Surface finishing for CNC parts, measured on the part
How to choose anodizing, plating, passivation, blasting, and marking, and what each one does to your dimensions. With real photos from the finishing line our parts go through, where coating thickness is checked on the part itself.
Measured: 18.4–18.7 µmThe same anodized plate: threads plugged before the tank, coating measured after. Photographed at our finishing partner.
Finish chooser
Common finishes at a glance
Start from the base material and what the finish must do. The thickness column is what changes your dimensions.
Finish
Base material
Thickness / size change
Used for
Anodizing, Type II
Aluminum
5–25 µm per surface
Clear, black, or color; corrosion protection
Hard anodizing, Type III
Aluminum
25–50 µm per surface
Wear resistance, electrical insulation
Nickel plating
Steel, brass, copper, aluminum*
5–50 µm
Corrosion, hardness, conductivity
Zinc plating
Carbon steel
5–12 µm typical
Low-cost corrosion protection
Black oxide
Steel
~1–2 µm
Dark finish for tooling and fixtures
Passivation
Stainless steel
No change
Restores corrosion resistance after machining
Electropolishing
Stainless steel
Removes 10–30 µm
Lower Ra, cleanability
Bead blasting
Most metals
No meaningful change
Even matte look; often before anodizing
Laser marking
Metals, most plastics
None to a few µm
Part numbers, serials, QR codes
* Nickel on aluminum needs a zincate pre-treatment; confirm at drawing review. Typical values; your drawing defines the requirement.
At the finishing line
What happens to your parts after machining
Finishing is done by managed partners, not in our machine shop. We visit the line, and every finished part comes back to us for inspection. These photos were taken there in August 2026.
01
Mask what must stay bare
Threads, fit bores, and sealing faces are plugged or taped, following the drawing.
02
Rack every part
Each part is clamped for electrical contact, so the coating builds evenly.
03
Clean and activate
Ultrasonic degreasing removes cutting oil; activation prepares the surface.
04
Anodize or plate
Parts go through the tanks with time and temperature controlled per process.
05
Measure the coating
Thickness checked on the part itself. This plate read 18.4–18.7 µm.
06
Back to us for inspection
Parts return to our factory and are checked against the drawing again.
Photos: anodizing and plating line of a Meili finishing partner. Process steps vary by finish.
Coating and size
Every coating changes the size of the part
Anodizing grows about half outward and half into the metal. Plating builds on every exposed surface. On a fit that matters, that is enough to turn a slip fit into a press fit.
We size for the coatingWhere the coating changes size, as with hard anodizing or plating, tight bores and shafts are machined with an allowance and measured before and after.
We mask what must stay bareThreads, fit bores, and sealing faces are plugged before the tanks when the drawing marks them.
We measure after finishingCoating thickness with an eddy-current gauge, then fits and threads checked again at our factory.
Example: 20 µm Type II anodize adds about 20 µm to a shaft diameter and takes about 20 µm off a bore diameter.
Masking plugs keep threads and bores bare.Every color batch is checked with a colorimeter against your reference.
01 · Aluminum
Anodizing, Type II and hard anodizing
Anodizing (anodising in British English) turns the aluminum surface into aluminum oxide. It only works on aluminum. Type II (5–25 µm) gives corrosion protection and a clear, black, or colored finish. Hard anodizing, Type III (25–50 µm, up to 50 µm through our partners) gives a dense, wear-resistant, electrically insulating layer, usually dark gray to black.
Color varies between batches and alloys. For cosmetic parts, send a color reference and approve a first sample. Fits and threads need masking or an allowance, because the coating grows into bores and onto thread flanks. How much anodizing really changes size →
Type II
Type III, hard
Thickness
Typically 12–25 µm
Typically 30–50 µm
Hardness
Moderate
High, wear resistant
Colors
Clear, black, colors
Dark gray to black
Best alloys
6061, 6082, 7075
6061, 7075; check 2xxx
Allowance
Usually none (grows about 3–5 µm per side)
Yes on precision sizes (10–20 µm per side, by alloy)
02 · Steel, brass, copper
Nickel, zinc, and black oxide
Nickel plating (5–50 µm) adds corrosion protection, hardness, and a uniform look; electroless nickel coats bores and recesses more evenly than electrolytic. Zinc plating (5–12 µm typical) is low-cost sacrificial protection for carbon steel, usually with a clear, yellow, or black passivate. Black oxide (~1–2 µm) is a dark finish for tooling and fixtures that needs oil to resist rust.
All plating builds on every exposed surface, so mark masked threads, press-fit bores, and contact faces on the drawing and state the thickness range when a fit depends on it.
Nickel plating tank at our finishing partner.Activation before plating.
