50 Microns of Coating, 50 Microns of Diameter Change: How Should Procurement Calculate the Right Pre-Machining Size Before Anodizing?

July 30, 2026
Posted by leeDelsy
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50 Microns of Coating, 50 Microns of Diameter Change: How Should Procurement Calculate the Right Pre-Machining Size Before Anodizing?

A part can come off the CNC machine at a perfect size. Then it goes out for anodizing, and suddenly it does not fit. The bore is too tight. The shaft is too fat. Nothing was machined wrong. The coating itself changed the size.

This happens more often than most buyers expect, and it costs real money when it does. So this guide walks through exactly why anodizing changes your dimensions, how much each type of anodize shifts them, and how to size a part correctly before it ever reaches the tank. By the end, you will have a simple, repeatable method for anodizing tolerance design that keeps parts assembling the first time.

A shaft and a bore shown side by side before and after anodizing

Quick Answer: A 0.002" (50 micron) coating changes a diameter by roughly 0.002" total. Outside diameters grow. Inside diameters shrink. This happens because anodizing is not a coating that sits on top of a part like paint. It is a conversion process, and it eats into the base metal while it builds outward at the same time.

That single rule solves most tolerance surprises. But the exact numbers depend on your anodize type, your alloy, and your process, so let's break down each part of the equation. First, we will look at why the dimension change happens at all. Then we will compare Type II against Type III anodize. After that, we will run the actual math for machining a part before it goes to anodizing. Finally, we will cover how to write the tolerance correctly on your drawing so nothing gets lost between your shop and your finisher.

Why Does Anodizing Change Your Part Dimensions in the First Place?
How Much Do Type II and Type III Anodizing Really Change Your Tolerances?
How Do You Calculate the Right Pre-Anodize Machining Size for ODs and IDs?
How Should You Specify Tolerances on Your Anodizing Drawing to Avoid Scrapped Parts?
Conclusion

Why Does Anodizing Change Your Part Dimensions in the First Place?

Anodizing is not paint, and it is not plating. Both of those processes simply add a layer on top of a part. Anodizing works differently. It uses an electrochemical process to turn a thin layer of the aluminum itself into aluminum oxide. Because it converts existing metal instead of just adding metal on top, the part changes shape from the inside out, not just from the outside in.

Here is the short version: about half of that oxide layer builds outward from the original surface. The other half eats inward, consuming aluminum that used to be there. Buyers and engineers often call this the 50/50 rule anodizing shops use for early planning, and it is a good starting point for any estimate.

The 50/50 rule is useful, but it is an estimate, not a guarantee. The actual split between building outward and penetrating inward shifts based on a few things:

Alloy grade — different aluminum series (2000, 5000, 6000, 7000) react differently in the tank.
Anodize type — hardcoat and standard anodize do not behave the same way.
Process parameters — bath chemistry, current density, and temperature all play a role.

As a rough guide, Type III hardcoat often runs closer to 45% build-up and 55% penetration, showing a slightly uneven anodizing build-up and penetration ratio rather than a clean 50/50 split. Type II anodize on 6061 aluminum can lean even further, sometimes closer to 20% build-up and 80% penetration. If you are working from a MIL-A-8625 dimensional impact standard, that spec sets requirements for the coating itself, so always confirm your supplier's actual ratio for your specific alloy before you commit to a machining size. If your parts also need a fine surface finish, our surface finishing services page covers how anodizing fits alongside other finishing options.

How Much Do Type II and Type III Anodizing Really Change Your Tolerances?

Not every anodize job moves your dimensions by the same amount. The size of the shift depends almost entirely on which type of anodize you specify, so this is the first decision that affects your tolerance plan.

Here is a simple breakdown showing Type II vs Type III anodizing dimensional change side by side:

Anodize Type	Coating Thickness	Dimensional Change Per Surface
Type II (standard)	0.0002"–0.001" (5–25 microns)	0.0001"–0.0005"
Type III (hardcoat)	0.001"–0.003" (25–75 microns)	0.0005"–0.0015"

For most commercial parts, Type II barely moves the needle. The change is small enough that many designs absorb it without any adjustment at all. Type III hardcoat is a different story. Because the coating itself is much thicker, the hardcoat anodizing tolerance allowance needs to be planned for on purpose, not discovered after the fact. A 0.002" hardcoat layer can shift a critical fit by several thousandths of an inch, and that is more than enough to seize a bearing bore or bind a sliding shaft.

