CNC Mirror Polishing: How Mirror Finishes Are Made
CNC mirror polishing is a controlled abrasive process that takes a machined surface down to Ra 0.2–0.8 μm without losing dimensional accuracy. This page explains the mechanism, the boundary conditions, and how to judge whether a part should be mirror polished, lapped, or left as machined.
What CNC mirror polishing actually removes
CNC mirror polishing is not a coating and not a chemical dip. It is the last stage of abrasive material removal. The tool, a felt or cloth wheel carrying diamond or alumina compound, sweeps the surface along a controlled path while the machine holds the part in a known orientation. Each pass shears off the peaks left by the previous operation.
The starting point matters more than the polishing itself. A part that comes off a Ø12 mm end mill at Ra 1.6–3.2 μm carries tool marks roughly 5–15 μm deep. The polishing step has to remove that entire damaged layer before it can begin to reflect. If the pre-machining finish is coarser, the polisher spends its budget on stock removal instead of on the final gloss.
Mirror finish is defined by what the surface does to light, but it is measured by what it does to a stylus or an optical profiler. Ra 0.2–0.8 μm is the band where a steel or aluminium surface reads as reflective under diffuse light. Below Ra 0.05 μm the surface starts to behave like an optical component, and the process stops being polishing and becomes figuring.
One useful mental model: polishing removes material in the nanometer-to-micrometer range per pass, so it can correct roughness but cannot correct geometry. If a bore is 0.02 mm out of round, no amount of polishing will make it round. It will only make it a shiny out-of-round bore.
- 1Removal scale0.1–2 μm per pass depending on compound grit and pressure
- 2Starting finishRa 1.6–3.2 μm from milling, or Ra 0.8–1.6 μm from fine turning
- 3Target finishRa 0.2–0.8 μm for a true mirror look on most metals
- 4What it cannot fixFlatness, roundness, position, or subsurface cracks
The four-stage path from machined to mirror
A reliable mirror finish is built in four stages, each with its own abrasive size. Skipping a stage does not save time; it pushes the work into the next stage and usually produces a wavy surface with scattered deep scratches. The stages are grinding, lapping or fine polishing, mirror polishing, and cleaning.
Stage one knocks down the machining marks. Loose abrasive at 15–30 μm, or a fixed abrasive pad, is used with the tool moving across the marks at an angle. The goal is a uniform matte surface with no visible directionality. If you can still see the cutter path under a 10× loupe, stage one is not finished.
Stage two refines the scratch pattern. Compound size drops to 6–9 μm, then to 3 μm. This is where flatness on a lapping plate is preserved, because the plate controls the geometry while the abrasive only removes the top layer. For a flat sealing face, this stage often does the real work, and the mirror polish is cosmetic.
Stage three is the mirror pass. Diamond compound at 1 μm and then 0.5 μm, applied to a soft wheel at low pressure. Speed is moderate: too fast and the compound dries out and scratches; too slow and the surface smears instead of cutting. The final pass should be almost silent, with the wheel barely kissing the part.
Stage four is cleaning, and it is where most shops lose the finish. Polishing compound embeds in soft metals and in any open porosity. Ultrasonic cleaning in a suitable solvent, followed by a rinse and a dry, removes residue that would otherwise show up as a haze or as a contamination source in a vacuum or medical application.
- 1Grinding15–30 μm abrasive, removes cutter marks, sets flatness
- 2Lapping6–9 μm then 3 μm, refines scratch pattern on a flat plate
- 3Mirror pass1 μm then 0.5 μm diamond on a soft wheel, low pressure
- 4CleaningUltrasonic removal of embedded compound before inspection
How the workpiece material changes the recipe
Aluminium is the easiest metal to bring to a mirror. It cuts fast, and alloys such as 6061 and 7075 respond well to diamond compound. The problem is softness. A polished aluminium surface scratches if you look at it wrong, so it usually needs anodizing or a clear coating immediately after polishing, and that coating adds a slight haze.
Stainless steel is the opposite. Grades 304 and 316 work-harden under the polishing wheel, so light pressure and fresh compound matter more than speed. If the wheel dwells in one spot, the surface hardens locally and the next pass skips over it, leaving a patch that will not come up. 17-4PH behaves better after a full heat treat.
