Radial Mortising in mortising in hubs it is advisable to attach to the carriage or bed some chucking device with a properly gradu ated index-plate by which the spacing may be made accurate and the cut radial. In such cases the variable-stroke chisel-bar is used. A hub-mor tising machine of high-grade construction has double chisels (p1. 16, fig. 5). The table carrying the standards which support the hub is operated by gearing; the hubs are held in a three-jawed universal chuck operated by a single screw in connection with accurately spaced dials. All gigging, spacing, and feeding are automatic.
Hollow mortising is done by what is known as a "hollow chisel," its cutting portion having four sides, with bevels upon the inside edges and a slight concavity upon each (fig. 6). Inside these runs an auger-bit. In working with this the anger is run out of the hol low chisel and makes a round hole as nearly as possible the size of the square one which it is desired to make; it is then withdrawn and the chisel run forward, cutting the square corners and leaving a perfectly square mor tise. To make a rectangular hole other than square more than one stroke can be made; thus a hole z inches by r inch can be made by two strokes of a i-inch chisel, a hole 3 inches by 1 inch with three strokes of the same chisel, and so on.
While most mortising is done by chisels, there are certain grades which require the removal of so much material that a rotating cutter is used. Of course most "end" or " open " mortises may be made by a cutter, in the same way as a tenon; in fact, the cut made after changing a single thick tenon into a double thin one is practically end-mortising. Mortising chisels may be either solid, as in Figure 7 (p1. 16), or made with stocks and separate chisels, as in Figure 6, these last permitting the use of a shorter and stiffer chisel.
Boring is clone by gimlets and augers, the first worked by hand only and the latter used both by hand and in machines. Figures zo, 22, 23, and 24 (p1. r3) show gimlets, and Figures 25 to 27 augers. Twist drills used for wood are of the same general appearance as those for metal (p1. 24, fig. to), and differ from augers principally in the cutting-edges of their ends.
drills for wood the cutting-edges are ground at an angle of from 20° to 30°, and in those for metal at an angle of from 50° to
Go°. The harder the material to be drilled, the nearer must the angle of the cutting-edges approach 90°. Of drills for wood the ordinary gimlet (fil. 13, figs. 20, 22-24) is the simplest form. Upon a slender cone a spiral furrow runs toward the left from the point to the top, the edges of the furrow being generally so little sharpened that actual detachment of the material is not effected, but simply a partial forcing apart of the fibres, which, however, suffices to allot the screw to be driven with greater facility. Hand rota tion of this simple form of drill is effected by a cross-handle placed on the tipper end. The screw-auger (figs. 21, 25, 27), besides being larger than the gimlet, differs from it in the sharper angle to which the helical cutting edge is ground, which therefore allows detachment of broad shavings. The handle requires the use of both hands, while also admitting of greater pressure.
Figure 18 shows a centre-bit. Besides the guide-point, which precedes the process of boring, there is on one side of the broad lancet-shaped end of the bit a cutter with a sharp edge set toward the periphery, and, on the other side a radial cutting-edge. The point secures the position of the rotating axis, while the lance-like cutter neatly separates around the cir cumference of the hole the fibres of wood to be removed, and the radial cutting-edge continuously lifts out these fibres by a helical cut. The bung-hole borer (fif,-. 19) is a modification of this. On one end of a trun cated cone are the point, cutter, and lance, by which a hole can be cut in the bottom of a filled barrel for the purpose of inserting a faucet without the escape of the liquid. As soon as the bit penetrates, the hole is closed by the cone, and then the tool is quickly exchanged for the faucet.
better handling the centre-bit there is gene rally used a brace (figs. 14-17). The bit is placed in its lower end, and the requisite pressure is exerted upon the disc-like upper end with the left hand or with the breast, while the right hand, grasping the crank-handle in the middle, moves in a circle.
are made in great variety of designs. The sim plest are plain boring-arbors with only a pulley and frame, and a stop to regulate the depth of hole.