Figs. 489.

Drills arc also frequently used in the drilling-lathe; this is a miniature lathe-head, the frame of which is fixed in the table vice; the mandrel is pierced for the drills, and has a pulley for the bow, therein resembling fig. 490, except that it is used as a fixture.

* This is analogous to the level of the Indian masons and carpenters; they squceze a few drops of water on the upper surface of the straight edge, which is made exactly parallel, and the escape of the fluid from either end, denotes that to be the lower of the two.

† See Technical Repos., 1822, vol ii, p. 149; also Bess's Cyclopedia.

The figure 490 just referred to, represents one variety of another common form of the drill-stock, in which, the revolving spindle is fitted in a handle, so that it may be held in any position, without the necessity for the breast-plate; the handle is hollowed out to serve for containing the drills, and is fluted to assist the grasp.

Fig. 491 represents the socket of an "universal drill-stock" invented by Sir John Robison; it is pierced with a hole as large as the largest of the wires of which the drills are formed, and the hole terminates in an acute hollow cone. The end of the drill-stock is tapped with two holes, placed on a diameter; the one screw a, is of a very fine thread, and has at the end two shallow diametrical notches; the other b, is of a coarser thread and quite flat at the extremity. The wire-drill is placed against the bottom of the hole, and allowed to lean against the adjusting-screw a, and if the drill be not central, this screw is moved one or several quarter-turns, until it is adjusted for centrality; after which the tool is strongly fixed by the plain set-screw b.

Fig. 492 is a drill-stock, contrived by Mr. William Allen: it consists of a tube, the one end of which has a fixed center and pulley much the same as usual; the opposite end of the tube has a piece of steel fixed into it, which is first drilled with a central hole, and then turned as a conical screw, to which is fitted a corresponding screw nut n; the socket is then sawn down with two diametrical notches, to make four internal angles, and lastly, the socket is hardened. When the four sections are compressed by the nut, their edges stick into the drill and retain it fast, and provided the instrument is itself concentric, and the four parts are of equal strength, the centrality of the drill is at once ensured. The outside of the nut. and the square hole in the key k, are each taper, for more ready application; and the drills are of the most simple kind, namely, lengths of wire pointed at each end, as in fig. 493.*

The sketch, fig. 492, is also intended to explain another useful application of this drill-stock, as an upriyht or pump-drill, a tool little employed in this country (except in drilling the rivet holes for mending china and glass, with the diamond-drill, fig. 70, vol. I.,) but as well known among the oriental nations as the breast-drill. The pump-drill is figured and explained on page 3 of the fourth volume of this work, to which the reader is referred; occasionally the pump-drill and the common drill-stock are mounted in frames, by which their paths are more exactly defined; but these contrivances are far from being generally required, and enough will be said in reference to the use of revolving braces, to lead to such applications, if considered requisite, for reciprocating drills. - See Appendix, Notes A. Z. to B.B. page 100.;.

Holes that are too large to be drilled solely by the breast-drill and drill-bow, are frequently commenced with those useful instruments, and are then enlarged by means of the hand-brace, which is very similar to that used in carpentry, except that it is more commonly made of iron instead of wood, is somewhat larger, and is generally made without the spring-catch.

Holes may be extended to about half an inch in diameter, with the hand-brace; but it is much more expeditious to employ still larger and stronger braces, and to press them into the work in various ways by weights, levers, and screws, instead of by the muscular effort alone.

Fig. 494 represents the old smith's press-drill, which although cumbrous, and much less used than formerly, is nevertheless simple and effective. It consists of two pairs of wooden standards, between which works the beam a b, the pin near a is placed at any height, but the weight to is not usually changed, as the greater or less pressure for large and small drills, is obtained by placing brace more or less near to the fulcrum a; and this part of beam is shod with an iron plate, full of small center holes the brace. The weight is raised by the second lever c d, the two being united by a chain, and a light chain or rope is also suspended from d, to be within reach of the one or two men engaged in moving the brace. It is necessary to relieve the weight when the drill is nearly through the hole, otherwise, it might suddenly break through, and the drill becoming fixed, might be twisted off in the neck.

• See Technical Repository, vol. ii, 1822, p. 147.

Figs. 494.

495.

The inconveniences in this machine are, that the upper point of the brace moves in an arc instead of a right line; the limited path when strong pressures are used, which makes it necessary to shift the fulcrum a; and also the necessity for re-adjusting the work under the drill for each different hole, which in awkwardly shaped pieces is often troublesome.

A portable contrivance of similar date, is an iron bow frame or clamp, shown in fig. 495; the pressure is applied by a screw, but in almost all cases, whilst the one individual drills the hole, the assistance of another is required to hold the frame; 495 only applies to comparatively thin parallel works, and does not present the necessary choice of position. Another tool of this kind, used for boring the side holes in cast-iron pipes for water and gas, is doubtless familiarly known; the cramp or frame divides into two branches about two feet apart, and these terminate like hooks, which loosely embrace the pipe, so that the tool retains its position without constraint, and it may be used with great facility by one individual.