It is customary in most machine shops to make all milling cutters of more than 1/2-inch face with teeth cut spirally as in Fig. 202. The amount of spiral given the teeth varies in different shops and on different classes of work.
The object of spiral teeth is to maintain a uniformity of cutting duty at each instant of time. With teeth parallel to the cutter axis, the tooth, on meeting the work, takes the cut its entire length at the same instant, and the springing of the device holding the work and of the cutter arbor causes a jump to the work. If the teeth are cut spirally, the cut proceeds gradually along the whole length of the tooth; and after it is started, a uniform cutting action is maintained, producing smoother work and a truer surface, especially in the case of wide cuts.
Milling cutters may be cut with either a right- or a left-hand spiral or helix, although it is generally considered good practice to cut a mill having a wide face with a spiral that will tend to force the cutter arbor into the spindle rather than to draw it out; then, again, it is better to have the cutting action force the solid shoulder against the box, rather than draw the adjusting nut against the box.
Where two very long mills are used on the same arbor and it is found necessary to cut them with a quick spiral, one cutter is sometimes made with a right-hand spiral and the other with a left-hand spiral, in order to equalize the strain and to reduce the friction resulting from the shoulder of the spindle pressing hard against the box.
Special care should be taken in cutting spiral milling cutters to see that the work does not slip. When a cut has been taken across the face of a cutter, it is best to lower the knee of the milling machine, thus dropping the work away from the mill while coming back for another cut; the knee can then be raised to its proper position, which is determined by means of the graduated collar on the elevating shaft of the machine.
As it is important that the face of the cutting tooth be radial and straight, it will be found necessary to use an angular cutter of the form shown in Fig. 203, since cutters of this form readily clear the radial face of the cut and so remain sharp longer and produce a smoother surface to the face of the tooth than an angular cutter of the form used for cutting teeth which are parallel to the cutter axis.
The angular cutters for spiral mills are made with either 40 degrees, 48 degrees, or 53 degrees on one side, and 12 degrees on the other. By setting the cutter, as shown in Fig. 203, so that the dis-tance A is one-twelfth the diameter, the face cut by the 12-degree side of the angular cutter will be nearly radial for the usual proportions. The setting for cutting the teeth of a spiral cutter must be made before turning the spiral bed to the angle of the spiral.
Spiral cutters with nicked teeth, Fig. 204, are especially adapted for heavy milling. As the chip is broken up, a much heavier cut can be taken than would be possible with an ordinary cutter. The nicking may be done as follows: An engine lathe is geared to cut a thread of the required pitch-two threads to the inch will be found satisfactory-and with a round-nosed tool 1/6 inch wide, a thread is cut of a depth that will not grind out before the teeth become too shallow to allow further grinding. This thread should be cut before milling the spaces to form the teeth. Milling Cutters with Interlocking Teeth. When two milling cutters of an equal diameter are to be used on the same arbor in such a manner that the end of one cutter is against the end of the other, the corners of the cutting teeth are likely to break away, leaving a projection-or fin-on the work, as shown in Fig. 205. In order to overcome this, part of the teeth are cut away on the sides of the cutters; that is, a tooth is cut away on one cutter, and the corresponding tooth on the other cutter is left full length to set into the recess formed by the cutting away of the tooth. In some shops it is customary to cut away every other tooth; while in others, two, three, or four teeth will be cut away and an equal number left. Fig. 206 represents a pair of mills having every other tooth cut away, while Fig. 207 represents a pair having four teeth cut away.
In order to cut away the teeth to make a cutter with interlocking teeth, the cutter should be placed on a plug or an expanding arbor, as described for milling teeth on the sides of side milling cutters. By means of a milling cutter having the proper width, the teeth may be milled away, although, in the case of a cutter having several teeth cut away, Fig. 207, it is well to use a narrow cutter, and after taking one cut, to turn the index head so that the next tooth is in position. This should be continued until the desired number of teeth have been cut away, after which the index head should be turned to pass over the required number of teeth, and the operation repeated.
Fig. 205. Bully Made Mill.
It is necessary, when making cutters with interlocking teeth (sometimes called dodged teeth) that the milling be deep enough to prevent the corresponding tooth on the other part of the cutter from striking the bottom of the recess. The parts of the cutter should bear against each other on the shoulders, or hubs.