Appearance
A manufacturer of platinum engagement rings (950 Pt, ring shank 2.5 mm x 20 mm, requiring a 1.5 mm through-bore for channel-set diamond mounting) used a PCD-tipped micro gun drill (1.5 mm, CERATIZIT WTX-Micro, Vc = 25 m/min, f = 0.003 mm/rev, oil at 80 bar). Bore Ra 0.3 microns inside. The bore was polished with 1 micron diamond paste on a cotton thread, removing 0.01 mm, then ultrasonically cleaned.
Jewellery Micro Drilling
Precious metal micro deep hole drilling for jewellery requires tooling capable of producing smooth, burr-free holes in highly ductile, galling-prone materials. 950 platinum (95% Pt, 5% Ru or Co) and 18K gold (75% Au, 25% Cu/Ag/Zn) have elongations of 25-40% and work-hardening rates that cause the material at the cutting edge to deform plastically rather than shear cleanly, producing a built-up edge (BUE) on the drill tip within 0.1-0.5 mm of cumulative cutting. The BUE increases the cutting force, raises the temperature, and causes the drill to produce a rough, torn bore surface with micro-burrs (0.01-0.05 mm) that must be removed by post-drilling polishing.
The micro gun drill for precious metals should have a PCD (polycrystalline diamond) tip — diamond does not react with gold or platinum at cutting temperatures below 600 C, and the PCD tip provides the sharpest cutting edge (3-5 micron edge radius) and the lowest coefficient of friction (0.01-0.03 against gold/platinum). The drilling parameters are: Vc = 20-30 m/min, feed f = 0.002-0.005 mm/rev, oil coolant at 60-100 bar. The peck depth should be 1-2 mm with a 0.5-second retract dwell to clear the stringy precious metal chips. After drilling, the bore is polished using a cotton thread or a silk cord coated with 1-3 micron diamond paste, pulled through the bore 5-10 times. The polishing removes micro-burrs and achieves Ra 0.1-0.3 microns. The component is then ultrasonically cleaned in a jewellery-specific cleaning solution (ammonia-based or citrus-based, 50 C, 5 minutes).
Drilling Parameters Comparison for Precious Metals
The table below compares micro gun drilling parameters for the most common jewellery precious metals.
| Parameter | 950 Platinum | 18K Yellow Gold | 18K White Gold | 14K Gold | Sterling Silver |
|---|---|---|---|---|---|
| Cutting speed Vc | 20-30 m/min | 25-35 m/min | 22-32 m/min | 28-40 m/min | 30-45 m/min |
| Feed f | 0.002-0.005 mm/rev | 0.003-0.006 mm/rev | 0.003-0.006 mm/rev | 0.004-0.008 mm/rev | 0.004-0.008 mm/rev |
| Tool tip | PCD | PCD | PCD | PCD or carbide | Carbide or HSS |
| Coolant pressure | 60-100 bar | 50-80 bar | 50-80 bar | 40-70 bar | 30-60 bar |
| Peck depth | 1-2 mm | 1-2 mm | 1-2 mm | 2-3 mm | 2-3 mm |
| Final bore finish | 0.1-0.3 Ra | 0.1-0.3 Ra | 0.1-0.3 Ra | 0.2-0.5 Ra | 0.3-0.6 Ra |
| Galling tendency | High | High | Medium-high | Medium | Medium-low |
Ring Shank vs Earring Post Drilling Comparison
The drilling requirements differ significantly between ring shank through-bores and earring post micro-channels.
| Feature | Ring shank through-bore | Earring post micro-channel |
|---|---|---|
| Bore diameter | 1.0-2.5 mm | 0.4-0.6 mm |
| Bore length | 10-25 mm | 14-17 mm |
| Aspect ratio | 5:1 to 15:1 | 25:1 to 35:1 |
| Concentricity tolerance | Not critical | 0.02 mm TIR |
| Primary purpose | Stone setting / sizing | Friction-back mechanism |
| Post-drilling polish | 1-3 micron paste | 1 micron paste |
| Cleaning method | Ultrasonic | Ultrasonic + solvent flush |
FAQ
Why is PCD the preferred tool material for drilling precious metals?
PCD (polycrystalline diamond) is chemically inert when machining gold and platinum at cutting temperatures below 600 C. Unlike carbide tooling, diamond does not form an adhesive bond with the precious metal at the tool-chip interface, preventing the built-up edge that causes rough bore surfaces. The PCD edge also maintains its sharpness (3-5 micron edge radius) for 5-10 times longer than carbide when machining precious metals, reducing tool change frequency and improving bore consistency.
What causes the high galling tendency in platinum drilling?
950 platinum has a high work-hardening rate and low thermal conductivity (71 W/mK, compared to 318 W/mK for silver). The heat generated at the cutting edge raises the temperature of the platinum locally, causing it to become more ductile and adhesive at the tool-chip interface. The platinum particles weld to the cutting edge under the combined pressure and temperature, forming a built-up edge that tears the bore surface. The PCD tip reduces galling by providing a lower coefficient of friction and a chemically inert surface.
How is concentricity maintained in earring post micro-channels?
Earring post micro-channels must be concentric with the post outer diameter within 0.02 mm TIR to ensure that the friction-back mechanism functions correctly. The post is gun-drilled on a Swiss-type CNC lathe using a PCD micro gun drill, with the post held in a precision guide bushing that supports it within 0.005 mm of the spindle axis. The drill is aligned to the spindle axis using a laser centring system before drilling. The concentricity is verified by mounting the drilled post between dead centres on a watchmaker's lathe and measuring the bore runout with a 0.001 mm dial indicator.
What is the purpose of the cotton thread polishing step?
The cotton thread or silk cord coated with diamond paste removes the micro-burrs (0.01-0.05 mm) that are left on the bore surface by the gun drilling process. These micro-burrs would prevent the diamond channel from sliding into the ring shank through-bore and would snag on the earring post friction-back mechanism. The polishing also smooths the bore surface to Ra 0.1-0.3 microns, which is required for the adhesive bonding of the channel-set stones and for the smooth operation of the friction-back earring mechanism.
Why must the coolant be NSF H1 registered and residue-free?
The oil coolant used in jewellery micro-drilling must be NSF H1 registered (food-grade, incidental contact) and residue-free because any residual oil left in the bore will prevent the adhesive from bonding during stone setting. A single drop of oil in a 1.5 mm diameter bore can render the adhesive joint unreliable, causing the stone to become loose. The ultrasonic cleaning step removes the coolant, but if the coolant leaves a residue, the cleaning may not be effective. Residue-free coolants evaporate completely at 50-80 C with no measurable residue (below 0.1 mg per bore).
Data are based on published research and industry experience as of 2026.