Surface Finish Guide: Ra Values, Processes & Cost Impact

Surface finish is often overlooked in early design stages, yet it significantly impacts part performance, aesthetics, cost, and lead time. Specifying the wrong finish can add 30–50% to your part cost and a week to your delivery. This guide explains everything you need to know about surface finishes in manufacturing.

Understanding Surface Roughness: Ra Values

Ra (Roughness Average) is the most common measurement of surface texture. It represents the arithmetic average of the absolute deviations from the mean surface line, measured in micrometers (μm) or microinches (μin).

Common Ra Values in Manufacturing

Standard CNC machining typically achieves Ra 1.6–3.2 μm without special operations. Going below Ra 1.6 requires additional finishing steps that add significant cost.

Cost Impact of Surface Finish

As a rough rule, each halving of Ra value doubles the finishing cost:

Common Surface Finishing Processes

As-Machined

The surface directly from CNC machining with tool marks visible. Suitable for non-cosmetic, non-contact surfaces. Ra typically 1.6–3.2 μm. No additional cost or lead time.

Bead Blasting

Glass or ceramic beads propelled at the surface create a uniform matte texture. Removes tool marks and creates a consistent satin appearance. Typical Ra: 2.0–4.0 μm. Cost: $2–$10/part. Lead time: Same day.

Bead blasting is the most popular cosmetic finish for aluminum prototypes. It hides machining marks while maintaining dimensional accuracy (removes less than 0.01mm of material).

Anodizing

Anodizing is an electrochemical process that converts the aluminum surface into a durable, corrosion-resistant aluminum oxide layer. It's available only for aluminum alloys.

#### Type II Anodizing (Decorative/Standard)

• Layer thickness: 5–25 μm
• Accepts dye colors (black, red, blue, gold, etc.)
• Moderate hardness and corrosion resistance
• Cost: $3–$15/part depending on size
• Lead time: 1–3 days
• Dimensional change: +0.005–0.015 mm per surface (half grows outward, half inward)

#### Type III Hard Anodizing

• Layer thickness: 25–75 μm
• Significantly harder than Type II (60–70 Rockwell C)
• Superior wear and corrosion resistance
• Limited color options (natural dark gray/bronze, or black)
• Cost: $10–$30/part
• Lead time: 2–5 days
• Dimensional change: +0.025–0.050 mm per surface

Critical note for tight-tolerance parts: Account for anodizing thickness in your dimensional design. If a bore requires a finished diameter of 10.000 ±0.025 mm, machine it to 9.97–9.98 mm before Type II anodizing.

Powder Coating

Electrostatically applied dry powder cured under heat, forming a hard, durable finish. Available on steel, aluminum, and other metals.

• Thickness: 40–120 μm
• Excellent UV and corrosion resistance
• Huge color range (RAL color chart)
• Cost: $3–$15/part
• Lead time: 1–3 days
• Minimum coverage: Powder coating covers all exposed surfaces; masking is needed for threads, mating surfaces, etc.

Electroplating

#### Nickel Plating

Adds a bright, corrosion-resistant metallic layer. Available in both electroless (uniform thickness on all surfaces) and electrolytic (thicker on exposed surfaces). Cost: $5–$20/part. Excellent for steel parts that need corrosion protection with a premium appearance.

#### Chrome Plating

Decorative chrome provides a mirror-bright finish. Hard chrome (industrial chrome) provides extreme wear resistance for hydraulic cylinders, bearing surfaces, and tooling. Note: Environmental regulations (REACH in EU, EPA in US) are restricting hexavalent chrome. Trivalent chrome alternatives are becoming standard.

#### Zinc Plating

The most cost-effective corrosion protection for steel parts. Available in clear, yellow, black, and olive drab chromate finishes. Provides 100–500 hours of salt spray protection. Cost: $1–$5/part.

Passivation (Stainless Steel)

A chemical process (nitric acid or citric acid bath) that removes free iron from the surface and enhances the natural chromium oxide protective layer. Essential for stainless steel parts in medical, food processing, and chemical applications.

Cost: $2–$8/part. Lead time: 1 day. No dimensional change.

Brushing/Polishing

Mechanical finishing using progressively finer abrasives. Creates directional grain patterns (brushed) or mirror finishes (polished). Labor-intensive and costly for complex geometries. Best for flat or cylindrical surfaces. Mirror polish on a complex CNC part can cost more than the machining itself.

Selecting the Right Finish

Decision Matrix

Tips for Specifying Surface Finishes

• Only specify Ra values on functional surfaces: Sealing surfaces, bearing fits, and mating interfaces need Ra callouts. Cosmetic surfaces are better specified by process (e.g., “bead blast + Type II anodize, black”).

• Call out critical surfaces on your drawing: Use surface finish symbols on the specific faces that matter. A general note saying “Ra 0.8 all over” will dramatically increase cost.

• Consider the base machining finish: Standard CNC delivers Ra 1.6–3.2 μm. If your finish process (like anodizing) will obscure tool marks anyway, don't pay extra for a finer machining finish.

• Account for masking costs: Every thread, bore, or mating surface that needs to stay uncoated requires masking, which adds labor cost. Consolidate masked areas where possible.

• Test finishes on prototypes: Colors and textures look different in person than on screen. Order a sample part with your intended finish before committing to a production run.

How FabVector Handles Surface Finishing

When you upload a part to FabVector, you can specify your desired surface finish directly in the quoting interface. Our AI engine factors finish requirements into the quote, including any masking, dimensional compensation, and lead time impact.

We work with finishing specialists who handle anodizing, powder coating, plating, and more — all coordinated through our platform so you receive fully finished parts ready for assembly. No need to manage separate vendors for machining and finishing.