Mirror Polishing
Mechanical finishing · Mechanical
Mirror polishing removes visible directional scratches and buffs the surface to high specular reflectance.
What is Mirror polish?
Stainless No. 8 and SPI or PIA A-1 mold polishing are familiar appearance classes, but optical mirrors require separate figure specifications.
How it works
The surface is leveled through successively finer abrasives, often beyond 1000 grit, then buffed with fine compound. Mold A-1 work uses diamond buffing after precision stone and paper sequences. Typical decorative mirror work is Ra 0.025–0.10 µm (1–4 µin); reflectance, waviness, pits, orange peel, and distortion must also be controlled because low Ra alone does not guarantee a true mirror.
Why specify it
- Typical Ra 0.025–0.10 µm (1–4 µin)
- Produces premium specular appearance
- Improves release on polished mold faces
- Makes hygienic surfaces easier to inspect
Where it falls short
- Very high manual labor and reject risk
- Reveals pits and waviness
- Rounds edges and lettering
- Decorative mirror polish is not an optical figure specification
Mirror polish at a glance
Mirror polish is strongest on cosmetic quality (95 of 100), at a high cost. It adds little corrosion protection. Surface hardness is not rated: the finish leaves the part's own surface.
Each score is 0–100 against every finish in this catalog. Corrosion comes from salt-spray life (ASTM B117, log scale) and hardness from the finish layer's Vickers or pencil hardness; cosmetic, lubricity and cost are FabDigit's engineering assessment. Economy on the radar is the inverse of cost.
Mirror polish key numbers
Typical values from the governing specifications and shop practice; bars are this finish against the highest value among mechanical finishing finishes.
- Thickness
- No coating; ASTM A480/A480M No. 8 decorative mirror is typically Ra 0.025–0.05 µm (1–2 µin), not a looser 0.10 µm shop polish.
- Electrical
- Unchanged
- Appearance
- Highly reflective, specular surface with no readily visible directional grain.
Specifications for Mirror polish
The documents a drawing cites to define this finish, with the Type / Class rows that apply. Each links to a page explaining the standard.
Drawing call-out
MIRROR POLISH DESIGNATED SURFACES TO Ra 0.05 µm MAX, ASTM A480/A480M No. 8 APPEARANCE, NO VISIBLE PITS OR DIRECTIONAL LINESWhich materials take Mirror polish?
Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library.
| Material family | Verdict | Notes |
|---|---|---|
| Yes | Common decorative mirror on brass with tarnish protection afterward. | |
| Yes | Polishes brightly but requires protection from tarnish for lasting appearance. | |
| Yes | Common substrate for No. 8 decorative finish. | |
| Yes | Tool steels can receive SPI or PIA mold polish classes. | |
| With care | Can be bright but soft alloys show waves and scratches readily. | |
| With care | Possible with non-sparking media and fire-safe dust control. | |
| With care | Possible but slow; contamination-controlled compounds are required. | |
| With care | Soft die-cast zinc cuts quickly and can embed media. | |
| Limited | Clear acrylic and similar resins can be buffed; most engineering plastics smear or haze. |
How Mirror polish is applied, and what it does to the part
- 1Precision level defects
- 2Progress through fine abrasive papers
- 3Pre-buff
- 4Final fine or diamond buff
- 5Clean and inspect under controlled lighting
| Dimensions | Leveling removes variable stock and can change form, flatness, and edge definition; machine with polish allowance and inspect after polishing. |
|---|---|
| Build-up per side | 0 µm — No buildup; variable stock is removed during leveling. |
| Masking | Usually needed. Mask all functional datums, sharp edges, engraving, and surfaces not included in the cosmetic zone. |
| Electrical | Unchanged |
| Appearance | Highly reflective, specular surface with no readily visible directional grain. |
| Added lead time | 3–10 working days on top of the bare part |
Design rules for Mirror polish
What to decide on the drawing before the part goes to the finisher.
- Specify the visual inspection lighting.Fine lines and haze depend on illumination.
- Add flatness or optical figure separately.Ra does not control macroscopic form.
- Use SPI or PIA class for mold faces.A-1 through D-3 classes communicate mold finishing method and appearance.
- Provide polish allowance.Defect leveling removes measurable stock.
Mirror polish versus the alternatives
Finishes engineers weigh against Mirror polish, and when each wins.
| Finish | Corrosion | Hardness | Cosmetic | Cost | Thickness | Choose it when |
|---|---|---|---|---|---|---|
Mirror polish this page | 10 | — | 95 | $$$$ | 0 µm | — |
| 8 | — | 82 | $$$ | 0 µm | when gloss is needed without full specular appearance | |
| 0 | — | 70 | $$$$ | 0 µm | when flatness and sub-0.1 µm Ra are functional requirements |
The radar overlays Mirror polish with Polishing, its most common alternative — chosen when gloss is needed without full specular appearance. Scores in the table are 0–100 across every finish in the catalog.
Cost, lead time and compliance
Relative cost
Mirror Polishing is a high-cost finish ($$$$): more expensive than about 84 % of the finishes FabDigit quotes.
Finishing is priced per part with a lot minimum, from FabDigit's own China-side rates cross-checked against US job-shop pricing. The exact figure appears on the instant quote once the part is uploaded.
Lead time and compliance
| Added lead time | 3–10 working days |
|---|---|
| RoHS | Compliant |
| Notes | Collect fine metal dust and polishing compound; segregate stainless tooling from carbon steel. |
What Mirror polish is used for
- Decorative stainless trim
- Plastic injection mold cavities
- Hygienic equipment
- Reflective hardware
Mirror polish — frequently asked
what Ra is mirror polish
Decorative mirror polishing commonly targets Ra 0.025–0.10 µm (1–4 µin). A low Ra does not by itself control waviness, pits, haze, or optical figure.
what is No. 8 stainless finish
ASTM A480/A480M No. 8 is a highly reflective polished stainless sheet appearance. State an Ra limit and approved sample when the exact visual result matters.
what is SPI A1 mold finish
SPI or PIA A-1 is the highest diamond-buffed mold finish class. A-1 through D-3 describe increasingly textured methods, from diamond buff through dry blasting.
mirror polish vs lapping
Mirror polishing prioritizes specular appearance and may round form. Lapping uses a controlled flat tool and loose abrasive to achieve flatness and often Ra below 0.1 µm.
does mirror polishing remove scratches
It removes scratches only by leveling surrounding material. Deep pits or machining grooves require substantial stock removal and may remain unacceptable.
Other mechanical finishing finishes
Aluminum Bright DipBright dipping is a chemical polishing pretreatment that levels microscopic peaks on aluminum to produce a reflective surface before anodizing.CorrosionHardnessCost$$
As MachinedAs machined is the baseline CNC condition with no secondary cosmetic finishing.CorrosionHardnessCost$
Bead BlastingBead blasting propels spherical media at a surface to make a uniform satin or matte finish.CorrosionHardnessCost$
Brushed FinishBrushing creates a directional grain with abrasive belts, wheels, or pads.CorrosionHardnessCost$$
DeburringDeburring removes loose or sharp burrs left by cutting, punching, molding, or casting.CorrosionHardnessCost$
Edge RoundingEdge rounding deliberately replaces a sharp edge with a controlled radius or broad edge break.CorrosionHardnessCost$
HoningHoning precision-finishes bores with expanding abrasive stones and a controlled cross-hatch.CorrosionHardnessCost$$$
Sand BlastingSand blasting uses angular abrasive to remove scale, rust, and coatings or to create a rough coating-ready surface.CorrosionHardnessCost$