Fig. 1963.
Suppose, however, that the cutter being dead true the cutting action ceases on the centre line, and therefore exists through the distance a only, and if we take a plain cutter of the same diameter as in [Fig. 1963] we see that its period of feed only extends through the length b, and it becomes apparent that to perform an equal amount of work the face cutter is longer under feed, and therefore does less work in a given time than the plain cutter, the difference equalling twice that between a and b in the two figures, because it occurs at the beginning and at the end of the cut.
There is, however, another question to be considered, inasmuch as that the face cutter must necessarily be of larger diameter than the plain one, because the work must necessarily pass beneath the washer (c, [Fig. 1915]), that is between the two cutters; hence the cutter is more expensive to make.
Fig. 1964.
We may in very short work overcome this objection by feeding the work, as at k in [Fig. 1964], the face l to be milled requiring to feed the length of the teeth instead of the distance h in the figure. In the end mill the amount of feed also is greater for a given length of finished surface than it is in the plain cutter, as will be readily understood from what has already been said with reference to face mills.
Face milling possesses the following points of advantage and disadvantage, in addition to those already enumerated: If the work is sprung by the pressure of the holding devices it is in a line with the plane of motion of the teeth, hence the truth of the work is not impaired. On the other hand, the teeth meet the scale or skin of the work at each cut, whereas in a cylindrical cutter this only occurs when the cutter first meets the work surface.
The strain of the cut has more tendency to lift the work table than in the case of a cylindrical cutter. The work must be held by end pressure; hence the chuck or holding jaws must be narrower than the work, rendering necessary more work-holding devices. Since, however, both sides of the work are simultaneously operated on, there is no liability of error in parallelism from errors in the second chucking, as is the case with plain cutters.