It is difficult, if not quite impossible, to drill a hole to an exact diameter. For most work, however, a variation of a few hundredths of an inch is of no account, but when greater accuracy is required the hole must be reamed. Holes to be reamed are first drilled a little smaller than the desired size (1/64 in. or even 1/100 in.), and then reamed out to exact size. They should never be drilled over 1/32 in. smaller than the size of the reamer. Reaming is especially necessary where two or more parts are to be bolted together, since the drill in passing through them will often cut more out of one part than another because of the variation in the structure of the metal.

Reaming may be done by hand or with a drilling machine; or the reamer may be held in the drilling machine, or in the drill spindle socket and turned by hand with a wrench. Reaming should be done very carefully, and it may be necessary to tap the reamer gently with a hammer or wrench to feed it. There should be no "wobbling" or irregular motion, but a very steady and slow motion under light pressure. In some cases the weight of the reamer and the wrench is sufficient to feed the tool through the hole.

In reaming with a drill press a power feed may be used in some cases, but great care must then be exercised to see that the reamer does not stick and break. Some reamers have a shallow screw thread cut on the small end which makes them self-feeding. Oil, drilling compound, or some other lubricant should always be used when reaming wrought iron or steel, but not when reaming cast iron or brass.