Don't Wait for Assembly to Discover Your Bearing Won't Fit: The Complete Anodizing Tolerances Guide?

Don't Wait for Assembly to Discover Your Bearing Won't Fit: The Complete Anodizing Tolerances Guide?

So your machined aluminum part came back from the anodizer — and your bearing won't press in. Your shaft won't slide. Your bolt won't thread. Everything measured perfectly before finishing. Now nothing fits. This scenario plays out in machine shops and engineering offices every single week. And almost every time, it was completely avoidable.

Anodizing changes your part's dimensions. Not by accident. Not randomly. It happens every single time — in a predictable, calculable way. The engineers who know this ship parts that assemble first try. The engineers who don't end up honing, scrapping, or re-machining. This anodizing tolerances guide exists so you never land in that second group again.

Before we get into the formulas and tables, it helps to understand why this happens. Anodizing is not like paint. You are not simply adding a layer on top of aluminum. Instead, an electrochemical reaction converts the aluminum surface itself into aluminum oxide. This oxide grows in two directions at once — outward from the original surface and inward into the base metal. That dual-direction growth is the root cause of every assembly surprise. Once you understand that, the rest of this guide clicks into place immediately.

Table of Contents

1. What Actually Happens to Dimensions During Anodizing?
2. Type I vs. Type II vs. Type III — How Much Does Each One Move Your Dimensions?
3. Why Do Holes Shrink More Than You Expect — And What Can You Do About It?
4. How to Design, Specify, and Communicate for Post-Anodizing Success?
5. Conclusion
   

What Actually Happens to Dimensions During Anodizing?

When aluminum is anodized, the surface does not simply get coated. The electrochemical process converts aluminum into aluminum oxide — and this new oxide layer grows in both directions simultaneously. Roughly one portion grows outward, adding to the outside dimension. The remaining portion grows inward, consuming the base metal beneath the original surface. This is not a flaw in the process. It is fundamental chemistry. Understanding this anodizing dimensional change is the first step toward designing parts that actually assemble.

Key fact — read this first: Anodizing is not a surface coating. It is a conversion of the aluminum itself. Approximately half of the oxide layer grows into the metal. The other half grows outward from it. That is why your dimensions change — and why the direction of change depends entirely on whether you are looking at an external surface or an internal bore.

The 33/67 Rule for Type II (Decorative/Sulfuric) Anodizing

For standard Type II sulfuric anodizing, the oxide layer splits as follows:

• ~33% of the total coating thickness grows outward from the original surface
• ~67% grows inward into the aluminum

So if your Type II coating is 15 µm thick, roughly 5 µm adds to the outside and roughly 10 µm eats into the base metal.

The 45/55 Rule for Type III (Hardcoat) Anodizing

Hard anodizing build-up follows a slightly different ratio:

• ~45% of the total coating grows outward
• ~55% grows inward

Hardcoat is also much thicker than Type II. That combination — thicker coating plus a larger outward percentage — means Type III causes significantly more dimensional change per surface.

Why This Ratio Matters for Internal vs. External Features

For an external surface (like a shaft OD), outward build-up increases the diameter. That can often be corrected with light polishing or pre-compensation. For an internal feature (like a bore or a hole), build-up occurs on both sides of the circumference simultaneously. The diameter shrinks by twice the per-surface change. That is exactly why holes are the most dangerous features to overlook — and why the next section focuses on them specifically.

Type I vs. Type II vs. Type III — How Much Does Each One Move Your Dimensions?

Not all anodizing is equal — and not all anodizing moves your dimensions by the same amount. The anodizing type you specify determines how thick the oxide layer will be, and therefore how much your part dimensions will shift. This section gives you the hard numbers. Bookmark it. You will use it every time you design a precision anodized part.

Quick-reference: Dimensional change by anodizing type

Bottom line: Type I barely touches your dimensions. Type II is manageable with planning. Type III demands explicit pre-compensation on every precision feature.

Type I (Chromic Acid): Minimal Change

Type I produces an extremely thin coating — typically less than 0.0001" per surface. For most engineering applications, this dimensional change is negligible. Type I is primarily used for fatigue-sensitive aerospace parts where coating thickness must be minimized. If you are running Type I, dimensional compensation is rarely required.

Type II (Sulfuric): 5–25 µm Total, ~33% Outward

Type II vs Type III anodizing tolerance is the comparison engineers ask about most often. Type II sits in the middle ground — thin enough to be forgiving, but thick enough to matter on precision fits. A 20 µm Type II coating adds approximately 6.6 µm outward per surface. For a bore, that means roughly 13.2 µm total diameter reduction. On a loose fit, that is usually acceptable. On an H7 tolerance hole, it requires planning.

Type III (Hardcoat): 25–125 µm Total, ~45% Outward

This is where engineers get into trouble. A mid-range hardcoat at 50 µm total adds approximately 22.5 µm outward per surface. For a bore, total diameter loss is approximately 45 µm — nearly 0.002". That is not a rounding error. That is a real interference on any precision fit.

