CNC Machining Surface Finishes Guide: How Buyers Should Choose
For a buyer, a surface finish is not a color choice added after machining. It changes corrosion life, wear, electrical behavior, cleanability, appearance, dimensions, inspection method, lead time, and total cost. This guide helps engineering and procurement teams choose a finish that protects the part without creating avoidable tolerance or cosmetic risk.
1. Define the job the finish must do
- Corrosion protection in indoor, outdoor, marine, chemical, or washdown service
- Wear, friction, hardness, conductivity, insulation, or cleanability
- Cosmetic color, gloss, texture, and acceptable visual variation
- Dimensional limits on bores, threads, sealing faces, and mating features
- Compliance, certificates, traceability, and salt-spray requirements
2. Buyer comparison table
| Finish | Typical materials | Primary benefit | Main purchasing risk |
|---|---|---|---|
| Type II anodize | Aluminum | Corrosion protection and color | Color varies by alloy, temper, geometry, and lot |
| Type III hard anodize | Aluminum | Wear resistance and electrical insulation | Greater buildup; tight fits require allowance or masking |
| Chemical conversion coating | Aluminum | Corrosion protection and electrical continuity | Class and compliance must be specified |
| Passivation | Stainless steel | Removes free iron and supports corrosion resistance | Does not hide scratches or replace correct alloy selection |
| Electropolishing | Stainless steel | Smoother, cleaner, more corrosion-resistant surface | Material removal affects sharp edges and dimensions |
| Black oxide | Carbon/alloy steel | Low dimensional change and dark appearance | Limited corrosion protection without oil or seal |
| Zinc plating | Steel | Economical sacrificial corrosion protection | Thread allowance, white rust, and hydrogen embrittlement controls |
| Nickel plating | Steel, copper alloys | Wear, corrosion, and appearance | Thickness distribution varies on recesses and edges |
| Powder coating | Aluminum and steel | Durable color and environmental protection | Thick film can bridge holes and obscure edges |
| Bead blasting | Many metals | Uniform matte appearance | It is a texture process, not corrosion protection |
3. Coating buildup and fit calculations
When a coating adds thickness t to each surface, an external diameter can increase by approximately 2t and an internal diameter can decrease by approximately 2t. Example: a 10.000 mm bore with 12 µm buildup per side can become about 9.976 mm if it is not masked or post-machined. A 9.980 mm shaft would no longer assemble.
Do not simply tighten the machining tolerance. Define whether the drawing dimension applies before or after finishing, identify masked surfaces, and agree on the coating-thickness range used in the tolerance stack.
4. Finish thickness planning table
| Process | Indicative thickness | Dimensional planning |
|---|---|---|
| Type II anodize | Often about 5–25 µm | Part of the layer penetrates and part builds outward; confirm supplier process |
| Hard anodize | Often about 25–75 µm | Plan bearing fits, threads, sealing diameters, and edge condition |
| Zinc plating | Often about 5–25 µm | Control threads and recessed coverage |
| Electroless nickel | Often about 5–50 µm | More uniform than electrolytic plating, but specification still controls |
| Powder coating | Often about 50–150 µm | Mask holes, grounds, threads, and precision interfaces |
These are planning ranges, not acceptance criteria. The purchase order and drawing should state the governing specification and required thickness.
5. Cosmetic expectations need measurable language
“No scratches” or “perfect black” is not a complete acceptance standard. State the viewing distance, lighting, viewing angle, cosmetic zones, allowable defect size and count, color standard, gloss range, and whether parts from different production lots must match. Approve a physical limit sample when appearance is critical.
6. Common buyer mistakes
- Selecting a finish before confirming base alloy compatibility.
- Applying nominal dimensions to coated parts without stating pre- or post-finish condition.
- Leaving threads, bearing seats, electrical contacts, and sealing faces unmasked.
- Using bead blast as if it were a corrosion-resistant coating.
- Requiring a cosmetic standard without defining inspection conditions.
- Requesting aggressive salt-spray performance without defining the test standard, duration, and acceptance rule.
- Ignoring heat-treatment and plating restrictions for high-strength steel.
7. Practical selection examples
Outdoor aluminum enclosure
Start with a corrosion-resistant aluminum alloy, then consider Type II anodize or conversion coating plus paint. Define color, sealing, masked grounding points, and post-finish dimensions for gasket grooves and connector bores.
Stainless food-equipment component
Specify the stainless grade, surface roughness, weld condition, passivation or electropolishing standard, cleanability requirements, and certificate expectations. Passivation alone does not create a polished hygienic surface.
Hardened steel shaft
Clarify hardness, grinding sequence, corrosion target, plating thickness, hydrogen-embrittlement relief, and which journals must remain uncoated. Inspect critical diameters after all finishing steps.
8. What to include in an RFQ
- Base material, temper, hardness, and approved equivalents
- Named finish standard, type, class, color, gloss, and thickness range
- Pre-finish and post-finish dimensions
- Masking drawing for threads, bores, contacts, seals, and datum targets
- Surface preparation and required machined Ra before finishing
- Cosmetic zones, defect criteria, and approved reference sample
- Corrosion or adhesion test standard and reporting requirement
- Quantity, annual demand, packaging, and part-to-part protection
9. Inspection and approval plan
Critical jobs may require coating-thickness measurement, color or gloss measurement, adhesion testing, salt-spray reports, material certificates, masking verification, and a dimensional report after finishing. Agree on sampling before production; do not discover at final inspection that the drawing and coating specification conflict.
Frequently asked questions
Should holes be machined oversize before coating?
Only after a tolerance-stack review. The required allowance depends on process, thickness range, penetration, masking capability, and whether post-finishing is allowed.
Can anodized parts from different lots match exactly?
Not always. Alloy chemistry, temper, surface texture, rack position, bath condition, and geometry influence color. Use defined color limits and approved samples.
Which finish is cheapest?
The lowest unit price is not always the lowest total cost. Masking, inspection, rework risk, packaging, and dimensional recovery often dominate the final cost.
Request a finish and tolerance review
Send Zync Precision your CAD model, drawing, material, quantity, environment, cosmetic expectations, and critical dimensions. We can review machining allowance, masking, inspection, and finish compatibility before quotation.