[Illustration: Fig. 137. Using a Try-Square as a Guide in Boring.]
Auger-bits are sharpened with an auger-bit file, Fig. 142, p. 90, a small flat file with two narrow safe edges at one end and two wide safe edges at the other. The "nibs" should be filed on the inside so that the diameter of the cut may remain as large as that of the body of the bit. The cutting lip should be sharpened from the side toward the spur, care being taken to preserve the original angle so as to give clearance. If sharpened from the upper side, that is, the side toward the shank, the nibs will tend to become shorter.
The _plug-cutter_, Fig. 131, is useful for cutting plugs with which to cover the heads of screws that are deeply countersunk.
_Center-bits_, Fig. 132, work on the same principle as auger-bits, except that the spurs have no screw, and hence have to be pushed forcibly into the wood. Sizes are given in 16ths of an inch. They are useful for soft wood, and in boring large holes in thin material which is likely to split. They are sharpened in the same way as auger-bits.
_Foerstner bits_, Fig. 133, are peculiar in having no spur, but are centered by a sharp edge around the circumference. The size is indicated on the tang, in 16ths of an inch. They are useful in boring into end grain, and in boring part way into wood so thin that a spur would pierce thru. They can be sharpened only with special appliances.
_Expansive-bits_, Fig. 134, are so made as to bore holes of different sizes by adjusting the movable nib and cutter. There are two sizes, the small one with two cutters, boring from 1/2" to 1-1/2" and the large one with three cutters boring from 7/8" to 4". They are very useful on particular occasions, but have to be used with care.
_Reamers_, Fig. 135, are used for enlarging holes already made. They are made square, half-round and six cornered in shape.
_Countersinks_, Fig. 136, are reamers in the shape of a flat cone, and are used to make holes for the heads of screws. The rose countersink is the most satisfactory form.
The _washer-cutter_, Fig. 138, is useful not only for cutting out washers but also for cutting holes in thin wood. The size is adjustable.
The primitive celt, which was hardly more than a wedge, has been differentiated into three modern hand tools, the _chisel_, see above, p. 53, the _ax_, Fig. 139, and the _adze_, Fig. 141.