For design engineers specifying surface finish on a CNC machined aluminium part, writing ‘black anodise’ on a drawing without specifying Type II or Type III is a tolerance-time bomb. Type II anodising grows 4–12 µm of oxide layer — manageable with the right pre-machining stock allowance. Type III hard anodise grows 50 µm, half inward and half outward: a 20 mm bore becomes 19.95 mm after Type III. On a bore tolerance of ±0.025 mm, that dimensional growth renders the part out of spec before a single test is run. The fix costs $200–$800 in rework per part — entirely avoidable with a complete surface finish specification.
Surface finish is also not a cost-neutral choice. Powder coating adds 15–25% to part cost but provides excellent corrosion protection on steel and aluminium. Electropolishing on stainless steel adds 20–40% but produces the biocompatible, low-surface-energy surface required for pharmaceutical and medical equipment. Specifying a finish that is more expensive or more complex than the application requires is common — understanding what each finish actually provides, and what dimensional changes it imposes, is how engineers control both cost and dimensional compliance.
Surface Finish Options for CNC Machined Parts: Full Comparison
| Finish | Material | Thickness/Depth | Dimensional Change | Corrosion Resist. | Cost Add | Melhor para |
|---|---|---|---|---|---|---|
| Como maquinado | Todos | Ra 0.8–3.2 µm tool marks | Zero | Baixa | $0 | Engineering validation, internal parts, structural |
| Bead blast | All metals | Surface texture only | Negligible (<0.01 mm) | Low (cosmetic only) | $5–$20/part | Uniform matte appearance; cosmetic pre-treatment |
| Type II anodise | Al only | 4–12 µm oxide layer | +4–12 µm outward (≈50/50 in/out) | Bom | $8–$25/part | Consumer electronics, general Al parts; colour options |
| Type III hard anodise | Al only | 25–125 µm (std 50 µm) | +25 µm outward (50 µm total) | Excellent; wear-resistant | $20–$60/part | Aerospace, wear surfaces, hydraulic cylinders |
| Revestimento em pó | Steel, Al | 60–150 µm polymer layer | +60–150 µm | Excelente | $12–$40/part | Industrial, outdoor, coloured structural parts |
| Passivation (ASTM A967) | Stainless | Chemical only; no thickness | Zero | Excellent (restores Cr2O3) | $5–$15/part | Required after all SS machining; food/medical |
| Electropolimento | Stainless, Al | Removes 5–40 µm | −5–40 µm (removing material) | Excellent; low surface energy | $15–$60/part | Pharmaceutical, medical, sanitary applications |
| Zinc plate (electroplate) | Aço | 5–25 µm zinc | +5–25 µm | Good (sacrificial) | $8–$20/part | Indoor steel, fasteners, general corrosion protection |
| Hard chrome (decorative) | Steel, Al | 5–30 µm Cr | +5–30 µm | Excelente | $25–$80/part | Wear-resistant shafts, sliding components |
| Black oxide | Aço | 1–2 µm | Negligible | Low (oil-impregnated) | $5–$12/part | Internal steel parts, reduced light reflection |
| Chemical conversion (Alodine) | Al | <1 µm conductive oxide | Negligible | Good (conductive) | $5–$12/part | EMI enclosures; pre-treatment before paint/primer |
Lewei Precision provides all standard CNC surface finishes through our in-house and partner finishing network. Our maquinagem de precisão process accounts for finish-specific dimensional stock in the machining programme — we machine to the pre-finish dimension, not the drawing nominal, to ensure post-finish dimensions are within tolerance.
Type II vs Type III Anodising: The Dimensional Impact Engineers Miss
The anodising process converts aluminium surface material into aluminium oxide (Al2O3). Approximately 50% of the oxide layer grows outward from the original surface; 50% grows inward by converting existing aluminium. This ’50/50 rule’ (MIL-A-8625F) has critical dimensional consequences:
| Especificação | Oxide Thickness | Outward Growth | Dimensional Impact on ±0.025 mm Bore | Required Pre-Anodise Stock Allowance |
|---|---|---|---|---|
| Type II Clear (undyed) | 4–8 µm | 2–4 µm outward | Bore becomes ~4 µm smaller — within ±0.025 mm | Leave 0.003–0.005 mm oversized on precision bores |
| Type II Black (dyed) | 8–12 µm | 4–6 µm outward | Bore ~6 µm smaller — within ±0.025 mm with planning | Leave 0.005–0.008 mm oversized |
| Type III 25 µm spec | 25 µm | 12.5 µm outward | Bore ~13 µm smaller — may violate ±0.025 mm spec | Leave 0.012–0.015 mm oversized |
| Type III 50 µm std | 50 µm | 25 µm outward | Bore ~25 µm smaller — VIOLATES ±0.025 mm spec | Leave 0.025–0.030 mm oversized — or post-finish ream |
| Type III 75 µm heavy | 75 µm | 37.5 µm outward | Bore ~38 µm smaller — violates ±0.05 mm spec | Leave 0.038–0.045 mm oversized — post-finish bore required |
The engineering action: for any critical bore, shaft, thread, or press-fit feature that must hold tolerance after anodising — specify the post-anodise dimension on the drawing AND the anodise specification. The machinist machines to a pre-anodise stock allowance; the finish brings the part to drawing nominal.
