this
is to lay the work flat, either on the bench or in the vise, and sight
first from the front and then from the side of the work, to see that
the bit is perpendicular both ways. The test may also be made with the
try-square, Fig. 137, or with a plumb-line, either by the worker,
or in difficult pieces, by a fellow worker. The sense of
perpendicularity, however, should constantly be cultivated. (2)
Another precaution is that, in thru boring, the holes should not be
bored quite thru from one side, lest the wood be splintered off on the
back. When the spur pricks thru, the bit should be removed, the piece
turned over, and the boring finished, putting the spur in the hole
which is pricked thru in boring from the first side. It is seldom
necessary to press against the knob of the brace in boring, as the
thread on the spur will pull the bit thru, especially in soft wood.
Indeed, as the bit reaches nearly thru the board, if the knob is
gently pulled back, then when the spur pricks thru the bit will
be pulled out of its hole. This avoids the necessity of constantly
watching the back of the board to see if the spur is thru. (3) In stop
boring, as in boring for dowels or in making a blind mortise, care
should be taken not to bore thru the piece. For this purpose an
auger-bit-gage, Fig. 219, p. 116, may be used, or a block of wood of
the proper length thru which a hole has been bored, may be slipped
over the bit, or the length of bit may be noted before boring, and
then the length of the projecting portion deducted, or the number of
turns needed to reach the required depth may be counted on a trial
piece. Tying a string around a bit, or making a chalk mark on it is
folly.

[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 th