CNC Surface Finish Guide: How to Specify Ra, Anodising & Cosmetic Finishes

A drawing arrives with a familiar note:

SURFACE FINISH: Ra 0.8 µm ALL OVER

It looks precise.

It may also make the component considerably more expensive than necessary.

Surface finish is one of those engineering specifications that can quietly influence tooling, feed rates, finishing passes, inspection and secondary processing across an entire component.

A bearing surface may genuinely require a fine controlled finish.

The hidden underside of the same component probably doesn't.

At Axion Precision, our CNC milling service includes machining in aluminium, stainless steel, titanium and engineering plastics alongside secondary operations including polishing, anodising, powder coating and grinding. AXION

The goal isn't to produce the smoothest possible component.

It's to produce the surface the application actually requires.

What does Ra mean on an engineering drawing?

Ra means arithmetical mean surface roughness.

In simplified terms, it describes the average deviation of a real surface from its mean line across the measured sampling length.

For metric engineering drawings, Ra is normally expressed in micrometres (µm).

So:

Ra 3.2 µm is rougher than Ra 1.6 µm.

Ra 1.6 µm is rougher than Ra 0.8 µm.

Ra 0.8 µm is rougher than Ra 0.4 µm.

The important point is that these aren't simply different grades of visual quality.

Surface texture can affect:

• friction;

• wear;

• sealing;

• lubrication;

• bearing behaviour;

• fatigue;

• coating;

• appearance; and

• how two components mate.

That's why surface finish belongs to functional engineering—not just cosmetics.

What surface finish can CNC machining achieve?

There isn't one universal “CNC finish”.

It depends on:

• material;

• cutting tool;

• tool condition;

• tool geometry;

• feed;

• speed;

• machine rigidity;

• workholding;

• toolpath;

• cutter engagement; and

• whether a dedicated finishing pass is used.

As a useful commercial reference, Protolabs currently states that Ra 6.3 to Ra 0.8 µm is typical for general CNC machining, with its normal CNC offering around Ra 1.6 µm or better. Protolabs

That makes Ra 1.6 µm a very familiar engineering requirement.

But familiar doesn't mean it should be placed everywhere automatically.

When is Ra 3.2 µm enough?

For many ordinary mechanical surfaces, a relatively standard machined finish is perfectly functional.

Think about:

• bracket faces;

• clearance features;

• external non-cosmetic walls;

• mounting structures;

• internal pockets;

• fixture components; and

• surfaces that don't slide, seal or locate precisely.

If the surface's only job is to exist within the required dimensional envelope, specifying an exceptionally fine finish provides little benefit.

This is where a general drawing note can be useful.

Rather than marking every face individually, the drawing can establish a sensible default finish and then call out finer requirements only where necessary.

That keeps the drawing readable and tells the machinist where additional attention actually matters.

When should you specify Ra 1.6 µm?

Ra 1.6 µm is a common precision-machining finish and can be appropriate for many mating or functional surfaces.

Examples might include:

• general locating faces;

• precision machined components;

• gasket interfaces depending on gasket type;

• controlled cosmetic machining;

• mating mechanical surfaces; and

• features where ordinary roughing marks are undesirable.

It is also a useful starting point where you want a good quality machined surface without moving into grinding or polishing territory.

But again, the component determines the requirement.

A face sealing against an elastomer, a metal-to-metal seal and a decorative aluminium panel are three completely different surface-engineering problems even if somebody has written “Ra 1.6” against all three.

When might Ra 0.8 µm be appropriate?

As roughness requirements become finer, the manufacturing process becomes more sensitive.

Ra 0.8 µm may be justified on features such as:

• certain bearing fits;

• sliding interfaces;

• precision shafts;

• sealing surfaces;

• hydraulic or pneumatic features; and

• components where friction or wear behaviour matters.

Achieving the requirement may involve a dedicated finishing pass with reduced feed and carefully controlled tooling.

On some geometries, another process such as grinding may make more sense.

Axion's CNC capability includes precision milling plus secondary grinding and finishing options, allowing the process route to follow the functional requirement rather than assuming every finish must come directly off an end mill. AXION

Why a finer Ra value can increase CNC cost

Imagine face milling a large aluminium plate.

If the requirement is simply dimensional accuracy with a normal machined finish, the cutter can operate at an efficient production feed.

Now require a much finer surface.

The machining strategy may need:

• a separate finishing tool;

• an additional pass;

• reduced feed;

• carefully controlled step-over;

• fresh cutting edges;

• additional inspection; or

• a secondary finishing process.

