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1. What is a drill bit used for?
The drill bit is a cutting tool with rotary motion designed to produce a cylindrical hole by removing material. In horology, where dimensions are frequently below one millimetre and precision is measured in hundredths, or even microns, the tool must ensure perfectly cylindrical and straight holes, free of burrs, and above all reproducible from one part to another. The choice of drill bit — through its geometry, its material and its coating — directly determines the quality of the resulting hole, the surface finish, the dimensional accuracy and the tool’s lifespan. It is therefore a modest-looking tool, yet a decisive one in the manufacturing chain of movement components as well as external parts (case, dial, hands).
2. The main drill bit profiles
There are several geometries, each suited to a specific type of operation.
The helical drill bitis the most universally used form. It consists of a shank (fixing part in the chuck), a cylindrical body running with two Helical flutes intended for chip evacuation, and a point forming the two main cutting edges, connected by a transverse edge (the web). The point angle is typically 118° for general use; it is increased to 130° or 140° for hard Steels and stainless Steels, and reduced to 90° to 100° for soft alloys and plastics. The helix angle influences chip evacuation: a low helix (modified N type, 10°-19°) is suitable for Brass and bronze, a normal helix (25°-30°) for Steel, and a high helix (35°-45°) for Aluminium and polymers.
Helical Drill bit
The centering Drill bit (or center Drill bit) is used to initiate drilling by creating a precise cone that will then guide the helical drill bit. Its short, rigid geometry prevents buckling (off-centre drilling). The standardized centering drill bit combines, on the same shaft, a cylindrical point and a cone, generally at 60°.
Pointing drill bit (or centering drill bit)
The lance drill bit, sometimes called a flat-tip drill bit or “snake’s tongue” in traditional horological vocabulary, remains a classic of manual operations. Forged then sharpened by hand by the horologist, it has a symmetrical lance-shaped point that penetrates the material without lateral drift. It was historically operated with the bow, then on the hand drill; today it still holds its place for delicate touch-ups and the repair of small holes where very fine control of the entry point is desired.
The stepped drill bit allows a hole with several successive diameters to be produced in a single pass, for example to jointly house a screw head and its clearance threading.
3. Materials and coatings
The material of the tool is chosen according to the material to be drilled, the desired cutting speed, and the expected service life.
Carbon steel, historically used in horology for very small hand-ground drill bits, retains excellent cutting ability provided it is worked at low speed. Its tempering temperature, close to 200 °C, nevertheless limits its use in modern high-speed machining.
High-speed steel, designated HSS (High Speed Steel) or HS in European standardization, is the current standard for the majority of small-diameter drill bits. It allows significantly higher cutting speeds thanks to its heat resistance (up to about 550 °C) and offers a good compromise between hardness, toughness, and cost.
Cobalt high-speed steel (HSS-Co, typically containing 5 to 8% cobalt) exhibits increased hot Hardness; it is preferred for treated Steels, stainless Steels and tough alloys frequently encountered in horological External parts (case, dial, hands) (cases, case backs, bezels).
Solid Carbide, made from sintered Tungsten carbide with a cobalt binder, is now widely used on high-speed machining centers. Its very high Hardness and thermal stability allow cutting speeds up to 10 times higher than those of HSS. However, its brittleness requires great spindle rigidity, careful guidance and the absence of impacts, which rules out manual use.
Polycrystalline Diamond (PCD) and cubic boron nitride (CBN) remain reserved for very specific applications: drilling ceramics, Sapphire or composite Materials used in certain External parts (case, dial, hands) components.
Modern coatings significantly extend the tool life and often allow dry drilling or low Lubrication. Titanium nitride (TiN, golden in color), titanium-Aluminium nitride (TiAlN, grey to anthracite), chromium nitride (CrN), and the diamond-like carbon coating (DLC (Diamond-Like Carbon)) are the most common. Each has its own surface Hardness and heat resistance, to be chosen according to the material-speed pair selected.
4. Implementation methods
Depending on the precision of the operation and the desired pace, the Drill bit can be operated by various equipment, ranging from the most manual to the most automated.
The Hand chuck (drill bit holder held and driven by hand) remains the reference tool for touch-ups and occasional drilling at the Bench. The Horologist (watchmaker) fits it with a spear Drill bit or a very small diameter helical Drill bit and turns the chuck between the fingers, or with the help of a Bow. The tactile sensitivity allows very fine control of the feed and limits breakage of fragile drill bits.
The sensitive Bench Drill provides motorized rotation, but the feed remains manual and very gradual. Its reduced travel and rigidity make it suitable for holes ranging from a few tenths of a millimeter to a few millimeters, common in horology.
The pillar Drill offers superior power and angular precision, with an adjustable table and descent controlled by a lever or handwheel. It is used for larger diameters or for repeated drilling on a series of blanks, and readily accepts standard HSS Drill bits.
The Horologist (watchmaker)’s Lathe, the pivoting Lathe or the mechanic’s Lathe allow, with the help of a movable tailstock, drilling along the Axis of a workpiece set in rotation — an ideal situation for obtaining a hole perfectly coaxial with a cylinder, for example on a rod, a Pipe or a Screw.
Numerically controlled machines — CNC Lathes, machining centres and programmable Drills — ensure the rates and repeatability required in industrial production. They allow the use of carbide Drill bits at very high speed, with internal or external high-pressure coolant delivery, and the combination of drilling with other operations (Milling, tapping, Reaming) in a single workpiece setup, guaranteeing remarkable dimensional consistency.
5. Parameters and best practices
Beyond the choice of tool, the success of horological drilling relies on three essential parameters: the cutting speed (expressed in metres per minute, depending on the tool-material pairing), the feed per revolution (often less than 0.02 mm for very small diameters) and Lubrication. A neat fluid cutting Oil or a suitable emulsion prevents seizing, dissipates heat and improves the surface finish of the hole. The concentricity of the setup, the rigidity of the spindle and the quality of the sharpening also remain essential conditions: a misaligned or poorly sharpened Drill bit will produce an oval, tapered or offset hole, defects that are unacceptable on a horological component.
Ultimately, the drill bit is never a simple interchangeable consumable: its geometry, material, and method of implementation form a coherent system upon which the quality of any drilling depends. Mastery of these parameters distinguishes professional horological practice, whether artisanal or industrial.
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