Fig. 1917.
If the machine is belted so that it can be revolved in either direction, both sides of the cutter may be utilised by taking the cutters off the arbor, turning them around and then replacing them in their original positions on the same. Thus in [Fig. 1917] we have at a a left-hand cutter that if reversed upon its arbor would be a right-hand one as at b, and it is obvious that the direction of revolution must be in each case as denoted by the arrows f g, which are in opposite directions. In this case the direction of work feed must be reversed, the work for a feeding in the direction of c, and that for b in the direction of d. It is to be observed, however, that the cutter could not be reversed if it was driven by an arbor that screwed upon the driving spindle of the milling machine. For if the machine has a right-hand thread then the cutter must revolve in the direction of g, and the work feed must be in that of c; whereas if the machine spindle drives its chucks, arbors, &c., by a left-hand thread, then the direction of cutter revolution must be as at f, and that of work feed as at d. But if the cutters are upon an arbor that is driven by a conical seat in the machine spindle, or by any other means enabling the arbor to revolve in either direction without becoming released from that spindle, then the cutter may be simply turned around and the feed direction reversed, as already explained. The reason for reversing the direction of feed when the direction of cutter revolution is reversed is as follows:—
Fig. 1918.
In [Fig. 1918] a and b represent two pieces of work of which b is to be fed in the direction of arrow c, so that the pressure of the cut tends to force the work back from under the cutter, whereas in the case of the work a, feeding in the direction of d, the teeth act to pull the work beneath the cutter, which causes tooth breakage.
Fig. 1919.