Copper and brass polish to a deep, warm mirror quickly, but they oxidize within days. For a display part, specify a lacquer or a thin plating right after polishing. Beryllium copper is a special case: the polishing dust is a health hazard, so it needs wet methods and controlled waste, and many shops will decline it.
Titanium and Inconel are slow. Titanium galls and smears unless the compound is aggressive and the pressure is kept low. Inconel is abrasive to the wheel itself and can take three to four times the polishing time of stainless. For these materials, ask whether a fine turned finish at Ra 0.4 μm is acceptable instead of a full mirror.
Plastics behave differently again. PMMA and PC can be brought to optical clarity, but they heat and smear easily, and any embedded abrasive shows as a white scratch. PEEK and POM polish to a satin sheen rather than a hard mirror, and the result is usually good enough for a seal or a wear surface.
- 1Aluminium 6061 / 7075Fast to polish, needs anodize or coating to stay clean
- 2Stainless 304 / 316Work-hardens, use light pressure and fresh compound
- 3Copper / brassPolishes quickly, tarnishes in days without protection
- 4Titanium / InconelSlow, heat-sensitive, often better left at Ra 0.4 μm
Where CNC mirror polishing stops working
The tool has to reach the surface. A deep pocket with a 3 mm corner radius and a 40 mm depth gives the wheel almost no room, and the polished band will be narrow and uneven. If a part is designed with blind pockets, sharp internal corners, or long narrow slots, the mirror finish will be partial, and telling the customer that up front is better than shipping a part with an unexplained dull corner.
Free-form surfaces are a better fit. A 5-axis machine can keep the polishing tool normal to the surface across a curved contour, which is exactly what a hand polisher cannot do consistently. This is where CNC mirror polishing earns its cost on molds, impellers, and medical instrument bodies.
Edge quality is the second limit. Polishing rounds edges, and a sharp edge cannot survive the process. If a part needs a crisp 90° edge next to a mirror face, the edge has to be masked or the polishing has to stop short, which leaves a visible transition line. Designers should decide which one they want before the part is made.
The third limit is size. On our 4,000 mm travel machines we can polish large panels and long extrusions, but the polishing head reaches only the areas the machine can orient to. A 2 m long part with a mirror face on one side is routine; the same part with mirror on all four sides needs a different setup and more time.
- 1Deep pocketsTool access limits the polished area, expect partial coverage
- 2Sharp edgesPolishing rounds them, mask or accept a transition line
- 3Free-form surfaces5-axis keeps the tool normal, best case for CNC polishing
- 4Very large partsUp to 4,000 mm, but only where the head can reach
How to verify a mirror finish without arguing about it
Visual comparison is the fastest check and the least useful in a dispute. Two people looking at the same surface under different lights will disagree. If the finish matters, put a number on it. A portable stylus profilometer gives Ra, Rz, and Rmax in seconds, and Rz often tells you more than Ra because it captures the deepest scratches.
For a mirror surface, measure in at least three places and record the orientation of each trace. Ra is direction-dependent on a polished surface: a trace across the polishing direction reads higher than one along it. Reporting a single number without direction is a common source of rejected parts.
Gloss is a separate property and needs a separate instrument. A gloss meter at 60° will distinguish a true mirror from a bright but hazy surface that still reads Ra 0.3 μm. If the part is for appearance, specify gloss units as well as Ra. If it is for a seal or a low-friction contact, Ra and Rz are the numbers that matter.
We inspect 100% of parts before shipment and can supply dimensional and surface reports on request. For mirror-polished parts, the report typically includes Ra traces at marked locations plus a visual record under controlled lighting, so the acceptance criteria are documented before the parts leave the shop.
