Of these tools there are various forms. The following figure represents a taper countersink such as is employed for holes to receive flush rivets or countersunk head-bolts, this form of tool being mainly employed for holes above 5/16 of an inch in diameter. It should be made, tempered, and used as directed for pin-drills. In tempering these tools, or any others having a pin or projection to serve as a guide in a hole, the tool should be hardened right out from the end of the pin to about 3/8 of an inch above the cutting edges. Then lower the temper of the metal (most at and near the cutting-edges), leaving the pin of a light straw-color, which may be accomplished by pouring a little oil upon it during the lowering or tempering process. The object of this is to preserve it as much as possible from the wear due to its friction against the sides of the hole. For use on wrought-iron and steel, this countersink (as also the pin-drill) may have the front face hollowed out, after the fashion of the lip-drill, and as shown by the dotted line.

Countersink-Pin Drills