The Sensitive shown in Figure 5 (15/. 26), was designed with particular reference to the accomplishment of rapid and accurate work with small drills, and at the same time to obviate as far as possible all dan ger of their breakage. By small drills are meant drills from 0 of an inch in diameter, the size of an ordinary lead-pencil, down to .o15 of an inch in diameter, the size of a cambric sewing-needle. One of the chief requisites of a drilling-machine for this purpose is a true- and light-running spindle. The usual mode of running a spindle is to attach a pulley to it direct and give it motion by a belting from another driven pulley. The objections to the old style were, first, the strain on the spindle by the belt tension, ren dering it liable to spring; and secondly, the wear on the boxes or journals in which the spindle ran, occasioned by the strain and pull of the belt being always from one direction, with the consequent certainty of wearing the boxes out of round.
Through the top arm of the drill-frame (fig. 4) is inserted a hollow sleeve, whose lower end comes nearly flush with the frame—that is, it only extends through a sufficient length to provide a good bearing for the pulley to run on. The drill-spindle runs on the inside of this sleeve, and the pulley, with a hub somewhat longer than the upper end of the sleeve, on the outside. The spindle has a small groove or key-way cut lengthwise.
In the upper end of the pulley-hub a bushing is made fast, and in this bushing is the key fitting the spline in the spindle, which drives it when the pulley is revolved and at the same time provides for vertical spindle motion. Hence there is no belt tension on the spindle.
The next important point is in securing a feed of so sensitive a nature that the operator can at all times judge of the exact power applied and the resistance of the work to the drill-point. This feature is covered by balancing the weight of the spindle-chuck, etc., by a flat coiled spring. Through the lower or adjustable arm is run a pinion-shaft, whose pinion drives into a rack, which is in the form of a sleeve, in which the spindle runs, and is held in place vertically in the spindle by the nut and collars shown. On the outer end of the pinion-shaft is keyed a disc, into which the lever is set, while on the inside thereof is a projecting pin, over which is placed the end of a flat coiled spring. The spring is coiled in a spring box of the same diameter as the disc referred to, and to adjust the spring tension is arranged to revolve on a boss or projection on the frame. When the spring is so set that the tension thereon just balances the weight of the spindle, its attachment, lever, etc., the spring box is made fast by a set-screw to the frame, and the lightest touch on the lever will raise or lower the spindle. When in operation, the only resistance to the hand is that of the material being drilled, and the expert operator can force his drill up to its safety limit. By loosening a hand-nut on the back of the post the lower head can be set at any height, allowing for quick adjust ment for varying thicknesses of work.
One more very essential feature in light drilling is that the tool shall run steady without vibration, especially about the spindle. A recent improvement urakes the top arm in two pieces planed together, to slide on a tongue and groove, and provided with an adjusting screw and lock nut. With this arrangement an endless leather belt can be used to drive from the spindle-driving pulley on the back cone to the spindle pulley, and thus do .way not only with the vibration caused by the lacing run ning over the pulley at high speed, but also with the trouble and expense of lacings and the necessity of providing means for keeping the belt at the proper tension at all times. The drill-table swings around on the head of the column, so that the supplementary table underneath can he used, or by removing the latter stand a bell-shaped stand can be used in its place. The support for this lower table- and cup-stand is adjustable on a slide to any height. Drills having the essential good points of this one are made having two, three, four, and even more, spindles. The arrangement made for driving the two-, three-, and four-spindle drills with a single endless belt is a great convenience, doing away with the care required to keep up a short belt for each spindle.
Radial it frequently happens that several holes are to be drilled in the same direction in masses of metal which are heavy and difficult to move, the vertical drilling-machine is sometimes so arranged that within a certain range the drill-spindle can be shifted to every desired place without detriment to the motion to be imparted to it from the driv ing-shaft. If this is attained by radially pushing the head containing the drill-spindle upon a horizontal arm and by rotating this arm, this kind of machine is called a "radial" drill (6/. 24,.fig. 16). The arm rotates around a cast-iron column, having in its axis a vertical shaft, which receives at its lower end a rotation from the horizontal driving-shaft by a bevel-gear and transmits it on the upper end, by another bevel-gear, to a horizontal shaft (not plainly shown in the Figure), which participates in the motion of the arm as well as in the rectilinear displacement of the drill-spindle carriage. This shaft being connected (by spline and groove) with the bevel-gear driving the drill-spindle, the connection of movement between the driving shaft and drill-spindle remains the same for all possible positions of the drill. The drill-feed motion is derived from its rotation in the same man ner as in the machine represented by Figure 17, except that here the feed can also be executed by hand by turning the hand-wheel, at the right of the drill-spindle. The material to be drilled is secured to the horizontal or vertical surface of the frame, which is provided with T-grooves, or, if very large and heavy, is placed directly upon the floor.