Threads deserve their own mention here, since they are often the first feature to fail. External threads grow on both the major and minor diameter, while internal threads shrink at the pitch diameter. That shrink is exactly what causes an anodizing hole shrinkage calculation to matter so much for tapped holes. The safest move is to mask threads before anodizing, or to chase them again after the part comes out of the tank. Parts like these often show up in bearing housings, pump bodies, and other industrial machinery components, where a tight thread fit is not optional.

How Do You Calculate the Right Pre-Anodize Machining Size for ODs and IDs?

This is the part that actually protects your parts on the shop floor. Once you know your coating type and thickness, the math itself is simple.

The core rule: Machine outside diameters undersize, and machine inside diameters oversize, both by half the coating thickness per surface.

For an outside diameter (OD):

Pre-Anodize OD = Finished OD − Coating Thickness

For an inside diameter (bore or ID):

Pre-Anodize Bore = Finished Bore + Coating Thickness

Let's walk through a real example. Say you need a 25 mm bearing bore, and the drawing calls for a 0.002" Type III hardcoat. Using the pre-anodize machining allowance above, you would machine the bore to roughly 25 mm plus 0.002", not to the final 25 mm size. After the coating grows inward, the bore lands back at your target of 25 mm. If a machinist skips this step and cuts straight to the finished dimension, the bore comes out of the tank too small, and the bearing will not seat. That single mistake can scrap the whole part, since there is usually no safe way to remove that much material after the coating is already applied.

This same logic applies whether your parts are simple shafts or complex housings produced through precision CNC machining services, where tight-tolerance features are common and the anodizing thickness per surface has to be built into the program from the start, not added afterward.

How Should You Specify Tolerances on Your Anodizing Drawing to Avoid Scrapped Parts?

Doing the math correctly only helps if it actually reaches the shop floor. The drawing callout is what protects your tolerance in real production, so this step matters just as much as the calculation itself.

You have two solid options here, and each one shifts responsibility to a different party:

Option A — Specify pre-anodize dimensions. Something like: "Machine to these dimensions. Anodize per MIL-A-8625 Type III, 0.002" minimum. Final dimensions will be approximately +0.002" on ODs and −0.002" on IDs." This makes your machinist responsible for hitting the pre-size correctly.

Option B — Specify post-anodize (finished) dimensions. Something like: "All dimensions and tolerances apply after anodizing. Machine undersize or oversize as required to achieve finished dimensions." This puts the responsibility on your anodizing supplier to control the process and land on the final size.

For critical fits, the safest approach is to specify both, since this removes any ambiguity between machined size vs finished size anodizing on the print itself. A note like this covers you either way: "Critical bores: machine to [X] to achieve [Y] after 0.002" Type III anodize. Confirm with anodizer before production."

Before you send parts out, it also helps to have a short checklist ready for whichever shop you choose:

What is your typical build-up and penetration ratio for this alloy?
Do you mask threads, or chase them after coating?
Can you confirm dimensional change in writing before we run production?

If you are searching for anodizing services near me and comparing local suppliers, asking these three questions upfront will save far more time than comparing price alone. This kind of checklist is really just a working anodizing design guide you can reuse on every future job, whether the finished parts end up in industrial equipment or something as everyday as kitchen appliance components.

It is also worth understanding what a missed calculation actually costs. A scrapped precision bore is rarely a small loss, since the entire machining cost behind that part is gone. Rework through stripping and re-anodizing is sometimes possible, but it carries its own risk, because stripping removes base aluminum and can shift the dimension even further off target. Assembly delays and rush replacement orders add on top of that. In almost every case, planning the tolerance before machining starts is far cheaper than fixing the part after it comes out of the tank.

Conclusion

The Bottom Line on Anodizing and Your Tolerances

Anodizing changes your dimensions. That is simply how the process works, and ignoring it is the most expensive mistake a buyer or engineer can make on an aluminum part.

Here is the short version to keep on hand:

Know your coating type and thickness. Type II is thin. Type III is thick. The thicker the coating, the bigger the size change.
Apply the 50/50 rule as a starting point. Half builds outward. Half penetrates inward. ODs grow. IDs shrink.
Write your tolerance strategy directly on the drawing. State clearly whether dimensions apply before or after anodizing.
Confirm with your anodizer before machining starts. Alloy and process both shift the exact ratio, so never assume.

A 0.002" hardcoat coating adds roughly 0.002" to your OD and takes roughly 0.002" away from your bore. Plan for that change while the part is still on the machine, not after it comes out of the anodizing tank.