Anodising CNC Machined Aluminium: Tolerances, Masking & Design Guide
An aluminium component leaves the CNC machine perfectly within tolerance.
It is inspected.
Everything passes.
Then it is anodised.
The bearing no longer fits.
What happened?
Nothing necessarily went wrong with the machining.
The problem may be that the drawing treated anodising as if it were paint colour rather than part of the engineering specification.
Anodising creates an aluminium-oxide layer by electrochemically converting the component surface. Part of that layer grows into the original aluminium and part grows outward, so the finished component does not have exactly the same dimensions as the freshly machined component. Anodizing.org Members
For a decorative enclosure with generous clearances, the difference may be irrelevant.
For a component containing:
bearing seats;
precision bores;
sliding fits;
dowel holes;
fine threads;
sealing faces; or
closely toleranced assemblies,
it can determine whether the component works.
Axion Precision provides CNC milling with anodising and other post-processing options for aluminium components from prototypes through production quantities. The important point is to consider machining and finishing as one manufacturing process, rather than designing the machined part first and adding anodising at the end.
What actually happens when aluminium is anodised?
Unlike paint or powder coating, anodising does not simply deposit a separate layer on top of the aluminium.
The process electrochemically converts the aluminium surface into aluminium oxide.
The resulting layer is thicker than the amount of original aluminium consumed, which is why some of the coating projects outward from the original surface.
The Aluminum Anodizers Council gives a useful rule of thumb:
under nominal Type II conditions, approximately two-thirds of the anodic layer penetrates the original surface and one-third grows outward;
under hard-coat conditions, approximately half penetrates and half grows outward. Anodizing.org Members
Those are planning rules, not universal machining allowances.
Actual behaviour depends on factors including:
alloy;
anodising chemistry;
coating thickness;
current density;
temperature;
pretreatment;
component geometry; and
the anodiser's process.
That is why a precision drawing should define the required finished condition, rather than asking a machinist to guess a generic anodising allowance.
Does anodising really change a CNC part's dimensions?
Yes.
Imagine a simple external shaft.
If anodic coating grows outward from every exposed surface, the finished shaft becomes slightly larger.
Now consider a bore.
Coating grows inward from both sides of the bore wall.
The bore becomes smaller.
The same principle applies to other features:
External dimensions can increase.
Bores can decrease in diameter.
Slots can become narrower.
Grooves can become narrower.
Threads can become tighter.
Clearances can reduce.
This is insignificant for some components and critical for others.
The design question is therefore not simply:
“How thick is the anodising?”
It is:
“Which finished dimensions are sensitive to the coating?”
A practical hard-anodising example
Suppose an aluminium component receives a nominal 50 µm hard-anodised layer.
Using the common 50/50 hard-coat rule of thumb, approximately 25 µm might grow outward from each exposed surface.
Consider an external shaft originally machined to:
Ø20.000 mm
If both sides effectively gain around 25 µm radially, the finished diameter could increase by roughly:
0.050 mm
Now consider a bore.
If approximately 25 µm grows inward from each side, its diameter could reduce by roughly:
0.050 mm
That is enough to completely consume many precision-fit tolerances.
The Aluminum Anodizers Council gives the same underlying example in imperial units: with a 1.5 mil hard coating and nominal 50/50 growth, a hole's diameter reduction is approximately 1.5 mil because growth occurs on both sides. Anodizing.org Members
But don't simply put +0.05 mm onto every hard-anodised bore.
That would replace one bad assumption with another.
The actual coating requirement and the anodiser's process should determine the manufacturing allowance.
Coating thickness is not the same as dimensional growth
This distinction causes a surprising amount of confusion.
Suppose the drawing specifies:
Hard anodise, 50 µm
That does not necessarily mean every external surface grows outward by 50 µm.
Some of the oxide forms below the original surface.
The amount above the original surface is the portion that directly changes the external dimension.