03 · Stainless steel
Passivation and electropolishing
Passivation is a chemical clean, not a coating. It removes free iron left by machining and restores the chromium oxide layer that makes 304 and 316 corrosion resistant. It does not change dimensions, so it is safe on tight tolerances. Specify it per ASTM A967 or equivalent.
Electropolishing removes about 10–30 µm and lowers Ra, which helps cleanability on food, lab, and medical parts. State an Ra target and confirm tolerance-critical features before specifying it.
04 · Appearance
Bead blasting, brushing, and polishing
Bead blasting gives an even matte look that hides tool marks and is often done before anodizing; it can slightly round sharp edges. Brushing gives a directional satin grain on flat faces. Mechanical polishing goes from smooth to mirror and removes a little material each pass.
Recesses and inside corners rarely look exactly like flat faces. For cosmetic parts, approve a sample before the batch.
Blasting cabinets at our finishing partner.Finished rings, separated in trays.
05 · Traceability
Laser marking
Laser marking puts part numbers, serials, revision letters, and QR or data matrix codes into the surface permanently. On stainless steel, annealing gives a dark mark without removing material; on aluminum, steel, and brass, engraving gives a recessed mark that survives wear. On anodized aluminum, marking is done after anodizing.
Specify location, content, character height or code size, and orientation on the drawing. Changes after machining are rarely practical.
By material
Which finishes work with which materials
Compatibility comes from the base metal's chemistry. We confirm it at drawing review, before the quote.
Material
Common finishes
Notes
Aluminum
Anodizing Type II / III, bead blasting, brushing, polishing, nickel plating, laser marking
6061 and 7075 anodize best; 2024 and 5052 have limits with Type III.
Tarnishes without protection; nickel is the practical choice.
Titanium
Bead blasting, color anodizing (cosmetic), laser marking
Titanium anodizing is cosmetic, not protective.
Engineering plastics
Laser marking, flame polishing for clear PC
Anodizing, plating, and passivation do not apply.
Grades and machining notes for each material are in the materials guide.
Drawing notes
What to put on your drawing
Unclear finish notes are one of the most common reasons for re-quoting and rework. These eight lines answer almost every question a finisher will ask.
Finish name and standard"Anodize Type II, clear, per MIL-A-8625" is clearer than "anodize".
Color or appearanceClear, black, or a color reference (RAL, Pantone, or a physical sample).
Coating thicknessMinimum and maximum, when the coating affects a fit.
MaskingName each feature to keep bare, e.g. "Mask M8 thread, full depth".
Threads after finishingMasked before finishing, or re-tapped / gauged after.
Ra targetFor polished or electropolished faces with a function, e.g. Ra 0.4 for sealing. Typical values by process are in our surface finish chart (Ra values).
Inspection basisCoating thickness, hardness, adhesion, or salt spray hours, if required.
PackingIndividual bags or separators for cosmetic faces.
Machined parts cost far less from 20 piecesDrawing review, programming, and fixturing are paid once and shared across the batch. Ask for 5, 20, and 50 pcs prices in one quote.
Does anodizing change the size of a machined part?
Yes, but less than the coating thickness. Type II typically grows a part about 3–5 µm per side, and even a 20 µm coating measures only 5–8 µm per side, because pretreatment first removes about 10 µm of base metal. We usually leave no allowance for Type II; for hard anodizing (30–50 µm) precision sizes are machined with an allowance and measured before and after. Threads are plugged or re-machined.
How do you protect threads and fits during anodizing or plating?
Threaded holes, press-fit bores, and sealing faces are plugged or taped before the parts go into the tanks, as long as the drawing marks them. If a thread cannot be masked, it is re-tapped or gauged after finishing.
Can you match an anodize color to a sample?
Send a physical sample or a color reference. Every color batch is checked with a colorimeter against that reference at our finishing partner. Alloys can still shift the shade slightly, so for cosmetic parts we recommend approving a first sample before the batch.
How thick is hard anodizing?
Usually 25–50 µm per surface, specified on the drawing. We can measure coating thickness with an eddy-current gauge after finishing.
Who does the finishing, and who checks it?
Anodizing, plating, passivation, blasting, and heat treatment are done by managed partners. Finished parts come back to our factory, where we inspect them against the drawing again before packing.
Can stainless steel parts be passivated or electropolished?
Yes, through our finishing partners. Passivation does not change dimensions; electropolishing removes roughly 10–30 µm and should be specified with an Ra target when it matters.
Reviewed by Raymond Liang, co-founder, 15 years in CNC machining · Updated October 9, 2026
Send the drawing with your finish notes
We check finish compatibility, coating allowance, and masking at drawing review, then quote machining and finishing as one price, delivered.
