CNC Processing of Pure Silver

• 1
  What pure silver actually is at the spindle
• 2
  How the cut behaves: galling, burrs and built-up edge
• 3
  Workholding: where most silver parts are lost
• 4
  Which features belong on a mill and which do not
• 5
  What tolerance and finish you can hold
• 6
  How a silver job runs through the shop
• 7
  Fine silver vs sterling silver on the same part

Pure silver means 99.9% Ag or higher, usually written 4N or 3N. It conducts heat and electricity better than any other metal, it is soft, and it work-hardens slowly. At the cutting edge those two facts fight each other. The heat leaves the shear zone fast, which protects the tool, but the metal itself deforms easily, so a light finishing pass can smear instead of cut.

Most silver that reaches a CNC machine is not pure. Sterling silver is 92.5% Ag with 7.5% copper, and that copper changes everything. It raises hardness, improves chip formation and makes the alloy far more stable in thin sections. If your drawing says sterling, you are machining a different material from fine silver, and the feeds below should be adjusted accordingly.

Silver is also a precious metal, and that changes shop practice. Chips are swept and weighed, not thrown away. Offcuts are logged. A 30% scrap rate on aluminium is annoying; on silver it is a real cost line, so the process plan is usually written to minimize stock removal rather than to maximize cycle speed.

There is no silver in the general materials list we run day to day. We take these jobs as a specialty process, quote them individually, and confirm the alloy, temper and chip-recovery expectations before a tool touches the blank.

Fine silver has a strong tendency to gall. Fresh silver surfaces bond to tool steel under pressure, and once a built-up edge forms on the flank, the tool stops cutting and starts pushing. The result is a torn, burnished surface that looks polished and measures wrong. The fix is geometry, not force: sharp uncoated carbide or PCD with a high positive rake, a honed edge that is polished rather than coated, and enough feed per tooth to stay out of the rubbing zone.

Feeds and speeds sit in a narrow window. Roughing aluminium might run at 3,000 rpm with a 10 mm cutter; fine silver at the same diameter prefers roughly 800 to 1,500 rpm with a feed around 0.05 to 0.15 mm per tooth. Too slow and the tool rubs. Too fast and the workpiece heats, expands and gets cut oversize.

Burrs are the second problem. Silver does not break a chip cleanly, so a sharp edge on a silver part usually carries a rolled burr. Plan a finishing pass with a small depth of cut, then a light chamfer or a hand-deburr step. A 0.1 mm chamfer is often enough to remove the burr without changing the visual edge the designer wanted.

Cooling matters more than people expect. Silver conducts heat away from the cut so quickly that the tool lasts well, but the part also carries that heat into the fixture. Use a flood or mist coolant, keep the blank clamped evenly, and never run a finishing pass on a part that is still warm from roughing. Let it settle first.

• 1
  High positive rakeKeeps the edge shearing instead of ploughing.
• 2
  Sharp, uncoated edgesCoatings add friction and promote galling.
• 3
  Light finishing passes0.05 to 0.1 mm radial engagement holds size.
• 4
  Even clampingPoint contact deforms thin silver sections.

Silver is soft enough that a standard vise jaw will mark it. A 0.5 mm witness mark on a polished jewellery face is a scrapped part. Soft jaws machined to the blank profile, or a bed of sacrificial aluminium, spread the clamping load and leave no print. For very thin discs, vacuum fixturing or a low-melt fixturing alloy holds the part flat without touching the visible face.

Thermal movement is the quieter problem. Silver expands at roughly 19 × 10⁻⁶ per °C, close to copper and about twice aluminium. A 60 mm silver part that warms by 10 °C during roughing grows about 0.011 mm, which is more than a ±0.005 mm tolerance. That is why we rough, let the part return to shop temperature, then finish.

Thin walls behave differently again. A 0.8 mm silver wall will deflect under a normal finishing pass, spring back, and measure oversize in the middle of the span. Trailing support, a soft wax fill, or a stepped roughing strategy that leaves more material for the finish pass all reduce the spring-back.

Chip recovery is part of fixturing on these jobs. Chips collect in pockets, in the coolant tray and around the jaws. Every one of them has value, so the setup is designed so swarf can be swept completely rather than blown into a corner.

CNC processing of pure silver rewards parts with prismatic geometry: flat faces, pockets, slots, bosses, drilled holes and turned profiles. A 5-axis machine can reach the back of a curved jewellery form in one setup, which matters because re-clamping a soft part usually costs more than the extra axis time.

Very deep, narrow features are the wrong fit. A 0.5 mm slot that is 8 mm deep needs a tool so slender that it will deflect in silver and leave a tapered wall. If the design allows a wider slot, a shallower depth or an EDM step, the part becomes machinable and the cost drops sharply.

Mirror finishes are achievable but slow. A diamond fly-cut pass can reach Ra 0.2–0.8 μm on flat silver, which is usually good enough for decorative and optical contact surfaces. Complex curved surfaces are better finished by hand polishing or tumbling after machining, because chasing a mirror on a contoured wall with a ball tool is the slowest way to get there.

Engraving and fine marking work well. We can laser mark silver with a minimum character height of 1.5 mm, and machined engraving holds crisp edges. Both are better done after the final finish pass so the lettering is not blurred by a subsequent polish.

We hold ±0.005 mm (±0.0002 in) on silver parts, the same as our general machining capability, but the achievable number depends on the feature. A 20 mm bore in a solid block will hold that. A 0.6 mm wall on a ring will not, no matter how careful the setup is, because the wall deflects under its own cutting load.

Surface finish divides into three bands. As-machined silver sits at Ra 1.6–3.2 μm. A controlled finishing pass reaches Ra 0.8–1.6 μm, which is the normal target for functional faces. Diamond turning or fine fly-cutting pushes to Ra 0.2–0.8 μm on flat, unobstructed surfaces. Anything beyond that is a polishing operation, not a machining one.

Inspection is where silver jobs differ from steel jobs. We check raw material, monitor in process and inspect 100% before shipment, and reports are available on request. On a silver part we also weigh the incoming blank and the outgoing chips, because that is the only practical way to confirm the material balance on a precious-metal job.

If a feature cannot be measured without touching the part, we say so before cutting. Optical and vision checks handle most silver geometry, but a soft surface will not tolerate a stylus dragging across it, and a CMM touch probe on a polished face leaves a mark.