CNC Part Surface Finishing: A Complete Buyer's Guide

Why surface finish matters

Surface finish on CNC machined parts is not primarily cosmetic — it has direct functional consequences. Corrosion resistance, wear resistance, fatigue life, electrical conductivity, optical properties, and assembly fit are all affected by the surface condition and any applied coating or treatment. An anodized aluminum housing resists corrosion and provides electrical insulation; the same part left bare would corrode in a humid environment and could short out adjacent electronics. Surface finish callouts on engineering drawings are contractual requirements — a part delivered without the specified finish is nonconforming regardless of dimensional accuracy. Understanding the available finishes and their properties allows you to specify the right treatment for each application rather than defaulting to "anodized" for everything.

Common finishes for CNC machined parts

Type II anodizing (sulfuric acid) is the standard finish for aluminum, producing a 0.0002"–0.001" thick oxide layer that is integral to the part surface (not a coating that can peel). It provides good corrosion resistance and can be dyed in a wide range of colors. Hard anodizing (Type III) produces a thicker, denser layer (0.001"–0.003") with significantly higher hardness (60–70 HRC equivalent) and wear resistance, used for pistons, gears, and sliding surfaces — note that hard anodize significantly increases dimensional size. Powder coat is an organic coating applied electrostatically and cured in an oven, offering excellent corrosion and UV resistance for non-precision exterior surfaces; it is thicker (0.003"–0.006") than anodize and not suitable for tight-tolerance features. Passivation (per ASTM A380 or AMS 2700) is a chemical treatment for stainless steel that removes free iron from the surface and maximizes the natural chromium oxide layer — it provides no dimensional change and is required for most aerospace and medical stainless parts. Electroless nickel plating deposits a uniform 0.001"–0.003" layer of nickel-phosphorus alloy over steel, aluminum, or copper alloys, providing corrosion resistance, a hard wear surface, and consistent deposit even in blind holes and complex geometries. Zinc plating (electroplated zinc per ASTM B633) is a low-cost option for steel parts requiring moderate corrosion protection, often with a chromate conversion coating (clear, yellow, or black) for additional protection. Black oxide is a chemical conversion coating for steel that provides minimal corrosion protection (requires supplemental oil or wax), a matte black appearance, and virtually zero dimensional change — common on tooling, hardware, and firearms components. Bead blast produces a uniform matte texture by propelling glass or steel media at the part surface; it does not provide corrosion protection but improves cosmetic consistency by eliminating tool marks. Polished finishes are produced by progressively finer abrasive steps and are specified by Ra value or grit equivalent; they are required for optical components, medical implants, and fluid-contact surfaces where surface roughness affects flow or biocompatibility.

How to specify surface finish on drawings

Surface finish is specified on engineering drawings using the surface texture symbol (a check mark with additional callouts) per ASME Y14.36. The primary parameter is Ra (arithmetic mean roughness), measured in microinches (µin) or micrometers (µm). As-machined CNC milling surfaces typically produce Ra 63–125 µin; turning produces Ra 32–63 µin with standard inserts. Finer values (Ra 16, Ra 8, Ra 4) require additional passes, slower feeds, or secondary operations. When specifying a surface treatment such as anodizing, always call out the specification (e.g., "Anodize per MIL-A-8625 Type II Class 1") rather than just "anodize" — the specification defines thickness, hardness, and testing requirements. For plated parts, specify the coating material, minimum thickness, and relevant standard (e.g., "Electroless Nickel per MIL-C-26074 Class 1, 0.001" minimum thickness"). Include a general note for the default surface condition on surfaces not otherwise called out.

Finish compatibility with materials

Not every finish works on every material, and incompatible combinations can cause adhesion failure, embrittlement, or dimensional problems. Anodizing is specific to aluminum alloys — not all alloys anodize equally well; 2024 and 7075 can be anodized but produce darker, less uniform results than 6061. Passivation applies only to stainless steel and some nickel alloys. Electroless nickel can be applied to aluminum, steel, and copper alloys but requires proper pre-treatment for each substrate. Powder coat adhesion on aluminum requires chromate or zirconate pre-treatment; on steel it requires blast cleaning to Sa 2.5. Zinc plating is not compatible with high-strength steel over 200 ksi tensile strength due to hydrogen embrittlement risk — use mechanical plating or zinc-nickel alloy plating for high-strength fasteners and structural components. Always confirm the complete material-finish compatibility with your machining shop or finishing house before design release.

Lead time and cost impact of finishing

Adding a surface finish extends lead time and adds cost that many buyers underestimate when planning schedules. Most machine shops subcontract finishing to specialized shops — anodizing, plating, powder coat, and passivation are typically outsourced. This adds 2–5 business days for routine work and up to 2 weeks for specialty finishes like hard chrome or Nadcap-approved processes. Finishing costs for common treatments: Type II anodize typically adds $2–8/part for small to medium parts; hard anodize $5–15/part; powder coat $8–20/part depending on masking requirements; passivation $1–4/part; electroless nickel $5–20/part depending on size and thickness. Masking — protecting threads, tight-tolerance bores, and mating surfaces from coating — adds significant cost for complex parts. Always include finishing lead time and cost in your project plan, and confirm with your shop whether finishing is included in their quote or subcontracted separately.

In-house vs subcontracted finishing: what to ask your CNC shop

Some CNC shops have in-house finishing capability — typically bead blast, black oxide, and sometimes anodizing or powder coat. In-house finishing offers faster turnaround, tighter control, and simpler supply chain management (one point of contact, one purchase order). However, many finishing processes require specialized equipment, chemicals, and environmental permits that make them economical only at dedicated finishing shops. Ask your CNC supplier specifically: which finishes they perform in-house, which they subcontract, and who their finishing subcontractors are. For critical applications, you may want to approve the finishing subcontractor separately or direct-source the finishing. For Nadcap-required special processes (cadmium plating, hard chrome, heat treatment), the finishing supplier must have Nadcap accreditation for that specific process — verify this independently, as it flows down to the end buyer for aerospace and defense work.