Type III Anodizing: Hardcoat Thickness, Tolerances & Design Guide

1. What Is Type III Hardcoat Anodizing?

Anodizing is an electrochemical surface-conversion process. Aluminum is made the anode in an electrolyte, and an oxide layer develops at its surface. Unlike an applied paint layer, the anodic coating consumes some of the substrate while also growing outward. The underlying metal still carries the structural load.

Hardcoat is selected when surface wear is a meaningful design concern: a guide face rubbing against a mating component, a sleeve exposed to repeated motion, or an aluminum housing with functional contact surfaces. It is not a universal solution to impact damage, poor lubrication, excessive bearing pressure or an unsuitable base alloy.

The Aluminum Anodizers Council notes that hardcoat performs well in rubbing and sliding applications but can fail under point pressure when the softer aluminum underneath deforms. A hard surface therefore does not make an aluminum part mechanically equivalent to hardened steel. Source: AAC specifying guide.

Type and class describe different requirements

Under MIL-PRF-8625, Type III identifies the hard anodic coating. Class 1 is non-dyed; Class 2 is dyed. Natural coloration is possible in Class 1, so “non-dyed” does not mean colorless. Cite the required specification revision in the purchase documents. The DLA ASSIST record is the official place to check the document’s status.

2. Type III Anodizing Thickness and Specification Parameters

The useful question is not “How thick can hardcoat be?” but “What thickness will meet the functional requirement without creating a fit or geometry problem?” Very thick coatings can increase the challenge of finishing small features and holding a tight assembly tolerance.

Thickness references: AAC reference guide; specification values: MIL-PRF-8625F, Amendment 2, §3.7.2.1. These are external references, not a statement of MileMetal’s approved process capability.

Microns, mils and millimeters

A mil is one thousandth of an inch, not one millimeter. A thickness written as 2 mils is 0.002 in, or 50.8 µm. Mixing these units can produce an order-of-magnitude error in a quotation or machining allowance.

These conversions do not recommend a thickness. A specific value must be checked against the material, feature size, desired wear behavior and finishing route.

3. Type II vs. Type III: Choose by Function

Type II is often considered for decorative or general protective finishes, while Type III is evaluated for more demanding surface-wear requirements. Do not choose solely by the apparent shade of a sample or assume that a thinner coating has no dimensional effect.

For a broader comparison, see MileMetal’s hard anodizing vs. regular anodizing guide. This article concentrates on specifying and manufacturing Type III parts.

4. Calculate Coating Growth Before Machining

The total oxide thickness is not the same as the outward increase of a surface. The AAC gives an approximate hardcoat planning relationship of 50% penetration and 50% buildup per surface. Actual growth must be agreed with the anodizer for the alloy and process. Source: AAC thickness guide.

Worked example: a finished 20 mm outside diameter

Assume a design calls for a finished outside diameter of 20.000 mm and a nominal total coating thickness of 50 µm. With 50% outward growth, g is 25 µm, or 0.025 mm. The nominal machining target is therefore:

20.000 − 2 × 0.025 = 19.950 mm before anodizing.

For a bore that must finish at 20.000 mm under the same simplified assumptions, the nominal pre-anodize target is 20.050 mm. The oxide grows into the opening and reduces its size.

Allow for variation, not just the nominal value

Suppose the illustrative coating requirement is 50 ±10 µm. If the growth fraction stays at 50%, the outside-diameter increase ranges from 0.040 to 0.060 mm. A machined shaft at 19.950 ±0.005 mm would then finish between 19.985 and 20.015 mm. That range would not satisfy a finished requirement of 20.000 ±0.010 mm.

Also distinguish dimensions measured on the incoming machined part from those measured on the delivered finished part. Marking a bearing seat “after finish” removes an ambiguity that otherwise travels from the drawing to machining, inspection and assembly.

5. Alloy, Surface Preparation and Color

The alloy and its supplied condition are inputs to the finishing process. Do not replace a specified material with another grade simply because both are described as aluminum. A change that helps machining may alter the finish response or introduce a new qualification requirement.

