Fig. 496 will serve to show the general character, of various constructions of more modern apparatus, to be used for supplying the pressure in drilling holes with hand braces. It consists of a cylindrical bar a, upon which the horizontal rectangular rod b, is fitted with a socket, so that it may be fixed at any height, or in any angular position, by the set-screw c. Upon b slides a socket, which is fixed at all distances from a, by its set-screw d, and lastly, this socket has a long vertical screw e, by which the bract is thrust into the work.

The object to be drilled having been placed level, either upon the ground, on trestles, on the work bench, or in the vice, according to circumstances, the screws, c and d are loosened, and the brace is put in position for work. The perpendicularity of the brace is then examined with a plumb-line, applied in two positions, (the eye being first directed as it were along the north and south line, and then along the east and west,) after which the whole is made fast by the screws c and d. The one hole having been drilled, the socket and screws present great facility in re-adjusting the instrument for subsequent holes, without the necessity for shifting the work, which would generally be attended with more trouble, than altering the drill-frame by its screws.

Sometimes the rod a is rectangular, and extends from the floor to the ceiling; it then traverses in fixed sockets, the lower of which has a set-screw for retaining any required position. In the tool represented, the rod a, terminates in a cast-iron base, by which it may be grasped in the tail-vice, or when required it may be fixed upon the bench; in this case the nut on a is unscrewed, the cast-iron plate, when reversed and placed on the bench, serves as a pedestal, the stem is passed through a hole in the bench, and the nut and washer when screwed on the stem beneath, secure all very strongly together. Even in establishments where the most complete drilling-machines driven by power are at hand, modifications of the press-drill are among the indispensable tools: many arc contrived with screws and clamps, by which they are attached directly to such works as are sufficiently large and massive to serve as a foundation.

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Various useful drilling tools for engineering works, are fitted with left-hand screws, the unwinding of which elongate the tools; so that for these instruments which supply their own pressure, it is only necessary to find a solid support for the center. They apply very readily in drilling holes within boxes and panels, and the abutment is often similarly provided by projecting parts of the castings; or otherwise the fixed support is derived from the wall or ceiling, by aid of props arranged in the most convenient manner that presents itself.

Fig. 497 is the common brace, which only differs from that in fig. 496 in the left-hand screw; a right-hand screw would be unwound in the act of drilling a hole when the brace is moved round in the usual direction, which agrees with the path of a left-hand screw. The cutting motion produces no change in the length of the instrument, and the screw being held at rest for a moment during the revolution, sets in the cut; but towards the last, the feed is discontinued, as the elasticity of the brace and work, suffice for the reduced pressure required when the drill is nearly through, and sometimes the screw is unwound still more to reduce it.

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The lever-drill, fig. 498, differs from the latter figure in many respects, it is much stronger, and applicable to larger holes; the drill socket is sufficiently long to be cut into the left-hand screw, and the piece serving as the screwed nut, is a loop terminating in the center point. The increased length of the lever gives much greater purchase than in the crank-formed brace, and in addition the lever-brace may be applied close against a surface where the crank-brace cannot be turned round; in this case the lever is only moved a half circle at a time, and is then slid through for a new purchase, or sometimes a spanner or wrench is applied directly upon the square drill-socket.

The same end is more conveniently fulfilled by the ratchet-drill, fig. 499, apparently derived from the last; it is made by cutting ratchet teeth in the drill-shaft, or putting on the ratchet as a separate piece, and fixing a pall or detent to the handle; the latter may then be moved backward to gather up the teeth, and forward to thrust round the tool, with less delay than the lever in fig. 498, and with the same power, the two being of equal length. This tool is also peculiarly applicable to reaching into angles and places in which neither the crank-form brace, nor the lever-drill will apply. Fig. 500, the ratchet-lever, in part resembles the ratchet-drill, but the pressure-screw of the latter instrument must be sought in some of the other contrivances referred to, as the ratchet-lever has simply a square aperture to fit on the tang of the drill d, which latter must be pressed forward by some independent means.

Fig. 501, which is a simple but necessary addition to the braces and drill tools, is a socket having at the one end a square hole to receive the drills, and at the opposite, a square tang to tit the brace; by this contrivance the length of the drill can be temporarily extended for reaching deeply-seated holes. The sockets are made of various lengths, and sometimes two or three an used together, to extend the length of the brace to suit the position of the prop; but it must be remembered, that with the additional length the torsion becomes much increased, and the resistance to end-long pressure much diminished, therefore the sockets should have a bulk proportionate to their length.

The French brace, fig. 469, page 545, is also constructed in iron, with a pair of equal bevil pinions, and a left-hand center screw like the tools, fig. 497, 498, and 499: it is then called the corner-drill. Sometimes also, as in the succeeding figures 502 and 503, the bevil wheels are made with a hollow square or axis, as in the ratchet-lever, fig. 500; the driver then hangs loosely on the square shank of the drill tool, or cutter bar, and when the pinion on the handle is only one-third or fourth of the size of the bevil wheel with the square hole, it is an effective driver for various uses; the long tail or lever serves to prevent the rotation of the driver, by resting against some part of the work or of the work-bench.

All the before-mentioned tools are commonly found in a variety of shapes in the hands of the engineer, but it will be observed they are all driven by hand-power, and are carried to the work. I shall conclude this section with the description of a more recent drill-tool of the same kind, invented by Mr. A. Shanks of Glasgow.

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This instrument is represented of one-eighth size, in the side view, fig. 504, in the front view, 505, and in the section 506; it is about twice as powerful as fig. 503, and has the advantage of feeding the cut by a differential motion. The tangent screw moves at the same time the two worm wheels a and b; * the former has 15 teeth, and serves to revolve the drill; the latter has 16 teeth, and by the difference between the two, or the odd tooth advances the drill slowly and continually, which may be thus explained.

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* A principle first introduced in Dr. Wollaston's Trochiometer, for counting the turns of a carriage wheel.

The lower wheel a, of 15 teeth, is fixed N the drill-shaft, and this is tapped to receive the center-screw c, of four threads per inch. The upper wheel of 16 teeth is at the end of a socket d, (which is represented black in the section fig. 506), and is connected with the center-screw c, by a collar and internal key, which last fits a longitudinal groove cut up the side of the screw c; now therefore the internal and external screws travel constantly round, and nearly at the same rate, the difference of one tooth in the wheels serving continually and slowly to pro-the screw c, for feeding the cut. To shorten or lengthen the instrument rapidly, the side screw e is loosened; this seta the collar and key, free from the 16 wheel, and the center-screw for the time be moved independently by a spanner.

The differential screw-drill, having a double thread in the large worm, shown detached at /, requires 7 1/2 turns of the handle to move the drill once round, and the feed is one 61th of an inch for each turn of the drill; that being the sum of 16 by 4. See Appendix, Note B C, page 1004.