The bosses of drill-spindles are denoted by Figs. 850 and 852. The hole in a spindle-boss for tightly holding ends of drills, boring-rods, and other tools, may be either square or circular. In either case, the hole is tapered, in order to easily enter and detach drills whenever necessary. A regular conical hole is that which is most frequently provided, similar to those shown in the Figures.

The boss denoted by Fig. 850 is furnished with a steel fixing screw for holding the drill after being pushed into the hole, the length of which screw should be at right-angles to the drill-spindle's long axis. In the dotted lines which indicate the place and shape of the drill's end, may be seen a shallow flat bottom gap, in which the screw's point is tightly screwed with a tommy in the hole of the screw's head, which effectually prevents loosening of the drill while at work. The bottom of this gap requires to be right-angular to the length of the screw, to ensure an equable bearing, of the point when tightened. A boss having a screw of this class, should have a hole the sides of which are at an angle of only four or five degrees with each other, that the act of tightening the screw may not loosen the drill's end, which results if the hole is too taper. It is, however, necessary to remember that the nearer the sides of the hole resemble parallelism with each other, the greater is the difficulty of releasing the drill from the boss when requisite. But a nearly parallel hole is very effectual for obtaining a firm grip, which is necessary to make the drill rotate properly. Consequently, the trouble of hammering the drill to loosen its end if only slightly conical, is to be placed against the facility with which it can be tightly and accurately fixed.

To obtain a spindle-boss that will allow a drill-end to be immediately detached without hammering, and which will also exactly and tightly fit a drill-end within, it is necessary to provide the boss with a hole which is very taper, resembling that in Fig. 852; and instead of furnishing the boss with a screw, a thin key must be inserted as denoted in the Figure. By means of a key the entire conical part of the drill's end in the hole is made to fit exactly to every part of the hole occupied by the drill, the act of driving the key causing the cone to be wedged into the hole until in close contact. A very taper end cannot be made to fit thus closely by screwing the point of a screw thereupon ; but with a key a perfect contact is secured, however great the amount of tapering may be. The use of a key also prevents distortion of the conical hole in the boss, such distortion resulting from the series of repeated strains imposed while tightening a fixing screw. The only objection to the use of a key for these purposes, is the extra time required for entering and dislodging it. This frequent shifting of drills and boring-bars is but seldom requisite for large work ; therefore it may be said that keyed ends are suitable for large machines, and ends fixed with screw-points are suitable for small machines.

By Figs. 851 and 853 two drills' ends are shown detached from their respective spindles, the one denoted by Fig. 851 belonging to the boss shown by Fig. 850. In Fig. 853, which denotes an end belonging to Fig. 852, the shape of the key way is shown, having curved top and bottom surfaces.