Fig. 1851.
As regards the number of cutting points, suppose that there were a certain number, as three, shown in [Fig. 1849], all taking an equal cut; then, when the position indicated by the dotted lines was reached, where cutter h runs out, the entire duty would be only two-thirds as much as it was, and the die would shift laterally in the direction of the arrow enough to equalize this smaller amount of duty on all three, or make h, e, and d each cut two-thirds as much as at first. With four as shown in [Fig. 1850] when h reached the depression where its cut would run out, the entire duty would be three-fourths of what it was at first, and the die would travel laterally in the direction of the arrow sufficiently to equalise the pressure upon h and f, and upon e and g. With five, as shown in [Fig. 1851], in similar position the entire duty would be four-fifths as much; with six, five-sixths, and so on. Thus it can be seen that the variation between the least amount to be cut and the full amount is relatively less, the greater the number of cutting points that it is divided between, and hence the lateral movement would be less; therefore the general tendency of an increase in the number of cutting points would be to promote true work.
Hence, from these considerations it appears that it is not material whether the number is odd or even merely on that account; so four would be preferable to three only on account of being one more, and, in turn, five would be better than four, and six better than five, and so on. It is found, however, that bar iron usually inclines to the elliptical form, and that an even number is, therefore, preferable.
Thus far the cutting edges of the die have been assumed to be points equidistant about a circle—that is, it has been supposed to have absolute clearance, so that its movements would be regulated entirely by the depth of cut taken, in order to ascertain the inherent tendency to untruth caused by an odd or an even, a greater or a less, number of cutters. This tendency is, of course, modified in each case by the amount of clearance.
Fig. 1852.
