table being returned by hand; or the knee should be dropped so that cutter will clear the groove just cut, and then run the table back to the starting point. Most mechanics prefer to stop the machine, for in dropping the knee, there is more liability of error, as the depth of cut has to be set for each groove, and this also takes more time than it does to stop the machine.

The remaining pages of this chapter are devoted to illustrations and descriptive data of gear cutting and similar operations on milling machines. These operations show how different gear cutting jobs can be set up, and are given simply as suggestions for those not familiar with this class of work.