Illustration of Reaming

Reamers are used to widen a hole diameter while improving its roundness.

Reamers usually consist of four or more inserts which, compared to the two inserts of the twist drill, provide better hole tolerances.
The tools usually consist entirely of high-speed steel or cemented carbide or alternatively they have soldered or mechanically mounted inserts.
There are also reamers with only one insert that, in combination with sliding surfaces, allow for the desired tolerance