Machining tolerances after anodizing define the dimensions a finished CNC aluminum part must meet after its surface treatment. If a bore, thread or mating face controls assembly, mark the required condition on the drawing and agree how the supplier will check it. A bore can measure correctly when it leaves the machine and still be too small for its mating part after anodizing.
This guide follows a hypothetical CNC-machined aluminum housing with a precision bore and two blind threaded holes. It shows which dimensions belong on the customer drawing, which machining targets the supplier needs to develop, and what evidence to ask for before approving the first article.
1. Put machining tolerances after anodizing on the drawing
Start with the mating parts. Identify the bore that locates a pin or bushing, the face that controls seating, and the threads that hold the assembly together. Then mark the inspection condition for those features. If a finished fit matters, the drawing should make that fit assessable on the delivered part.
Use a feature-specific note when the drawing includes both coated and masked regions. A general instruction to anodize the part does not identify which faces must retain their machined dimensions, where coating can stop, or whether the supplier may leave a functional surface bare. If a manufacturing drawing includes a pre-anodize size, label that size clearly and keep the final acceptance requirement separately identifiable.
| Feature | Drawing decision | What to verify |
|---|---|---|
| Precision locating bore | Coating required in the bore. Finished diameter Ø20.000 ±0.010 mm in this example. | Confirm the process can deliver the required coated bore. Measure at the agreed depth and locations after finishing. |
| Two blind threaded holes | Mask only if bare threads are approved. Identify both holes and the permitted mask boundary. | Check the threads using the agreed gauging procedure after finishing. Verify full usable depth separately when required. |
| Mating face | State whether coating is required. Identify any final size or geometry that affects assembly. | Inspect the specified feature in the finished condition and reference the drawing datums where applicable. |
| Visible outer face | Mark cosmetic surfaces and any restrictions on rack contact. | Compare appearance and contact marks with the agreed acceptance criteria. |
The drawing identifies the functional outcome. The supplier then reviews workholding, tool access, coating and measurement as one process. Easiahome’s CNC design for manufacturing checklist covers the other checks to complete before releasing a drawing.
2. Calculate bore allowance using surface growth
Part of the anodic layer grows into the original aluminum surface and part builds outward from it. The outward growth changes the size of the feature. On a simple cylindrical bore, inward encroachment into the opening from both walls reduces its diameter. On a coated external cylinder, outward growth on opposite sides increases its diameter.
The design information in MIL-PRF-8625 discusses dimensional growth, including small holes and threads. The Aluminum Anodizers Council’s technical FAQ explains that alloy, process conditions and pretreatment can affect dimensional change. Use a range verified for the actual part geometry and finishing route when setting a machining target.
Worked example: a coated Ø20 mm bore
Suppose the buyer needs a final bore of Ø20.000 ±0.010 mm. For this illustration, assume the supplier has established an effective inward change of 0.008–0.012 mm per wall, measured from the pre-finish condition to the finished condition. This is an assumed dimensional change for the example. Confirm your own range with the anodizer and trial parts.
With comparable growth at opposite walls, the simplified calculation is finished bore diameter = pre-finish bore diameter − twice the effective growth per wall. A nominal pre-finish diameter of 20.020 mm offsets a nominal change of 0.010 mm on each wall.
| Check | Input or calculation | Result |
|---|---|---|
| Final drawing limits | 20.000 ±0.010 mm | 19.990–20.010 mm |
| Illustrative machining window | 20.020 ±0.004 mm before finishing | 20.016–20.024 mm |
| Smallest estimated finished bore | 20.016 − 2 × 0.012 mm | 19.992 mm |
| Largest estimated finished bore | 20.024 − 2 × 0.008 mm | 20.008 mm |
The arithmetic leaves only 0.002 mm of margin at each limit. Check that margin against actual finishing variation, bore form and the measurement method before committing to a production tolerance. A tighter drawing, a deeper bore or a different coating route can change the decision. Quote the complete route and verify the finished feature on trial parts.
