Surface finish is one of the most important details in a CNC machining RFQ. It affects appearance, corrosion resistance, wear behavior, coating thickness, inspection method, packaging and final part cost. Buyers should choose the finish according to material, function and visible surface requirements instead of treating finishing as a last-minute note.
This guide compares common surface finish options for custom CNC machined parts, including aluminum parts, stainless steel parts, titanium components and engineering plastic parts.
Why Surface Finish Changes a CNC Machining Quote
A machined part can often be supplied as-machined, but many functional and cosmetic parts need additional finishing. Each finish adds handling, lead time and dimensional considerations. Some finishes also require masking, threaded-hole protection, color control, surface roughness control or post-finish inspection.
For accurate quoting, buyers should include finish requirements in the drawing or RFQ package at the same time as material, quantity, tolerance and inspection requirements. If the finish is still open, list acceptable options so the supplier can compare cost and manufacturing risk.
Common Surface Finish Options for CNC Parts
| Finish | Common Materials | Main Purpose | RFQ Notes |
|---|---|---|---|
| As-machined | Aluminum, steel, stainless steel, brass, copper, plastics | Functional prototype or internal-use part | Define deburring, edge break and surface roughness if important. |
| Anodizing | Aluminum | Corrosion resistance, wear resistance, color or cosmetic appearance | Confirm color, Type II or hard anodizing, and masked surfaces. |
| Passivation | Stainless steel | Improve corrosion resistance after machining | Confirm grade, cleaning expectation and whether cosmetic polish is separate. |
| Bead blasting | Aluminum, stainless steel, titanium | Uniform matte appearance | Identify cosmetic faces and protect sharp functional edges if needed. |
| Polishing | Stainless steel, aluminum, brass, copper | Smoother appearance or lower roughness | State target surface roughness or visual standard when critical. |
| Plating | Steel, brass, copper, selected aluminum processes | Wear, conductivity, corrosion protection or appearance | Account for coating thickness on fits, threads and bores. |
| Powder coating | Aluminum, steel | Durable colored coating | Confirm color, texture, masking and any assembly clearance issues. |
Aluminum CNC Parts: Anodizing, Bead Blasting and Powder Coating
Aluminum CNC machining is often paired with anodizing because aluminum is lightweight, machinable and suitable for many cosmetic or protective finishes. Clear anodizing is common for functional parts. Black anodizing is common for equipment, fixtures, optical components and consumer-facing mechanical parts. Hard anodizing may be considered when wear resistance matters.
Bead blasting before anodizing can create a uniform matte surface, but it may soften sharp visual edges and change the appearance of tool marks. Powder coating is thicker than anodizing, so buyers should review threaded holes, slots, press-fit areas and mating surfaces before specifying it.
Stainless Steel CNC Parts: Passivation, Polishing and Bead Blasting
Stainless steel CNC machining is commonly selected for corrosion resistance, strength and clean appearance. Passivation is often used after machining to clean the surface and support corrosion performance. It is not the same as polishing. If the part also needs a bright, brushed or mirror-like appearance, that cosmetic requirement should be specified separately.
For precision stainless steel parts, note which dimensions must be inspected after finishing. Shafts, sealing surfaces, threaded features and bearing fits may need tighter control than general external faces.
Titanium CNC Parts: Clean Handling and Controlled Appearance
Titanium CNC machining is used for lightweight, corrosion-resistant and high-performance components. Titanium parts often require careful handling to avoid surface contamination or cosmetic damage. Bead blasting can make the surface more uniform, while polishing may be used when appearance or roughness is important.
Because titanium machining and finishing can be sensitive to heat, tool condition and handling, buyers should define functional surfaces and inspection requirements clearly.
Plastic CNC Parts: Deburring, Edge Quality and Surface Marks
Engineering plastics such as PEEK, Delrin and nylon are usually not anodized or passivated. For PEEK machining and other plastic CNC parts, the main surface concerns are burrs, sharp edges, tool marks, flatness, stress marks and cosmetic handling. Thin walls and small features can be affected by clamping and cutting pressure.
If the plastic part is visible in an assembly, include cosmetic expectations in the drawing. If the part is functional, define the features that control fit, sliding, sealing or insulation.
How Surface Finish Affects Tolerances
Some finishes add thickness or remove material. Plating and powder coating can affect threads, bores, slots and mating surfaces. Polishing can slightly change edges and surface geometry. Bead blasting can affect sharp cosmetic edges. Anodizing creates an oxide layer and may require masking for critical surfaces.
For precision components, buyers should mark dimensions that must be held after finishing. This is especially important for tight bores, bearing seats, threaded holes, sealing faces and assembly interfaces.
What Buyers Should Include in a Surface Finish RFQ
- Base material and material grade.
- Required finish, color, texture or surface roughness.
- Cosmetic faces and hidden non-cosmetic faces.
- Masked holes, threads, contact faces or grounding areas.
- Dimensions that must be inspected after finishing.
- Acceptable finish alternatives if cost or lead time matters.
- Packaging requirements to prevent scratches or coating damage.
Related CNC Machining Pages
For process and sourcing decisions, review custom CNC machining services, CNC milling service, CNC turning parts and the CNC machining quote checklist.