Each operation consumes machine or labour time.

Now multiply that requirement across every surface of the component.

A single unnecessary drawing note can turn what should be an efficient machining job into a much slower one.

This is why Axion's design-led approach includes Design for Manufacture review rather than treating every specification as isolated from manufacturing cost. AXION

Surface finish and dimensional tolerance are not the same thing

This distinction matters.

A surface can be dimensionally accurate but relatively rough.

It can also be beautifully polished but dimensionally wrong.

A ±0.01 mm tolerance controls size or position.

An Ra 0.8 µm requirement controls surface texture.

Neither automatically guarantees the other.

Consider a precision shaft.

Its diameter might require a tight tolerance because it interfaces with a bearing.

Its surface may also require a fine finish because the bearing or seal runs against it.

Those are two separate functional requirements and should be specified accordingly.

Conversely, putting Ra 0.8 on a component doesn't magically make the component more dimensionally precise.

Don't use Ra to specify appearance alone

This is another common trap.

Ra measures surface roughness.

It doesn't fully describe how a surface looks.

Two surfaces can have similar measured Ra values and look very different because of:

• machining direction;

• cutter pattern;

• waviness;

• reflectivity;

• material grain;

• bead blasting;

• polishing direction; or

• anodising.

If appearance matters, the drawing or purchase specification should communicate that separately.

For a premium visible component, consider defining:

• cosmetic faces;

• acceptable machining marks;

• grain direction where relevant;

• bead-blast requirement;

• anodising colour;

• gloss level;

• visual inspection standard; and

• protected areas.

For particularly appearance-sensitive work, an approved physical sample can be more useful than attempting to encode every aesthetic requirement into an Ra number.

Cosmetic faces should be marked on the drawing

Imagine a black-anodised aluminium electronics housing.

The top and front surfaces are visible to the customer.

The underside bolts to a chassis and is never seen.

Those surfaces shouldn't necessarily receive the same manufacturing attention.

Marking the cosmetic faces tells the manufacturer where handling marks, fixture contact, cutter transitions and finish consistency matter most.

This can influence:

• fixture design;

• machining sequence;

• protective handling;

• deburring;

• blasting;

• polishing; and

• final inspection.

Without that information, the supplier has to guess.

And guessing at cosmetic expectations is a great way for both customer and manufacturer to be disappointed.

What does bead blasting do?

Bead blasting propels fine media against the component surface.

On aluminium, it is commonly used to create a uniform matt or satin appearance and reduce the visual prominence of machining marks.

Xometry's current UK finishing guidance describes bead blasting as producing a uniform matt/satin-like surface and notes its common use before anodising. Xometry UK

This combination is particularly popular for:

• electronics housings;

• premium mechanical components;

• camera equipment;

• instrumentation;

• consumer-facing aluminium parts; and

• control hardware.

Axion also offers bead-blasted finishes on aluminium extrusion projects alongside anodising and powder coating. AXION

But bead blasting should not be treated as a magic “make it perfect” button.

Bead blasting can change edges and critical surfaces

Blasting is a physical process.

The media impacts the surface.

That means critical features deserve consideration.

For example:

• precision bores may need masking;

• sealing surfaces may need protection;

• threads may require plugs;

• very sharp edges can become visually softened;

• polished surfaces obviously need protection; and

• different geometries may receive media differently.

If the component contains a highly controlled bearing bore, don't simply write:

BEAD BLAST ALL OVER

unless you genuinely want that bore blasted too.

A better drawing defines which surfaces receive the cosmetic treatment and which remain protected.

Bead blasting isn't corrosion protection

Bead blasting changes surface texture.

It isn't, by itself, a protective coating.

For aluminium components requiring both a uniform matt appearance and corrosion resistance, blasting is often followed by anodising.

That leads to another common specification problem.

What is anodising?

Anodising is an electrochemical process that deliberately grows an aluminium oxide layer at the component surface.

It can improve:

• corrosion resistance;

• wear behaviour;

• surface hardness;

• appearance; and

• electrical insulation.

Standard decorative anodising is widely available in colours such as clear and black, while hard anodising is used where greater wear resistance or coating thickness is required.

Axion lists anodising among the finishing options available for both CNC-machined aluminium parts and custom aluminium extrusions. AXION

Anodising needs to be considered before machining

This is crucial for precision components.

Anodising creates an oxide layer with finite thickness.

Part of that oxide grows into the original aluminium surface and part grows outward.

For ordinary clearance features the dimensional effect may be insignificant.

