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.
[Illustration: Fig.138.