For nominal hard anodising, a useful early-design assumption may be approximately half the coating thickness growing outward. Anodizing.org Members
But for production drawings, confirm the actual finishing specification and supplier capability.
Decorative anodising versus hard anodising
Designers sometimes specify simply:
BLACK ANODISE
That may not contain enough engineering information.
There are major differences between a relatively thin decorative/protective anodic finish and a thicker hard anodic coating intended primarily for wear resistance.
ISO 7599:2018 specifically covers decorative and protective anodic oxidation coatings and notes that hard anodic coatings used primarily for engineering wear and abrasion resistance are outside its scope. ISO
Commercial UK finishing guidance commonly places decorative anodising around 5–25 µm and hard anodising around 25–50+ µm, although exact requirements depend on the application and specification. Protolabs
The thicker coating makes dimensional planning much more significant.
So don't use “anodise” and “hard anodise” interchangeably.
When should you choose decorative anodising?
Decorative/protective anodising is commonly appropriate when the priorities include:
corrosion protection;
colour;
appearance;
surface protection;
product identification; and
a durable aluminium finish.
Typical applications include:
electronic enclosures;
instrument housings;
control panels;
brackets;
product bodies;
optical equipment;
camera components; and
general engineering hardware.
Axion's CNC milling service supports aluminium grades including 6061, 6082, 7075 and 5083, with anodising available as a downstream finishing option. AXION
When should you consider hard anodising?
Hard anodising is more likely to be selected where the surface has a functional job to do.
Examples include:
sliding components;
wear surfaces;
guides;
mechanisms;
equipment exposed to abrasion;
pistons or actuator components;
tooling;
mechanical interfaces; and
demanding industrial hardware.
The objective is generally a thicker, harder oxide layer rather than primarily decorative colour.
But that extra thickness makes it especially important to plan:
fits;
threads;
masking;
edge geometry;
final inspection; and
mating-component clearances.
A hard-anodising note added after a precision component has already been designed can create substantial manufacturing problems.
The bearing-bore problem
Consider an aluminium housing containing a precision bearing seat.
The CAD model says:
Ø30 H7
The machinist makes the bore correctly.
Then the entire component receives hard anodising.
The bore becomes smaller.
The bearing no longer fits.
There are several possible strategies.
Option 1 — Compensate during machining
The bore can be machined oversize by a controlled amount so that anodising brings it into the required finished range.
This can work where the coating process is sufficiently controlled and the supplier understands the final requirement.
Option 2 — Mask the bore
Prevent anodising from reaching the bearing seat.
The bore remains bare aluminium and retains its machined dimensions much more closely.
Option 3 — Finish-machine after anodising
Leave material for a final machining operation after finishing.
This can recover a precise dimension but removes the anodic coating from the machined surface.
Each strategy produces a different finished component.
The drawing needs to tell the manufacturer which outcome matters.
Should bearing bores be masked?
Sometimes.
Not automatically.
If the anodised layer is functionally useful inside the bore and the finished diameter can be controlled through pre-machining compensation, leaving the bore coated may be desirable.
If maintaining a precise machined bearing fit is more important than having anodic coating on that surface, masking may make more sense.
If the bore also provides:
electrical contact;
a highly controlled datum;
a sealing interface; or
another specific surface function,
that can change the decision again.
The key is to treat the bore as a functional surface, not simply another coloured area.
Threads need the same consideration
Anodising also affects thread flanks.
That means an internal tapped hole can become tighter after coating.
External threads can grow.
Fine threads and small threads are particularly sensitive because their available clearance is small.
Possible strategies include:
anodising the thread and designing the allowance accordingly;
masking or plugging the thread;
tapping after anodising;
chasing the thread after anodising; or
using another controlled process agreed with the supplier.
Current UK anodising guidance explicitly notes that threaded holes may require plugging because of coating build. Protolabs
Don't simply write:
M4 × 0.7 – BLACK ANODISE ALL OVER
and assume the supplier knows whether “all over” includes the thread.
State what the finished thread needs to do.
