Fig. 1731.
In fitting the shank, the fitting or bearing marks will show most correctly when the shank is driven very lightly home, for if driven in too firmly the bearing marks will extend too far in consequence of the elasticity of the metal. If the hole in the spindle is not true with the axial line of the spindle, or if the sides of the hole are not a true square or are not equidistant from the axial line of the spindle, the drill must be fitted with one side of its square shank always placed to the same side of the square in the socket, and these two sides must therefore be marked so as to denote how to insert the drill without having to try it in the socket. Usually a centre-punch mark, as at e, [Fig. 1731], is made on the drill and another on the collar as at f.
To enable the extraction of the drill from the socket the latter is provided with a slot, shown in figure at c, the slot passing through the spindle and the end of the drill protruding into the slot, so that a key driven into the slot will force the drill from the socket. The key employed for this purpose should be of some soft metal, as brass or hard composition brass, so that the key shall not condense or press the metal of the keyway, and after the key is inserted it should be lightly tapped with a hammer, travelling in the direction of the line of the spindle and not driven through the keyway.
The drill should not be given a blow or tap to loose it in the spindle, as this is sure in time to make its socket hole out of true.
The thread shown on the end of the drill spindle in figure is to receive chucks for holding and driving drills.
The various forms of small drill chucks illustrated in connection with the subject of lathe chucks are equally suitable for driving drills in the drilling machine.
Fig. 1732.