different requirements in slab milling, cutting keyways in shafts, etc. Saws for slitting metal and slotting screws are essentially plain mill- ing cutters, although rarely regarded as such on account of their extreme thinness.
Plain milling cutters 34" or less in width are usually made with straight teeth, while those above that width have teeth of a spiral form. The object of the spiral is to give a shearing cut, reducing the stress upon the teeth, and preventing a distinct shock when each tooth engages the work as is the case with straight teeth. Consequently, a spiral tooth cutter on wide surfaces produces much smoother results
Fig. 47
than a straight tooth cutter. It requires less power to operate, and, in relieving the cutter of strain, the tendency to vibrate or chatter is reduced.
The teeth of cutters, especially those of a wide face, often have notches or nicks cut in them, the nicks following each other alter- nately. Cutters made in this manner can be run at coarser feeds than those with plain teeth, for the nicks break up the chips, and help to keep the cutters cool.
Side Milling Cutter. This type of cutter is like a plain milling cutter with the addition of teeth on both sides.
Side milling cutters are employed on a large variety of work, being used often in pairs with a space between, as shown in Fig. 47. When so used, they are known as "straddle mills." In work that has to be
