Surface Finishing

CNC Polish Services: 6 Checks Before You Order

This guide is for engineers and buyers sourcing CNC polish services on machined metal and plastic parts. It covers which finishes hit which Ra, when polishing is the wrong call, and the six checks to run before you release the order.

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Ra 0.2–0.8 μm achievableNo minimum order quantity100% inspection before shipmentISO 9001 / IATF 16949

CNC polish services on a 5-axis machined engine part

On this page

1
Key takeaways
2
Which polish level fits your part
3
How CNC polish services change a machined surface
4
Material choice changes polish time and cost
5
Geometry limits what any polishing shop can reach
6
How to write the polish callout on a drawing
7
What to verify before you place a polishing order
8
Six steps to release a polish order without surprises

Quick answer

Key takeaways

Polishing is geometry repair, not just shineIt removes tool marks. Deep chatter or a stepped toolpath will still show through.

Match the Ra number to the functionRa 3.2 μm is fine for brackets. Optical seats and sealing faces need Ra 0.2–0.8 μm.

Soft metals polish fast, hard steels cost moreAluminium and brass cut quickly. 17-4PH, 440C and Inconel need more passes and media changes.

Ask how they measureA Ra claim without a profilometer trace or comparator is a guess, not a spec.

Quote the finish in the RFQIf the drawing says only "polish", every shop will price a different operation.

Selection table

Which polish level fits your part

Use the function, not the drawing note, to pick the row.

Finish level	Typical Ra	Best for	Watch out for
As machined	Ra 1.6–3.2 μm	Brackets, housings, non-visual parts	Tool marks visible under raking light
Standard polish	Ra 0.8–1.6 μm	Visible covers, handles, mating faces	Edge rounding on thin walls
Fine polish	Ra 0.2–0.8 μm	Sealing faces, optical seats, bearings	Cost climbs fast below Ra 0.4 μm
Bead blast + polish	Ra 0.8–1.6 μm	Uniform satin look on castings	Blast hides, it does not flatten
Vibratory tumble	Ra 1.6–3.2 μm	Deburring many small parts at once	Cannot reach deep pockets

The short version

If the surface has a function, write the Ra number and the instrument. If it only has to look right, approve a physical sample first. Either way, name the faces and the limits before the shop quotes.

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What polishing actually does

How CNC polish services change a machined surface

Polishing is an abrasive process. Each step uses finer media than the last, cutting the peaks left by the previous operation until the surface is a set of shallow scratches too small to see. On a milled face, that means working through the tool mark pattern first, then refining it. On a turned OD, it means removing the feed helix.

This matters because polishing does not add material. It removes it. If a boss is 0.05 mm proud and the polished surface has to stay within ±0.005 mm, the shop has to mask that face or leave it unpolished. Engineers who write "polish all over" on a drawing with tight tolerances create a conflict the shop has to resolve by hand.

The starting surface decides the number of passes. A face milled at Ra 1.6 μm may reach Ra 0.4 μm in two or three steps. The same face with chatter marks needs several extra passes, and no shop can quote that accurately from a photo. Send the part or the setup sheet.

GreatLight runs polishing as part of a finishing sequence, not as a standalone errand. Parts come off 5-axis centers, get deburred, then go to the polish bench before anodizing or plating. Keeping it in one shop means the finish is measured against the same drawing the machinist worked from.

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Cut, don't coatAny coating fills the surface instead of flattening it.
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Ra is an averageA single deep scratch survives polishing and still reads low on some gauges.
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Direction mattersBrushed and mirror finishes reflect light differently on the same Ra.

Materials

Material choice changes polish time and cost

Aluminium alloys such as 6061 and 7075 polish quickly. They are soft, so media cuts fast, but they also smear. A polisher who leans too hard pushes aluminium across the surface and creates orange-peel texture. Light pressure and clean media solve it.

