[Fig. 1967] represents a common form of cutter of this class that is used for cutting spiral grooves on milling cutters up to 3 inches in diameter, which contain eight teeth per inch of diameter. The angle of the teeth on b is 12° to the side face a of the cutter, and the angle of the teeth at c is 40° to the face d.
The effect produced by making face b at an angle instead of leaving it straight, or in other words, instead of cutting the teeth on the face a, may be shown as follows:—
Fig. 1968.
Suppose that in [Fig. 1968] we have a sectional view taken through the middle of the thickness of a cutter for a rectangular groove, the circumferential surface being at a right angle to the side faces, and it is evident that the teeth, at every point in their length across the cutter, except at the extreme corner that meets the side faces as c, will have contact with the seat of the groove while passing through the angle f only (which is only one half of the angle e in [Fig. 1965]); or in other words, each tooth will have contact with the seat of the groove as soon as it passes the line g, which passes through the axis of the cutter; whereas, when the teeth are parallel with the side of the cutter, as was shown in [Fig. 1965], the teeth continue to have contact with the side walls of the groove after passing the line g.
By forming the cutter as in [Fig. 1967], therefore, we confine the action to the angle f, [Fig. 1968], the teeth having contact with the walls of the groove as soon as they pass the line g.
Fig. 1969.