Anodizing and fit: specify the finished hole, not just the colour
A pin that fits before finishing may not fit afterwards. Set the functional dimensions, masking boundary and inspection stage together.
Consider a hypothetical aluminum bracket: its locating pin enters smoothly during machining inspection, but will not enter after black anodizing. The purchase order said “black anodize”; the drawing did not say whether the hole size applied before or after treatment. Before asking the machinist to enlarge the next batch, decide which finished surfaces must carry the coating and which dimensions must still work at assembly.
Why a successful trial fit can disappear
Anodizing converts surface aluminum into an oxide layer that grows partly into the metal and partly beyond the original surface. AAC explains that the dimensional result depends on the process, alloy and pretreatment. [1] A machining report therefore cannot establish the dimensions after this later treatment. A coating certificate likewise does not establish the finished fit.
Growth beyond both original bore walls can reduce the space available to the pin. Pretreatment such as etching also affects the final dimensions, so the diagram shows a possible direction of change, not a net allowance. Do not halve a generic coating thickness and use it as a machining adjustment. Ask the processor to review the actual material, complete route and geometry; the designer must approve the finished condition.
- 1Original aluminum surface
- 2Anodic layer at the bore wall
- 3Area designated for masking
Sectional schematic; coating and displacement exaggerated. The two bore walls can affect available diameter. The masked area is a separate design choice, not a promise of unchanged dimensions.
Give each critical dimension a measurement stage
Start with the assembly requirement. Does the pin locate a removable cover, transmit load, or merely guide a screw? Record the required finished limits and any relevant surface or geometric requirements against the feature. A nominal diameter alone does not define an acceptance range. A hand trial with one pin also does not define interchangeability across future batches.
AAC’s specifying guide treats alloy, preparation and anodic finish as specification inputs. [2] For the hypothetical bracket, mark the locating bore as a finished critical feature and identify the pin specification or mating drawing revision. If another dimension is intentionally controlled before treatment, label that stage explicitly. This allows the machining and anodizing quotations to refer to the same deliverable.
Choose a route that preserves the required function
There are several routes to discuss, and none should be selected solely because it removes an assembly complaint. Keeping the coating in the bore may preserve a required surface condition, but needs an agreed allowance and demonstrated finished results. Masking may be appropriate when the design permits an area without anodic coating. Finishing the bore after anodizing is another proposal to assess, rather than an automatic repair instruction.
The comparison below is a hypothetical decision aid, not a processor capability statement. Ask who supplies the finished part, who approves the route and what evidence accompanies it. If machining and treatment are purchased separately, name the party that resolves an out-of-limit finished bore. Otherwise each supplier can deliver its own operation while the buyer still owns an unusable assembly.
| Proposed route | Decision before ordering | Acceptance evidence |
|---|---|---|
| Coat the bore | Decision before orderingAgree the finished limits and process allowance | Acceptance evidenceFinished bore results and required coating evidence |
| Mask the bore | Decision before orderingApprove the un-anodized final surface and boundary | Acceptance evidenceFinished bore results and masking-boundary check |
| Machine after anodizing | Decision before orderingApprove the area where coating will be removed and the final surface condition | Acceptance evidencePost-machining dimensions and accepted surface condition |
Draw the masking boundary where it matters
West-Tech describes selective masking and recommends defining critical boundaries; an area without anodizing may have another specified treatment. [3] On the bracket drawing, identify the full bore length to be protected, the entrance chamfer and the transition to the coated face. “Mask hole” is insufficient if the supplier cannot tell whether the chamfer belongs to the protected area.
Explain why the boundary matters. If the mating pin shoulder seats against the entrance, the transition belongs in the interface review. If it is only visible on an external face, agree an appearance criterion separately. Ask the processor to propose a feasible boundary and its acceptance limits. Do not infer a perfectly sharp edge from a clean CAD colour boundary, or require a boundary precision that no one has quoted.
Check the finished interface, alongside the coating
For the first article, request the marked bore’s dimensional results after the complete agreed route, with drawing revision, feature identification and measurement method. Discuss whether diameter at one location is enough, or whether length, form and access require additional checks. Coating thickness evidence should identify its agreed test location and method; do not assume that a reading on an accessible outer face proves every internal location.
Use the specified mating component or an agreed gauge for the functional check, under defined conditions. Keep that result alongside dimensional evidence: “the pin goes in” does not tell you whether the fit is too loose. Agree sampling and the release decision before the batch. A satisfactory first article supports that route and sample; it does not remove the need for the agreed production acceptance checks.
Ask for one coordinated answer before comparing price
Send the annotated drawing to the machining and finishing parties together. Request the proposed process, masking scope, final dimensional responsibility and inspection evidence in one response. If the answer only repeats “black anodizing available”, the finished-fit question remains open. Keep the uncertain item visible in the quotation comparison rather than treating it as included.
Compare price after these assumptions align. Separate a quoted masking operation, final inspection, first-article work and any proposed post-treatment machining. If the supplier suggests widening a drawing limit, that is a design decision for approval, not a purchasing clarification. When several routes meet the accepted function, the useful comparison becomes their total work, handoffs and acceptance evidence.
A question to send with the marked drawing
Open supplier questions
For the highlighted locating bore, please confirm whether the specified limits will be met after the complete proposed finishing route. State the alloy and drawing revision used, anodizing specification and agreed revision, coating requirements, bore/chamfer masking boundary, and any proposed machining after treatment. Please identify who is responsible for final dimensional acceptance and what first-article and batch inspection evidence is included. Flag any drawing change needed before manufacture; do not alter the limits without approval.
Sources and scope
- Aluminum Anodizers Council — Anodized Aluminum FAQ
AAC FAQ: dimensional change and pretreatment discussion; general guidance, not a machining allowance for this part.
- Aluminum Anodizers Council — Specifying Anodized Aluminum
AAC specifying guide: material and finish inputs. Its historical specification references do not establish a current contractual revision.
- West-Tech Finishing — What Is Masking in Anodizing and Why Is It Important?
West-Tech’s own masking explanation: selective areas and boundaries. It does not establish another supplier’s capability or the example’s acceptance limits.