Tapping after anodising has a trade-off
Suppose a threaded hole is anodised and then tapped afterwards.
The thread may now fit beautifully.
But the tap has removed the anodic layer from the thread surface.
That may be completely acceptable.
Or it may defeat part of the reason for specifying anodising.
This is why manufacturing decisions cannot be separated from function.
If corrosion protection, electrical insulation or wear performance matters inside the thread, stripping the coating during a secondary tapping operation may not be desirable.
Masking is useful—but it is not free
Masking can protect:
precision bores;
bearing seats;
dowel holes;
threads;
electrical contact areas;
bonding areas;
sealing surfaces; and
critical datums.
The anodiser may use:
plugs;
tapes;
liquid masking materials; or
purpose-designed fixtures.
Every masked feature introduces extra work.
A component containing twenty individually masked M3 holes will generally be more troublesome than a component with one masked bearing bore.
So don't mask every feature by default.
Mask where the function requires it.
Define masking on the drawing
A note saying:
MASK CRITICAL FEATURES
is not a useful engineering specification.
Which features?
Where does the masking boundary start?
Does the chamfer remain anodised?
Does the bottom of the bore remain anodised?
Is slight overspray acceptable at the boundary?
A better drawing identifies the actual surface or feature and clearly states the requirement.
For example:
MASK Ø20 BEARING SEAT — NO ANODIC COATING
The precise wording should follow your company's drawing standards and the agreed finishing specification, but the principle is straightforward:
Remove ambiguity before the part reaches the anodising line.
Electrical contact areas often need masking
Anodised aluminium is electrically insulating compared with bare aluminium.
That is useful in some applications.
It is a problem in others.
Consider an aluminium electronics enclosure that relies on a metal-to-metal connection for:
grounding;
bonding;
shielding; or
continuity.
Anodising the contact surface may interfere with that electrical path.
Those surfaces may need to remain conductive through masking or another appropriate finishing strategy.
So electrical engineers should review anodising requirements alongside mechanical engineers.
A beautiful black enclosure that no longer provides the intended electrical connection is not a successful finish.
Don't forget earthing points
This deserves explicit mention because it is easy to miss.
Suppose an M4 screw attaches a protective earth lug to an anodised enclosure.
The screw may mechanically clamp perfectly.
But if the mating faces are anodised, the electrical connection may not perform as expected.
The solution might involve:
masking a defined contact area;
using a suitable conductive surface treatment;
using a serrated washer or dedicated bonding feature where permitted by the electrical design; or
another validated connection method.
The correct solution depends on the relevant electrical and safety requirements.
The important mechanical-design lesson is simply:
Do not assume an anodised aluminium surface is electrically equivalent to bare aluminium.
Sealing surfaces also deserve attention
Imagine an O-ring sealing against a machined aluminium face.
Anodising may improve corrosion resistance.
But the sealing function depends on more than coating thickness.
You may need to consider:
surface roughness;
porosity;
coating integrity;
edge condition;
dimensional change;
sealing chemistry; and
whether the coating is sealed.
For critical sealing applications, define the required finished surface condition rather than assuming that “anodised” automatically produces a suitable seal face.
Surface preparation changes appearance
Anodising does not hide the underlying metal surface in the same way as a thick paint or powder coat.
The condition of the aluminium before anodising strongly influences the finished appearance.
ISO 7599 explicitly highlights the importance of pretreatment in obtaining the required final appearance or texture. ISO
This means machining marks can remain visible.
So can:
scratches;
handling marks;
dents;
tool transitions;
polishing variation; and
differences in bead blasting.
If a component is highly cosmetic, surface preparation should be part of the drawing and quotation.
Bead blasting before anodising
Bead blasting is often used to create a more uniform matt appearance before anodising.
It can reduce the visual prominence of light machining marks and create a consistent texture.
But blasting is itself a manufacturing process.
It can influence:
surface roughness;
sharp edges;
engraving;
tiny features;
cosmetic appearance; and
potentially critical surfaces if not protected.
