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.