For a close-tolerance bore, bearing seat, precision thread or sliding interface, it may matter a great deal.

That means the manufacturer needs to know before machining:

• anodising type;

• required thickness where critical;

• which surfaces are coated;

• which surfaces are masked;

• whether dimensions apply before or after coating; and

• whether critical fits require compensation.

Don't manufacture a precision aluminium assembly and only afterwards decide it should be hard anodised.

Finishing is part of the manufacturing specification.

Should bearing bores be anodised?

Sometimes yes.

Sometimes absolutely not.

The answer depends on what the bore does.

Hard anodising can provide useful wear resistance in certain aluminium interfaces.

But coating thickness changes the effective diameter, and the resulting oxide surface has different characteristics from bare aluminium.

A bore accepting a press-fit bearing may need:

• dimensional compensation before anodising;

• masking;

• post-finish machining; or

• another deliberately engineered strategy.

There isn't a universal rule.

The drawing should communicate the functional requirement, and the manufacturing route should be selected accordingly.

What about threads and anodising?

Threads deserve the same thought.

For large, relatively loose threads, ordinary anodising may cause little practical difficulty.

For small or close-fitting threads, coating build-up can affect assembly.

Options can include:

• machining allowance;

• masking;

• thread chasing where appropriate;

• changing the fit; or

• inserting threaded hardware after finishing.

The correct route depends on coating thickness, thread size and assembly requirement.

Again, finishing belongs in the design conversation early.

Hard anodising vs standard anodising

Standard decorative anodising and hard anodising shouldn't be treated as interchangeable colour choices.

Hard anodising is generally selected when the surface needs greater:

• wear resistance;

• hardness;

• abrasion resistance; or

• durability.

That can be valuable for:

• sliding components;

• tooling;

• industrial mechanisms;

• automation equipment; and

• components subject to repeated handling or abrasion.

But if the part only needs an attractive black corrosion-resistant finish, specifying hard anodising may introduce cost and dimensional considerations without providing useful benefit.

Use the finish that solves the problem.

Powder coating is a different proposition

Powder coating applies a comparatively substantial polymer coating rather than growing a thin oxide layer from the aluminium itself.

It can provide:

• strong colour consistency;

• durable cosmetic appearance;

• good environmental protection;

• textured finishes; and

• a wide selection of colours.

Axion offers powder coating for CNC components and custom aluminium extrusion profiles. AXION

Its thickness makes masking especially important around:

• bearing seats;

• threads;

• electrical contact areas;

• precision fits;

• sealing faces; and

• assembly datums.

A precision-machined part shouldn't be designed as if powder coating has zero thickness.

When should you polish a CNC component?

Polishing can be functional, cosmetic or both.

Possible reasons include:

• premium appearance;

• reducing surface roughness;

• easier cleaning;

• reducing sites where contamination can collect;

• optical requirements; or

• preparation for another finishing operation.

But polished surfaces can become labour-intensive quickly.

Complex pockets and inaccessible corners may require substantial manual work.

If only the exterior face needs a polished appearance, specify that face.

Don't require a mirror finish inside a hidden pocket unless the application genuinely needs it.

Stainless steel finishing is a different conversation

Aluminium dominates discussions around anodising, but stainless steel brings its own finishing requirements.

Depending on application, a stainless component might be:

• left as-machined;

• bead blasted;

• mechanically polished;

• passivated;

• electropolished; or

• ground.

The correct choice depends strongly on service environment.

For example, a decorative stainless bracket and a hygienic process component may both be made from 316L but require completely different surface specifications.

Material grade alone doesn't define the finish.

Surface finish on turned components

Surface finish can be particularly important on rotational components.

Axion's CNC turning service is used for shafts, bushes, collars, threaded parts and other rotational geometry.

Features such as:

• seal running surfaces;

• bearing journals;

• sliding shafts;

• piston features; and

• precision diameters

may require both dimensional and surface-texture control.

A shaft can measure perfectly and still perform poorly against a seal if the running surface is unsuitable.

Likewise, demanding a fine finish across every diameter on a complex turned component can unnecessarily increase cycle time.

Specify by function.

Don't confuse a beautiful surface with a good component

A polished or black-anodised component can look extremely impressive.

That says surprisingly little about whether it has been manufactured correctly.

Quality still depends on:

• material;

• dimensional accuracy;

• geometry;

• thread quality;

• surface integrity;

• inspection;

• cleanliness;

• coating control; and

• assembly performance.

Cosmetics matter where cosmetics matter.

