Another method of forming the half-round countersink is shown in [Fig. 1188], in which the cone is of the same angle as the lathe centres; the back a is ground away to avoid its contact with the work and give clearance, while clearance to the cutting edge is obtained by filing or grinding a flat surface b at the necessary angle to the upper face of the cone. In this case it is assumed that the centre-drilling and countersinking are true one with the other. Yet another form of countersink is shown in [Fig. 1189], consisting of a cone having three or four teeth. It may be provided with a tit, which will serve as a guide to keep the countersink true with the hole, and this tit may be made a trifle larger in diameter than the hole, and given teeth like a reamer, so as to ream the hole out while the countersinking is proceeding.
Unless one side of a half-round reamer is filed away so as to give the cutting edge alone contact with the bore of the hole, an improper strain is produced both upon the work and the countersink.
Fig. 1190.
In [Fig. 1190], for example, is shown, enlarged for clearness of illustration, a hole, and a half-round countersink in section, and it is evident that if the countersink is set central to the hole, it will have contact at a and at b, and a cannot enter the metal to cut without springing towards c.
Fig. 1191.