Anodizing CNC-machined aluminum requires design decisions that affect tolerances, appearance, and cost. With experience finishing precision parts across aerospace and medical sectors, proper specification prevents costly redesigns.
Anodizing is an electrochemical process that converts aluminum surface into durable aluminum oxide, providing corrosion resistance and surface hardness. Success requires careful tolerance planning, geometry considerations, and material selection to achieve consistent results.
Learn how to choose anodizing types, set thickness allowances, and design for consistent finishes—backed by real CNC tolerance insights.
Table of Contents
How Does the Aluminum Anodizing Process Work?
Anodizing creates a protective aluminum oxide layer through controlled electrochemical conversion, not surface coating. This process integrates the protective layer into the aluminum itself, preventing peeling while adding corrosion resistance and surface hardness for CNC-machined parts.
Key process characteristics:
- Electrochemical conversion in sulfuric acid bath with electrical current
- Creates microscopic pores that can be sealed or dyed for color
- Type II adds 0.0002-0.0007″ thickness; Type III reaches up to 0.002″
- Maintains sharp edges and fine machining details
From our experience finishing precision parts across medical, aerospace, and audio sectors, this surface conversion delivers measurable benefits: corrosion resistance increases dramatically over bare aluminum, surface becomes significantly harder, and wear resistance improves substantially. The process maintains sharp edges and fine details that CNC machining creates, making it ideal for components where both function and appearance matter.
Design Takeaway: Since anodizing converts existing aluminum rather than adding a separate coating, it won’t mask machining marks or hide surface imperfections – plan your pre-anodizing surface finish accordingly.
How Do I Choose the Right Anodizing Type for My Parts?
Type II anodizing provides standard corrosion protection with minimal thickness impact, while Type III hard anodizing offers maximum wear resistance but affects tight tolerances. Your choice depends on whether you prioritize dimensional accuracy or maximum durability.
Type comparison for design decisions:
- Type II: Standard protection, maintains tight tolerances, decorative colors available, lower cost
- Type III: Maximum hardness for wear surfaces, thicker coating impacts dimensions, limited colors
- Both provide excellent corrosion resistance for most applications
From our experience with precision parts across aerospace and medical sectors, we recommend Type II for housings, brackets, and components where appearance and moderate protection matter. Type III works best for sliding surfaces, pivot points, and high-stress applications where durability outweighs dimensional constraints – like aircraft fittings or medical device mechanisms.
Consider your operating environment and function: outdoor exposure or mechanical wear favor Type III’s superior hardness. Indoor electronics, audio equipment, or general industrial components typically need only Type II protection while preserving critical dimensions and color options.
Design Takeaway: Choose Type II when maintaining tight tolerances is critical; select Type III when maximum durability justifies thicker coating and potential design modifications.
How Much Thickness Does Anodizing Add to Parts?
Type II anodizing adds 0.0002-0.0007″ total thickness, while Type III adds 0.0005-0.002″. This coating grows both inward and outward from the original surface, requiring tolerance adjustments for critical dimensions and clearance fits.
Thickness planning considerations:
- Coating builds approximately half inward, half outward from original surface
- Type II: Adjust critical dimensions by ±0.0003″
- Type III: Adjust tight-tolerance features by ±0.001″
- Threaded holes and shafts both affected by coating thickness
In our CNC production experience, holes shrink while external features grow. A 0.250″ hole becomes approximately 0.249″ after Type II anodizing, while a shaft grows to 0.251″. This significantly impacts press fits, bearing seats, and threaded connections where even 0.0005″ affects assembly.
Plan machining allowances before anodizing: machine holes slightly oversize and external features slightly undersize. Critical interfaces may require post-anodizing machining or selective masking to maintain precise fits – we help customers identify these features during design review.
Design Takeaway: Always specify pre-anodizing dimensions on drawings and clearly identify which surfaces need coating thickness compensation versus those requiring exact final dimensions.
What Design Rules Should You Follow for Anodizing?
Design parts with uniform sections, adequate drainage, and accessible surfaces for consistent anodizing coverage. Sharp internal corners and deep recesses can create uneven coating or incomplete coverage that affects part performance.
