Surface finishing after machining

CNC Machining Powder Coating Service: How to Specify It Without Losing Your Tolerances

This guide is for engineers and buyers who machine a part, then want it powder coated, and need to know what the coating does to fits, threads and cure-sensitive features. Read it and you can write a coating note that a shop can actually follow.

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±0.005 mm machining toleranceFrom one prototype to 10,000+ partsQuotation in 12 hoursIATF 16949:2016

On this page

1
Key takeaways
2
When Powder Coating Fits the Part, and When It Does Not
3
What a CNC Machining Powder Coating Service Actually Changes on the Part
4
Which Materials Survive the Cure Oven
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How to Mark Up the Drawing So the Coater Gets It Right
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Supplier Checks Before You Place the Order
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Step by Step: From Machined Part to Coated Part

Quick answer

Key takeaways

Coating adds thicknessTypical dry film is 60–120 μm; every coated surface grows by roughly half that per side.

Mask before, not afterThreads, bores, sealing faces and ground datums need masking tape or plugs fitted at the coating stage.

Cure heat is the real limitStandard powders cure near 180–200 °C, which is fine for aluminium and steel, risky for filled plastics.

One shop, one tolerance chainMachining and coating under one roof keeps the ±0.005 mm callouts and the finish spec in the same inspection report.

Cheap coatings hide in the fine printAsk for film thickness range, masking method and cure schedule in writing before you approve the sample.

Decision table

When Powder Coating Fits the Part, and When It Does Not

Match the part to the finish before you send the drawing out.

Part condition	Powder coating	Better alternative	Why
Large flat covers, brackets, housings	Good fit	—	Covers wide area at low cost per part
Threaded holes M6 and under	Risky	Mask or tap after coating	Film bridges the crest and jams the nut
Bores with H7 fit	Risky	Mask the bore	0.06–0.12 mm growth closes the clearance
Sealing faces, O-ring grooves	Risky	Mask or use anodizing	Coating thickness varies across a face
Outdoor steel frames	Good fit	—	Holds up to UV and rain for years
Thin aluminium sheet under 1 mm	Risky	Anodizing or wet paint	Cure heat can distort thin sections
Parts needing Ra 0.8–1.6 μm finish	Poor fit	Anodizing or bead blasting	Powder leaves an orange-peel texture
Prototype runs of one to five parts	Acceptable	Wet paint for speed	Setup cost per color is shared

The short version

Powder coating is a good fit for housings, brackets and frames that need a durable color finish, and a poor fit for threads, tight bores and heat-treated structural parts unless you mask and control the cure. Decide which faces stay bare before you send the drawing out.

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What the process does

What a CNC Machining Powder Coating Service Actually Changes on the Part

Powder coating is a dry process. The shop sprays an electrostatically charged powder onto a grounded metal part, then bakes it so the powder melts, flows and cross-links into a solid film. There is no solvent to flash off, so a single coat can reach 60–120 μm without runs. That is much thicker than most wet paints, and thickness is the first thing that matters to a machined part.

The number to remember is half. If the dry film is 80 μm, each coated surface grows about 40 μm. A shaft that was turned to Ø20.000 mm comes back at roughly Ø20.080 mm. On a free surface that is harmless. On a bearing seat, a dowel hole or a snap fit, it is the difference between a press fit and a scrapped assembly.

Powder also flows into corners and around edges. Coverage is better than wet paint on complex geometry, but the film is thinnest on sharp outside edges and thickest in inside corners. If your print calls a flatness or parallelism tolerance on a coated face, expect the coating to sit inside that tolerance band.

Parts that come off a CNC machine at Ra 0.8–1.6 μm will not stay that smooth after coating. The powder leaves a light orange-peel texture, usually equivalent to Ra 3–6 μm. That is fine for a housing, and wrong for a sliding surface. Specify coating only on the faces that need protection and leave the rest bare.

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Film growth is per sideAdd half the dry film thickness to every coated dimension.
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Edges coat thin, corners coat thickDo not put a tight flatness callout on a coated face.
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Texture replaces your machined finishRa 0.8–1.6 μm becomes a matte orange-peel surface.

Materials and heat

Which Materials Survive the Cure Oven

The cure step is where most problems start. Standard thermoset powders need roughly 180–200 °C for 10–20 minutes at part temperature, not oven air temperature. Aluminium alloys such as 6061, 6061-T6, 7075, 6082 and ADC12 handle that without trouble. So do the stainless grades 303, 304, 316 and 316L, and the common steels 1018, 1045, 4130 and 4140.

Two groups need a second look. The first is heat-treated aluminium. 7075 in the T6 condition loses some strength if it sits at 200 °C for a long soak. If the part is structural, tell the coater to use a low-temperature powder that cures at 150–160 °C, or accept a shorter cure and check adhesion on a sample first.

The second group is plastics. ABS, PC, PMMA, POM and PA will not survive 180 °C. PEEK will, and carbon fibre parts vary by resin system. If a plastic part needs color, powder coating is the wrong process. Use wet paint or a molded-in color instead.

Thin sheet is a quieter risk. Aluminium under about 1 mm can warp during cure because the coating shrinks as it cools. The fix is simple: support the part on a fixture, or switch to anodizing. For 5-axis work on large frames up to 4,000 mm, we usually coat after all machining and inspection are complete.

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Safe at 180–200 °C6061, 7075, 6082, 304, 316L, 1018, 4140, ADC12.
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Use low-temperature powderStructural 7075-T6 and thin aluminium under 1 mm.
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Do not powder coat plasticsABS, PC, POM, PA and PMMA soften well below cure temperature.

