Appearance
A manufacturer of mechanical watch movements drilled jewel bearing holes in synthetic ruby (9 Mohs, 0.3 mm thick) using a diamond-tipped micro-drill (0.3 mm, 10-20 micron diamond grit in metal bond) at 60 000 rpm (Vc = 57 m/min), feed 1 micron/rev, DI water at 3 bar. Penetration in 2-3 seconds, olive-hole profile (barrel-shaped for oil retention). Position tolerance plus/minus 0.005 mm, Ra 0.05-0.1 microns inside the hole. Inspection at 50x optical microscope.
Watch Jewel Bearing Drilling
Watch jewel bearing drilling is one of the most demanding micro-drilling operations in precision manufacturing. The jewel — a synthetic ruby or sapphire disc (corundum, Al2O3, Mohs hardness 9) — must have a central hole that is precisely positioned, has a specific internal profile, and has an extremely smooth surface finish.
The Olive-Hole Profile The olive-hole profile — a bore that is 0.005-0.010 mm larger in diameter at the mid-thickness than at the entry and exit — is the defining characteristic of a watch jewel bearing. The wider mid-section acts as an oil reservoir, retaining the lubricating oil by capillary action. The narrower entry and exit meter the oil flow to the pivot. The profile is produced by a diamond micro-drill with a cutting edge that has a slight bulge at the mid-point of the cutting length. As the drill penetrates the 0.15-0.50 mm thick jewel, the mid-section reams the bore to a larger diameter at the mid-thickness.
Drilling Parameters The diamond-tipped micro-drill (0.15-1.0 mm diameter, 3-10 mm length) is made by embedding synthetic diamond grit (10-30 micron) in a metal bond (bronze or cobalt) on a steel shank. The drilling parameters are: spindle speed 30,000-80,000 rpm, feed 0.5-3 microns/rev, deionised water or light oil mist coolant. The drill penetrates a 0.3 mm thick ruby blank in 2-3 seconds. The hole position tolerance is +/- 0.005 mm, and the internal surface finish is Ra 0.05-0.1 microns.
Comparison Table: Watch Jewel Bearing Specifications by Movement Type
| Parameter | Basic Automatic (7-17 jewels) | Chronometer (17-25 jewels) | Tourbillon (20-35 jewels) | Ultra-Thin (15-21 jewels) | Grande Complication (30+ jewels) |
|---|---|---|---|---|---|
| Jewel hole diameter (mm) | 0.3-0.6 | 0.2-0.5 | 0.15-0.4 | 0.3-0.6 | 0.15-0.5 |
| Jewel thickness (mm) | 0.20-0.35 | 0.15-0.30 | 0.15-0.25 | 0.15-0.25 | 0.12-0.25 |
| Olive bulge (mm) | 0.005-0.008 | 0.005-0.010 | 0.008-0.012 | 0.005-0.008 | 0.008-0.012 |
| Position tolerance (mm) | +/- 0.005 | +/- 0.003 | +/- 0.002 | +/- 0.005 | +/- 0.002 |
| Surface finish Ra inside (µm) | < 0.1 | < 0.08 | < 0.05 | < 0.1 | < 0.05 |
| Drill type | Diamond micro-drill | Diamond micro-drill | Diamond micro-drill | Diamond micro-drill | Diamond micro-drill |
| Spindle speed (rpm) | 30,000-60,000 | 40,000-80,000 | 60,000-80,000 | 30,000-60,000 | 60,000-80,000 |
| Feed per rev (µm) | 1-3 | 0.5-2 | 0.5-1 | 1-3 | 0.5-1 |
| Coolant | DI water | DI water | Light oil mist | DI water | Light oil mist |
| Inspection magnification | 50x | 100x | 150x | 50x | 150x |
| Typical pivot clearance (µm) | 5-10 | 3-8 | 2-5 | 5-10 | 2-5 |
Precious Metal and Gemstone Micro-Drilling
Beyond jewel bearings, watchmaking and jewellery require micro-drilling of precious metals (18K gold, platinum, palladium, silver) for components such as bridges, plates, and decorative elements, and of gemstones for bead stringing and setting.
