Editing Reamer (section)

== Reaming versus drilling to size ==
The geometry of a hole drilled in metal by a [[twist drill]] may not be accurate enough (close enough to a true cylinder of a certain precise diameter) and may not have the required smooth [[surface roughness|surface finish]] for certain engineering applications. Although modern twist drills can perform excellently in many cases—usually producing sufficiently accurate holes for most applications—sometimes the stringency of the requirements for the hole's geometry and finish necessitate two operations: a drilling to slightly undersize, followed by reaming with a reamer. The planned difference between the drill diameter and the reamer diameter is called an ''[[Allowance (engineering)|allowance]]''. (It allows for the removal of a certain small amount of material.) The allowance should be < 0.2&nbsp;mm (.008 in) for soft materials and < 0.13&nbsp;mm (.005 in) for hard materials. Larger allowances can damage the reamer. The drilled hole should not be enlarged by more than 5% of the drilled diameter. Drilling followed by reaming generally produces hole geometry and finish that is as close to theoretical perfection as possible. (The other methods of hole creation that approach nearest to perfection under certain conditions are [[Boring (manufacturing)|boring]] [especially single-point boring] and [[Cylindrical grinder#ID grinder|internal cylindrical grinding]].)