Fig. 1980.
When the cutter has arrived at the end of the work it will have marked on it a line, as in [Fig. 1979], this line representing the spiral it will cut with those change gears, and all that remains to do is to swing the work over so that this line is parallel with the face of the cutter, as shown in [Fig. 1980]. If the diameter of the cutter is small we may obviously secure greater accuracy by placing a straight-edge upon the side of the cutter so as to have a greater length to sight by the eye in bringing the line fair with the cutter. This being done it remains to merely set the cutter in its required position with reference to the work diameter.
Fig. 1981.
If an error be made in setting the angle of the work to the cutter the form of groove cut will not correspond to that of the cutter. This is shown in [Fig. 1981], in which the cutter being at an angle to the groove the latter is wider than the cutter thickness, and it is obvious that by this means different shapes of grooves may be produced by the same cutter. In proportion, however, as the cutter is placed out of true the cutting duty falls on the cutting edges on one side only of the cutter, which is the leading side c in the figure, while the duty on the other side, b, is correspondingly diminished.
The simplest method of holding work to be operated upon in the milling machine is either between the centres or in the vice that is provided with the machine. The principles involved in holding work in the vise so as to keep it true and avoid springing it for milling machine work, are the same as those already described with reference to shaping machine vises.
In milling tapers the work, if held in centres, should be so held that its axial line is in line with the axes of both centres, for the following reasons:—