CNC Machining Polishing Service: What to Check Before You Award the Job

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  Key takeaways
• 2
  Seven checks for a CNC Machining Polishing Service
• 3
  Why an Ra target needs a face and a direction
• 4
  Stock removal versus your tolerance window
• 5
  Media, compound and grit sequence
• 6
  How the finish will be measured and reported
• 7
  Lead time, MOQ and quote format
• 8
  Seven steps to qualify a polishing supplier

Ra is an average. Two surfaces with the same Ra value can look and behave differently because the peak count and the lay direction differ. A milled face polished along the tool marks reads lower than the same face polished across them. If the drawing just says "polish to Ra 0.8 μm", the polisher will pick the easiest direction and the part may still fail a wipe test.

Write the callout per face. A sealing face, an optical bore and a visible cover need different numbers. GreatLight works to Ra 0.2–0.8 μm for fine finishes, Ra 0.8–1.6 μm for high cosmetic finishes, and Ra 1.6–3.2 μm as machined. Put the target number on the face that sees the seal, the light, or the customer's hand.

Direction matters more on curved geometry. A 5-axis path leaves a scallop pattern that a single-direction polish cannot flatten without cutting into the radius. Tell the polisher which axis the part was cut on, or send the setup sheet. It saves a rework cycle.

Polishing is a subtractive step. On 6061 aluminium you can lose 0.02–0.05 mm on a sharp edge before the surface looks right. On hardened 440C or 17-4PH the same visual result may take 0.005 mm or less, but the pressure needed can round a thin wall.

This is where a single-source shop wins. If the part was machined to ±0.005 mm and then handed to a separate polisher, nobody owns the stack-up. When GreatLight machines and polishes in the same plant, the CAM programmer leaves the right amount of stock on the faces that will be polished and holds the critical bores untouched.

Ask the polisher two questions: how much material do you expect to remove, and how will you protect the faces that must not change? A shop that cannot answer both is guessing. For a part with a 0.1 mm wall or a thread, that guess is expensive.

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  Thin wallsBelow 0.8 mm, hand pressure can deflect the part. Ask for backing or a fixture.
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  Threads and splinesMask them or polish before cutting. Loose abrasive rounds the crest.
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  Sharp edgesState the edge break you want. Polishing will radius it whether you asked or not.

A credible polishing plan names the sequence, not just the final finish. For stainless steel that is usually a coarse abrasive to remove the machining lay, a medium step to blend, then a fine compound on a soft wheel. Skip the medium step and you get a mirror with visible scratches under raking light.

Compound choice follows the material. Aluminium loads a wheel fast, so the polisher needs a lubricated compound and lower wheel speed to stop the surface smearing. Titanium and Inconel work-harden, so heavy pressure is the wrong move. Plastics like POM and PMMA need low speed and a clean wheel or the surface burns.

Ask what the shop does between material lots. A wheel used on carbon steel and then on 304 stainless will leave ferrous contamination that shows up as rust spots after a humidity test. Good shops keep separate wheels per material family and mark them.

A visual sign-off is not a measurement. A portable roughness tester gives Ra, Rz and Rmax in seconds, and the polisher should own one. Ask for a trace on the polished face, not on a scrap coupon that was polished alongside.

On large parts the measurement position matters. A 4,000 mm frame will not polish evenly from one end to the other. Ask for readings at three points along the length so you can see the spread. If the spread is wide, the process is not controlled.

For medical and automotive work, the report also needs a dimensional check after polishing. GreatLight inspects 100% before shipment, with raw material check, in-process monitoring and final inspection, and reports on request. That covers the polished surfaces as well as the machined features.

Polishing adds a step that cannot always be parallel to machining. Hand polishing of a complex 5-axis part can take longer than the cut itself. A quote that treats polishing as a free add-on is either using a very light cosmetic pass or has not thought it through.

Ask for the lead time split by operation: machining days, polishing days, inspection days. GreatLight can start production within 24 hours of a released order, and typical parts ship in 3–5 days, but a mirror finish on a large frame will sit at the polishing bench longer than a bead-blasted bracket.

MOQ is the other filter. Some polishing houses only run batches because setup cost dominates. If you need one prototype for a design review, a no-MOQ shop is the practical choice. GreatLight runs from one prototype to 10,000+ part runs with no minimum order quantity.