3. Decide how to protect thread fit
Thread fit depends on the flanks and the thread geometry. The simple bore calculation above applies to a plain cylinder. For an internal thread, agree how the supplier will preserve the specified thread fit and depth after surface treatment.
- Mask the threaded holes when the design permits bare aluminum in those locations. Identify the holes and the limits of the uncoated area. Consider whether corrosion protection, appearance or electrical requirements affect that decision.
- Coat the threads with a planned allowance when coating is required inside the holes. Ask the supplier to confirm feasibility with its anodizer, including the thread geometry and coating behavior in blind features. Check the finished thread with the agreed gauges.
- Approve a post-finish operation separately if the intended fit requires further work. Define the permitted extent and any requirements for surfaces where the operation removes coating.
For the example housing, the buyer might add: Mask the two threaded holes identified in Detail B. Keep the mask boundary within the area shown. Verify thread fit after finishing to the drawing requirements. Use the note only when the engineering drawing actually permits the proposed mask. Include the required full-thread depth for blind holes where fastener engagement matters.
4. Agree how the finished part will be inspected
For machining tolerances after anodizing, check the features that control assembly in the condition specified on the drawing. For a precision bore, agree on the measurement method, the locations along the bore and whether size and form have separate requirements. A single diameter reading can miss a difference between the entrance and deeper sections. Use a calibrated method that is suitable for the tolerance and access available.
For threaded holes, agree which gauges and acceptance rules apply. If the design also specifies a minimum length of complete thread, plan a suitable depth check. If the order requires coating thickness evidence, agree where thickness can be measured and how inaccessible internal surfaces will be addressed. A thickness reading on an external face does not establish the coating on every internal surface.
For the first article, ask for the drawing revision, sample identification, the inspection condition and the measured results against the marked features. If useful, request paired pre-finish and finished bore readings on the same samples to establish the dimensional change. Easiahome’s buyer’s guide to CNC inspection reports explains how to review the results and trace them to the correct drawing.
5. Send an RFQ that covers machining, finishing and inspection
Ask suppliers to price the same finished part. One quotation may include masking, coating and post-finish measurement, while another may cover machining and a standard finish. The difference matters when a coated bore controls an assembly fit.
- Attach the current STEP model and the controlled 2D drawing. Include the alloy and temper, quantities and drawing revision.
- State the coating specification and the required type, thickness, color or other details as applicable to the order.
- Mark the critical finished sizes, threads, datums, masked regions and cosmetic surfaces.
- Identify the required inspection records and whether you need a trial or first article before production.
- Ask the supplier to itemize special masking, inspection and trial work, and flag any tolerance assumptions in its quote.
For the complete file list, see Easiahome’s guide to preparing drawings for a CNC machining quote.
Frequently asked questions
Does anodizing make a machined hole smaller?
On a bore whose walls receive coating, the layer’s growth into the opening generally reduces the diameter. The net change also depends on the pretreatment, how well the bore receives coating and the process used. Verify it on the actual geometry when the fit is critical.
Should I dimension a part before or after anodizing?
Mark the condition that governs each critical feature. If the delivered fit matters, define the finished requirement and ask the supplier to propose its pre-finish target. Where both dimensions are contractual, distinguish them clearly on the applicable drawing.
Can I use the same allowance for Type II and Type III anodizing?
Confirm the effective growth for the specified process, alloy and geometry. The total film thickness and outward growth can differ with the finishing route, so reuse an allowance only when the process evidence supports it. See the hard coat anodizing guide for more context on Type III.
Should every threaded hole be masked?
Choose the mask plan based on the thread fit, permitted coating coverage and the part’s service conditions. Show the exact regions on the drawing and agree when to check thread fit.
Request a review of your anodized CNC part
Send your STEP file and 2D drawing through the Easiahome contact page. Highlight any precision bores, threads or mating faces that must meet tolerance after anodizing. Include the coating specification, quantity and inspection requirements so the team can review the machining, masking and final inspection scope before quoting.
