Anodizing Thickness: How Much Does It Add?

Anodized and raw CNC parts with coating thickness inset

A CNC-machined aluminum part can meet its machining tolerances before finishing and still fall out of tolerance after anodizing. But anodizing can change what inspection finds afterward. The process forms aluminum oxide from the metal surface. It creates a coating whose growth must be considered during dimensional planning. When engineers understand anodizing thickness, they account for that change when specifying machined features, tolerances, coating requirements, and final inspection criteria.

For CNC aluminum parts, JLCCNC uses different typical coating thicknesses depending on the anodizing finish. Natural-color anodizing is around 0.008 mm, while other anodizing colors are around 0.015 mm. Hard coat anodizing is typically around 0.025–0.030 mm, and conductive anodizing is around 0.003 mm. These are typical values for planning; the required finish and thickness should be confirmed for the specific part and drawing.

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What Does Anodizing Thickness Mean

Anodizing thickness is the thickness of the aluminum oxide layer formed during anodizing. It is not the same as dimensional growth: part of the oxide develops into the aluminum surface, while the remaining portion extends beyond the original surface. Under MIL-PRF-8625F design guidance, each coated surface can be expected to increase in dimension by approximately half of the applied coating thickness, although actual growth varies with process conditions and alloy.

Coating Thickness vs Dimensional Growth

Anodizing thickness is the measured thickness of the aluminum oxide layer formed during anodizing. It can be measured in micrometres or mils using methods such as eddy-current measurement under ASTM B244. Coating thickness and dimensional growth are not the same value: part of the oxide layer develops within the original aluminum surface, while the remainder extends outward. The amount of dimensional change therefore depends on the portion of the coating that builds beyond the original surface.

How Much Does Anodizing Change Part Dimensions

How to Calculate Anodizing Dimensional Change

To estimate how much anodizing changes a finished dimension, first determine the specified coating thickness and then estimate the portion that builds outward from the original aluminum surface. A simple engineering calculation may use a growth factor for the selected anodizing process, but the actual growth ratio varies with alloy and process conditions. For tight-tolerance CNC parts, the anodizer's demonstrated process behavior should be used instead of treating a generic growth factor as a guaranteed result.

Consider a CNC aluminum part specified for hard coat anodizing at a typical coating thickness of 0.025–0.030 mm. Using a 50% outward-growth estimate for planning, approximately 0.0125–0.015 mm of the coating would build outward from each coated surface. Across two opposite coated surfaces, the estimated overall dimensional shift would be approximately 0.025–0.030 mm.

For a 20.000 mm finished bore with a 0.025–0.030 mm hardcoat, the 0.5× growth assumption gives approximately 0.0125–0.015 mm of outward growth on each wall. The bore therefore closes by about 0.025–0.030 mm, giving an initial pre-anodize machining target of approximately 20.025–20.030 mm. The final machining allowance should still be confirmed against the anodizer's demonstrated process capability when the bore tolerance is tight.

Effects on Holes, Threads, and Mating Fits

The direction of the change depends on the feature. An outside diameter becomes larger. A coated bore becomes smaller as oxide develops on both walls. Threads need added care. MIL-PRF-8625F warns that anodizing affects both internal and external thread dimensions. For a 60° thread, MIL-PRF-8625F states that the pitch diameter increases by four times the surface-growth amount. Under a 0.5× growth assumption, a 0.025–0.030 mm coating corresponds to about 0.050–0.060 mm of pitch-diameter change. Because the functional effect differs between internal and external threads, thread fit should be evaluated in the final anodized condition rather than by applying the same clearance rule to both.

Type II Anodizing Thickness Range

Typical Type II Thickness

MIL-PRF-8625F lists 0.00007–0.0010 in (approximately 1.8–25.4 µm) as the Type II coating thickness range in its design information. This range should not be treated as a universal production target. The required coating condition should be taken from the drawing, purchase order, or applicable specification.

For JLCCNC's typical CNC aluminum finishing, natural-color anodizing is around 0.008 mm (8 µm), while other anodizing colors are around 0.015 mm (15 µm). These values are useful for dimensional planning, but a project with a defined coating requirement should be controlled to that specified requirement rather than to a generic Type II value.

Factors That Affect Type II Coating Thickness

Actual anodizing thickness varies with aluminum alloy, surface preparation, current density, bath conditions, processing time, and other process controls. For dimensional-critical parts, the specified coating thickness and the anodizer's demonstrated process capability should determine the machining allowance rather than the finish name alone.

Type II vs Type III Thickness Comparison

JLCCNC's values below are typical planning references rather than the full thickness ranges defined by MIL-PRF-8625F. When a military or customer specification applies, the drawing or purchase requirement controls the required coating thickness.

For a broader comparison of conventional anodizing and hard anodizing, see JLCCNC's guide to anodizing vs. hard anodizing.

Conductive Anodizing Thickness

Conductive anodizing is typically around 0.003 mm (3 µm) at JLCCNC. Its coating thickness is lower than the typical thickness used for conventional anodizing or hard coat anodizing, which can matter when the part has tight electrical-contact or dimensional requirements. The required conductive finish and critical dimensions should be defined on the drawing when contact performance or fit is important.

How Anodizing Affects CNC Critical Dimensions

Compensating Critical Dimensions Before Anodizing

Critical features should be machined with enough allowance for the specified anodizing thickness as well as the anodizer’s expected process variation. This approach allows the final coated diameter, width, or mating surface to remain inside its required tolerance. The anodizing supplier’s process data can help with establishing the machining target when tolerances leave little room for variation.

Managing Critical Features Through Anodizing

Masking or plugging can keep anodic oxide off selected bores, threads, contact areas, and features that must retain their machined dimensions. Once processing has been completed, critical finished dimensions should be inspected against the drawing. ASTM B244-09(2021) provides a nondestructive eddy-current method for measuring anodic coating thickness on aluminum alloys. For Type III coatings, MIL-PRF-8625F permits ASTM B244 or ASTM E376 for thickness conformance, with the reported thickness calculated from the average of at least eight measurements. The measurement method and probe setup should still be appropriate for the coating, substrate, feature geometry, and required accuracy.

How to Specify Anodizing on a CNC Drawing

CNC drawing with anodizing thickness specifications

Specify the Anodizing Type and Thickness

For an anodized CNC part, the drawing should identify the applicable anodizing specification, coating type and class where required, color when relevant, and any specific coating thickness requirement. Do not rely on “Type II” or “Type III” alone when a particular coating thickness is critical to the part's function.

For close-tolerance parts, identify the final coated dimension and, where useful for manufacturing control, the corresponding pre-anodize machining dimension. Assembly-critical bores, threads, shafts, and mating faces should clearly state whether the dimensional limits apply after anodizing. If selected surfaces must remain uncoated, specify the masking or plugging requirement directly on the drawing.

Define Critical Dimensions After Anodizing

Dimensions that are critical at assembly or inspection should state whether their limits apply after anodizing. The drawing can also identify surfaces that demand masking and features that must be free of coating. This gives the machinist, anodizer, and inspector one finished requirement to follow. This is particularly important for threaded features and close-tolerance holes, where MIL-PRF-8625F provides specific guidance on dimensional effects.

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Conclusion: Anodizing Thickness and Dimensional Control

Anodizing can affect the final dimensions of a CNC aluminum part, particularly when tight tolerances are involved. Defining the finished anodized requirement before machining helps account for dimensional changes during finishing and reduces the risk of rework. For parts with demanding dimensional requirements, JLCCNC can review the machining and anodizing requirements together before production. Upload your CAD file to get a fast quote starting from $1, with lead times as short as 3 days.