Machining marks, dents and inconsistent preparation can remain visible after finishing. Anodizing should not be treated as a repair operation for damaged surfaces. Where appearance matters, define the prefinish texture, inspection lighting, viewing distance and approved sample rather than relying on the word “smooth.”

Natural hardcoat coloration varies with alloy and thickness. A dark coating may be dyed, naturally colored, or a different finish entirely. Photographs are useful for communicating shape and appearance, but cannot verify coating type, thickness or wear performance.

6. Decide Sealing, Masking and Rack Contact Locations

Source: MIL-PRF-8625F, Amendment 2, §3.8.2. Agree on the accepted sealing treatment and any application-specific restrictions with the finishing supplier.

Masking is especially important where the assembly needs electrical continuity, a close fit or a controlled thread. Identify those surfaces on a marked drawing. “Mask as required” shifts a design decision to the process operator and may produce a part that looks correct but cannot be assembled.

• Grounding lands: identify the contact area and its required protection or conductivity condition.
• Bearing seats and precision bores: state whether they are coated, masked or finished by an approved subsequent operation.
• Threads: specify the finished gauging requirement and review coating access and buildup.
• Rack contact zones: agree where process contact marks may remain.
• Mask boundaries: define where the transition can occur so it does not cross a sealing or sliding surface.

A threaded insert, mixed-metal assembly or trapped recess can complicate processing. Review the assembly sequence before committing to a finished-part drawing. Rework after anodizing may expose aluminum and require a separate engineering disposition.

7. Product Geometry Examples from the MileMetal Library

These six existing MileMetal product images show relevant CNC aluminum geometries: housings, pockets, ports, mounting faces and precision bores. They are presented as design examples; the photographs do not certify that every pictured part received Type III anodizing.

Use the gallery to identify the surfaces that matter on your own part. A housing may need a cosmetic exterior, a controlled bearing bore and an electrically conductive mounting land—all within the same component. Those requirements should be separated on the drawing rather than covered by one general finish note.

8. Inspect the Coating and the Finished Component

Thickness is only one acceptance characteristic. A part can have the correct coating reading on an accessible face and still fail at a bore, thread, masked interface or wear surface. Agree on measurement locations and the final inspection stage before production starts.

A hardness number without its method, load, specimen and coating condition is weak evidence for a functional claim. For a wear-critical component, request the relevant wear test rather than assuming that a quoted hardness guarantees service life. Likewise, laboratory corrosion exposure does not translate directly into years of field performance.

Problems to address before repeat production

• Interference after finishing: check pre-machining allowance and coating variation before changing the mating part.
• Different shades between lots: check material traceability and the agreed preparation and color criteria.
• Patchy or damaged surfaces: review incoming condition, cleaning, racking and process control with the finisher.
• Unexpected contact marks: move approved rack zones away from functional or prominent cosmetic surfaces.
• Poor fit in internal features: inspect the actual feature; an external-face thickness reading does not prove internal coverage.

9. Put the Complete Requirement on the Drawing

A useful finish note must tell the supplier what result is required and how that result will be accepted. Keep the finished drawing as the contractual reference, with any supplier machining allowances recorded separately and controlled.

This example is not a universal specification. In particular, the illustrative 50 ±10 µm requirement may be incompatible with a tight fit unless the dimensional stack is reviewed. Replace all zone references with actual drawing details, and use the sealing condition and specification revision required by the application.

Send a quotation package that can be reviewed

1. Part definition: drawing revision, PDF and available STEP file.
2. Material: alloy, temper, product form and substitution rules.
3. Finish: specification, type, class, thickness, color and sealing.
4. Functional surfaces: finished fits, masked zones, electrical contacts and wear faces.
5. Acceptance: thickness locations, visual criteria, test reports and traceability.
6. Order scope: sample quantity, production quantity, repeat demand, packaging and requested date.

Compare quotations on that same basis. More masking, tighter finished tolerances and additional test documentation can change cost even when the overall part size remains unchanged. If the geometry or alloy changes after first-article approval, review the finishing assumptions again.

10. Frequently Asked Questions