The smallest holes are those required in watch-work, and the general form of the drill is shown on a large scale in fig. 488; it is made of a piece of steel wire, which is tapered off at the one end, flattened with the hammer, and then filed up in the form shown at large in fig. 570, p. 547; lastly, it is hardened in the candle. The reverse end of the instrument is made into a conical point, and is also hardened; near this end is attached a little brass sheave for the line of the drill-bow, which in watchmaking is sometimes a fine horse-hair, stretched by a piece of whalebone of about the size of a goose's quill stripped of its feather.
The watchmaker holds most of his works in the fingers, both for fear of crushing them with the table vice, and also that he may the more sensibly feel his operations; drilling is likewise performed by him in the same manner. Having passed the bow-string around the pulley in a single loop (or with a round turn), the center of the drill is inserted in one of the small center holes in the sides of the table vice, the point of the drill is placed in the mark or cavity made in the work by the center punch; the object is then pressed forward with the right hand, whilst the bow is moved with the left; the Swiss workmen apply the hands in the reverse order, as they do in using the turn-bench*.
and which include some of the marbles. Glass may also be drilled with fig. 470, or 471, lubricated with turpentine. The sandstones are readily bored in Hunter's patent stone boring-machine (see p. 543, also Con v. Civ. Eng. 1842, p. 146), and the granites are not bored, but crushed by the jumper, or chisel point, see p. 170, Vol. 1. the compact mineral*, such as 4, 5, 6 of the table, the grinding tools may be used with sand, but emery is more effective; this powder may be also employed for minerals not exceeding the hardness of 7 and 8; but emery being somewhat inferior in hardness to the ruby, this gem and the diamond, marked 9 and 10 in the table, require either diamond dust, or splinters of the diamond, the outside skin and natural angles of which, are much harder than the inside substance. See the ninth chapter of Vol. I. generally, especially pages 178 to 180.
•SeeVol. IV., page 16.
Clockmakers, and artisans in works of similar scale, fix the object in the tail-vice, and use drills, such as fig. 488, but often larger and longer; they are pressed forward by the chest which is defended from injury by the breast-plate, namely, a piece of wood or metal about the size of the hand, in the middle of which is a plate of steel, with center holes for the drill. The breastplate is sometimes strapped round the waist, but is more usually supported with the left hand, the fingers of which are ready to catch the drill should it accidentally slip out of the center.
As the drill gets larger, the bow is proportionably increased in stiffness, and eventually becomes the half of a solid cone, about 1 inch in diameter at the larger end, and 30 inches long; the catgut string is sometimes nearly an eighth of an inch in diameter, or is replaced by a leather thong. The string is attached to the smaller end of the bow by a loop and notch, much the same as in the archery bow, and is passed through a hole at the larger end, and made fast with a knot; the surplus length is wound round the cane, and the cord finally passes through a notch at the end, which prevents it from uncoiling.
Steel bows are also occasionally used; these are made something like a fencing foil, but with a hook at the end for the knot or loop of the cord, and with a ferrule or a ratchet, around which the spare cord is wound. Some variations also are made in the sheaves of the large drills; sometimes they are cylindrical with a fillet at each end; this is desirable, as the cord necessarily lies on the sheave at an angle, in fact in the path of a screw; it pursues that path, and with the reciprocation of the drill bow, the cord traverses, or screws backwards and forwards upon the sheave, but is prevented from sliding off by the fillet. Occasionally indeed, the cylindrical sheave is cut with a screw coarse enough to receive the cord, which may then make three or four coils for increased purchase, and have its natural screw-like run without any fretting whatever; but this is only desirable when the holes are large, and the drill is almost constantly used, as it is tedious to wind on the cord for each individual hole. The structure of the bows, breast-plates, and pulleys, although often varied, is sufficiently familiar to be understood without figures. - See Appendix, Note AY, page 1002.
When the shaft of the drill is moderately long, the workman can readily observe if the drill is square with the work as regards the horizontal plane; and to remove the necessity for the obser-ion of an assistant as to the vertical plane, a trifling weight is sometimes suspended from the drill shaft by a metal ring or hook, the joggling motion shifts the weight to the lower extremity; the tool is only horizontal when the weight remains central*. In many cases, the necessity for repeating the shaft and pulley of the drill is avoided, by the employment of holders of various kinds, or drill-stocks, which serve to carry any required number of drill-points. The most simple of the drill-stocks is shown in fig. 489; it has the center and pulley of the ordinary drill, but the opposite end is pierced with a nearly cylindrical hole, just at the inner extremity of which a diametrical notch is filed. The drill is shown separately at a; its shank is made cylindrical, or exactly to fit the hole, and a short portion is nicked down also to the diametrical line, so as to slide into the gap in the drill-stock, by which the drill is prevented from revolving; the end serves also as an abutment whereby it may be thrust out with a lever. Sometimes a diametrical transverse mortise, narrower than the hole, is made through the drill-stock, and the drill is nicked on both sides; and Mr. Gill proposes that the cylindrical hole of 489, should be continued to the bottom of the notch, that the end of the drill should be filed off obliquely, and that it should be prevented from rotating, by a pin inserted through the cylindrical hole parallel with the notch; the taper end of the drill would then wedge fast beneath the pin. †