We have now to examine the case of a left-hand groove, and in [Fig. 1975] we have such a groove in a cylinder l. Let it be supposed that the direction of its revolution is as denoted by arrow f, and if the cutter is started at h (as it must be to cut a left-hand groove if the work revolves as at f), then the side c moves over towards the cutter, and the dragging or crowding action occurs on that side; whereas if the direction of revolution is as at k, and the cutter starts at n and feeds to h, then side b of the groove moves towards the cutter; hence face c of the groove is cut the smoothest. Obviously then the direction of cutter and work revolution and of feed, in cutting angular grooves in which one angle of the cutter is at a greater degree of angle than the other to the side of the cutter, should be so arranged that the work revolves towards that side of the cutter on which its teeth have the greater angle, whether the spiral be a right-hand or a left-hand one. In cutting grooves not truly circular the same principle should be observed.
Fig. 1976.
In [Fig. 1976], for example, it is better if the side b is the one that moves towards the cutter, the direction of revolution being as denoted by the arrow, whether the groove be a right-hand or left-hand (supposing, of course, that the cutter starts from end e of the work).
Obviously, also, the greater the degree of spiral the more important this is, because the work revolves faster in proportion to the rate of feed, and therefore moves over towards the outer faster.
In cutting spirals it is necessary first to put on such change gears as are required to revolve the work at the required speed for the given spiral, and to then set the work at such an angle that the cutter will be parallel to the groove it cuts, for if this latter is not the case the groove will not be of the same shape as the cutter that produces it.
Fig. 1977.