Real Example: A 0.002" Hardcoat Adds ~0.0009" Per Surface

Let's make this concrete. Your part has a 0.002" (50.8 µm) hardcoat anodizing specification. Using the 45% outward rule:

• Outward build per surface = 0.002" × 0.45 = 0.0009"
• Total diameter reduction on a bore = 0.0009" × 2 = 0.0018"

This is consistent with MIL-A-8625 tolerance guidance for Type III coatings. Always account for this in your tolerance stack-up — without exception.

For a broader look at how surface finish specifications interact with dimensional tolerances, it is worth reviewing your full finishing callout before releasing drawings to production.

Why Do Holes Shrink More Than You Expect — And What Can You Do About It?

Holes are the most dangerous features to anodize without planning. External diameters grow — and that growth is usually on one surface. Internal diameters shrink — and that shrinkage happens on both sides of the bore at the same time. This anodizing hole shrinkage doubles the dimensional impact compared to what you might expect from looking at a single surface. If you have ever had a press-fit bearing refuse to enter an anodized bore, this is exactly why.

The double-impact rule for bores: Coating builds up on the left wall and the right wall simultaneously. So the total diameter loss = per-surface build-up × 2.

Formula:

• Type II diameter compensation = T × 0.33 × 2
• Type III diameter compensation = T × 0.45 × 2

Where T = specified total coating thickness.

Build-Up on Both Sides = Double the Dimensional Impact

This is the single most important concept in this entire guide. When you anodize a bore, the coating grows inward from the cylindrical wall on every side simultaneously. The hole diameter shrinks by the sum of the outward build-up on both opposing walls. There is no way around this geometry. It applies to every cylindrical bore, every threaded hole (if unmasked), and every slot with parallel walls.

Through Holes vs. Blind Holes: What Anodizes Differently

Through holes receive consistent electrolyte flow and current density. They typically anodize uniformly along their full length. Blind holes are different. Solution circulation becomes restricted as depth increases. The practical rule: a blind hole will only anodize consistently to a depth approximately equal to the hole's diameter. Beyond that point, coating thickness drops off — sometimes to zero. This means:

• Do not count on consistent coating in deep blind holes
• Do not rely on anodizing for corrosion protection in the deep sections of blind holes
• If coating consistency matters, design through holes where possible

The ±3 Micron Reality: Anodizing Thickness Variation

No anodizing process is perfectly uniform. In practice, coating thickness varies by approximately ±3 microns across a part's surface — even with tightly controlled process parameters. This variation must be included in your tolerance stack-up. For anodized finish tolerance on IT7 or tighter fits, this variation alone consumes a meaningful fraction of your available tolerance budget. Plan accordingly.

Can You Hold IT6 or IT7 Tolerances?

Yes — but only with careful planning. For a 25mm bore at IT7, your total tolerance is 21 µm. Coating thickness variation of ±3 µm per surface means ±6 µm total diameter variation from the anodizing process alone. That leaves only 15 µm for machining variation. It is achievable, but it requires:

• Tight anodizing thickness specification on the drawing
• Clear "after anodizing" tolerance callout
• Possible masking or post-anodizing honing for the most critical features

How to Design, Specify, and Communicate for Post-Anodizing Success?

Knowing the numbers is only half the job. The other half is acting on them — in your CAD model, on your engineering drawing, and in your communication with your CNC machining supplier and anodizer. There are three core strategies engineers use to handle anodizing dimensional change. Each has a different risk profile. Choosing the right one depends on how tight your tolerance is and how much variation you can tolerate in the finished part.

Strategy decision table — choose based on your fit class:

Rule of thumb: If your fit class is H7 or tighter, never skip the compensation step. If it is an interference fit, mask the feature — full stop.

Option A: Pre-Compensation — Machine Oversize Now, Hit Tolerance After Anodizing

This is the most common approach for precision bores. You calculate how much the hole will shrink, then machine it that much larger before anodizing. The pre-anodize machining allowance calculation works as follows:

1. Confirm your specified anodize thickness (T)
2. For Type II: diameter compensation = T × 0.33 × 2
3. For Type III: diameter compensation = T × 0.45 × 2
4. Machine your bore to: final target diameter + compensation value

Worked example:

• Target bore after anodizing: 25.000mm
• Specified Type III coating: 0.050mm total
• Compensation = 0.050 × 0.45 × 2 = 0.045mm
• Machine bore to: 25.045mm before anodizing

Option B: Specify Final Tolerances and Let the Anodizer Adjust

For production runs with an established anodizing supplier, you can specify "after anodizing" tolerances on your drawing and let the supplier's process knowledge drive the pre-compensation. This works well when:

• Your supplier has process data for your alloy and coating spec
• You have validated the process on a pilot run
• Your tolerance allows ±3 µm of coating variation headroom

This approach shifts responsibility clearly to the supplier — which is why drawing callouts matter so much.

