The first process, milling with a properly formed revolving cutter, as in ordinary milling machine work, is applicable not only to the work mentioned above, but also to the cutting of spiral gears and a portion of the work upon worm gears. The cutter must be shaped exactly to the form of the space between the teeth of the gear to be cut. It must revolve at a speed suitable to the kind of metal to be cut, and must be supported by a spindle of ample dimensions properly supported in well-fitting journal boxes, set in housings of such dimensions and weight as to insure rigidity and the elimination of vibration. The work to be cut must be properly mounted so as to avoid vibration as much as possible, and be provided with feeding mechanism by which a rate of feed may be produced according to the speed of the cutter and the kind of metal to be cut.

For ordinary uses this process produces satisfactory results upon spur gears, internal gears, and racks. While it is used also for much bevel gear work and answers the requirements of ordinary work, there are conditions in the form of teeth of bevel gears that do not exist in that of spur gears. It has been previously explained that the dimensions of the tooth parts of a bevel gear are measured at the outer end, or at the largest part of the tooth, while the lines of the tooth are radial, meeting at the apex of the cone base, from which the gear takes its form. It may therefore be readily understood that it is quite impossible to form a revolving cutter so as to cut to the correct theoretical dimensions of the tooth through its entire length. It is the practice to form the contour of the cutter so that it is a compromise between the correct forms of the two ends of the teeth, but rather closer to the form at the outer or larger end, the form being practically correct at a point one-third of the face of the tooth from its outer end. As a rule, the width of face of a bevel gear should not be over five times the thickness of the teeth at the outer end. It is usually considerably less. If the face is too wide, the inner ends of the teeth will be cut away too much as the width of the cut is uniform from one end to the other; and this results in thin and useless teeth for a considerable part of their length from the inner end, as there is no contact with or bearing upon the teeth of the engaging gear at this point. For this reason, resort is had to filing the faces of the teeth at the large end, after the gears have been run together so as to show by the marks thus produced.