It is sometimes advisable to cut the teeth of end mills spirally, as shown in Fig. 240. As there is no support at the outer end of this form of mill, it will be necessary to cut the teeth of a spiral that will have a tendency to force the mill into the collet rather than to draw it out. Fig. 240 represents a left-hand end mill cut with a right-hand spiral.
This form of end mill is useful when it is necessary to cut into the work with the end of the mill, and then move along, as in the case of dies, cams, and grooves. The teeth, being sharp on the outside, cut a path from the point of entrance, and, being coarse, allow a heavy cut, especially in cast iron. Fig. 241 shows two views of an end mill with center cut.
After the teeth on the end have been cut with an angular cutter, a thin, straight-faced cutter of small diameter should be run through; close to the face of the cutter tooth, making a cut as shown at A; this cut should be of sufficient depth to permit backing-off the inner edge of the tool, as shown at B. This clearance allows the mill to cut away the slight projection left in the center of the mill when it is fed into a piece of work, Fig. 242.
Fig. 241. Form of End Mill with Center Cut.