Option C: Mask Critical Holes

For interference fits, bearing press fits, or any feature where post-anodizing variation cannot be tolerated, masking is the answer. Your anodizer plugs or tapes the feature before processing. The masked area gets no coating at all. This is the correct approach for:

• Bearing bores with H6 or tighter tolerance
• Press fits that would be destroyed by even 0.001" change
• Precision threads (see note below)

Thread note: Standard practice for anodized parts with threads is to mask threads before anodizing — or install thread inserts (Helicoils) after anodizing — or chase threads post-anodize to restore fit. Never leave precision threads unmasked on a hardcoat part.

The Stack-Up Calculation: Including Coating Thickness Compensation

This is the step most engineers skip — and it is the one that causes the most assembly surprises. Your tolerance stack-up must include:

• Machining tolerance on the bore
• Coating thickness compensation (the calculated diameter change)
• Coating thickness variation (±3 µm per surface = ±6 µm on diameter)
• Mating part tolerances

If the sum of these variations exceeds your fit class tolerance, you need a tighter machining spec, a tighter anodizing spec, or masking. There is no fourth option.

What to Put on Your Drawing

Clear drawing callouts prevent most anodizing tolerance problems before they start. Include all of the following on your drawing:

• Anodizing specification: e.g., "ANODIZE PER MIL-A-8625 TYPE III CLASS 2, 0.002" MINIMUM THICKNESS"
• Tolerance application: "ALL DIMENSIONS AND TOLERANCES APPLY AFTER ANODIZING UNLESS OTHERWISE NOTED"
• Masking callout: "CRITICAL BORES [A, B, C] — MASK BEFORE ANODIZING. DO NOT ANODIZE."
• Pre-compensation note (if using Option A): "BORE D: MACHINE 0.045mm OVERSIZE TO COMPENSATE FOR TYPE III ANODIZE BUILD-UP"

When You Did Not Compensate Enough: Post-Anodizing Recovery

Even with the best planning, sometimes a hole comes back too small. Your options at that point are limited — but they exist:

• Honing or lapping: Can remove 0.0005"–0.001" from a hardcoat bore. Works well for small corrections on through holes.
• Stripping and re-anodizing: Chemical stripping removes the oxide layer with minimal aluminum attack. Then re-anodize with corrected pre-compensation. This adds cost and lead time — and risks part geometry if not done carefully.
• Re-machining the mating part: For press-fit bearings or shafts, you may be able to adjust the mating component OD instead. Expensive, but sometimes faster than reworking the anodized part.

Prevention is always cheaper. A ten-minute compensation calculation before machining saves hours of rework after anodizing.

Alloy Matters Too

Different aluminum alloys anodize differently. High-copper alloys like 2024 produce thinner, less consistent coatings. Cast alloys with high silicon content — common in industrial machinery components — can yield uneven coating thickness across a surface. For electronics manufacturing housings where cosmetic uniformity and tight dimensional control both matter, alloy selection is part of the engineering decision. Always validate your compensation approach with your anodizer for your specific alloy and coating specification before committing to a full production run.

Conclusion

Here is what every engineer needs to walk away with from this guide:

Three core rules — apply them every time:

1. Know your anodizing type. Type I is negligible. Type II requires moderate planning. Type III (hardcoat) demands explicit pre-compensation on every precision feature, every time.
2. Calculate your compensation. Use the 33% outward rule for Type II and the 45% outward rule for Type III. Double the per-surface change for bore diameter loss. Include ±3 µm coating variation in your stack-up.
3. Communicate it on your drawing. Specify your anodizing type, coating thickness, and whether tolerances apply before or after anodizing. Call out masked features explicitly. One clear drawing note prevents hours of rework.

The bottom line: Anodizing dimensional change is predictable. It is calculable. It is manageable. The only parts that fail assembly are the ones where no one ran the numbers.

Before your next anodized part goes to the machine shop, spend ten minutes on the compensation calculation. Your assembly line — and your production schedule — will be better for it.

External Links — Recommended Resources

[Anodizing tolerances guide][^1]

[hard anodizing build-up][^2]

[Type II vs Type III anodizing tolerance][^3]

[anodizing hole shrinkage][^4]

[anodized finish tolerance][^5]

[^1]. This technical guide provides a detailed overview of how different anodizing types (Type II, Type III, and Chromic ) affect part dimensions, offering essential tolerance guidance for engineers to ensure precise fit and function after surface treatment.

[^2]. This technical resource explains the critical ratio between outward build-up and inward penetration in hard anodizing, providing essential data for engineers to manage dimensional changes in precision-machined aluminum components.

[^3]. This comprehensive comparison details the differences between Type II and Type III anodizing, focusing on how their distinct thickness and hardness profiles impact design tolerances and performance in demanding industrial applications

[^4]. This authoritative FAQ guide provides critical technical insights into how anodizing affects internal diameters, specifically explaining why hole shrinkage must be calculated by doubling the build-up amount to ensure precision fit in machined components.

[^5]. This professional technical resource from Top Anod outlines the essential factors governing anodized finish tolerances, detailing how film thickness, alloy composition, and process parameters must be precisely controlled to meet the stringent dimensional and aesthetic requirements of aerospace and industrial applications.