Stainless is the common request and the one with the most variation. 303 and 304 respond well to standard polish. 316L is slower but predictable. 17-4PH and 440C are harder and hold a mirror longer, but they wear out belts and compounds faster, so the per-part price is higher.

Titanium and Inconel are different again. Titanium galls easily and needs slow speeds with fresh abrasive. Inconel work-hardens if the polisher lingers in one spot, so the shop has to keep the pressure moving and accept more passes.

Plastics are the wild card. Acrylic (PMMA) and polycarbonate can be brought to optical clarity, but heat builds fast and turns the surface cloudy. PEEK and POM polish to a matte satin at best. If you need a clear window, say so early, because the setup is different from metal polishing.

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AluminiumFast, but smears under heavy pressure.
2
Stainless 303/304Predictable. 17-4PH and 440C cost more per part.
3
TitaniumGalls. Needs slow speed and fresh abrasive.
4
Acrylic and PCCan go clear, but heat is the enemy.

Geometry

Geometry limits what any polishing shop can reach

Polishing tools are small, but they are still rigid bodies with a contact patch. A Ø2 mm internal corner cannot be polished with a wheel that fits a Ø20 mm radius. The shop reaches in with a mounted point or a felt bob, and the result is a different texture from the surrounding face.

Deep pockets are the second limit. A pocket 80 mm deep with a 10 mm opening blocks light and blocks the tool. The bottom may polish, the walls stay as machined. If the whole pocket has to match, drilling a cross-hole for access is sometimes the only route.

Thin walls flex. A 0.8 mm aluminium wall pushed by a polishing pad deflects, and the finished wall is no longer flat. Shops handle this by backing the wall or reducing pressure, both of which add time. Tell the shop about thin sections before quoting.

Sharp edges are the quiet problem. Polishing rounds every edge it touches. If the drawing calls for a sharp corner or a crisp cosmetic break, the shop has to mask it or polish up to a stop line. A 0.3 mm edge break is normal; a knife edge is not.

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Internal cornersBelow about Ø3 mm, expect a hand finish, not a wheel finish.
2
Deep pocketsIf depth exceeds 5× the opening, walls likely stay as machined.
3
Thin wallsUnder 1 mm on aluminium, expect deflection and extra fixturing.
4
Edge breakPlan 0.2–0.3 mm unless the drawing says otherwise.

Specification

How to write the polish callout on a drawing

The fastest way to get a repeatable quote is to put the Ra number and the inspection method in the same note. "Polish to Ra 0.8 μm, measured with a contact profilometer across the sealing face" leaves no room for interpretation. "Polish as required" leaves all of it.

State the area. A note that says "polish" on a part with 40 faces will be read differently by every estimator. Use a surface finish symbol on the specific faces you care about, and add "balance as machined" so the shop knows where to stop.

Add the sample rule. If the visual appearance matters more than the number, ask for a first-article sample before the run. Comparing a physical part against a photo on a screen rarely ends well, especially on brushed and satin finishes.

Finally, name the measurement instrument. A profilometer reads Ra, Rz and Rmax. A comparator plate reads appearance. A gloss meter reads reflection. These three can disagree on the same part, so pick one and write it down.

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Number plus methodRa value and the instrument belong in the same note.
2
Mark the facesUse finish symbols, not a blanket note.
3
Sample firstApprove a physical part on visual finishes.
4
One instrumentAgree on Ra, Rz or gloss, not all three.

Supplier checks

What to verify before you place a polishing order

Ask who does the polishing. Some CNC shops outsource it to a job shop down the road. That adds a truck trip, a second handling step and a second chance for a dinged edge. GreatLight keeps surface finishing in house across three wholly-owned plants in Dongguan and Singapore.

Check the quality system against your industry. A general machine shop with ISO 9001:2015 is fine for brackets. Medical work needs ISO 13485:2016, automotive needs IATF 16949:2016, and any shop handling your drawings needs ISO 27001:2022 if you care about document security.

