Anodizing is one of the most common finishing processes for CNC machined aluminum parts. It improves corrosion resistance, creates a harder surface, and allows for color options. But anodizing isn't just something that happens after machining—it affects decisions made during the design and machining phases. If you're planning to anodize aluminum components, there are several factors to consider before the first chip flies.
Understanding how anodizing interacts with machining helps you specify parts correctly, avoid dimensional issues, and get the surface quality you need. This article covers what to think about when CNC machining aluminum that will be anodized, from the perspective of a machine shop.
Choosing the Right Aluminum Alloy
Not all aluminum alloys anodize the same way. The alloy you select affects both the anodizing process and the final appearance.
Common Alloys for Anodizing
The 6000-series alloys, particularly 6061-T6, are widely used for anodized parts. They machine well, anodize consistently, and produce a uniform finish. The 5000-series alloys also anodize effectively and offer good corrosion resistance.
The 2000-series and 7000-series alloys contain higher copper or zinc content, which can create a less uniform anodized finish. These alloys may show color variation or a darker appearance after anodizing. If cosmetic appearance matters, stick with 6061 or 5052.
Material Consistency
When anodizing multiple parts that need to match, use aluminum from the same lot or heat when possible. Variations in alloy composition, even within specification, can result in slight color differences after anodizing. This becomes especially important for visible assemblies where parts sit side by side.
Accounting for Dimensional Changes
Anodizing builds up a layer on the aluminum surface. Unlike paint or powder coating, roughly half the anodized layer grows into the base material and half grows outward. This affects final dimensions.
Anodizing Thickness
Type II anodizing (sulfuric acid anodizing) typically produces a coating between 0.0002" and 0.001" thick. Type III (hard anodizing) creates a thicker layer, usually 0.001" to 0.003" or more.
Because the coating grows both inward and outward, the net dimensional change is approximately half the total coating thickness added to each surface. For a 0.001" Type II coating, expect about 0.0005" growth per surface.
Adjusting Machined Dimensions
Critical dimensions need to account for anodizing buildup. If a hole must be 0.500" diameter after anodizing with a 0.001" coating, machine it approximately 0.501" diameter. The anodizing will reduce the hole size by about 0.001" (0.0005" per side).
External dimensions grow. A shaft machined to 0.500" diameter will measure approximately 0.501" after the same anodizing process.
Communicate clearly on your drawing which dimensions are critical and whether they apply before or after anodizing. The drawing typically serves as the controlling specification, so note "dimensions after anodizing" or "dimensions before anodizing" to avoid confusion.
Tolerance Considerations
Anodizing thickness varies slightly across a part and between parts in the same batch. If you're working with tight tolerances, factor in both the nominal coating thickness and its variation.
For features with tolerances tighter than ±0.002", specify whether the tolerance applies before or after anodizing. Some parts may require post-anodize machining or masking of critical surfaces to maintain tight tolerances.
Surface Finish and Cosmetic Appearance
Anodizing doesn't hide surface imperfections—it highlights them. The anodized layer is transparent, so any scratches, tool marks, or blemishes on the machined surface will remain visible and often become more pronounced.
Machined Surface Quality
If appearance matters, specify an appropriate surface finish on your drawing. A finish of 63 Ra or better generally anodizes well for functional parts. For cosmetic or visible surfaces, 32 Ra or finer produces better results.
Tool marks, chatter, and feed lines will show through anodizing. Parts that will be visible in an assembly may need bead blasting or other surface preparation before anodizing to create a uniform matte appearance.
Handling and Fixturing Marks
CNC machining requires holding the part in a vise, fixture, or chuck. These contact points can leave marks. On parts where all surfaces will be visible after anodizing, discuss fixturing strategy with your machine shop.
Sometimes a secondary operation or alternative workholding method minimizes visible marks. Other times, small witness marks are unavoidable and acceptable. Clarify your expectations early.
Bead Blasting and Surface Prep
Many anodized parts receive bead blasting before anodizing to create a uniform, matte surface. This process removes minor tool marks and creates a consistent texture.
If you want a bead-blasted finish, specify it on the drawing or in your RFQ notes. The machine shop needs to know whether they should optimize for a machined finish or if bead blasting will follow.
Masking and Selective Anodizing
Some features shouldn't be anodized. Threaded holes, precision bores, mating surfaces, and electrical contact areas often require masking.
Threads
Anodizing adds thickness to threads, which can cause assembly issues. Internal threads may become undersized and difficult to engage. External threads grow and may not fit into mating parts.
Options include masking threads during anodizing, chasing threads after anodizing, or machining threads oversize to account for coating buildup. The best approach depends on thread class, quantity, and application.
Specify on your drawing which threads are critical and whether they should be masked or chased after anodizing.