If only the outside of an enclosure needs the matt cosmetic treatment, don't automatically blast every precision feature inside it.
Define cosmetic faces.
Mark cosmetic faces on the drawing
This is one of the simplest improvements a design engineer can make.
If one face of a component will sit behind a PCB and another will be visible on the front of a £20,000 instrument, they do not have the same cosmetic importance.
Tell the manufacturer.
Useful drawing information might identify:
primary cosmetic faces;
secondary cosmetic faces;
surfaces where rack marks are permitted;
areas where colour variation is acceptable;
areas requiring a defined texture; and
surfaces that must remain free from handling damage.
“COSMETIC PART” is far less useful than identifying which surfaces the customer actually sees.
Why rack marks exist
The anodising process needs electrical contact with the component.
That means the component must be held or “racked” somewhere.
The contact point can leave a local mark where the anodic coating is absent or different.
The rack point has to go somewhere.
For a purely functional bracket, that may not matter.
For a premium visible component, it can matter a lot.
A good cosmetic drawing therefore gives the anodiser somewhere acceptable to make contact.
Suitable locations might include:
an internal thread;
a hidden bore;
an internal face;
a sacrificial feature; or
another non-cosmetic area agreed with the finisher.
Don't demand:
NO RACK MARKS ANYWHERE
without providing a physical way to process the component.
Colour consistency needs realistic expectations
Black anodising is common on precision CNC components because it provides a clean technical appearance.
But anodised colour is influenced by more than the dye.
Factors can include:
alloy composition;
material batch;
heat treatment;
surface preparation;
coating thickness;
process conditions; and
sealing.
Two components made from different aluminium alloys should not automatically be expected to appear identical after colour anodising.
Even nominally identical alloys from different stock can show some variation.
If colour matching is commercially important, discuss:
common material batch;
controlled pretreatment;
batch processing;
approved samples; and
acceptable variation
before production begins.
Alloy choice affects anodising
This is another reason material selection and finishing should not be treated independently.
Axion machines aluminium grades including 6061, 6082, 7075 and 5083. AXION
Those alloys are chosen for different combinations of:
strength;
machinability;
corrosion behaviour;
availability;
weldability; and
mechanical performance.
They can also respond differently during anodising.
If the component has a demanding cosmetic requirement, ask whether the chosen engineering alloy is also appropriate for the desired finish.
Do that before releasing the drawing.
Not after the first batch returns from the anodiser.
Don't choose 7075 just because it is stronger
This is a broader DFM point.
7075 is an excellent high-strength aluminium alloy.
But if the component does not need that strength, another alloy may offer a better combination of:
material cost;
machinability;
corrosion resistance;
availability; and
finishing characteristics.
Likewise, don't automatically choose 6061 or 6082 just because they anodise well if the structural requirement points elsewhere.
Material selection should consider the complete manufacturing and service requirement.
Axion's CAD and design-for-manufacture support is intended to resolve decisions like this while changes are still inexpensive.
What about hard anodising sliding surfaces?
Hard anodising can be very useful for wear surfaces.
But specifying it doesn't remove the need to think about the mating pair.
A sliding system depends on both surfaces.
Consider:
mating material;
contact pressure;
lubrication;
speed;
contamination;
surface roughness;
coating thickness;
edge loading; and
operating environment.
A thick hard-anodised layer on one component may improve wear resistance, but the final geometry still has to provide the intended running clearance.
For a precision sliding assembly, calculate the clearance in the finished coated condition.
Press fits can change dramatically
Imagine an aluminium housing and a steel dowel.
Before anodising, the hole provides the intended interference or transition fit.
After anodising, the bore becomes smaller.
The fit becomes tighter.
Depending on the dimensions involved, assembly force may increase substantially or the pin may simply stop fitting.
This is why the tolerance on a critical fit should usually be thought of as a finished-part requirement.
