CNC Reflective Cup Processing Guide

• 1
  What a reflective cup actually does
• 2
  How profile error becomes beam error
• 3
  Material choice and its limits
• 4
  Five-axis toolpaths and why they matter
• 5
  Polishing, coating and what the beam shows
• 6
  Inspection and beam testing
• 7
  From drawing to finished cup
• 8
  Reflective cup material comparison

A reflective cup is a bowl-shaped body with a polished inner surface. The source sits at or near the focus, light leaves the source in every direction, and the walls bend the off-axis rays forward. The output depends on the profile: parabolic walls collimate a point source, elliptical walls converge onto a second focus, and free-form walls shape an asymmetric beam for a road sign or a machine-vision light.

Metal cups beat coated glass in two places. They tolerate heat and vibration, and they can carry mounting bosses, cooling fins and alignment features in the same block. That is why automotive headlamps, stage lights, medical examination lamps and LED modules often use a machined metal reflector rather than a molded one.

• 1
  Point source, tight focusThe spot stays small only if the source sits within a fraction of a millimeter of the design position.
• 2
  Area source, softer beamA 2 mm LED die smears the focus. The profile is then adjusted for a wider, even field.

Ray behavior is linear. A wall that is 0.05 mm too shallow moves light by roughly twice that distance at the target, because the surface normal tilts and the reflected ray swings twice as far as the surface slope. On a 40 mm cup aimed at 3 m, a 0.05 mm profile deviation can shift the edge of the beam by tens of millimeters.

That is the reason a reflective cup drawing usually carries two different tolerance classes. The mounting face, the locating bore and the focal datum are held tight so the cup lands in the right place. The optical wall is held to a profile tolerance, not to a simple diameter tolerance, because the whole curve matters.

Tangency matters too. Where the parabola meets the rim, a sharp step scatters a ring of stray light around the beam. Where the cup meets the mounting plate, a burr lifts one side and tilts the axis. Both faults are visible on a beam pattern before any CMM report is printed.

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  Profile toleranceApplied to the optical surface over its full height, not to a single diameter.
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  Focal datumOne face or bore that every other feature is measured from.
• 3
  Rim conditionDeburred and broken by 0.2–0.3 mm to stop edge glare.

Aluminium is the default. 6061-T6 machines cleanly, takes a diamond or single-crystal cutter well, and anodizes into a durable reflective layer. 7075 gives higher stiffness for thin walls, but its zinc content makes bright anodizing less predictable, so it is normally used where strength matters more than color.

Copper and brass polish to the highest reflectivity in the visible band and pull heat away from the source faster than aluminium. C110 and C36000 both hold a fine finish. The trade-off is weight and cost, and copper tarnishes quickly unless it is plated or lacquered within a day or two of polishing.

For hot or corrosive duty, stainless 304 or 316 machines well but is harder to bring to a mirror finish. 17-4PH covers higher temperature. Titanium TC4 and Inconel are possible, though tool wear and polishing time climb sharply and the part rarely justifies the cost unless weight or corrosion rules out everything else.

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  Aluminium 6061-T6General lighting, anodized finish, best cost-to-finish balance.
• 2
  Copper C110Highest visible reflectivity, needs plating or lacquer.
• 3
  Stainless 304/316Corrosive or washdown environments, matte-to-satin finish.
• 4
  Titanium TC4Weight-critical optics, slow to polish.

A deep cup has walls that lean away from the tool. On a three-axis machine the cutter has to reach down the wall, and the shank rubs the opposite side long before the tip reaches the bottom. The result is chatter, a poor finish and a profile that drifts.

Five-axis machining tilts the tool along the wall so the cutting edge stays engaged and the shank stays clear. Continuous tilting keeps the effective cutting speed steady from rim to base, which is what holds Ra 0.2–0.8 μm on a curved surface. On a 40 mm cup with a 12 mm ball-nose cutter, stepover of 0.05–0.1 mm is a workable starting point for the finishing pass.

Roughing removes most of the volume with a smaller tool and leaves 0.3–0.5 mm of stock on the optical wall. Semi-finishing brings that to 0.05–0.1 mm. The finishing pass runs at high spindle speed, low feed per tooth and full coolant, and it should be a single continuous pass, because a stop mark on a mirror surface cannot be polished out without changing the profile.

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  Tool tilt20–40° from the wall normal keeps the shank clear and the edge cutting.
• 2
  Stepover0.05–0.1 mm on the finish pass for a 12 mm ball-nose cutter.
• 3
  Single passAvoid tool retracts inside the optical zone.

Machining sets the shape. Polishing sets the reflectivity. On aluminium, a diamond-turned surface can reach Ra 0.2 μm straight off the tool, which is enough for many LED cups. Where the requirement is higher, a sequence of diamond paste grades from 9 μm down to 1 μm is applied by hand or on a CNC polishing path, with the profile checked between steps.

Over-polishing is a real failure mode. A soft pad rounds the rim and flattens the curve near the base, so the beam widens and a dark ring appears around the center. Polishing time should be capped and the profile verified on an optical comparator or a profilometer, not judged by eye alone.

Coating follows. Clear or hard anodizing gives a durable dielectric layer with 85–90% reflectivity in the visible band. Electroless nickel gives a harder surface for abrasive duty. Silver or gold plating pushes reflectivity higher but needs a protective topcoat, and the plating thickness must be uniform or the profile shifts again. Bright anodizing holds color better than it holds figure, so keep it thin where the beam is tight.

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  Diamond turningRa 0.2 μm possible on aluminium with a single-crystal cutter.
• 2
  Hard anodizeWear resistance, slight reflectivity loss.
• 3
  Silver platingHighest reflectivity, requires a topcoat.

A reflective cup cannot be signed off with calipers. The optical surface needs a form measurement: a profilometer trace along one or more meridians, or a CMM scan compared against the CAD surface. Wall thickness is checked at several heights, because a thin wall that varies by 0.1 mm will distort when the cup is clamped into its housing.

The functional check is a beam pattern. The cup is mounted in a representative housing with the production source, projected onto a screen at a fixed distance, and photographed. Beam width, hot spots and stray rings show up immediately, and they often reveal a problem that dimensional inspection passed.

At GreatLight every part goes through raw material check, in-process monitoring and final inspection before shipment, with reports on request. Typical tolerance is ±0.005 mm and fine finishes run to Ra 0.2–0.8 μm. Photometric testing is done on the customer's source, since the beam depends on the emitter as much as on the cup.

• 1
  Profile traceTwo or more meridians against the CAD curve.
• 2
  Wall scanThickness checked at rim, mid-height and base.
• 3
  Beam photoFixed distance, production LED, documented result.