Ask what happens to a reject. A polishing shop that cannot rework a scratched part is a scrap risk on your PO. The practical question is whether they can step back one grit and re-polish without losing the dimension. If the answer is no, the part is scrap.

Confirm the inspection plan. GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring and a final inspection. Reports are available on request. If your drawing has a Ra callout, that is the report you want to see.

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In-house or outsourcedHandoffs add damage risk and lead time.
2
CertificationsMatch ISO 9001, IATF 16949, ISO 13485 or ISO 27001 to your sector.
3
Rework pathAsk if a scratched part can be re-polished.
4
Inspection reportRequest the measurement, not just a pass stamp.

Workflow

Six steps to release a polish order without surprises

Run these in order. Each one removes a common reason for rework.

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1. Define the function of the surfaceWrite down what the face does: seal, slide, reflect, or look good. A sealing face and a cosmetic cover need different Ra targets even on the same part.
2
2. Set the Ra and the instrumentPick a number from the table above and name the gauge. Ra 0.8–1.6 μm for visible covers, Ra 0.2–0.8 μm for sealing and optical seats.
3
3. Mark the faces and the stopsUse finish symbols on the drawing. Add "balance as machined" and list any face that must stay sharp or stay within ±0.005 mm.
4
4. Flag geometry the tool cannot reachCall out corners under Ø3 mm, pockets deeper than 5× the opening, and walls under 1 mm. Ask for a hand-finish note on those areas.
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5. Send material and quantity togetherState the alloy or polymer grade, the quantity, and whether the parts are one prototype or a 10,000-piece run. GreatLight has no minimum order quantity.
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6. Approve a sample before the runOn visual finishes, approve a physical first article. On Ra finishes, approve the inspection report. Then release the balance.

FAQs

Questions buyers ask about CNC polish services

Can polishing hold a tight tolerance?

Polishing removes material, so any face with a tolerance tighter than the stock you remove is at risk. On a face held to ±0.005 mm, the shop either masks it or polishes before the final finishing pass. Tell the shop which faces are critical, and the sequence gets planned around them.

As a rule, cosmetic faces can lose 0.02–0.05 mm during a hand polish. Functional faces should be machined to size after polishing, or left out of the polish area entirely.

Which Ra can GreatLight reach?

Fine polish reaches Ra 0.2–0.8 μm on aluminium, brass and stainless. Standard polish lands at Ra 0.8–1.6 μm, which covers most visible covers and mating faces. As-machined surfaces run Ra 1.6–3.2 μm.

The number depends on geometry and material as much as on the polishing operation. A flat aluminium plate reaches Ra 0.2 μm more easily than a deep stainless pocket.

Does polishing work on plastics?

Yes, with limits. Acrylic and polycarbonate can be brought to near-optical clarity with graduated grits and a final compound. Heat is the main risk, so the shop keeps the speed low and does not dwell.

POM, PA and PEEK take a satin finish but rarely a mirror. Carbon fibre parts are usually polished only on a resin-rich surface, and the weave pattern will still show through.

How does polishing compare to bead blasting or anodizing?

Bead blasting creates a uniform matte texture and hides small defects. Polishing flattens the surface and reflects light. Anodizing is a coating that adds a hard oxide layer, and it follows whatever texture is underneath.

A common sequence is polish then clear anodize for a bright, corrosion-resistant finish. If the part is bead blasted first and then anodized, the result is satin, not glossy.

What lead time should I plan for?

GreatLight returns a quotation and a free DFM analysis within 12 hours. Production can start within 24 hours, and parts ship in 3–5 days. The historical late-delivery probability is below 2%.

Add time for the sample step if you are approving a visual finish. That loop is usually faster than discovering a texture mismatch after the full run.

How do you protect the part during handling?

Uploads are secure and confidential, and an NDA is available on request. On the shop floor, polished faces get masked or bagged before the part moves to the next operation or into shipping.

Edge damage during transit is the most common defect on a polished part. Protective film and individual wrapping cost little compared with re-polishing a full batch.