TAPS
Set of three taps
What are taps used for?
The tap is a rotary cutting tool used to produce the internal thread of a hole — an operation known as tapping. Engaged in a pilot hole of the appropriate diameter, it progressively cuts, by chip removal, a helix that will then form the seating for a screw. In watchmaking, where diameters frequently drop below one millimetre and where the breakage of a tap inside a workpiece is most often irreversible, tapping requires specific tools, dedicated standards and a particularly codified working procedure.
In its general form, the watchmaking tap is presented as a high-speed steel (HSS) or hardened tool-steel shank, ending in a threaded portion split by two or three longitudinal flutes. These flutes free the cutting edges and ensure chip evacuation. The shank of the tool, with a square-section end, allows it to be driven by a tap holder — tap wrench, chuck or bench tapping stand. Usual diameters range from a few tenths of a millimetre to a few millimetres, with codified pitches and thread profiles.
The threading used in watchmaking complies largely with the NIHS standards (Normes de l’Industrie Horlogère Suisse, Swiss Watch Industry Standards), which define the geometry of the thread and the series of standardised dimensions. Alongside the ISO metric thread, reserved for the most substantial components, watchmaking mainly uses the so-called “S” pitch — a series of triangular 60° threads designed specifically for very small diameters: bridge screws, strap screws, regulator screws, and so on. This codification ensures the interchangeability of threaded components across the industry and the availability, from specialist suppliers, of taps, dies (see screw plate) and screws to the same specifications. The Thury pitch, older, retains a residual presence in the restoration of antique pieces.
For watchmaking diameters, tapping is traditionally carried out with a set of three successive taps: taper (roughing), plug (intermediate) and bottoming (finishing). The first — taper tap — has a long tapered lead-in section, takes the first chips and starts the thread without straining the tool. The second — plug tap — has a shorter chamfer lead; it deepens and regularises the thread. The third — bottoming tap — has only a minimal lead, or none at all; it finishes the thread to the nominal diameter and down to the bottom of the hole, which proves particularly important in blind holes, where only the bottoming tap allows a usable thread to be obtained over the full available length. Progression in three passes distributes the cutting effort over three tools, limits the torque exerted at each stage and considerably reduces the risk of breakage — a risk all the more critical as the extraction of a fragment of broken tap from a watch component often forces the outright replacement of the part.
At the bench, the watchmaker begins by drilling the pilot hole to the prescribed diameter, generally codified in the NIHS tables for each pitch dimension. The workpiece is then immobilised — between vice jaws, on a tapping stand or in a specialised fixture — so that the tap penetrates strictly perpendicular to the plane of the hole. The tool, mounted in its holder, is engaged without haste; each forward turn is punctuated by a partial reverse intended to break the chip and to prevent the flutes from clogging. Suitable lubrication — watchmaking cutting oil — accompanies the gesture. The sequence taper, plug, bottoming is scrupulously followed.
Modest-looking tools, watchmaking taps concentrate the demands of a trade in which every threadable assembly rests on rigorous standardisation — notably that of the NIHS series — and on the mastery of a gesture where patience always prevails over force.