Fig. 1192.
But when the lathe has made rather more than one-half a revolution, the forcible contact at b will be relieved, and either the work or the countersink will move back towards d. This may be remedied by setting the countersink to one side, as in [Fig. 1191], or by cutting it away on one side, as in [Fig. 1192], when the half-round reamer will, if the work be rigidly held while being countersunk, act as a cutting tool. But it is more troublesome to hold the work rigidly while countersinking it than it is to simply hold it in the hands, and for these reasons the square centre is an excellent tool to produce true countersinking.
Fig. 1193.
[Fig. 1193] represents a square centre, the conical end being provided with four flat sides, two of which appear at a b, or it may have three flat sides which will give it keener cutting edges, and will serve equally well to keep it true with the drilled hole. But it is questionable whether it is not an advantage not to have the cutting edges so keen as is given by the three flat faces, because the less keen the cutting edges are, the more true the countersinking will be with the hole, the extra pressure required to feed the square centre tending to cause it to remain true with the hole notwithstanding any unequal density of the metal on different sides of the hole. An objection to the square centre is that it involves more labor in the grinding to resharpen it, and is not so easy to grind true, but for fine work this is more than compensated for in the better quality of its work.
Fig. 1194.