Precision Fits
Press fits, slip fits, and other precision mating features usually need masking. The anodizing shop applies plugs, tape, or other masking materials to prevent coating buildup on these surfaces.
Identify these features clearly on your drawing. Call out which surfaces require masking or note "no anodize" on specific dimensions.
Electrical Contact Surfaces
Anodizing creates an insulating layer. Surfaces that require electrical conductivity must be masked or machined after anodizing.
Color and Finish Options
Anodizing allows for various colors, though natural clear anodizing is most common for functional parts. Color anodizing uses dyes absorbed into the porous anodized layer before sealing.
Color consistency varies. Black anodizing is generally uniform. Other colors, especially lighter shades, can show variation between parts or batches. If color matching is critical, discuss expectations with both the machine shop and anodizing vendor.
Hard anodizing (Type III) typically produces a dark gray to black finish and is chosen for wear resistance rather than appearance.
Communicating Requirements to Your Machine Shop
Clear communication prevents problems. When requesting a quote for CNC machined aluminum parts that will be anodized, provide complete information.
Essential RFQ Information
A complete RFQ for machined and anodized parts should include:
- Engineering drawing or CAD file: A detailed drawing with dimensions, tolerances, and notes. STEP files are useful for programming but the drawing typically serves as the controlling specification.
- Material specification: Call out the specific aluminum alloy (e.g., 6061-T6). Mention if material certification or traceability is required.
- Quantity: Specify the number of parts needed. Prototype and production quantities may use different machining approaches.
- Tolerances: General tolerances and any critical dimensions. Note whether tolerances apply before or after anodizing.
- Surface finish requirements: Specify Ra values or finish callouts for critical surfaces.
- Anodizing specification: Type II or Type III, color if applicable, and coating thickness if critical.
- Masking requirements: Identify threads, bores, or surfaces that should not be anodized.
- Required completion date: When you need the finished parts.
- Inspection requirements: Any specific inspection, documentation, or reporting needed.
- Special notes: Cosmetic requirements, assembly considerations, or other relevant details.
Drawing Notes for Anodized Parts
Include relevant notes directly on your drawing:
- "All dimensions after anodizing" or "All dimensions before anodizing"
- "Type II anodize, clear" or "Type III hard anodize, natural"
- "Mask threads" or "Chase threads after anodizing"
- "Bead blast before anodizing"
- "All surfaces cosmetic" (if appearance is critical)
These notes reduce ambiguity and help the machine shop plan the job correctly.
Prototype Versus Production Considerations
The quantity you need affects both machining and anodizing.
Prototype Quantities
Smaller quantities allow for faster turnaround but may have higher per-piece costs. Anodizing shops often have minimum batch charges, so anodizing a few prototype parts can be relatively expensive per piece.
For prototypes, consider whether anodizing is necessary for testing or if it can wait until production. If you're validating fit and function, you might machine and test parts first, then anodize production quantities later.
Production Quantities
Larger quantities justify more elaborate fixturing and process optimization. Anodizing costs per piece decrease with volume.
If you're transitioning from prototype to production, review any dimensional or cosmetic issues that appeared during the prototype phase. Adjustments to machining dimensions, surface finish, or masking requirements are easier to implement before committing to a large production run.
Working With Your Machine Shop and Anodizer
Some machine shops handle anodizing in-house. Others work with trusted anodizing vendors. Either way, coordination between machining and finishing is important.
If your machine shop manages anodizing, they'll coordinate the process and ensure parts meet your specifications. If you're managing anodizing separately, make sure both the machine shop and anodizer understand the requirements.
Discuss any concerns or questions early. Experienced shops can suggest design modifications or specification adjustments that improve results or reduce cost.
Practical RFQ Checklist for Anodized Aluminum Parts
Use this checklist when requesting a quote for CNC machined aluminum parts that will be anodized:
- Engineering drawing or STEP file provided
- Aluminum alloy specified (e.g., 6061-T6)
- Quantity clearly stated
- General and critical tolerances identified
- Dimensions noted as before or after anodizing
- Surface finish requirements specified
- Anodizing type and color indicated (Type II, Type III, clear, black, etc.)
- Masking or post-anodize machining requirements noted
- Cosmetic or appearance expectations communicated
- Required delivery date provided
- Any inspection or documentation requirements listed
Request a Quote for Your Anodized Aluminum Parts
Planning a project that involves CNC machining aluminum for anodizing? Anco Precision machines aluminum components with anodizing in mind, accounting for dimensional changes, surface finish requirements, and finishing specifications.
When you're ready to request a quote, send your drawing or STEP file, material specification, quantity, tolerances or critical requirements, and required delivery date. Clear information helps us provide an accurate quote and deliver parts that meet your expectations.
Contact Anco Precision to discuss your project and get your machined and anodized aluminum parts into production.