Powder Coating: Thickness and Tolerance Impact
Powder coating applies 60–150 µm of polymer to the part surface. Unlike anodising (which converts material), powder coating adds material on top. For parts with critical dimensions on coated surfaces — enclosure lid interfaces, thread clearances, or mating surfaces — the added thickness must be masked before coating or accounted for in pre-coating machining stock.
- Thread clearance: metric threads in coated steel housings must be chased (re-tapped) after powder coating if the coating enters thread relief areas — powder in threads prevents fastener engagement
- Mating surface interfaces: if two powder-coated aluminium panels must mate with a precision seal, mask the sealing faces during coating or increase mating face clearance by 0.15–0.30 mm to accommodate the coating thickness
- Press-fit bores: mask press-fit bores before powder coating — coating inside a press-fit bore changes the interference fit by 60–150 µm, which is catastrophic for designed press-fit interference of 0.01–0.05 mm
Passivation vs Electropolishing for Stainless Steel
| Imóveis | Passivation (ASTM A967) | Electropolimento |
|---|---|---|
| Processo | Citric or nitric acid removes free iron, restores Cr2O3 passive layer | Electrochemical removal of 5–40 µm of surface material |
| Dimensional change | Zero — purely surface chemistry, no material removal | Removes 5–40 µm (can affect tight tolerances — specify on drawing) |
| Surface finish improvement | None — does not improve Ra | Yes — removes microscopic peaks; Ra improves 30–50% |
| Resistência à corrosão | Restores standard SS passive layer | Superior — smoothed surface has fewer corrosion initiation sites |
| Biocompatibility | Adequate for instrument-grade surfaces | Superior — required for implantable and pharmaceutical wetted surfaces |
| Custo | $5–$15/part | $15–$60/part |
| Required for | All machined SS parts in food, medical, pharmaceutical | Medical implants, pharmaceutical process equipment, sanitary surfaces |
| Application note | Mandatory after all SS machining — not optional | Specify pre/post-electropolish dimensions; dimensional change must be planned |
Surface Finish Selection Decision Tree
- Aluminium, cosmetic/consumer, colour required → Type II anodise (specify pre-anodise dimension on precision features)
- Aluminium, wear/hydraulic/aerospace → Type III hard anodise (specify pre-anodise stock allowance; post-finish ream precision bores)
- Aluminium, EMI shielding enclosure → Alodine (chemical conversion coating — conductive, <1 µm, no dimensional impact)
- Steel, indoor general engineering → Zinc plate or black oxide with oil
- Steel, outdoor/industrial → Powder coat (mask threads, press fits, and mating precision surfaces)
- Stainless steel, any application → Passivation mandatory (ASTM A967); electropolish additionally for pharmaceutical or medical
- Any metal, engineering validation only → As-machined (save finish cost until design is frozen)
Perguntas mais frequentes
What is the difference between Type II and Type III anodising?
Type II (decorative/standard) anodising produces an aluminium oxide layer of 4–12 µm — 2–6 µm of outward dimensional growth. Type III (hard anodise) produces 25–125 µm of oxide, standard 50 µm — 12.5–62.5 µm of outward dimensional growth. Type III provides 10× more wear resistance than Type II but imposes dimensional changes that must be accounted for in pre-anodise machining stock. Type II is suitable for corrosion protection and cosmetic applications. Type III is required for wear contact surfaces, hydraulic cylinder bores, aerospace sliding interfaces, and applications requiring maximum surface hardness (HV 400–600 vs HV 60–70 for untreated aluminium).
Does powder coating affect part tolerances?
Yes. Powder coating adds 60–150 µm to every coated surface. On precision mating surfaces, thread bores, press-fit interfaces, and seal faces — the coating thickness directly affects fit. Solutions: mask precision features before coating (adds $5–$25 per masking step); machine to a pre-coating stock allowance on critical features (coordinate with the coating supplier for expected thickness on your part geometry); or re-machine / re-tap after coating (adds one operation per feature). Always specify masking requirements on the drawing or purchase order — not verbally.
Is passivation required after CNC machining stainless steel?
Yes — passivation per ASTM A967 (citric acid or nitric acid) is required after all Maquinação CNC operations on stainless steel for food-contact, medical, and pharmaceutical applications. CNC machining disrupts the natural chromium oxide passive layer on stainless steel by embedding iron from tooling, coolant, and fixture contact. These iron deposits cause surface rust and galvanic corrosion in service. Passivation dissolves the free iron and restores the full Cr2O3 passive layer, providing the corrosion resistance the alloy is specified for. Passivation adds $5–$15 per part and 1–2 days of lead time — always include it in the manufacturing process for SS food and medical parts.
Conclusion: Specify the Complete Finish — Including the Dimensional Impact
- ‘Black anodise’ is not a complete finish specification — specify Type II or Type III, thickness (µm), and pre-anodise stock allowance on all precision features
- Type III hard anodise grows 25 µm outward at 50 µm spec — a ±0.025 mm bore will be out of tolerance after Type III without pre-anodise stock allowance
- Passivation is mandatory after CNC machining stainless steel for food/medical/pharmaceutical — not optional
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