For counterboring small holes or for facing the metal around their ends, the form of counterbore shown in [Fig. 1746] is employed. The pin must be an accurate fit to the hole, and to capacitate one tool for various sizes of holes the bit is made interchangeable. The stock has a flat place on it to receive the pressure of the screw that secures the counterbore, and the end of the stock is reduced in diameter, so that the counterbore comes against a shoulder and cannot push up the stock from the pressure of the feed; the end of the counterbore is bored to receive the tit pin, thus making it permissible to exchange the pin, and use various sizes in the same counterbore.
Fig. 1747.
Fig. 1748.
Twist drills for use in wood work are given a conical point, as was shown with reference to lathe drills, and when the holes are to be countersunk, an attachment, such as shown in [Fig. 1747], may be used. It is a split and threaded taper, so that by operating the nut in one direction it may be locked to the drill, while by operating it in the other it will be loosened, and may be adjusted to any required distance from the point of the drill, as shown in [Fig. 1748].
