CNC Machined Parts Surface Finishes: From Basic Smoothing to Specialty Coatings

Introduction

The surface finish of a CNC-machined part plays a critical role in defining its performance, appearance, and longevity. From reducing friction in mechanical systems to improving corrosion resistance in harsh environments, selecting the correct finish can directly impact the function and lifecycle of your component.

Understanding the capabilities of each surface treatment—from as-machined finishes to advanced thermal coatings—is essential for optimizing product performance and manufacturing value.

Basic Surface Finishes for CNC Machined Parts

These finishing processes are typically applied immediately after machining to remove sharp edges, enhance aesthetics, or meet baseline dimensional specifications. Though “basic,” they are essential in ensuring parts meet functional and safety standards.

1. As-Machined Finish

The as-machined finish is the default state of a CNC part after cutting, milling, or turning. It retains visible tool marks and exhibits a surface roughness of approximately Ra 3.2 µm.

This finish is ideal for internal mechanical components or non-cosmetic parts where surface precision matters more than visual appearance. Many internal industrial components rely on this finish to avoid unnecessary cost while maintaining tight tolerances.

2. Deburring and Tumbling

Tumbling is a mechanical process where machined parts are placed in a vibratory chamber filled with abrasive media. This removes burrs and sharp edges, improving handling safety and assembly precision. Surface roughness typically improves to Ra 1.6–3.2 µm.

It’s especially useful for small- to medium-sized batches of metal and plastic parts with complex geometries or edge features.

3. Bead Blasting and Sandblasting

Sandblasting or bead blasting uses high-pressure air and abrasive particles to clean and texture the surface. It provides a uniform matte or satin appearance and removes minor surface contaminants or machining marks.

Typical applications include:

• Pre-treatment before coating or anodizing

• Visual uniformity in visible mechanical parts

• Grip-enhancing texture in handheld components

Ra values usually range from 1.6 to 3.2 µm, depending on media and pressure.

4. Polishing

CNC polishing is a manual or mechanical process used to achieve smooth, shiny surfaces with minimal roughness. This is essential for medical tools, reflectors, or cosmetic-facing components where finish is part of the product experience.

• Mechanical polishing can reduce roughness below Ra 0.2 µm

• Electro-polishing (for stainless steel): creates ultra-smooth, passivated surfaces

Applications include medical-grade surgical components and high-precision optical fixtures.

Functional Surface Coatings and Treatments

Beyond basic smoothing, CNC-machined components often require specialized surface treatments to enhance corrosion resistance, electrical performance, thermal endurance, and wear protection. These treatments are especially relevant in aerospace, medical, automotive, and electronics sectors.

5. Anodizing (Type I, II, III)

Anodizing is an electrochemical process that forms a controlled oxide layer on aluminum parts. It improves hardness, wear resistance, and corrosion performance while allowing color tinting for visual or organizational purposes.

• Type I (Chromic Acid): Thin layer, minimal dimensional change

• Type II (Sulfuric Acid): Decorative, color-capable, ~10–25 µm thick

• Type III (Hard Anodizing): High-wear applications, thickness up to 100 µm

Used in aerospace brackets, drone housings, and electronics enclosures.

6. Electroplating (Nickel, Chrome, Zinc)

Electroplating adds a metal layer onto the surface of CNC parts, offering enhanced aesthetics, wear resistance, and electrical conductivity.

• Nickel plating enhances corrosion resistance and lubricity

• Chrome plating gives a mirror-like appearance and surface hardness

• Zinc plating prevents steel corrosion in general-purpose parts

Electroplated parts are widely used in consumer devices, automotive trim, and hydraulic fittings.

7. PVD Coating (Physical Vapor Deposition)

PVD coating applies thin-film metallic or ceramic layers to increase hardness, reduce friction, and add color or reflectivity. Titanium nitride (TiN), zirconium nitride (ZrN), and DLC (diamond-like carbon) are typical examples.

• Coating thickness: 1–5 µm

• Hardness: up to 2500 HV

• Color: gold, silver, black, bronze, etc.

Used in cutting tools, high-wear consumer components, and surgical instruments requiring both hygiene and visual precision.

8. Thermal Spraying and Cerakote

Thermal spray coatings, such as plasma, arc, or flame spraying, apply molten particles onto the surface, creating a bonded layer that protects against heat, wear, and chemical attack. Cerakote is a ceramic-polymer composite used especially in firearms, aerospace, and marine components.

• Withstands temperatures up to 1000°C

• Ideal for parts under high-stress abrasion or chemical environments

Selecting the Right Surface Finish: Technical and Economic Considerations

Choosing an appropriate surface finish depends on multiple factors beyond visual appeal. Engineers must weigh performance expectations, dimensional tolerances, part geometry, and cost implications.

Key Selection Criteria:

1. Material Compatibility: Not all treatments apply to all materials. For example, anodizing is suitable only for aluminum, while PVD is ideal for steel, titanium, and high-speed tool alloys.

2. Dimensional Impact Hard anodizing can add up to 100 µm per side, which may exceed critical tolerance thresholds. Polishing or electropolishing, in contrast, removes material and should be factored into dimensional design.

3. Functional Requirement

   • Friction reduction → Polishing or PVD

   • Corrosion protection → Anodizing, plating, or PTFE coating

   • Cosmetic finish → Bead blasting, chrome plating, or lacquer coating

   • Heat resistance → Thermal spray or ceramic coatings

4. Cost vs Performance Trade-off Surface treatments can increase part costs by 10–50%, depending on complexity and post-processing sequence. However, they often extend product life severalfold, lowering long-term maintenance and failure rates.

Industry Applications by Finish Type

Surface Finish Performance Comparison Table

Conclusion

CNC machined parts can be transformed significantly through proper surface finishing—from standard smoothing for functional tolerances to advanced coatings that protect, insulate, and beautify. Selecting the right finish not only affects part performance but also total cost of ownership, user satisfaction, and compliance with sectoral standards.

For precision-driven industries such as aerospace, defense, medical, and electronics, surface treatment is not an optional aesthetic—it is a critical engineering decision. Whether applying hard anodizing to aerospace aluminum components or chrome plating to high-polish automotive trim, the correct process choice can deliver long-term mechanical and economic value.

At Neway, we support customers from surface finish consultation through to full production, offering CNC machining services paired with surface treatments that meet exacting requirements across materials and markets.

FAQ

1. What’s the difference between bead blasting and sandblasting in CNC finishing?

2. Does anodizing affect part dimensions in precision CNC components?

3. Which surface treatment offers the best corrosion resistance for marine use?

4. Can I combine multiple finishes on a single CNC-machined part?

5. What’s the best finish for high-temperature CNC machined parts?