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CNC Machining Surface Finish Guide: Ra Chart, As-Machined, Bead Blast, Anodize, and Chem Film
A practical engineering reference for specifying surface finish on CNC machined parts. Covers Ra values and the standard machined surface finish chart, as-machined ranges from milling and turning, glass-bead blasting, sulfuric (Type II) and hardcoat (Type III) anodize, and chromate conversion (chem film) — with the tolerance, cost, and DFM impact of each. Written from the perspective of Olympus Machining, an ITAR-registered precision CNC machine shop in Hanover, Pennsylvania.
Surface Finish (Ra) Quick Reference
Ra (roughness average) is the arithmetic mean of profile deviations from the mean line over a sampling length. It is the most common way to specify surface finish on engineering drawings, reported in microinches (µin) in the U.S. and micrometers (µm) elsewhere. 1 µm equals approximately 40 µin.
| Ra (µin) |
Ra (µm) |
Typical Process |
Visual / Functional Use |
| 500 |
12.5 |
Rough mill, saw cut |
Non-critical, hidden surfaces |
| 250 |
6.3 |
Rough turn, face mill |
Weldments, bracket back faces |
| 125 |
3.2 |
Standard mill / turn (default) |
General machined surface — visible tool marks |
| 63 |
1.6 |
Finish pass, sharp insert |
Mating faces, sealing surfaces with gasket |
| 32 |
0.8 |
Finish pass + ball end / fine turn |
O-ring grooves, dynamic seal lands, bearing fits |
| 16 |
0.4 |
Grinding, hand polish |
Hydraulic seal surfaces, precision shafts |
| 8 |
0.2 |
Lapping, fine grinding |
Mirror finish, optical, medical implant |
| 4 |
0.1 |
Lapping, polishing |
Specialty optical / mating gauge surfaces |
Values are typical achievable Ra by process; actual results depend on material, tooling, fixturing, and operator skill.
As-Machined Finishes (No Secondary Operation)
The default surface finish on a CNC machined part with no secondary operation is 125 µin Ra (3.2 µm). This is the industry baseline assumed on drawings that do not call out a finish. Tool marks are visible and the surface direction follows the cutter path — circular on faced or turned surfaces, linear on side-milled walls.
Typical As-Machined Bands
- 3-axis milling, aluminum, standard end mill: 63–125 µin Ra routine, 32 µin Ra with finishing pass.
- Multi-axis milling, ball end finishing: 32–63 µin Ra on contoured surfaces.
- CNC turning, sharp insert, finish pass: 16–63 µin Ra on diameters.
- Stainless and titanium: usually one band rougher than aluminum at the same parameters; finish improves with lower feed and sharper geometry.
Specifying tighter than 32 µin Ra in the as-machined state is achievable but raises cycle time materially. For finer than 16 µin Ra, plan on a secondary polishing, grinding, or lapping operation.
Bead Blasting (Glass Bead / Aluminum Oxide)
Bead blasting is a media-blast process that produces a uniform matte finish by impacting the surface with glass beads, aluminum oxide, or stainless shot. It is primarily cosmetic — it hides tool marks, evens out color variation between machined surfaces, and gives the part a clean, manufactured look common on consumer electronics, aerospace brackets, and instrument housings.
- • Finish: uniform matte gray; final Ra typically 60–120 µin depending on media and pressure.
- • Material removal: typically under 0.0005 inch; sharp edges round by 0.001–0.003 inch.
- • Tolerance impact: negligible for ±0.005 inch and looser; mask features tighter than ±0.001 inch or finish-machine afterward.
- • Use before: anodize or chem film — bead blast first to control cosmetic appearance of the final coating.
- • Common spec: "Glass bead blast, MIL-STD-1504 or equivalent, before anodize."
Type II Anodize (Sulfuric)
Type II anodize per MIL-A-8625 Type II is a sulfuric-acid electrochemical conversion of the aluminum surface to aluminum oxide. It is the standard cosmetic anodize for consumer products, electronics enclosures, and general industrial hardware on 6061, 7075, and other wrought aluminum alloys.
- • Coating thickness: 0.0002–0.0010 inch (5–25 µm).
- • Class 1: undyed, clear; Class 2: dyed (black, red, blue, gold, etc.).
- • Dimensional growth: roughly half the coating thickness grows outward per surface — about 0.0001–0.0005 inch on diameters.
- • Properties: corrosion-resistant, electrically insulating, accepts dye, paintable.
- • Tolerance impact: compensate for growth on press-fit, slip-fit, and threaded features tighter than ±0.001 inch. Mask threads and bores if dimensional control is critical.
- • Common spec: "Anodize per MIL-A-8625 Type II Class 2 Black, sealed."
Type III Anodize (Hardcoat)
Type III anodize per MIL-A-8625 Type III, commonly called hardcoat, is a denser and thicker oxide formed at lower bath temperatures. It is specified where the part must resist abrasion, survive sliding contact, or carry military hardcoat callouts on weapons, optics, and aerospace components.
- • Coating thickness: 0.0010–0.0040 inch (25–100 µm), commonly specified at 0.002 inch.
- • Hardness: 60–70 HRC equivalent on the oxide layer.
- • Dimensional growth: roughly 50 percent grows into the part and 50 percent outward — at 0.002 inch coating, expect about 0.001 inch added per surface (0.002 inch on a diameter).
- • Color: natural is dark gray to black on most alloys; Class 1 (undyed) or Class 2 (dyed black is most common).