Precious Metal Micro-Drilling Drilling holes in 18K gold and platinum presents unique challenges. Gold is extremely ductile and tends to produce large burrs at the hole exit. The burrs are difficult to remove because gold is soft and aggressive deburring can damage the surrounding surface. Platinum work-hardens at the cutting edge, requiring consistent feed without dwell — any pause in the feed causes the cutting edge to rub against the work-hardened layer, accelerating tool wear. The drilling parameters for precious metals are: Vc = 20-50 m/min (for 18K gold), feed f = 0.5-3 microns/rev, using uncoated micrograin carbide or PCD (polycrystalline diamond) drills with a polished rake face and a point angle of 120-140 degrees. The coolant is a light oil mist or compressed air (water-based coolant can cause tarnishing of some gold alloys).
Gemstone Drilling Drilling natural gemstones (amethyst, topaz, quartz, tourmaline) for bead stringing requires diamond-tipped drills with water cooling. The drill is a diamond electroplated tube (similar to a trephine bit) or a solid diamond-tipped pin drill. The drilling parameters depend on the gemstone hardness: for quartz (Mohs 7), Vc = 30-50 m/min, feed = 0.5-2 microns/rev, water coolant at 2-5 bar. For harder stones (topaz Mohs 8, sapphire Mohs 9), the speed is reduced to Vc = 20-30 m/min and the feed is reduced to 0.3-1 micron/rev. Gemstone drilling produces a slurry of water and stone dust that must be contained and disposed of as hazardous waste (the dust can contain heavy metals and crystalline silica).
Comparison Table: Micro-Drilling Parameters for Watchmaking and Jewellery Materials
| Material | Hardness | Drill Type | Vc (m/min) | Feed (µm/rev) | Speed (rpm) | Coolant | Burr Control | Tool Life (holes) |
|---|---|---|---|---|---|---|---|---|
| Synthetic ruby (Al2O3) | 9 Mohs | Diamond metal-bond | 57-95 | 0.5-3 | 30,000-80,000 | DI water | None (brittle) | 5,000-20,000 |
| Synthetic sapphire | 9 Mohs | Diamond metal-bond | 50-80 | 0.5-2 | 30,000-60,000 | DI water | None (brittle) | 5,000-15,000 |
| 18K gold (yellow) | ~80 HV | Micrograin carbide | 20-40 | 0.5-3 | 5,000-15,000 | Oil mist | Significant | 500-2,000 |
| 18K gold (white) | ~120 HV | Micrograin carbide | 15-30 | 0.5-2 | 5,000-12,000 | Oil mist | Significant | 300-1,000 |
| Platinum 950 | ~110 HV | Micrograin carbide | 15-25 | 0.5-2 | 4,000-10,000 | Oil mist | Moderate | 200-800 |
| Palladium 950 | ~100 HV | Micrograin carbide | 20-35 | 0.5-3 | 5,000-12,000 | Oil mist | Moderate | 300-1,000 |
| Sterling silver | ~70 HV | Carbide or HSS | 30-60 | 1-5 | 8,000-20,000 | Oil mist | Significant | 1,000-5,000 |
| 316L stainless (watch case) | ~200 HV | Carbide (TiAlN) | 40-60 | 2-8 | 6,000-12,000 | Oil | Moderate | 1,000-3,000 |
| Titanium Grade 5 | ~350 HV | Carbide (TiAlN) | 20-40 | 1-5 | 4,000-8,000 | Oil | Low | 500-1,500 |
| Quartz gemstone | 7 Mohs | Diamond electroplate | 30-50 | 0.5-2 | 10,000-20,000 | Water | None (brittle) | 2,000-5,000 |
Tourbillon Cage Bridge and Watch Case Drilling
High-end watchmaking requires drilling operations on tourbillon cage bridges and watch cases that push the limits of precision micro-machining.
Tourbillon Cage Bridge Drilling The tourbillon cage is a rotating assembly that contains the escapement and balance wheel, designed to average out positional errors due to gravity. The cage bridge — a thin metal structure that supports the top pivot of the balance staff — requires drilled holes at compound angles for the pivot jewels and the cage screws. The bridge is typically made from germanium silver or nickel silver (for lower-cost tourbillons) or from hardened steel (for high-end tourbillons). The holes are 0.3-0.8 mm diameter, drilled at angles of 10-30 degrees relative to the bridge surface. The drilling is performed on a 5-axis CNC micro-machining centre with carbide micro-drills at Vc = 20-40 m/min, feed = 1-5 microns/rev.
Watch Case Drilling Watch case drilling — for screw holes, crown holes, and pusher holes — is performed on 316L stainless steel, titanium (Grade 2 or Grade 5), or precious metals (18K gold, platinum). The holes are 0.5-3.0 mm diameter, 2-25 mm deep. The drilling parameters for 316L cases are: Vc = 40-60 m/min, f = 0.02-0.08 mm/rev, TiAlN-coated carbide drills, oil coolant at 30-50 bar. The most critical quality requirement is burr control on the internal surfaces of the case — a burr at the edge of a screw hole can prevent the case back from sealing correctly, compromising the water resistance.
