Passivation (stainless)
Specify the applicable process and acceptance requirements for the selected stainless grade. Confirm cleaning and required records; do not treat passivation as the specification for a deposited coating.
CNC Surface Finishes for Machined Parts
Specify why the surface needs treatment, which material and process apply, and how appearance, masking and final dimensions will be accepted. Agreed finishes are coordinated with the machining order.
When this guide applies
Use this CNC surface finishes guide to translate function and appearance into a controlled finishing requirement. Start with the purpose: wear, corrosion exposure, contact, cleaning, identification or appearance. Identify the material grade, affected surfaces and final acceptance state.
State the process and applicable specification / edition, class or type where relevant, color or appearance reference, masking and the documents required. For stainless passivation, ASTM A967/A967M is one specification to identify when applicable; it does not define aluminum anodizing classes.
Finishing and specialist work are coordinated only within the agreed quotation / order scope. Confirm process compatibility, production responsibility and the final inspection requirement before release. If a surface texture or cosmetic condition matters, specify it; an unspecified machined surface has no universal Ra or appearance guarantee.
Anodizing aluminum
| Process category | Specify | Appearance input | Functional input | Acceptance to confirm |
|---|---|---|---|---|
| Sulfuric anodizing (Type II where specified) | Applicable process specification, alloy and required coating | Color, gloss and approved reference where needed | Corrosion, contact, masking and fit requirements | Appearance, coating requirements and finished dimensions |
| Hardcoat (Type III where specified) | Process specification, alloy, coating requirement and any sealing | Acceptable color / variation for the actual alloy | Wear, functional surfaces and protected fits | Coating and final geometry using the agreed method |
| Other specified anodizing | The actual process specification and the reason for its use | Project-specific appearance reference | Compatibility and relevant function | Availability, scope and required records confirmed before ordering |
Note
Coating thickness and outward growth are different quantities. Agree process-specific allowances and masking. External diameters and bores require different allowance directions; any model change needs the drawing owner’s agreement. See functional tolerances and final acceptance state.
Bead blasting media
| Media category | Purpose to review | Surface input | Sensitive features |
|---|---|---|---|
| Glass bead | A specified matte or satin appearance | Approved sample, affected faces and roughness if functional | Mask threads, sealing/contact faces and other sensitive areas as agreed |
| Abrasive grit | Surface preparation for a specified subsequent process | Required preparation and cleanliness specification | Protect critical geometry and confirm substrate compatibility |
| Ceramic or other media | The required texture and process compatibility | Media / appearance acceptance and any following treatment | Agree protection and inspection for fine features |
Note
Media, substrate, pressure, time and geometry affect texture and material removal. A media name does not guarantee Ra. Confirm contamination, embedding and cleaning requirements for the actual function; do not apply a universal hole-size cutoff.
Passivation, powder coat, and electropolish
Powder coating
Specify coating system, preparation, color / gloss, cure compatibility and thickness requirements. Mask functional fits and threads, and confirm the finished dimensions.
Electropolishing
Specify the substrate, affected surfaces, appearance and any material-removal limits. Review edges, threads and functional sizes after treatment.
As-machined surface
Call out surface texture, direction, burr condition and cosmetic acceptance where needed. Toolpath, material and geometry affect the result; no single Ra applies to every feature.
