CNC Machining Finishing Services: What to Check Before You Order

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  Key takeaways
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  What separates a capable finishing supplier from a cheap one
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  Why a finish callout is an engineering decision
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  How to read Ra numbers on a CNC machining drawing
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  Matching the finish to the material and the job
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  Coating thickness, masking and the tolerance stack
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  Inspection, traceability and certification scope
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  Cost, lead time and order quantity
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  Seven steps to specify and approve a finish

Milling and turning leave tool marks, burrs and a surface that is directionally rough. Those marks are not defects by themselves. They become defects when the part has to seal, slide, conduct current or take paint. The finishing step exists to convert a machined surface into one that meets a function.

The cost of getting this wrong is rarely cosmetic. A valve bore with the wrong texture leaks at pressure. A coated shaft with 20 μm more nickel than the drawing allows will not enter its bearing. An anodized thread loses clearance on both flanks. Each of these shows up at assembly, not at goods-in.

This is why the finish callout has to be written before the first chip is cut. If you add Ra 0.4 μm after machining, the shop may have to re-fixture the part, and geometry that was already in tolerance can move. Tell us the finish with the model, and we plan the passes around it.

Ra is the arithmetic mean deviation of the profile from its mean line. It says nothing about the shape of the peaks. Two surfaces can both read Ra 1.6 μm while one has rounded hills and the other has sharp spikes. For a static gasket face, that difference rarely matters. For a sliding seal, it decides whether the lip wears in or wears out.

Standard as-machined output from our mills and lathes sits at Ra 1.6–3.2 μm. Fine finishing reaches Ra 0.8–1.6 μm with a finishing pass, a smaller nose radius and reduced feed. Below that, down to Ra 0.2–0.8 μm, you are usually moving to a secondary operation: lapping, honing, polishing or a very light controlled pass on the same setup.

Two details change the reading more than the process does. First, the cut-off length: a 0.8 mm cut-off on a 1.6 μm surface can report 1.2 μm, while a 2.5 mm cut-off on the same surface reports 2.0 μm. Second, direction. A turned finish is circumferential; a milled finish is usually linear. If your drawing does not say which way the stylus travels, the incoming inspection may fail a part that is functionally fine.

Write it as: Ra 0.8 μm, measured across the lay, 0.8 mm cut-off, three readings per face. That sentence prevents most disputes.

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  Say the directionAcross the lay versus along it can double the reported value.
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  Say the cut-off0.8 mm and 2.5 mm are different filters, not different surfaces.
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  Say whereOne face, one band, or the whole part. Blanket callouts cost money.

Material changes the achievable finish. Aluminium alloys such as 6061 and 7075 cut cleanly and hold a fine turned finish well. Stainless 316 and 17-4PH work-harden at the surface, so a heavy finishing pass can tear rather than smooth. Titanium TC4 and Inconel move heat into the tool, and the finish suffers if the shop pushes feed to save time. Copper and brass machine to a very low Ra easily but tarnish within days, so a coating or a sealed bag matters.

Geometry sets a second limit. A deep 8 mm bore with a 3:1 depth-to-diameter ratio is hard to polish at the bottom. Internal corners left by a Ø6 mm end mill cannot be bead blasted evenly. Blind slots trap media from tumbling, and you will find it later during cleaning. If the finish is critical in a recess, the recess needs to be reachable by a tool or a nozzle.

Function picks the process, not the other way round. Decorative covers go to bead blasting or brushing, then clear anodize. Wear surfaces go to hardcoat anodize, electroless nickel or black oxide depending on the load and the environment. Electrical housings may need conductive anodize or masked contact pads. Medical and food-contact parts usually need passivation or a finish that survives repeated cleaning.

A single part often carries two or three of these at once. That is normal. What matters is that each face is called out, so the masking plan can be built before the part is loaded.

Plating and anodizing add or convert material at the surface. Electroless nickel typically builds a uniform layer and shifts every coated dimension outward. Anodize grows roughly half into the part and half out, which means a 25 μm hardcoat layer moves the surface about 12 μm. On a Ø10 H7 bore with a 15 μm fit clearance, that is enough to turn a slip fit into a press fit.

The practical rule is simple: any face that must hold size after coating gets masked, or the pre-coat dimension gets adjusted. Threads are the classic case. A coated M6 external thread will not enter a standard nut. Either mask the thread and coat around it, or cut the thread undersize before coating. Both work. Leaving it to the bench does not.

Masking is also a cost driver. Silicone plugs, lacquer and tape are applied and removed by hand. A part with ten masked features takes far longer than a part with one. If you can redesign a contact pad to sit on an already-masked face, or move it to a face that will not be coated, you save the operation.

Ask for the pre-coat and post-coat dimension on any critical fit. We list both on the inspection report when a drawing calls for it.

Visual inspection catches colour and gloss, and nothing else. Thickness needs a gauge. Roughness needs a profilometer with the right cut-off. Adhesion needs a tape or bend test on a coupon. If the drawing calls for a number, the shop needs the instrument, and the report needs to carry the reading.

Traceability matters when a batch fails six months later. A finish record that ties the run to a bath, a date and an operator is what lets you narrow a problem. Without it, you re-inspect everything. We keep in-process monitoring and final inspection records for each run, and we issue them on request.

Certificates have scope. ISO 9001:2015 covers a general quality system. IATF 16949:2016 is what automotive programs ask for. ISO 13485:2016 applies to medical device work, and ISO 27001:2022 covers information security, which matters when your drawings are confidential. Check that the certificate covers the process you are buying, not just the company name.

For regulated work, ask whether the finishing step is inside the certified scope or subcontracted. Both can be acceptable. You just need to know which one you are buying.

Finishing is usually a second queue. Machining may take three days and plating may add two more, with a gap for transport if the plater is off-site. When a supplier quotes a single number for the whole job, ask how it splits. The answer tells you where the schedule risk sits.

Setup dominates small runs. A plating line or an anodize bath charges for the rack, the masking and the pre-treatment, not for the surface area of one part. That is why a single prototype can cost more to finish than to machine. At volume, the same operation becomes a small fraction of unit cost.

There is no minimum order quantity here. One prototype and a 10,000-part run both go through the same process control. What changes is the sampling frequency and the racking method. For a prototype, we may hand-mask and inspect every face. For a run, we build a fixture and sample on a defined schedule.

Quotation and DFM feedback come back within 12 hours, and production can start within 24 hours of approval. Parts normally ship in 3–5 days. If a finish needs an outside bath with a longer cycle, we say so in the quote rather than after the order.