Essential anodizing design guidelines:
- Minimum 0.030″ radius on internal corners prevents coating buildup and stress concentration
- Include drainage holes for trapped solution – minimum 1/8″ diameter in low areas
- Avoid deep pockets with length-to-width ratios exceeding 3:1
- Position contact points on non-critical surfaces for electrical connections
From finishing complex aerospace and medical components, sharp internal corners accumulate thicker coating layers, potentially causing stress cracking over time. Deep blind holes or narrow slots may receive incomplete coverage due to poor solution circulation, creating weak spots in corrosion protection.
Consider contact witness marks: small marks occur where parts connect to processing fixtures. Position these on hidden surfaces or areas that won’t affect function or appearance. We work with customers during design review to optimize part orientation and minimize visible marking.
Design Takeaway: Add generous radii to internal corners and include drainage features early in design – retrofitting these changes after prototyping increases cost and extends timelines significantly.
Can You Anodize Threaded Holes and Small Features?
Yes, but anodizing coating buildup affects thread fit and may close small features partially. Plan for thread restoration or selective masking on critical threaded connections and narrow slots.
Thread and small feature impact:
- Internal threads: Coating buildup reduces clearance, may prevent proper engagement
- External threads: Increased diameter can cause assembly interference
- Small slots and keyways: Width reduces by coating thickness on both sides
- Fine details remain sharp but gain thickness from oxide layer
From our experience with precision assemblies across medical and aerospace sectors, threaded connections often need attention after anodizing. Standard hardware threads typically require restoration for proper fit, while custom threads can be designed slightly oversized to accommodate coating buildup. We help customers identify critical threads during design review.
Small features like connector slots, keyways, and narrow grooves also shrink from coating on all surfaces. Critical small features often need selective masking during anodizing or post-process restoration to maintain function. Consider whether tight-fitting features truly need anodizing protection or can be left bare.
Design Takeaway: Identify critical threaded and small features early – plan for thread restoration work or consider design alternatives like threaded inserts in anodized housings to avoid post-processing complexity.
What Anodizing Colors Are Available and How Durable?
Natural (clear) and black anodizing offer maximum durability, while decorative colors like blue, red, and gold provide good appearance but may fade with UV exposure. Choose based on your application environment and aesthetic requirements.
Color selection for different applications:
- Natural/Clear: Best corrosion protection, no fade risk, professional appearance
- Black: Excellent durability, hides surface imperfections, most versatile
- Decorative colors: Enhanced appearance but consider fade potential outdoors
- Color matching: Process parts together for consistent appearance across assemblies
From finishing components for audio equipment, medical devices, and outdoor applications, we recommend natural anodizing for maximum long-term performance. Black works excellently when you need professional appearance plus durability. Decorative colors enhance product aesthetics but work best for indoor applications or when periodic refinishing is acceptable.
Consider your brand and market positioning: consumer products often benefit from color differentiation, while industrial equipment typically uses natural or black for reliability perception. Color consistency varies between production runs, so plan accordingly for multi-part assemblies or future replacement parts.
Design Takeaway: Prioritize natural or black anodizing for demanding environments; choose decorative colors when aesthetic value justifies potential maintenance considerations and color variation risks.
Conclusion
Anodizing success depends on choosing the right type, planning thickness allowances, and designing geometry for consistent coverage. Natural and black finishes offer maximum durability, while decorative colors enhance aesthetics with trade-offs. Early design consideration prevents costly post-processing and assembly issues.
Contact us to explore anodizing and CNC manufacturing solutions tailored to your aluminum component requirements.
Frequently Asked Questions
Yes, both alloys anodize well, but 7075 may show slightly different color appearance due to alloy composition. Process parameters and thickness remain consistent between both materials for most applications.
Yes, specify “Type II” or “Type III” per MIL-A-8625 along with color requirements. Include notes about which surfaces need exact final dimensions versus those that can accept coating thickness variation.
No, anodizing follows the existing surface contour and may actually highlight machining marks. Plan appropriate surface finish (Ra 63 μin or better) before anodizing for optimal appearance on visible surfaces.
Yes, threaded inserts work excellently in anodized holes and avoid thread engagement issues. Machine holes to insert manufacturer specifications, then install inserts after anodizing completion for reliable assembly.
Typical anodizing adds 3-5 business days to project timeline, including setup, processing, and quality verification. Complex geometries requiring masking or post-process work may extend timeline further.
Machine holes 0.0005-0.001″ oversize for Type II anodizing, 0.001-0.002″ for Type III. External features should be machined 0.0005″ undersize to compensate for coating buildup on all surfaces.