FAQ
How is the olive-hole profile produced in watch jewel bearings?
The olive-hole profile is produced by a diamond micro-drill with a specialised cutting edge geometry. The drill has a cutting edge that is not a straight line along the drill axis but has a slight bulge — approximately 0.005-0.012 mm larger in diameter — at the mid-point of the cutting length. As the drill penetrates the jewel blank (a 0.15-0.50 mm thick synthetic ruby or sapphire disc), the mid-section of the cutting edge reams the bore to a slightly larger diameter at the mid-thickness of the jewel. The entry and exit of the bore are cut by the smaller-diameter ends of the cutting edge, resulting in a barrel shape. The drill geometry is produced by grinding the diamond-metal bond material to the required profile. The bulge size is controlled within +/- 0.001 mm by optical measurement of the drill profile before mounting. The drill must be inspected after every 100-200 holes to verify that the cutting edge profile has not worn, which would reduce the olive bulge. The olive-hole profile is verified by measuring the hole diameter at the entry plane, the mid-thickness plane, and the exit plane using a calibrated optical microscope with a cross-hair reticle or a laser confocal microscope. The diameter at the mid-thickness should be 0.005-0.012 mm larger than at the entry and exit. The olive profile is quantified by the "olive ratio" — the ratio of the mid-thickness diameter to the entry diameter. For a standard watch jewel bearing, the olive ratio should be 1.02-1.05 (2-5% larger at the mid-thickness). The olive profile serves a specific function: it acts as an oil reservoir that retains the watch lubricating oil by capillary action, and it meters the oil flow to the pivot-which rotates in the hole at speeds of up to 28,800 beats per hour (4 Hz). Without the olive profile, the oil would be rapidly displaced by the rotating pivot, and the bearing would run dry after a few months of operation.
What are the main challenges of drilling precious metals for jewellery?
Drilling precious metals — particularly 18K gold and platinum — presents three main challenges: burr formation, work hardening, and tool cost. Burr formation: gold is one of the most ductile metals, and the material displaced by the drill at the hole exit forms a large burr that can be several times the hole diameter. The burr is difficult to remove because gold is soft and any mechanical deburring tool (a file, a scraper, or a abrasive brush) tends to gouge the surrounding surface rather than cutting the burr cleanly. The recommended deburring method for gold is laser deburring (a pulsed laser ablates the burr without contacting the surface) or electrochemical deburring (the gold burr is dissolved in an electrolyte under anodic current). Work hardening: gold and platinum work-harden at the cutting edge — the material becomes harder as it deforms, making each subsequent pass more difficult. The drill must maintain a consistent feed rate without any dwell or hesitation. If the feed stops for even a fraction of a second, the cutting edge rubs against the work-hardened layer and the drill temperature rises rapidly, accelerating wear. CNC-controlled drilling with ballscrew-driven axes (not pneumatic) is essential for gold micro-drilling because the feed must be precisely controlled without any hesitation. Tool cost: PCD drills for precious metal drilling cost $50-150 each, and the tool life is 200-2,000 holes depending on the material and feed control. The drill must be replaced at the first sign of wear (increased burr size, decreased surface finish) because a worn drill in gold produces burrs that are difficult to remove. The cost per hole for precious metal drilling is $0.05-0.50 (including tool cost and deburring), which is significant for high-volume jewellery production.
How does micro-drilling of watch cases affect water resistance?
Watch case water resistance depends on the quality of the seals at the case back, the crown, and the pushers. The drilled holes for the crown and pushers must be within tight tolerance (typically +0.00/-0.02 mm for the crown tube hole, +0.00/-0.01 mm for pusher holes) to ensure that the seal (a round cross-section O-ring or a square-section gasket) compresses correctly. If the hole is too large, the O-ring does not compress sufficiently and water can leak past it. If the hole is too small, the crown tube may not fit or may pinch the O-ring during assembly. The hole must also be free of burrs on both the external and internal surfaces — any burr on the external surface can create a gap under the gasket, and any burr on the internal surface can cut the O-ring during assembly. The drilling operation for watch case holes is typically followed by a reaming operation (using a carbide reamer with a tolerance of +/- 0.003 mm) to achieve the final hole size. The hole entrance is then chamfered with a 0.1-0.2 mm chamfer to prevent burr formation during O-ring insertion. After drilling and reaming, the case is subjected to a water resistance test per ISO 2281: the case is pressurised to the rated depth (typically 30-300 m for a dress watch, 100-2000 m for a dive watch) and tested for water ingress by condensation or by vacuum decay. The water resistance test also checks the crown tube and pusher holes indirectly — if the hole tolerances were not correct, the seals would leak. The drilling process for dive watch cases (rated to 200 m or deeper) is more stringent: the hole tolerances are tighter (by 0.005-0.010 mm), and every case is individually pressure tested (not a sample). The drilling machine for dive watch cases must be in a temperature-controlled environment (+/- 1 °C) because the thermal expansion of the case material can affect the hole size.