- • Tolerance impact: significant. On critical fits, machine the feature undersize by the expected outward growth, or mask threads, bores, and bearing seats.
- • Common spec: "Hardcoat anodize per MIL-A-8625 Type III Class 1, 0.002 inch thick, unsealed."
Chem Film (Chromate Conversion Coating)
Chem film per MIL-DTL-5541 (formerly MIL-C-5541) is a chemical conversion coating that forms a thin chromate layer on aluminum. Unlike anodize, it is electrically conductive and adds negligible dimensional buildup, making it the standard for electronics chassis, RF enclosures, bonded assemblies, and as a paint primer.
- • Coating thickness: typically under 0.0001 inch — dimensionally negligible.
- • Type I: hexavalent chromium (gold/yellow color). Type II: trivalent chromium / RoHS-compliant (clear or pale).
- • Class 1A: maximum corrosion protection. Class 3: low electrical resistance for bonded grounds.
- • Properties: conductive, corrosion-resistant, accepts paint and adhesives, no measurable dimensional impact.
- • Tolerance impact: none for practical purposes — drawing dimensions apply as-machined.
- • Common spec: "Chem film per MIL-DTL-5541 Type II Class 3, clear."
Finish Selection: Tolerance and Cost Impact
| Finish |
Dimensional Effect |
Relative Cost |
When to Specify |
| As-machined 125 Ra |
None |
Baseline (1.0×) |
Default; no functional or cosmetic requirement |
| As-machined 63 Ra |
None |
+5–15% |
Gasket faces, mating surfaces |
| As-machined 32 Ra |
None |
+25–75% |
O-ring grooves, dynamic seals, bearing fits |
| Bead blast |
~0.0005 inch removed; edges rounded |
+10–25% |
Uniform matte cosmetic, prep for anodize |
| Type II anodize |
+0.0001–0.0005 inch per surface |
+15–40% |
Cosmetic color, corrosion, insulation |
| Type III hardcoat |
+0.0005–0.0020 inch per surface |
+30–75% |
Wear, sliding fits, military hardware |
| Chem film (MIL-DTL-5541) |
None (under 0.0001 inch) |
+10–25% |
Electrical grounding, paint prep, low buildup |
| Polish / mirror (≤8 Ra) |
Small material removal possible |
+50–150% on affected feature |
Optical, medical, decorative |
Cost ranges are typical for low-to-mid volume CNC machined parts; complex geometry, masking, and certifications can push costs higher.
DFM Tips for Specifying Surface Finish
- • Only call out finer than 125 Ra where functionally required — every step finer adds cost.
- • Call out the finish per surface, not on the title block — a global 32 Ra requirement multiplies cycle time across every face.
- • Always note "after plating / anodize" or "before plating / anodize" on coated tolerances. Otherwise the inspector has to guess.
- • Mask threads and critical bores for Type III hardcoat to avoid undersized threads or lost slip fits.
- • Specify chem film when conductivity matters — anodize is an insulator and will fail a continuity check.
- • Call out color and dye class on Type II anodize. "Black anodize" alone leaves the supplier to guess Type II vs III.
- • For repeat cosmetic batches, supply a master sample part. Color and matte appearance vary with bath chemistry and operator.
How Olympus Handles Surface Finish
Olympus Machining is an ITAR-registered (CAGE 9V9P0), CMMC Level 1 precision CNC machine shop in Hanover, Pennsylvania. We hold 63–125 µin Ra as-machined on most milled and turned surfaces, and routinely deliver 32 µin Ra with finishing passes for sealing and bearing surfaces. Bead blasting is in-house; Type II anodize, Type III hardcoat, chem film, passivation, black oxide, and electroless nickel are sourced from qualified U.S. finishing partners with full traceability.
Every order ships with a Certificate of Conformance and the relevant finishing certifications. AS9102 Rev C First Article Inspection is available on request, with surface finish measured by profilometer where called out on the drawing.
Related capabilities: precision CNC machining, CNC milling, CNC turning, quality assurance, and AS9102 First Article Inspection.
Frequently Asked Questions
What surface finish is standard for as-machined CNC parts?
125 µin Ra (3.2 µm) is the industry baseline. Most shops, including Olympus, hold 63–125 µin Ra routinely, and 32 µin Ra with a finishing pass on request.
What is the difference between Type II and Type III anodize?
Type II is a thin (0.0002–0.0010 inch) cosmetic and corrosion finish. Type III hardcoat is thicker (0.0010–0.0040 inch), much harder, and grows roughly 50 percent into the part — critical fits must be machined undersize to compensate.
Does bead blasting affect part tolerances?
Minimally. Bead blast removes less than 0.0005 inch and rounds sharp edges by a few thousandths. Mask features tighter than ±0.001 inch or finish-machine after blasting.
When should I specify chem film instead of anodize?
Specify chem film (MIL-DTL-5541) when the part must remain electrically conductive — grounded chassis, RF enclosures, bonded assemblies — or when no measurable dimensional buildup is acceptable.
How does surface finish affect CNC machining cost?
125 Ra is the cost baseline. 63 Ra typically adds 5–15 percent, 32 Ra adds 25–75 percent, and mirror finishes (under 8 Ra) usually require a separate polishing operation and can double the cost of the affected feature.
Need a Quote With Specific Surface Finish Requirements?
Send a STEP file and drawing with finish callouts. We'll return a quote with finish recommendation, tolerance review, and lead time.
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Last reviewed: June 10, 2026