Key decisions
- Coating thickness and outward dimensional growth are different quantities.
- Bores and outside diameters involve changes on two opposing surfaces.
- Confirm final fit, masking and local coating measurement rather than relying on a generic growth rule.
Treat anodizing as part of the component definition
An anodized part is not simply a finished machined part with a color added. The surface treatment can change the interfaces that locate, slide or seal, and the preparation before anodizing can affect surface condition. For a close fit, the machining and finishing requirements must be planned together.
Define what the finish is intended to achieve: appearance, wear behavior, corrosion protection or another requirement. Then specify the applicable process requirements and the final dimensions. A short note such as black anodize may be sufficient for an early visual discussion, but it leaves too many variables open for a controlled production order.
The published NASA process specification and Aluminum Anodizers Council guide provide useful examples of coating classifications and design considerations. Their process-specific requirements do not automatically apply to every commercial component. The purchase order must establish the actual agreed scope.
Distinguish total coating thickness from surface growth
Anodizing converts part of the aluminum surface while building an oxide layer. Total coating thickness therefore differs from the outward movement of the surface. The AAC and NASA discussions of hard anodizing describe approximately half-thickness growth as a useful context-specific model; it is not a universal rule for every anodizing process, alloy and geometry.
For dimensional planning, define a growth fraction g and coating thickness t only after reviewing the actual process. A simple single-surface allowance is g × t. An outside diameter with two opposing treated surfaces increases by approximately 2 × g × t, while a uniformly treated bore decreases by that amount. This model does not include material removed during pretreatment.
| Feature | Possible dimensional effect | Planning question |
|---|---|---|
| External locating diameter | Growth on opposing surfaces increases diameter | Is the finished maximum size compatible with the mating bore? |
| Internal bore or slot | Growth toward the opening reduces clearance | Is local internal coating behavior known rather than assumed from an outside face? |
| Thread | Surface change alters mating geometry | Should the thread be masked, compensated or produced by another approved sequence? |
| Datum or seating face | Surface preparation and coating affect the final contact | Which condition establishes the inspection datum? |
| Masked electrical or fit interface | The selected area remains outside the main coating route | What boundary, protection and final acceptance are required? |
Worked example: budget a bore after coating
The following arithmetic uses hypothetical, process-specific assumptions. The assumptions are not a production specification or a universal growth rule.
The engineering response is to obtain relevant process information and revise the plan. Options may include a different permissible finish requirement, masking, an approved post-finish operation or a change to the fit allocation. Simply machining to the nominal compensation and hoping the coating stays centered does not establish conformance at the extremes.
A nominal 10 mm finished bore
Assume coating thickness t = 20 ± 4 micrometers and, solely for this exercise, a fixed growth fraction g = 0.5 on both sides of a bore. Diametral reduction is then 20 ± 4 micrometers. A precoat bore of 10.020 ± 0.005 mm gives a nominal finished bore of 10.000 mm and a worst-case arithmetic variation of ±0.009 mm. If the final limit is ±0.010 mm, only 1 micrometer of that simplified budget remains. Pretreatment removal, internal coating variation and measurement effects have not been included, so the calculation is insufficient to approve the real process.
Resolve local geometry and masking before release
NASA notes that coating inside a narrow or complex cavity need not equal coating on the outside. A thickness result from an accessible external surface therefore cannot automatically establish the dimensions of an internal interface. Mark critical locations where a local requirement matters and agree on how it can be verified.
Electrical rack contact also needs a location. If the contact area would be unacceptable on a visible face or functional interface, show the allowed area on the drawing. Masking boundaries should likewise be dimensioned or illustrated clearly enough that different operators can interpret them consistently.
Post-finish machining changes the delivered surface system. It may expose aluminum or remove part of the intended protection. The designer should assess the resulting corrosion, electrical and appearance consequences and specify any additional treatment. A finishing supplier should not be expected to choose that tradeoff without approval.
Separate cosmetic approval from dimensional acceptance
A color sample can help define appearance, but it does not prove coating thickness or final fit. Specify whether a visual master controls color, texture, gloss or acceptable variation, and establish viewing conditions when the application needs them. An image on a screen is a weak substitute for an agreed physical sample where close matching matters.
For dimensions, distinguish precoat checks used to control manufacturing from final checks used for acceptance. A part may satisfy its machining allowance and still fail after finishing. If only selected final features require a report, identify them explicitly rather than leaving the supplier to infer the critical surfaces.
Send Beigain the finish specification with the model and drawing at the initial RFQ stage. Introducing masking, local thickness checks or a cosmetic master after the quotation can change both the route and the documentation effort.
- Specify alloy, temper and the relevant anodizing requirements.
- Identify the final dimensions and any separately controlled machining allowances.
- Define critical coating locations, masked areas and allowed rack contacts.
- Confirm how internal features and threads are treated and inspected.
- Separate color or texture approval from thickness and dimensional evidence.
- Assess exposed surfaces created by post-finish machining.
- Record the approved finish route and drawing revision with the order.
Questions about this topic
Does anodizing always increase a dimension by half the coating thickness?
No. Half-thickness growth is a context-specific approximation often discussed for hard anodizing. The real allowance depends on process, alloy, pretreatment and geometry, and diameters involve two surfaces.
Can the anodizer preserve a precision bore by masking it?
Masking may be a useful option, but the uncoated surface and boundary need design approval. Define the final size, protection and acceptance requirements instead of assuming masking solves every issue.
Can an outside coating measurement predict an internal bore size?
Not reliably without relevant process evidence. Internal current distribution and access can produce different coating behavior. Critical internal features need their own planning and verification.
Should a dimensional report be made before or after anodizing?
Precoat inspection can control the process, while final acceptance should follow the drawing requirement for the delivered condition. Specify both where the tolerance budget makes them necessary.
Technical references
- Aluminum Anodizers Council: Anodizing Reference Guide — Industry-association reference for coating types, alloy response and hard-anodize growth context; historical specification references are not automatic contractual requirements.
- NASA/JSC PRC-5006 Rev. D: Process Specification for the Anodizing of Aluminum Alloys — Primary process specification discussing surface preparation, contact locations and geometry-dependent hard-anodize dimensions. Its project-specific defaults are not adopted as Beigain capabilities.
Continue reading
- Surface Finishing Services
- Aluminum CNC Machining
- Surface finishing and critical dimensions: a specification guide
- Quality Control & Inspection
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