What inspection methods are used for watch jewel bearing holes?
Watch jewel bearing holes are inspected by a combination of optical microscopy, air gauging, and (for high-end movements) laser confocal microscopy. Optical microscopy: the drilled jewel is inspected at 50-150x magnification using a calibrated microscope with a cross-hair reticle. The hole position is measured relative to the jewel outer diameter (the hole centre must be within +/- 0.005 mm of the jewel centre). The hole diameter is measured at the entry and exit planes. The olive-hole profile is verified by focusing through the jewel thickness and measuring the diameter at the mid-thickness plane — the mid-thickness diameter must be 0.005-0.012 mm larger than at the entry/exit. Air gauging: the jewel is mounted on an air gauge spindle that measures the hole diameter by flowing air through a calibrated nozzle into the hole. The air flow rate is proportional to the clearance between the nozzle and the hole wall, providing a measurement that is accurate to +/- 0.001 mm. The air gauge is calibrated with a master jewel of known hole diameter. Air gauging is faster than optical measurement and is used for production sampling (typically 10-20% of jewels per lot). Laser confocal microscopy: for high-end movements (chronometer, tourbillon, grande complication), the jewel bearing hole is measured by a laser confocal microscope that scans the hole profile in three dimensions. The 3D scan provides the hole diameter at any depth, the olive profile shape, the surface roughness (Ra, Rz), and the edge condition (presence of any micro-chipping). The laser confocal measurement is non-contact and takes 30-60 seconds per jewel. The measurement data is recorded and stored with the jewel serial number for traceability. After inspection, each jewel is mounted in a brass or gold chaton (setting) by pressing or gluing, and the chaton is pressed into the watch plate or bridge. The assembled bearing (jewel + chaton + plate) is inspected again by optical microscope to verify that the hole centre is aligned with the plate reference.
What is the difference between synthetic ruby and synthetic sapphire for watch jewels?
Synthetic ruby and synthetic sapphire are both forms of corundum (aluminium oxide, Al2O3) with the same crystal structure and the same Mohs hardness of 9. The difference is the trace element that gives the material its colour: ruby contains 0.5-3% chromium (Cr3+ substituting for Al3+ in the crystal lattice), which absorbs green and blue light and gives ruby its red colour. Sapphire does not contain chromium; it can be colourless (white sapphire) or contain other trace elements (iron and titanium for blue sapphire, iron for yellow sapphire, etc.). For watch jewels, the preferred material is synthetic ruby (red) because the colour provides visual contrast for inspection — the jewel bearing is visible through the movement, and the red colour allows the watchmaker to see the oil in the olive-hole reservoir (the oil appears as a darker spot in the red jewel). Synthetic white sapphire is also used in some movements, particularly in the decorative jewels of display-back watches where the red colour is considered aesthetically undesirable. The manufacturing process for synthetic ruby and sapphire is the same: the Verneuil flame fusion process (also called the flame-fusion method) melts high-purity aluminium oxide powder with the appropriate dopant (chromium for ruby, no dopant or iron/titanium for sapphire) in an oxy-hydrogen flame at 2050 °C, and the molten drops fall onto a rotating pedestal, building up a single-crystal boule. The boule is cut into thin discs (jewel blanks) by diamond sawing, and the blanks are lapped and polished to the final thickness. The drilling characteristics of ruby and sapphire are essentially identical because they have the same hardness and crystal structure. The diamond micro-drill parameters are the same for both materials, and the drill life is similar (5,000-20,000 holes per drill). Some watchmakers report that ruby is slightly easier to drill than sapphire because the chromium dopant slightly weakens the crystal structure, but the difference is not significant enough to change the drilling parameters.
The information provided in this article is for general informational purposes only. Data and recommendations are based on published research and industry experience as of 2026.