Suppose, for example, that in [Fig. 1919] p is a piece of work fastened to the table t, feeding in the direction of a, the cutter w revolving in the direction of arrow b, n representing the feed nut operated by the feed screw s. Now while the table is being pulled in the direction of a, the sides c of the feed screw thread will bear against the sides of the thread in the nut, and whatever amount of looseness there may be between the threads of the screw and nut will in this case be on the sides d of the threads. So soon, therefore, as the wheel meets the work p, it will suddenly pull the work forward to the amount of the play or looseness on the sides d of the threads, and this in addition to the feed given by the rotating screw s, would cause the wheel to lock upon the work surface.
In all milling operations, therefore, the work is fed against the cutter as at b, in [Fig. 1918], unless, in the case of twin mills, it is fed (as at e and f in the same figure) in the middle of the cutters, in which case it is preferable to present it as at f, so that the pressure of the cut will tend to hold the work down to the table, and the table down upon its guideways. This position of the work presents some advantages for small work which will be explained hereafter.
Fig. 1920.
Fig. 1921.
[Fig. 1920] represents angular cutters, the teeth being at an angle to the cutter axis. These cutters are made right and left as at a and b in [Fig. 1921], the teeth of a being cut in the opposite direction to those at b, so as to be able to cut equal angles on the work when these angles lie in opposite directions, as c and d in the figure. Furthermore these cutters are sometimes screwed to their arbors, and can therefore be revolved in one direction only, which prevents their being turned around end for end, even though the machine be so belted as to be capable of revolving its spindle in either direction.
The angular cutters shown in [Fig. 1921] have their teeth arranged for a Brainard milling machine, in which the live spindle has a right-hand thread for driving the chucks, arbors, &c.; hence the direction of cutter revolution, and the arrangement of the teeth are as in the figure.