Drawing notes

How to Mark Up the Drawing So the Coater Gets It Right

A good coating note answers three questions: which surfaces, how thick, and what stays bare. The cleanest way is a separate view with coated faces shaded, plus a short table. Words alone leave room for interpretation, and interpretation is where rework comes from.

Call out the masking explicitly. Threads, reamed holes, bearing bores, keyways, splines and electrical grounding points all belong on the mask list. If a thread must be coated for corrosion reasons, specify that it gets chased with a tap after cure, and give the class of fit. Otherwise the film will bridge the thread crest and a nut will bind halfway down.

Set a film thickness range rather than a single number. Most industrial powders run 60–120 μm, and a coater who is told 80 μm exactly will either reject good parts or spray too thin. For outdoor steel, 100–150 μm is common. For a decorative cover, 60–80 μm is enough.

Finally, state the color standard. A RAL number or a physical sample is fine. A photo on a phone is not. Color also changes with gloss level, so specify matte, satin or gloss separately. If the part is an assembly, note whether the mating parts must match within a visible tolerance.

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Shade the coated facesOne view plus a table beats a paragraph of text.
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List every masked featureThreads, bores, keyways, splines, grounding points.
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Give a range, not a point60–120 μm covers normal powder variation.
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Name the color standardRAL number or physical sample, plus gloss level.

Supplier checks

Supplier Checks Before You Place the Order

Powder coating is a mature process, so the difference between suppliers is rarely the powder. It is the masking discipline, the oven calibration and the inspection paperwork. Those three items are what you are actually buying.

Start with the masking method. Ask how they protect a thread: tape, silicone plug, or a high-temperature cap. Tape is fine for a few parts and slow for thousands. A supplier who masks by hand on a 5,000-piece run will have variation. Ask to see a first-article part with the mask still on it.

Second, ask for the cure record. A calibrated oven logs part temperature, not just air temperature. If the supplier cannot show a cure profile for your alloy, they are guessing. For IATF 16949:2016 or ISO 13485:2016 work, that record is part of the traceability package, not an optional extra.

Third, look at how they inspect. A film thickness gauge reading on five points per part tells you more than a visual check. At GreatLight, machined and coated parts go through 100% inspection before shipment, with raw material checks, in-process monitoring and a final report on request. Coating thickness, color and masking are all recorded.

One more question worth asking: can they machine the part too? A shop that machines and coats under one roof owns the whole tolerance chain. A part that leaves one building and returns from another can pick up shipping damage and lose its inspection record. We run machining and finishing in the same 7,600 m² facility in Dongguan.

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See the mask on a real partTape, plug or cap, and how it is applied at volume.
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Ask for a cure profilePart temperature versus time, from a calibrated oven.
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Check the inspection recordFilm thickness points, color check, masking verification.

Workflow

Step by Step: From Machined Part to Coated Part

A workable sequence for a first order.

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1. Finish all machining firstCut, drill, tap and deburr before coating. Hold ±0.005 mm on any feature that will be masked. Do not plan a second machining pass after cure unless you have left stock for it.
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2. Write the coating note on the drawingShade coated faces, list masked features, set film thickness 60–120 μm, name the RAL color and gloss level. Add the cure temperature limit if the alloy is heat treated.
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3. Pre-treat the surfaceThe coater degreases, then applies a conversion coating or a phosphate layer for steel, or a chromate-free pre-treatment for aluminium. Skip this and adhesion fails at the first chip.
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4. Mask threads, bores and datumsFit silicone plugs in threaded holes and bores. Tape ground faces. Keep the mask line clean so it does not lift the film edge later.
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5. Spray and cureElectrostatic spray to the target film, then cure at 180–200 °C for 10–20 minutes at part temperature. Use 150–160 °C powder for heat-treated aluminium.
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6. Inspect and recordMeasure film thickness at five points per part, check color against the standard, confirm every masked feature is clean. Photograph the first article.
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7. Pack without rubbingParts should not slide against each other in the box. Foam or dividers between coated faces. A scratch in transit costs the same as a scratch in the oven.

FAQs

Common questions

Can you powder coat a part and still hold ±0.005 mm?

Yes, on the features that stay bare. Machining holds ±0.005 mm; the coating then adds 30–60 μm per side on the coated faces.

The practical rule is to machine coated dimensions undersize by the expected film growth, or mask the tight features entirely. We normally mask them, because film thickness varies across a part by 10–20 μm even in a good shop.

How much does powder coating add to the part thickness?

A standard industrial powder builds 60–120 μm of dry film, so each coated side grows 30–60 μm.

Heavy-duty outdoor coatings go to 100–150 μm. If your assembly has a gap of 0.2 mm, that gap can close after coating. Check the stack before you release the drawing.

Will powder coating fill or damage my threads?

It will bridge the thread crest if the hole is not masked. An M6 thread has roughly 0.5 mm of crest-to-root depth, and 80 μm of film is enough to make a nut bind.

Mask the threads, or specify that they are chased with a tap after cure. Never assume a coater will do this unless it is written on the drawing.

What is the minimum order quantity?

There is no minimum order quantity. We run from one prototype to 10,000+ part runs, and coating setup is shared across the batch.

For a single prototype, expect the color to come from a standard stock rather than a custom match, because custom powder has its own minimum batch.

How long does a coated order take?

Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours, and parts normally ship in 3–5 days.

Coating adds a cure cycle, so a coated batch sits one step longer than a bare machined batch. We keep the historical late-delivery probability below 2%.

Which certifications cover the coating work?

Our quality system holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022.

That means the coating step is documented in the same traceability chain as the machining step. Inspection reports are available on request, and uploads stay confidential under an NDA if you need one.