Function matters everywhere.

The best specification communicates both without confusing one for the other.

How to specify cosmetic aluminium parts

For a premium aluminium component, a useful drawing note might communicate:

Material: Aluminium 6082-T6
General finish: As machined
Cosmetic surfaces: Faces A, B and C
Secondary finish: Fine bead blast + black anodise
Mask: Bearing bores and electrical contact surface
Critical surface: Ra 0.8 µm before finish where indicated
Deburr: Remove sharp edges without damaging cosmetic faces

The exact wording and standards should, of course, match the product's requirements.

The important point is the structure.

It tells the manufacturer:

• what matters;

• where it matters; and

• what should remain untouched.

That's far more useful than:

BLACK ANODISE — GOOD COSMETIC FINISH

How to specify functional surfaces

For functional components, identify the surface by what it actually needs to achieve.

For example:

Bearing journal: Ø20 h6, Ra 0.8 µm

General turned surfaces: Ra 3.2 µm unless otherwise stated

Sealing face: Ra 1.6 µm, no radial scratches

Cosmetic outer diameter: Fine bead blast after machining

This prevents a demanding specification on one critical feature from accidentally becoming a requirement across the entire part.

Think about machining direction

Ra alone doesn't describe the direction of surface texture.

For some applications, lay direction matters.

A turned surface naturally tends to produce circumferential tool marks.

A milled surface may show directional or patterned cutter marks.

For a seal or sliding interface, the direction of those marks can influence performance.

Where this matters, specify the appropriate surface-texture requirement rather than relying solely on an Ra number.

If it doesn't matter, don't complicate the drawing unnecessarily.

Inspection should match the requirement

A visual cosmetic requirement and a measured roughness requirement aren't inspected the same way.

A specified Ra value may require a profilometer or other appropriate surface-measurement method.

A cosmetic anodised face may instead need controlled visual inspection against an agreed acceptance standard or reference sample.

If the requirement is critical enough to specify, consider how it will actually be verified.

This becomes particularly important for production quantities.

A requirement that cannot be objectively accepted or rejected is likely to create disputes.

Surface finish and DFM

Surface finish is an excellent example of why design and manufacturing shouldn't operate independently.

An engineer sees:

Ra 0.8

A machinist may see:

extra finishing pass + slower feed + different tooling + extra inspection.

A designer sees:

black anodise

A manufacturing engineer may see:

bead blast + masking + anodising + dimensional compensation + cosmetic handling.

Neither perspective is wrong.

They simply describe different consequences of the same drawing note.

Axion's CAD and mechanical design support applies DFM principles specifically to close this gap between design intent and production.

The objective isn't to relax important requirements.

It's to remove expensive requirements that aren't important.

A practical surface-finish checklist for engineers

Before releasing a CNC drawing, ask:

Which surfaces actually perform a functional job?

Give those the required roughness.

Which surfaces are visible to the customer?

Mark them as cosmetic.

Does every face really need the same Ra value?

Usually not.

Will the part be anodised, plated, painted or powder coated?

Specify it before machining.

Do any precision features need masking?

Identify them clearly.

Do dimensions apply before or after finishing?

Remove ambiguity.

Does appearance need an approved sample?

For premium cosmetic work, consider one.

Can the requirement be inspected objectively?

If not, improve the specification.

This short review can prevent a surprising amount of manufacturing cost and confusion.

CNC machining and surface finishing with Axion Precision

Axion Precision is based in Poole, Dorset and supports engineering businesses locally and across the UK with CNC machining from prototype development through production. AXION

Our CNC milling capability includes 3-, 4- and 5-axis machining in materials including aluminium, stainless steel, titanium and engineering polymers, with finishing options including polishing, anodising, powder coating, wet spray painting and grinding. AXION

For aluminium profiles and production components, Axion can also combine custom aluminium extrusion with secondary CNC machining and finishing. AXION

If you're unsure whether a component genuinely needs Ra 0.8 µm, whether a bearing bore should be masked before anodising, or how best to specify a cosmetic face, include that information with the RFQ.

It's much easier to engineer the finishing strategy before the component is manufactured.

Request a CNC machining quote

Send Axion Precision your:

• STEP model;

• PDF engineering drawing;

• quantity;

• material and temper;

• required Ra values;

• cosmetic-face requirements;

• anodising or coating specification; and

• masking requirements.

We'll review machining and finishing as one manufacturing process rather than treating the coating as something to worry about after the parts have already been made.

Upload your CAD files and request a quote from Axion Precision