[Figs. 1753], [1754], [1755], and [1756], however, represent a bar having a device for boring tapers in a drilling or boring machine. It consists of a sleeve a fixed to the bar s, and having a slideway at an angle to the bar axis. In this slideway is a slide carrying the cutting tool and having at its upper end a feed screw with a star feed. [Fig. 1753] shows the device without, and [Fig. 1754] with, the boring bar. a is a sleeve having ribs b to provide the slideway c for the slide d carrying the cutting-tool t. The feed screw f is furnished with the star g between two lugs h k. A stationary pin bolted upon the work catches one arm of the star at each revolution of the bar, and thus puts on the feed. To take up the wear of the tool-carrying slide, a gib m and set-screws p are provided, and to clamp the device to the boring-bar it is split at q and furnished with screws r. The boring-bar s, furthermore, has a collar at the top and a nut n at the bottom. The tool, it will be observed, can be closely held and guided, the degree of taper of the hole bored being governed by the angle of the slideway c to the axis of the sleeve.
Chapter XX.—HAND DRILLING AND BORING TOOLS AND DEVICES.
Hand Drilling and Boring Tools.—The tools used for piercing holes in wood are generally termed boring tools, while those for metal are termed drilling tools when they cut the hole from the solid metal, and boring tools when they are used to enlarge an existing hole. Wood-boring tools must have their cutting edges so shaped that they sever the fibre of the wood before dislodging it, or otherwise the cutting edges wedge themselves in the fibre. This is accomplished, in cutting across the grain of the wood, in two ways: first, by severing the fibre around the walls of the hole and in a line parallel to the axial line of the boring tool, and removing it afterwards with a second cutting edge at a right angle to the axis of the boring tool; or else by employing a cutting edge that is curved in its length so as to begin to cut at the centre and operate on the walls of the hole, gradually enlarging it, as in the case of Good’s auger bit (to be hereafter described), the action being to cut off successive layers from the end of the grain or fibre of the wood. Tools for very small holes or holes not above one-quarter inch in diameter usually operate on this second principle, as do also some of the larger tools, such as the nail bit or spoon bit and the German bit.
Fig. 1757.
