Tapping means producing an internal thread, that is, a helical groove, in a hole so that a screw or threaded rod can be screwed into it. The tool is called a tap: it looks like a fluted screw whose edges cut the thread. Its external counterpart is the threading of a rod.
You start with drilling to the “tapping drill” diameter, roughly equal to the nominal diameter minus the pitch: for an M8 with a 1.25 mm pitch, you drill 6.8 mm. The tap then enters at slow speed, by hand or, more often, on a machining centre with a tapping cycle that synchronises rotation and feed. Cutting taps remove a chip; forming taps create the thread by deformation, without chips, in ductile materials such as aluminium or mild steel. For large diameters, hard materials or delicate holes, thread milling with a thread mill is an alternative: a broken tool is less costly there than a snapped tap in a hole, which is often very hard to extract.
Tapping allows a demountable assembly on plates, flanges, enclosures and brackets. Thread engagement is worth about one diameter in steel, more in soft materials such as aluminium or plastics. Where holding strength is insufficient, a threaded insert reinforces the thread. This operation is part of CNC machining services.
In a quote request. State the full designation (for example M8, M8 × 1 fine pitch, G 1/4 pipe thread, UNC), whether the hole is through or blind, the useful thread depth and the tolerance class (6H is the most common in metric). Say whether a go/no-go gauge check is required, and whether a surface treatment follows: it changes the thread dimensions. The guide on the manufacturing specification lists what a workshop needs to know.