CNC Jewelry: How Precision Machining Turns CAD Into Wearable Metal

• 1
  How a CAD Model Becomes a Machined Ring
• 2
  Tolerance and Surface Finish on Small Parts
• 3
  Precious Metals, Titanium, and Stainless in the Spindle
• 4
  Which Jewelry Geometries Suit CNC and Which Do Not
• 5
  Finishing, Stone Setting, and Inspection After Machining
• 6
  Matching the Process to the Jewelry Part

Every CNC jewelry job starts as a solid model, not a sketch. The designer builds the ring, pendant, or clasp in CAD with real wall thickness, real fillet radii, and a defined stone seat geometry. That model is what the shop quotes from, and it is also where most cost problems are born. A 0.3 mm wall that looks fine on screen may deflect under a 3 mm end mill.

CAM software then slices that solid into passes. The programmer picks a tool, a stepover, a stepdown, and a feed rate, and the software outputs G-code. For a simple band, that path might be 8,000 lines. For a pavé setting with 60 seats, it can pass 200,000 lines. The machine does not know what jewelry is. It only follows coordinates.

This is the practical difference between hand work and CNC jewelry. A bench jeweler reads the metal as they cut and adjusts by feel. The machine repeats the coordinates exactly, which is why the tenth piece matches the first. It also means a wrong coordinate is repeated exactly, ten times.

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  Model firstWall thickness, fillet radii, and stone seats must exist in the CAD solid before quoting.
• 2
  Tool decides detailThe smallest internal radius you can cut equals the tool radius, no smaller.
• 3
  Path length drives timeMore passes means more spindle hours, which is the main cost driver.

On a 20 mm ring band, ±0.005 mm is a real and useful number. It holds a stone seat concentric, keeps a hinged clasp closing flush, and lets two halves of a machined locket mate without hand fitting. It does not mean every feature on the part is held that tight. It means the features the drawing calls out are held that tight.

Surface finish is a separate decision. As-machined aluminum or brass lands around Ra 1.6–3.2 μm, which reads as a fine satin texture under light. A finishing pass at Ra 0.8–1.6 μm removes most of the visible tool marks. Going to Ra 0.2–0.8 μm usually means a smaller stepover, a slower feed, or a subsequent polish, and it costs spindle time.

Here is the tradeoff engineers keep hitting. A mirror polish on a flat face is easy and cheap to inspect. A mirror polish inside a deep channel is hard, because the tool cannot reach and the polisher works blind. If the design needs a high shine inside a narrow groove, expect the shop to quote it as a hand operation, not a machining one.

• 1
  Call out only what mattersTight tolerance on a decorative face adds cost with no functional gain.
• 2
  Finish follows accessOpen surfaces polish predictably; deep channels do not.
• 3
  Inspect before platingPlating hides small burrs and can trap polishing compound.

Gold and silver cut easily but behave differently from steel. They are soft, so they smear rather than shear at aggressive feeds, and chips can weld to the cutting edge. A sharp tool with a generous rake angle and a light stepdown keeps the cut clean. Silver tarnishes fast after machining, so parts usually move to finishing the same day.

Platinum and palladium are heavier and gummier. They demand rigid setups and slower surface speeds. Titanium, especially Ti-6Al-4V, is the opposite problem: it is strong, holds heat at the cutting edge, and work-hardens if the tool rubs instead of cutting. A 5-axis machine with high-pressure coolant handles it, but the cycle time is longer and the tool wear is real.

Stainless steels such as 316L and 17-4PH sit in between. They are common for men's bands, watch cases, and clasps because they hold a brushed or bead-blasted finish well and resist skin oils. They machine slower than brass, and 17-4PH in the H900 condition needs carbide tooling and a conservative stepover.

• 1
  Soft metals smearGold and silver want sharp tools, light stepdown, and fast chip evacuation.
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  Titanium work-hardensNever let the tool rub; keep the feed per tooth up and the speed down.
• 3
  Stainless holds finish316L and 17-4PH take bead blasting and brushing predictably.

CNC suits jewelry that has a defined geometry and repeats. Signet rings, ID bracelets, cufflinks, watch bezels, earring posts, and machine-finished pendants all cut well. The reason is simple: the shape can be described numerically, so the tool path is stable and the tenth part matches the first.

CNC struggles with organic, undercut, or hollow forms. A hand-carved leaf with a deep undercut cannot be reached by a 3-axis tool, and even 5-axis access has limits set by tool length and stiffness. A very thin hollow sphere is another problem: the wall deflects during the finishing pass and the surface ripples.

There is a middle ground worth knowing. Many production jewelry lines machine a master pattern in aluminum or brass, then use that pattern for rubber molding and casting. The CNC step gives a crisp, repeatable master; the casting step gives low unit cost at volume. That combination is often cheaper than machining every finished piece.

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  Good fitRepeating geometry, flat or gently curved faces, machinable stone seats.
• 2
  Poor fitDeep undercuts, thin hollow shells, hand-carved organic surfaces.
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  Hybrid routeMachine the master, cast the production run, machine only the critical faces.

A machined ring is not finished when it leaves the spindle. It has burrs at the edges, a witness line where the roughing tool stopped, and possibly a small chamfer that needs to be uniform. Bead blasting, tumbling, brushing, and polishing each remove a different amount of material, so the drawing needs to say which one applies and whether the tolerance is measured before or after.

Plating is common on base-metal fashion lines and on some stainless pieces. Electroless nickel gives a hard, uniform layer that reaches into recesses. Gold and silver plating give color. The catch is edge coverage: a sharp internal corner plates thinner than a rounded one, so a 0.2 mm corner radius often decides whether the gold reads as continuous.

Inspection closes the loop. A 100% check before shipment catches the parts that drifted. For a jewelry run, that usually means dimensional checks on the critical features, a visual check under magnification for burrs and tool marks, and a fit check if the part mates with another component. Reports are available on request.

• 1
  Decide finish before tolerancePolishing removes material; state the measuring point clearly.
• 2
  Round the internal cornersA 0.2 mm radius helps plating coverage and reduces burrs.
• 3
  Check under magnificationTool marks and micro-burrs disappear at arm's length and show up in a photo.