Fig. 168.—Forms of Drills Used in Hand and Power Drilling Machines.
Larger drills than 3⁄4-inch are usually made with a tapered shank as shown at [Fig. 168], B. At the end of the taper a tongue is formed which engages with a suitable opening in the collet, as the piece used to support the drill is called. The object of this tongue is to relieve the tapered portion of the drill from the stress of driving by frictional contact alone, as this would not turn the drill positively and the resulting slippage would wear the socket, this depreciation changing the taper and making it unfit for other drills. The tongue is usually proportioned so it is adequate to drive the drill under any condition. A small keyway is provided in the collet into which a tapering key of flat stock may be driven against the end of the tongue to drive the drill from the spindle. A standard taper for drill shanks generally accepted by the machine trade is known as the Morse and is a taper of five-eighths of an inch to the foot. The Brown and Sharp form tapers six-tenths of an inch to the foot. Care must be taken, therefore, when purchasing drills and collets, to make sure that the tapers coincide, as no attempt should be made to run a Morse taper in a Brown and Sharp collet, or vice versa.
Sometimes cylindrical drills have straight flutes, as outlined at [Fig. 168], A. Such drills are used with soft metals and are of value when the drill is to pass entirely through the work. The trouble with a drill with spiral flutes is that it will tend to draw itself through as the cutting lips break through. This catching of the drill may break it or move the work from its position. With a straight flute drill the cutting action is practically the same as with the flat drill shown at [Fig. 168], E and F.
If a drill is employed in boring holes through close-grained, tough metals, as wrought or malleable iron and steel, the operation will be facilitated by lubricating the drill with plenty of lard oil or a solution of soda and water. Either of these materials will effectually remove the heat caused by the friction of the metal removed against the lips of the drill, and the danger of heating the drill to a temperature that will soften it by drawing the temper is minimized. In drilling large or deep holes it is good practice to apply the lubricating medium directly at the drill point. Special drills of the form shown at [Fig. 168], B, having a spiral oil tube running in a suitably formed channel, provides communication between the point of the drill and a suitable receiving hole on a drilled shank. The oil is supplied by a pump and its pressure not only promotes positive circulation and removal of heat, but also assists in keeping the hole free of chips. In drilling steel or wrought iron, lard oil applied to the point of the drill will facilitate the drilling, but this material should never be used with either brass or cast iron.
The sizes to be provided depend upon the nature of the work and the amount of money that can be invested in drills. It is common practice to provide a set of drills, such as shown at [Fig. 169], which are carried in a suitable metal stand, these being known as number drills on account of conforming to the wire gauge standards. Number drills do not usually run higher than 5⁄16 inch in diameter. Beyond this point drills are usually sold by the diameter. A set of chuck drills, ranging from 3⁄8 to 3⁄4 inch, advancing by 1⁄32 inch, and a set of Morse taper shank drills ranging from 3⁄4 to 11⁄4 inches, by increments of 1⁄16 inch, will be all that is needed for the most pretentious repair shop, as it is cheaper to bore holes larger than 11⁄4 inches with a boring tool than it is to carry a number of large drills in stock that would be used very seldom, perhaps not enough to justify their cost.
Fig. 169.—Useful Set of Number Drills, Showing Stand for Keeping These in an Orderly Manner.