tap
A cylindrical tool used to cut an internal screw thread in a hole, with cutting edges around the lower end and an upper end to which a handle is fitted to turn the tool.
tap: the tool that cuts threads inside a hole
A tap is a hardened steel tool that cuts helical grooves, or threads, into the inner surface of a hole. The cutting edges are ground into the lower end of a cylindrical shaft, arranged to match the pitch and profile of the thread being formed. The upper end (called the shank) is either squared to fit a tap wrench or fitted with a handle so the operator can turn the tool by hand as it bites into the workpiece.
Taps come in three standard profiles within a metric or imperial thread family: taper taps have a conical lead that makes starting easier and spreads the cutting load; plug taps have a shorter lead and cut most of the thread; bottoming taps have minimal flute length and are used only to finish near the base of a blind hole. A complete tapping job usually involves all three in sequence, though in through-holes a single plug tap often suffices.
The tool works by a combination of cutting and displacement. As the tap rotates and advances into the hole, the flutes (the helical grooves in the tap itself) curl and remove metal chips. The cutting edges themselves are the lands between the flutes. The flute design must clear chips efficiently: too small and chips jam; too large and the tap breaks. Most general-purpose taps have 2 to 4 flutes and are made from high-speed steel (HSS), while carbide taps are used for production runs in softer materials like aluminum.
Common failure modes and setup
Taps break most often from thread binding, misalignment, or oversized starter holes. If the hole is too small, chips accumulate and the tap jams; if too large, the tap chatters and fractures. Backing the tap out every half turn or full turn helps expel chips in manual tapping. On a machine, cutting fluid (usually a soluble oil for steel, straight mineral oil for aluminum) is essential to prevent both chip welding and thermal stress. Hand tapping in steel is inherently slower than machine tapping but remains standard in shops, repair work, and one-off fabrication.
The term tap derives from its tapping action, a repeated striking or screwing motion. Industrial standards specify tap sizes by thread pitch and nominal diameter: M12 x 1.75 (metric) or 1/2-13 UNC (unified inch) denote the workpiece thread form the tap will cut. Oversized taps called spiral-flute or spiral-point taps are common in CNC drilling and tapping operations because they handle chip evacuation better than standard parallel-flute designs.