Boring (alésage in French) is the finishing operation on an existing hole (drilled, cast or roughed out) that gives it a precise diameter, good roundness, an exact position and a fine surface finish. The French word also means the resulting hole: the bore of a bearing housing, a cylinder body or a casing.
Boring is done on a lathe or on a machining centre. The boring tool, with one or two cutting edges, is mounted on a bar or an adjustable head set to the micrometre: it removes a very fine chip and corrects both the diameter and the position of the hole. The reamer, a multi-tooth tool, suits small diameters and common fits. For even finer requirements, you move to internal grinding or honing. With good tooling, quality IT7 or even IT6 is routinely achieved, which means a tolerance band of roughly ten to a few tens of microns depending on the diameter.
Boring is essential for bearing seats, plain bearings, slide guides, cylinder or pump bodies and holes that must be perfectly aligned over a long length. Several coaxial bores are then machined in the same clamping to guarantee their alignment. Precision bores are requested under the CNC machining capability.
In a quote request. Give the diameter with its ISO tolerance (for example Ø 25 H7), the roughness, any cylindricity or coaxiality requirements and the depth. If the bore fits with a shaft, state both tolerances: the resulting fit is what matters. Mention the measuring temperature if the tolerance is very tight, because thermal expansion then becomes a factor in its own right. The guide on machining tolerances details ISO fits.