The machining dimension is then a manufacturing variable determined by the chosen finishing strategy.
Which dimensions should apply after anodising?
For precision parts, this should be explicit.
If the drawing says:
Ø25.000 +0.015 / 0
does that dimension apply:
before anodising?
after anodising?
after anodising and sealing?
after final grinding?
The supplier should not have to infer the answer.
A sensible drawing system can identify which critical dimensions apply after surface treatment.
The machinist and anodiser can then plan backwards from the final requirement.
This is especially important for:
bearing bores;
dowel holes;
shafts;
precision slots;
sliding fits;
sealing diameters; and
mating interfaces.
Avoid over-tolerancing cosmetic surfaces
Suppose an anodised enclosure has:
one H7 bearing bore;
two dowel holes;
four M4 mounting threads; and
25 other non-critical dimensions.
Only the functional features may require close control after finishing.
Putting ±0.01 mm on the entire component can create unnecessary:
machining time;
inspection;
process planning;
masking;
scrap risk; and
cost.
Use tight tolerances where they buy functional performance.
Let general geometry use sensible manufacturing tolerances.
This makes both CNC machining and finishing easier to control.
What should a good anodising drawing specify?
The exact specification depends on your industry and company standards, but the manufacturer generally needs enough information to understand:
The anodising requirement
Is it decorative/protective anodising or hard anodising?
Applicable standard
For example, where appropriate, ISO 7599 provides a current framework for specifying decorative and protective anodic coatings. ISO
Required coating thickness
State the relevant requirement rather than simply “anodise”.
Colour
Black, clear or another required colour where applicable.
Surface preparation
As-machined, bead blasted, brushed or another defined condition.
Masked surfaces
Clearly identify where anodising is prohibited.
Post-finish dimensions
Identify critical dimensions that apply after finishing.
Cosmetic surfaces
Show which faces have appearance requirements.
Rack-contact areas
Where appearance is critical, identify acceptable contact locations if necessary.
Electrical contact areas
Define surfaces that must remain conductive.
Inspection requirements
If a finished bore or shaft must meet a particular tolerance after anodising, say so.
The objective is not to cover the drawing in notes.
It is to eliminate the ambiguities that can produce unusable parts.
A practical RFQ example
Suppose you're buying a batch of 6082-T6 aluminium housings.
Each contains:
a precision bearing bore;
six M4 tapped holes;
an O-ring groove;
an electrical bonding pad; and
a visible external surface requiring black anodising.
A useful RFQ should not merely say:
6082-T6 / BLACK ANODISE
Instead, the engineering package should make clear:
which anodising process is required;
required coating thickness or applicable specification;
whether the bearing bore is coated or masked;
whether threads are coated;
whether the O-ring surface is coated;
which area remains bare for electrical bonding;
which external surfaces are cosmetic;
which dimensions apply after finishing; and
required quantity.
That information lets the supplier quote the actual manufacturing process rather than adding assumptions.
Why this matters to purchasing teams
Anodising mistakes are not only engineering problems.
They become purchasing problems.
A quotation that appears inexpensive may exclude:
masking;
bead blasting;
cosmetic handling;
controlled coating thickness;
post-anodise inspection;
thread protection;
special racking; or
rework of critical dimensions.
Supplier A and Supplier B may therefore be quoting completely different manufacturing routes from the same incomplete drawing.
A technically complete RFQ creates a much more meaningful commercial comparison.
Should machining and anodising come from the same supplier?
There can be advantages to placing the complete finished component with one manufacturing supplier.
The CNC supplier can plan:
pre-anodise machining allowances;
masking requirements;
surface preparation;
finishing;
final inspection; and
packaging
as one process.
That reduces the risk of a machined part being declared “within drawing tolerance” by one supplier and then becoming out of tolerance after finishing at another.
Axion's precision CNC machining service includes downstream surface finishing such as anodising, polishing, powder coating, painting and grinding so the manufacturing route can be considered through to the finished component. AXION
Anodising versus powder coating
These finishes solve different problems.
