Anodizing is one of the most common finishing processes for CNC machined aluminum parts. It provides corrosion resistance, improves surface hardness, and allows for color options that range from clear to black and various dyed colors. However, anodizing introduces specific considerations that affect both the machining process and how you communicate your requirements to a machine shop.
When you're planning aluminum parts that will be anodized, several factors come into play before the first chip is cut. Material selection, dimensional tolerances, surface finish requirements, and clear communication with your machining partner all influence the final result. Understanding these considerations from the machine shop's perspective helps ensure your parts meet expectations.
Material Selection for Anodized Aluminum Parts
Not all aluminum alloys anodize the same way. The alloy you choose affects both the anodizing process and the final appearance of your parts.
Common Alloys and Anodizing Characteristics
6061 aluminum is widely used for parts that will be anodized. It machines well, anodizes consistently, and produces a uniform gray appearance when clear anodized. This alloy is suitable for most structural and functional applications where appearance consistency matters.
7075 aluminum, known for its high strength, anodizes with a slightly different appearance than 6061. The higher copper content can result in a less uniform color, particularly when clear anodized. If cosmetic appearance is critical, this characteristic should be considered during material selection.
5052 aluminum anodizes well and is often chosen for applications requiring excellent corrosion resistance. It's commonly used in marine environments and chemical processing equipment.
2024 aluminum contains copper as a primary alloying element, which makes it difficult to anodize with consistent cosmetic results. While it can be anodized for corrosion protection, the appearance will typically be less uniform than 6061 or 5052.
Specifying Material on Your RFQ
When requesting a quote for aluminum parts that will be anodized, specify both the alloy and temper. For example, "6061-T6" provides complete information. If cosmetic appearance after anodizing is important, mention this requirement in your RFQ so the machine shop can recommend the most appropriate alloy if you're uncertain.
Dimensional Considerations and Tolerances
Anodizing adds a measurable layer to aluminum surfaces. Understanding how this affects your dimensions is essential for parts with tight tolerances.
How Anodizing Affects Dimensions
The anodizing process converts the surface aluminum into aluminum oxide, which grows both into the base material and outward from the original surface. Type II anodizing (the most common) typically builds up approximately 0.0001" to 0.0003" per surface. Type III (hard anodizing) builds a thicker layer, often 0.0005" to 0.002" or more per surface.
This means that external dimensions grow slightly, while internal dimensions (holes, slots, pockets) shrink slightly. For a hole, the diameter decreases by roughly twice the anodize thickness since the coating forms on all surfaces.
Accounting for Anodize Buildup
When dimensioning parts for anodizing, you have two approaches:
- Dimension the part as-anodized: The drawing shows final dimensions after anodizing. The machine shop machines the part oversize or undersize as needed to account for the anodize buildup. This approach works well when the drawing represents the functional requirements of the finished part.
- Dimension the part as-machined: The drawing shows dimensions before anodizing, with notes indicating which features are critical after anodizing. This approach requires clear communication about post-anodize requirements.
The drawing typically serves as the controlling specification. If critical dimensions must be held after anodizing, call this out clearly on the drawing or in accompanying notes. Specify whether tolerances apply before or after the anodizing process.
Threaded Features
Threads require special attention when parts will be anodized. The anodize coating reduces the effective thread size, which can cause assembly issues.
For internal threads, standard practice is to mask threads before anodizing or chase them after anodizing to restore proper fit. For external threads, masking is common, or threads may be machined oversize to compensate for the anodize buildup.
Indicate on your drawing or RFQ which threaded features must remain functional after anodizing. Specify if threads should be masked, chased, or machined to accommodate the coating.
Surface Finish and Cosmetic Requirements
Anodizing does not hide surface imperfections—it often makes them more visible. The anodize layer is translucent, and any scratches, tool marks, or blemishes on the machined surface will show through and may appear more pronounced after anodizing.
Machining for Cosmetic Surfaces
If appearance matters, specify surface finish requirements on your drawing. A callout such as "32 Ra" or "63 Ra" provides measurable guidance. For visible surfaces where cosmetic appearance is critical, consider specifying finer finishes or noting "cosmetic surface" on the drawing.
Machining operations affect surface appearance differently. Turning and milling operations leave characteristic tool marks. For parts where uniform appearance is important, secondary operations such as bead blasting or polishing may be performed before anodizing to create a consistent surface texture.
Handling and Fixturing Marks
CNC machining requires workholding, which can leave marks on the part surface. For parts that will be anodized, these marks may be visible in the final finish, particularly on cosmetic surfaces.