- 1RaAverage roughness, useful but hides isolated deep scratches
- 2Rz / RmaxPeak-to-valley, catches the scratch that ruins a seal
- 3Gloss unitsFor appearance parts, measured at 60° with a gloss meter
- 4Trace directionRecord it, Ra changes with measurement orientation
Which surface process fits which part
Use this to pick a process before quoting. Ra values are typical, not guarantees.
| Process | Typical Ra | Best for | Watch out for |
|---|---|---|---|
| As machined | Ra 1.6–3.2 μm | Brackets, housings, non-contact faces | Visible tool marks under light |
| Bead blasting | Ra 1.6–3.2 μm | Cosmetic covers, castings | Hides defects, no reflectivity |
| Vibratory tumbling | Ra 0.8–1.6 μm | Deburring many small parts at once | Rounds edges, uneven on large flats |
| CNC mirror polishing | Ra 0.2–0.8 μm | Visible covers, seal faces, optics mounts | Cannot fix geometry, needs clean start |
| Lapping | Ra 0.1–0.4 μm | Flat sealing faces, gauge blocks | Flat parts only, slow on complex shapes |
| Electropolishing | Ra 0.2–0.5 μm | Stainless, sanitary tubing and fittings | Line-of-sight limits, bath chemistry risk |
When mirror polishing is the right call
Choose CNC mirror polishing when the surface is visible, when it slides against another surface, or when it must release contamination. Choose lapping when flatness and Ra below 0.1 μm both matter on a flat face, and choose electropolishing for stainless parts with complex internal geometry. If the part only needs to look clean, bead blasting costs less and hides more.
Questions engineers ask before specifying a mirror finish
Does CNC mirror polishing change the part dimensions?
Yes, but by a small and controllable amount. Total material removed from a milled surface to a mirror is typically 10–30 μm, which is inside the ±0.005 mm tolerance band on most features if the polishing step is planned into the process. The risk is on edges and on thin walls, where removal is uneven.
If a dimension is critical, polish first and finish-machine the critical feature afterward where the setup allows it. Otherwise, tell us which dimensions are critical and we will leave polishing stock on the non-critical faces only.
Can any machined surface be brought to a mirror?
No. The surface must be reachable by the polishing tool, and the geometry must tolerate a rounded edge. Deep blind pockets, sharp internal corners, and long narrow slots will only be partially polished. Hardened tool steel above 55 HRC also polishes slowly and may need a different abrasive sequence.
If you send a drawing, we will mark the faces that can reach a true mirror and the faces that will come out satin. That is better decided before machining than after.
Does a mirror finish improve corrosion resistance?
On stainless steel, a smoother surface gives contaminants fewer places to sit, so it helps in sanitary and food-contact applications. It does not replace passivation or electropolishing, which address the chemistry of the surface rather than its roughness.
On aluminium, a mirror surface without anodizing can actually corrode faster in some environments because there is no thick oxide layer and the surface is more reactive. Specify anodizing or a clear coating if the part sees moisture.
How does polishing affect fatigue life?
Polishing removes the surface layer that contains machining marks, and those marks are where fatigue cracks usually start. A smoother surface can raise fatigue life, but the benefit depends on the alloy and the load direction, and it disappears if the polishing itself leaves deep scratches.
For highly loaded parts, compressive surface treatments such as shot peening do more for fatigue than polishing alone. Polishing after peening would remove the beneficial compressive layer, so the order of operations has to be planned.
What surface finish should I put on the drawing?
Specify Ra and a measurement direction, and add a note if Rz or gloss matters. A single Ra callout with no direction gives the shop room to interpret, which is fine for cosmetic parts and risky for seal faces.
For most visible metal parts, Ra 0.2–0.8 μm is the practical mirror band. Asking for Ra 0.05 μm on a large milled surface will raise cost sharply and is usually unnecessary unless the part is optical.
Can polishing be combined with other finishes?
Yes. Mirror polishing is often followed by clear anodizing, electroless nickel, or a thin decorative plating. Each of these adds 2–10 μm and slightly reduces gloss, so the polished surface should be specular enough to still look like a mirror after coating.
Masking is the other combination. If part of the surface must stay matte or must keep a sharp edge, we mask it before the polishing stage and remove the mask afterward.
Send a drawing, get a finish recommendation
We review the geometry, the material, and the finish callout, then tell you which faces can reach a mirror and which cannot. Quotation and DFM analysis come back within 12 hours.
12-hour quoteRa 0.2–0.8 μm100% inspectionNDA on request
More process notes from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
