CNC Treatment: How Silver Parts Are Machined

• 1
  What CNC Treatment Actually Does to Silver
• 2
  Tool Geometry and Cutting Parameters That Work
• 3
  Silver Alloys and When Machining Is the Wrong Choice
• 4
  Surface Finish, Tolerance, and Realistic Limits
• 5
  A Practical Sequence for Silver CNC Treatment
• 6
  Finishing Options After Cutting
• 7
  Silver Alloys: Machinability at a Glance

CNC treatment is a subtractive process. A rotating cutter removes material in controlled passes, and the machine moves the tool along a programmed path. The part is not coated or reshaped by heat. Everything that happens is shear and friction at the cutting edge.

Silver is soft. Pure silver sits around 25 HV, and sterling silver 925 lands near 75 HV. Compare that with 6061-T6 aluminium at roughly 95 HV, or 304 stainless at 190 HV. A sharp cutter takes silver easily, but the same softness makes chip evacuation the hard part.

The cut itself is not the problem. Rubbing is. When a dull edge slides across silver instead of shearing it, the surface work-hardens and tears. You get a smeared finish and a burr that is difficult to remove without gouging the part.

That is the engineering meaning of CNC treatment on silver: you are managing pressure and heat, not fighting hardness. Tool geometry, feed rate, and coolant choice matter more than spindle power.

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  Shear, not abrasionA sharp edge cuts cleanly; a dull edge rubs and tears.
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  Low hardnessSilver cuts fast, so feed and speed windows are wide.
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  Heat is the limitSilver conducts heat away quickly but galls at the tool tip.

Use two-flute or three-flute carbide end mills with a high rake angle. A rake of 15° to 20° lowers cutting pressure and helps the chip curl away from the wall. Coatings are optional. Uncoated polished carbide often leaves a better finish on silver than a TiAlN coating, which can drag.

Spindle speed for silver runs high. A common starting point for a 6 mm end mill is 8,000 to 12,000 rpm with a feed of 0.05 to 0.10 mm per tooth. At those values the chip is thin and the tool stays cool. Drop the feed and the edge rubs instead of cutting.

Cutting fluid matters. A water-soluble flood coolant at 6% to 8% concentration keeps the chip moving and stops cold welding. For deep pockets, use high-pressure through-tool coolant. Air blast alone is usually not enough on silver because the chips are dense and pack into corners.

Depth of cut should stay modest. Radial engagement of 30% to 40% of tool diameter with an axial depth of 0.5 to 1.0 × diameter gives good chip thinning without overloading the edge. If you hear a high-pitched squeal, you are rubbing. Increase feed per tooth by 20% and listen again.

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  Rake angle 15°–20°Lowers cutting pressure and improves chip curl.
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  0.05–0.10 mm per toothThin chips keep the edge cool and reduce galling.
• 3
  Flood coolant 6%–8%Stops cold welding and flushes dense silver chips.

Sterling silver 925 machines the most predictably. It is 92.5% silver and 7.5% copper, which raises hardness enough to form a chip instead of smearing. Coin silver 900 behaves similarly but is softer and tends to burr more on fine edges.

Fine silver 999 is the hardest to machine well. At near-pure composition it galls quickly, and thin walls deform under clamping pressure. If a design needs fine silver for conductivity, plan for light passes, sharp tooling, and soft jaws with a large contact area.

Silver-filled brazing alloys and silver-tungsten composites are a different story. They are abrasive and wear tools fast. Use diamond-coated or PCD tooling, and expect slower feeds. These materials are usually better suited to grinding or EDM than to milling.

If the part is a high-volume decorative item with no tight tolerance, casting or stamping will beat CNC treatment on cost. Machining earns its place when the geometry is complex, the tolerance is tight, or the quantity is low.

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  925 sterlingBest balance of machinability and edge retention.
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  Fine silver 999Galls easily; use light passes and soft jaws.
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  Silver-tungstenAbrasive; consider grinding or EDM instead.

As-machined silver with a sharp cutter and good coolant lands around Ra 1.6–3.2 μm. That is a matte surface with visible tool marks. Fine finishing passes at 0.02 mm radial engagement can reach Ra 0.8–1.6 μm, which reads as a soft satin finish.

Mirror finishes below Ra 0.2 μm are possible but rarely come straight off the machine. They need polishing or lapping after cutting. On silver, polishing removes material quickly, so leave 0.02 to 0.05 mm of stock for the finishing step.

Tolerance is achievable to ±0.005 mm on a rigid setup with temperature control. Silver moves with heat, though. A part that measures on size at 20 °C can shift a few micrometres after handling. For critical dimensions, let the part stabilize before final inspection.

Thin features are the real limit. Walls under 0.5 mm deflect under cutting force and clamp pressure. If your design needs them, expect to add support material or accept a slower, lighter process.

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  Ra 1.6–3.2 μmTypical as-machined finish on sterling silver.
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  Ra 0.8–1.6 μmReachable with a light finishing pass.
• 3
  ±0.005 mmHolds on rigid setups with thermal control.

Start with a stress-relief anneal if the stock was cold-rolled. Silver work-hardens during rolling, and machining a stressed blank releases that stress unevenly. A 30-minute anneal at 600 °C to 650 °C followed by slow cooling gives a uniform grain structure.

Face and square the stock first. Silver bar and plate are rarely flat enough to hold in a vise without rocking. Take a 0.2 mm facing pass on both sides to establish a reference, then clamp on that face for the rest of the job.

Rough with a 0.5 mm radial stepover at 0.05 mm per tooth, leaving 0.3 mm of stock on all walls. Then run a semi-finish pass at 0.1 mm stock and a finish pass at 0.02 mm. The semi-finish pass is what protects the final surface from the tool wear that builds up during roughing.

Deburr by hand with a fine ceramic stone, not a file. A file cuts too aggressively on silver and will round the edges you just spent time holding. Finish with bead blasting at low pressure if you want a uniform matte look.

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  Anneal first600 °C–650 °C for 30 minutes on cold-rolled stock.
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  Face both sidesEstablishes a flat reference for clamping.
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  Leave 0.3 mmRoughing stock protects walls from tool wear.
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  Stone, don't fileFiles round edges quickly on soft silver.

Silver tarnishes. That is chemistry, not a machining defect. If the part will be visible, plan a finish that slows oxidation. Rhodium plating and clear lacquer are common, and both add a thin layer that changes the final dimension slightly.

Bead blasting gives a uniform matte surface and hides small tool marks. Use fine glass beads at 2 to 3 bar. Higher pressure peens the surface and can distort thin sections.

Polishing with a soft wheel and rouge brings out the high reflectivity people expect from silver. It also rounds sharp edges, so mask or protect any edge that must stay crisp. Tumbling in a wet ceramic media is gentler and better for batches.

Laser marking works on silver and can add logos or part numbers. Minimum character height is 1.5 mm. Below that the mark becomes hard to read and can look muddy on a polished surface.

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  Rhodium or lacquerSlows tarnish; adds a thin dimensional layer.
• 2
  Bead blast at 2–3 barUniform matte finish; avoid high pressure.
• 3
  Laser markingMinimum character height 1.5 mm on silver.