If certain surfaces must be free of fixturing marks, indicate this on your drawing. The machine shop can plan setups to minimize marks on critical surfaces or use soft jaws and protective materials where appropriate. Keep in mind that completely eliminating all handling marks may not be practical for every part geometry.
Communicating Anodizing Requirements to Your Machine Shop
Clear communication about finishing requirements prevents misunderstandings and ensures the machined part is prepared correctly for anodizing.
What to Include in Your RFQ
When requesting a quote for aluminum parts that will be anodized, provide the following information:
- Engineering drawing or CAD file: A drawing or STEP file shows the part geometry and serves as the controlling specification. Include all relevant views, sections, and detail callouts.
- Material specification: Specify the aluminum alloy and temper (e.g., 6061-T6, 7075-T651).
- Quantity: Indicate whether this is a prototype run or production quantity, and specify the number of parts needed.
- Tolerances: General tolerances should be noted on the drawing. Call out any critical dimensions and specify whether tolerances apply before or after anodizing.
- Surface finish requirements: Specify surface finish where it matters, particularly for cosmetic surfaces.
- Anodizing details: Indicate the type of anodizing (Type II, Type III), color if applicable, and any masking requirements for threads or specific features.
- Required completion date: Provide a realistic timeline for when you need the parts.
- Inspection requirements: If you require specific inspection documentation, dimensional reports, or material certifications, specify these in your RFQ. Some customers may request compliance with particular industry standards—communicate these requirements upfront.
- Special notes: Include any additional information that affects machining or finishing, such as "cosmetic surfaces," "no fixturing marks on top surface," or "threads must be functional after anodizing."
Notes on the Drawing
The most effective way to communicate finishing requirements is through notes on the engineering drawing. Consider including:
- "Part to be anodized Type II per MIL-A-8625, clear"
- "Dimensions apply after anodizing unless otherwise noted"
- "Mask threads before anodizing" or "Chase threads after anodizing"
- "Cosmetic surface—minimize tool marks and handling marks"
- "Critical dimension after anodizing: [specify dimension and tolerance]"
These notes provide clear direction and reduce the need for follow-up questions.
Prototype Versus Production Considerations
Whether you're ordering prototype parts or production quantities affects several aspects of the machining and finishing process.
Prototype Runs
Prototype quantities allow you to verify fit, function, and appearance before committing to larger production runs. For parts that will be anodized, prototypes help confirm that dimensional tolerances account correctly for anodize buildup and that the surface finish meets cosmetic requirements.
When ordering prototypes, consider requesting one or two parts with anodizing to evaluate the final appearance and fit, particularly if cosmetic finish or tight tolerances are involved. This can reveal issues that aren't apparent from the machined part alone.
Production Runs
Production quantities benefit from process refinement based on prototype results. If dimensional adjustments are needed to account for anodize buildup, these can be incorporated into the production setup.
For production runs, consistent material sourcing becomes important. Variations in aluminum alloy composition, even within the same alloy designation, can affect anodizing appearance. If color consistency across production batches is critical, communicate this requirement.
Practical RFQ Checklist for Anodized Aluminum Parts
Use this checklist when preparing your request for quote:
- Engineering drawing or STEP file provided
- Aluminum alloy and temper specified
- Quantity clearly stated
- General tolerances noted on drawing
- Critical dimensions identified with tolerances
- Clarification whether dimensions apply before or after anodizing
- Surface finish requirements specified for critical surfaces
- Anodizing type and color indicated (Type II, Type III, clear, black, etc.)
- Threaded features identified with masking or post-anodize requirements
- Cosmetic surface requirements noted
- Required delivery date provided
- Inspection or documentation requirements specified
- Any special handling or fixturing notes included
Working with a Machine Shop That Understands Anodizing
Not all machine shops have extensive experience with parts destined for anodizing. A shop familiar with the process understands how to machine parts to account for anodize buildup, how to prepare surfaces for cosmetic finishing, and how to communicate effectively with anodizing vendors.
When evaluating machine shops for your anodized aluminum parts, ask about their experience with anodized components. Shops that regularly produce parts for anodizing can often identify potential issues during the quoting process and suggest solutions before machining begins.
Request a Quote for Your Anodized Aluminum Parts
If you have an active project involving CNC machined aluminum parts that will be anodized, Anco Precision can help. We machine aluminum components for customers across various industries and understand the considerations involved when parts will undergo finishing processes.
To request a quote, send your drawing or STEP file when available, along with material specification, quantity, tolerances or critical requirements, and your required delivery date. We'll review your project and provide a quote based on your specifications.
Clear communication about your anodizing requirements from the start helps ensure your parts are machined correctly the first time. Contact Anco Precision with your project details, and we'll work with you to produce aluminum parts ready for successful anodizing.