Specialty finishes
| Finish | Substrates | Key characteristics |
|---|---|---|
| Black oxide | Specified steel / stainless grades | Confirm process, corrosion expectation, supplementary protection and appearance |
| Electroless nickel | Suitable specified substrates | Confirm preparation, coating specification, thickness, masking and final fit |
| Zinc plating | Suitable specified steel parts | Confirm coating specification, corrosion duty, threads and any required post-treatment |
| Brushing / graining | Specified compatible metals | Agree direction, texture, reference sample and protected faces |
| Laser marking / engraving | Suitable specified substrates | Confirm mark content, location, contrast, depth and functional restrictions |
Finishes by material
| Material | Common finishes | Primary purpose |
|---|---|---|
| Aluminum 6061/7075 | Type II/III anodize, blast, powder | Corrosion, wear, cosmetics |
| Stainless 304/316 | Passivation, electropolish, blast, black oxide | Corrosion, cleanability, cosmetics |
| Carbon / alloy steel | Black oxide, zinc, EN, powder | Rust protection, wear, industrial look |
| Titanium | Color anodize, passivation, blast | ID color, oxide stability, matte prep |
| Copper / brass | EN, lacquer, polish | Tarnish control, wear, conductivity care |
| Engineering plastics | As-machined, selective blast / polish | Texture and optics — avoid metal coating assumptions |
Note
These are categories for discussion, not a preapproved finish list for every grade. Confirm substrate, preparation, process and service environment. A black finished-part photograph does not identify the process used.
Dimensional impact (per surface)
| Operation | Dimensional relationship | What to agree |
|---|---|---|
| Anodizing | Oxide formation and outward growth are distinct; effects depend on the specified process | Masking, coating requirement, bore / external-feature allowance and final check |
| Powder coating | Deposited material changes an exposed fit or thread | Required coating, protected features and final acceptance dimensions |
| Plating | Deposited material changes exposed dimensions and local geometry | Thickness requirements, coverage, masking and any approved post-finish operation |
| Passivation | Do not apply an assumed deposited-coating allowance | Applicable process, cleaning and verification for the actual part |
| Electropolishing | Material removal can affect edges, bores and outside dimensions | Allowed removal, protected geometry and post-process inspection |
| Blasting / mechanical finishing | Surface removal and edge change depend on process and access | Appearance, texture, sensitive surfaces and any final geometric check |
Note
No fixed stock adjustment applies to every coating, surface or material. Define which dimensions are accepted after finishing, agree the allowance with the process provider and drawing owner, and return approved changes to the controlled revision.
Illustrative diameter check: For a cylindrical surface with a uniform, confirmed net growth g into the surrounding free space, the outside diameter changes by +2g and a bore diameter by -2g. If g is 0.010 mm, those changes are +0.020 mm and -0.020 mm respectively. Here g is the net movement of each surface from its original position, not total anodic-oxide thickness. This geometric example excludes nonuniform build-up and local edge effects; it is not a process allowance. Agree actual growth, variation and final-fit checks with the process provider.
What goes wrong if finish is unspecified
Wrong cosmetic expectation
As-machined tool marks ship when the buyer expected matte blast or dyed anodize — rework or scrap follows if the drawing was silent.
Fit and thread interference
Coating can increase an outside diameter or reduce a bore opening. Agree masking, process-specific allowances and the final acceptance state for fits and threads.
Substrate mismatch
Specifying aluminum anodizing for a steel part, or ASTM A967/A967M stainless-steel passivation for aluminum, creates a substrate/specification mismatch. Aluminum conversion and etch-passivation processes have their own specifications. Name the substrate and process together.
Inspection after coating
Critical sizes measured before vs after finish disagree unless the drawing states which condition applies ([Tolerance Standards](/precision-tolerance-standards/)).
What to include in your finish RFQ
- Material grade and condition, current drawing revision and available model.
- Finish purpose, process and applicable specification / edition, type or class when required.
- Affected faces, color / gloss or appearance reference, and any required sample approval.
- Protected threads, bores, sealing / contact faces and the masking plan to confirm.
- Critical dimensions and surface texture, with the stage and state of final acceptance.
- Quantity, target delivery, required finish records and any open compatibility question.
STEP and a marked PDF are recommended when available. For .iges, DWG, DXF or .sldasm, contact Jaquent to confirm conversion, delivery and dependencies; the RFQ page lists direct upload formats. Technical details and attachments remain optional. Upload CAD for DFM Review. With a complete RFQ, we typically respond within one business day; complex multi-process projects may take 1–2 business days.
Ready to start your CNC project?
Share the finish requirement, protected surfaces and final acceptance state so machining and finishing can be reviewed together.