Anodising converts the aluminium surface itself.
Powder coating applies a polymer coating over the surface.
Powder coating can provide:
strong colour coverage;
substantial environmental protection;
a broad choice of finishes; and
a comparatively thick coating.
But that thickness can make dimensional masking even more important around:
threads;
precision bores;
locating features;
mating surfaces; and
electrical contacts.
Anodising is often attractive where a relatively thin, durable finish is desired while retaining the metallic character of the aluminium.
The choice should follow the product requirement rather than appearance alone.
What about aluminium extrusions?
The same finishing principles extend beyond fully machined billets.
Axion also supplies custom aluminium extrusions with anodising and secondary machining for applications including machine frames, housings, heat sinks and transport structures. AXION
This can be particularly useful when a production component starts as an extrusion but contains:
precision mounting holes;
machined end faces;
tapped features;
bearing locations; or
other secondary CNC operations.
Again, process order matters.
Should the profile be machined before anodising?
Should a particular face remain bare?
Do tapped holes need protection?
Should a critical datum be finish-machined afterwards?
These decisions belong in the manufacturing plan.
Anodising for marine and South Coast engineering
For companies in Poole, Bournemouth and the wider Dorset/South Coast manufacturing area, aluminium is particularly common in:
marine equipment;
electronics;
instrumentation;
specialist vehicles;
automation;
outdoor equipment; and
general precision engineering.
Axion is based in Poole, Dorset and supports local and UK-wide engineering customers with CNC manufacturing and DFM. AXION
In coastal applications, corrosion resistance can be an important reason for choosing an anodic finish—but the actual material, coating specification, sealing, mating metals and operating environment still need engineering consideration.
“Marine use” is not, by itself, a complete surface-finish specification.
A pre-release anodising checklist
Before releasing an anodised CNC aluminium component, check:
Have you specified the alloy?
Not simply “aluminium”.
Do you know why it is being anodised?
Appearance, corrosion, wear, insulation—or several of these?
Is the anodising type clear?
Decorative/protective and hard anodising are not interchangeable.
Is coating thickness defined where necessary?
Especially for functional coatings.
Which dimensions matter after finishing?
Mark them.
Are there bearing or dowel fits?
Decide whether they will be coated, compensated or masked.
Are there small or fine threads?
Define how they should be treated.
Are there electrical contact areas?
Anodising may need to be excluded.
Are there sealing surfaces?
Define the required finished condition.
Which surfaces are cosmetic?
Tell the supplier.
Where can the component be racked?
Particularly for premium visible parts.
Does colour matching matter?
Discuss batch and material implications.
Has the finishing requirement been reviewed before machining?
This is the big one.
Catching a coating problem in CAD is cheap.
Catching it after 500 finished parts arrive is not.
CNC aluminium machining and anodising with Axion Precision
Axion Precision supports aluminium components from prototype through production using 3-, 4- and 5-axis CNC milling across commonly specified engineering aluminium grades. AXION
Surface finishing can then be incorporated into the manufacturing route, including:
anodising;
polishing;
powder coating;
wet painting;
paint infill; and
grinding.
For components where coating affects a fit, tolerance or functional surface, Axion's CAD and mechanical-design support can also review the component from a DFM perspective before production.
That is the ideal time to decide whether a bearing bore needs an allowance.
Not when the bearing is sitting on the bench and no longer goes in.
Request a quote for anodised CNC aluminium parts
If you need anodised aluminium components, send Axion Precision:
STEP CAD model;
PDF engineering drawing;
aluminium grade;
quantity;
anodising requirement;
required coating thickness where applicable;
colour;
cosmetic-face requirements;
masked areas;
critical post-finish dimensions; and
required delivery.
If you are unsure how a specified anodic coating will affect a bore, shaft, thread or mating fit, flag the feature rather than building an unverified coating allowance into the CAD.
The machining and finishing strategy can then be reviewed around the finished component requirement.
