In the case of the plain mill, it is obvious that only one side of the nut is operated upon at a time, and as the whole of the pressure of the cut falls on one side of the work it acts to spring or bend the mandrel or arbor used to hold the nut, and this spring is sufficient, if several nuts are milled at once on the same arbor, to make the arbor bend and cause the nuts in the middle to be thicker than those at the ends of the arbor. In the case of hand-forged nuts in which there may be more metal to take off some nuts or some sides of nuts than off others, the extra spring due to an increased depth of cut will make a sensible difference to the size the work is milled to. In the case of the end mill the pressure of the cut falls in line with the arbor axis and downwards; hence the arbor spring is less and does not affect the depth of the cut.
Fig. 1961.
Fig. 1962.
In the case of the face mills the pressure of the cut falls on both sides of the work, and the spring is mainly endways of the nut arbor; hence, it does not affect the depth of the cut nor the truth of the work. Furthermore, in both the end and the face mills, the work will be true notwithstanding that the cutter may not be quite true, because each point of the work surface is passed over by every tooth in the cutter, so that the work will be true whether the cutter runs true or not; whereas in the plain mill or cutter each tooth does its individual and independent proportion of finishing. This is shown in [Figs. 1961] and [1962]. In [Fig. 1961] we have the plain mill, and it is obvious that the tooth does the finishing on the vertical line b, that being the lowest point in its revolution. After a tooth has passed that point the work in feeding moves forward a certain distance before the next tooth comes into action; hence to whatever amount a tooth is too high it leaves its mark on the work in the form of a depression, or vice versâ, a low tooth will leave a projection.
In [Fig. 1962] we have a piece of work being operated on by a face mill, and it is obvious that while the teeth perform cutting duty throughout the distance a, yet after the work has fed past the line a it is met by the cutter teeth during the whole time that the work is feeding a distance equal to a on the other side; hence the prolonged action of the teeth insures truth in the work. On the other hand, however, it is clear that the work requires to feed this extra distance before it is finished.