In cutting spiral grooves this is of increased importance, for the following reasons: In [Fig. 1969] we have a cutter shown in section, and lying in a spiral groove. Now suppose a tooth to be in action at the bottom of the groove, and therefore on the line g g, and during the time that it moves from that line until it has moved above the level of the top of the groove, the work will have performed some part of a revolution in the direction of the arrow, and has therefore moved over towards that side of the cutter; hence, if that side of the cutter had teeth lying parallel, as shown at b in [Fig. 1965], the walls of the groove would be scored as at c in that figure, whereas by placing the teeth at an angle to the side face, they recede from the walls after passing line g, and therefore produce smoother work.
Fig. 1970.
A cutter of this kind must, for cutting the teeth of cutters, be accurately set to the work, and the depth of cut must be accurate in order to cut the grooves so that one face shall stand on a radial line, and the top of the teeth shall not be cut to a feather edge. If the teeth were brought up to a sharp edge the width of the groove at the top would be obtained with sufficient accuracy by dividing the circumference of the work by the number of flutes or teeth the work is to contain, but it is usual to enter the cutter sufficiently deep into the work to bring the teeth tops up to not quite a sharp edge. The method of setting the cutter is to mark on the end of the work a central line r, [Fig. 1970], and make the distance e in same figure equal to about one tenth the diameter of the work.
Fig. 1971.
Obviously the cutter is set on opposite sides of the work centre, according to which side of the groove is to have the radial face. Thus for example, in [Fig. 1970], the cutter is set to the left of line r, the radial face of the groove being on the left, while in [Fig. 1971] the cutter is set on the right of line r, because the radial face is on the right hand side of it, the work consisting (in these examples) in cutting up a right and a left-hand mill or cutter.
The acting cutter j may in both cases be used to cut either a right or a left-hand flute, according to the direction in which the work w is revolved, as it is fed beneath the cutter j.