vided 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 fiveeighths 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 coliet, or vice versa.

Sometimes cylindrical drills have straight flutes, as outlined at Fig. 62, 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. 62, 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. 62, D, 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. Tables will be found in the last chapter giving drill speeds and feeds and other data relative to the use of this tool.

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. 63, 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 16 inch in diameter. Beyond this point drills are usually sold by the diameter. A set of chuck drills ranging from 3% to 34 inch, advancing by 1/2 inch, and a set of Morse taper shank drills ranging from 3/4 to 14 inches, by increments of 16 inch, will be all that is needed for the most pretentious repair shop, as it is cheaper to bore holes larger than 14 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.

In grinding drills, care must be taken to have the lips of the same length, so that they will form the same angle with the axis. If one lip is longer than the other, as shown in the flat drill at Fig. 62, E, the hole will be larger than the drill size, and all the work of cutting will come upon the longest lip. The drill ends should be symmetrical, as shown at Fig. 62, F.

It is considered very difficult to drill a hole to an exact diameter, but for most work a variation of a few thousandths of an inch is of no great moment. Where accuracy is necessary, holes must be reamed out to the required size. In reaming, a hole is drilled about 2 inch smaller than is required, and is enlarged with a cutting tool known as the reamer. Reamers are usually of the fluted form shown at Fig. 64, A. Tools of this nature are not designed to remove considerable amounts of metal, but are intended to augment the diameter of the drill hole by only a small fraction of an inch. Reamers are tapered slightly at the point in order that they will enter the hole easily, but the greater portion of the fluted part is straight, all cutting edges being parallel. Hand reamers are made in either the straight or taper forms, that at A, Fig. 64, being straight, while B has tapering flutes. They are intended to be turned by a wrench similar to that employed in turning a tap, as shown at Fig. 66, C. The reamer shown at Fig. 64, C, is a hand reamer of the taper form widely used by

blacksmiths. The form at D has spiral flutes similar to a twist drill, and as it is provided with a taper shank it is intended to be turned by power through the medium of a suitable collet.

As the solid reamers must become reduced in size when sharpened, various forms of inserted blade reamers have been designed.

A

B

C

D

E

F

Fig. 64.-Illustrating Standard Forms of Hand and Machine Reamers.

One of these is shown at Fig. 64, E, and as the cutting surfaces become reduced in diameter it is possible to replace the worn blades with others of proper size. Expanding reamers are of the form shown at Fig. 64, F. These have a bolt passing through that fits into a tapering hole in the interior of the split reamer portion of

the tool. If the hole is to be enlarged a few thousandths of an inch, it is possible to draw up on the nut just above the squared end of the shank, and by drawing the tapering wedge farther into the reamer body, the cutting portion will be expanded and will cut a larger hole.

Reamers must be very carefully sharpened or there will be a

tendency toward chattering with a consequent production of a rough surface. There are several methods of preventing this chattering, one being to separate the cutting edges by irregular spaces, while the most common method, and that to be preferred on machine reamers, is to use spiral flutes, as shown at Fig. 64, D. Special taper reamers are made to conform to the various taper pin sizes which are widely used in holding parts together in an

automobile. A taper of 16 inch per foot is intended for holes where a pin, once driven in, is to remain in place. When it is desired that the pin be driven out, the taper is made steeper, generally 4 inch per foot, which is the standard taper used on taper pins.

When threads are to be cut in a small hole, it will be apparent that it will be difficult to perform this operation economically on a lathe, therefore when internal threading is called for, a simple device known as a "tap" is used. There are many styles of taps, all conforming to different standards. Some are for metric or foreign threads, some conform to the American standards, while others are used for pipe and tubing. Hand taps are the form most used in repair shops, these being outlined at Fig. 65, A and B. They are usually sold in sets of three, known respectively as taper, plug, and bottoming. The taper tap is the one first put into the hole, and is then followed by the plug tap which cuts the threads deeper. If it is imperative that the thread should be full size clear to the bottom of the hole, the third tap of the set, which is straight-sided, is used. It would be difficult to start a bottoming tap into a hole because it would be larger in diameter at its point than the hole. The taper tap, as shown at A, Fig. 65, has a portion of the cutting lands ground away at the point in order that it will enter the hole. The manipulation of a tap is not hard, as it does not need to be forced into the work, as the thread will draw it into the hole as the tap is turned. The tapering of a tap is done so that no one thread is called upon to remove all of the metal, as for about half way up the length of the tap each succeeding thread is cut a little larger by the cutting edge until the full thread enters the hole. Care must be taken to always cher a tap straight in order to have the thread at correct angles to the surface.

In cutting external threads on small rods or on small pieces, such as bolts and studs, it is not always economical to do this work in the lathe, especially in repair work. Dies are used to cut threads on pieces that are to be placed in tapped holes that have been threaded by the corresponding size of tap. Dies for small work are often made solid, as shown at Fig. 66, C, but solid dies