In [Fig. 1977] we have a spiral so set, the centre of the cutter and of the groove being in the line o o, and the work axis (which is also the line in which the work feeds beneath the cutter) being on the line c c. The degrees of angle between the centre of the cutter, or line o o, and the axis of the work, or line c c, are the number of degrees it is necessary to set the work over to bring the cutter and the groove parallel, this number being shown to be 20 in the example.
Fig. 1978.
To find this angle for any given case we have two elements: first, the pitch of the spiral, or in other words, the length or distance in which it makes one complete turn or revolution; and second, the circumference of the work; for in a spiral of a given pitch the angle is greater in proportion as the diameter is increased as may be seen in [Fig. 1978], in which the pitch of the spirals is that in [Fig. 1977], while the angle is obviously different.
To find the required angle for any given case we may adopt either of two plans, of which the first is to divide the circumference of the work in inches by the number of inches which the spiral takes to make one turn. This gives us the tangent of angle of the spiral.
The second method of setting the work to cut a given spiral is to chuck the work and put on the necessary change gears. The cutter is then set to just touch the work and the machine is started, letting the work traverse beneath the cutter just as though the work was set at the required angle to the cutter:
Fig. 1979.