CNC machining requires an understanding of surface roughness

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  Understanding surface roughness: what Ra measures and what it hides
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  Why cutter marks, feed and tool wear set your finish
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  When to cut the finish and when to add a process
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  How surface roughness changes sealing, wear and fit
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  How to call out surface roughness without over-specifying
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  Surface roughness values and where they fit

Ra is the arithmetic mean deviation of the profile from its center line, measured over a sampling length. It is an average, so it flattens the picture. A surface with one deep scratch and a mostly smooth field can post the same Ra as a uniform surface with shallow chatter. That is why Ra alone rarely tells you whether a seal will hold.

Rz is the average peak-to-valley height across the sampling length. Rt is the single largest peak-to-valley distance on the whole traced profile. If a mating face must not crush a gasket, Rz and Rt matter more than Ra. A part can meet Ra 1.6 μm and still have one 12 μm spike that punches through a thin coating.

Sampling length matters too. A 0.8 mm cut-off on a turned shaft ignores long-wavelength waviness that a 2.5 mm cut-off would catch. When you compare two inspection reports, check the cut-off and the evaluation length before you argue about the numbers.

Stated simply: Ra is a headline, not the whole story. Use it as a starting spec. Add Rz or Rt when function depends on peaks, and state the cut-off so both sides measure the same thing.

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  RaAverage deviation. Fast to measure, easy to compare, hides outliers.
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  RzAverage peak-to-valley. Better for sealing and coating adhesion.
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  RtMaximum peak-to-valley on the trace. Worst-case spike height.

On a milled face, the visible pattern is the step left between successive passes of the cutter. On a turned diameter it is the helical thread left by the feed. The theoretical peak-to-valley height depends on feed per tooth and the tool corner radius, so a 0.8 mm corner radius at 0.1 mm/tooth leaves a much shallower scallop than a sharp corner at the same feed.

That is the lever you pull first. Reduce feed per tooth and the scallop gets shallower. Increase the corner radius and the same feed produces a smoother surface. Both changes cost cycle time, so the real question is always how much money the tighter finish is worth.

Tool wear moves the number during a run. A carbide insert that holds Ra 1.2 μm on part 10 may drift to Ra 2.0 μm by part 200. For long runs we track the finish at set intervals and change the insert on a count, not on a hunch.

Vibration is the other big input. A thin wall or a long overhang lets the tool deflect, and the profile picks up chatter that no feed change will fix. Sometimes the answer is a support, a different toolpath, or a stress-relief step before the finishing pass.

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  Feed per toothLower feed leaves a shallower scallop, but adds cycle time.
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  Corner radiusLarger radius smooths the same feed; check clearance first.
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  Tool wearFinish drifts across a run. Set an insert change count.
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  VibrationChatter marks ignore feed settings. Fix the setup.

As-machined aluminum typically lands around Ra 1.6–3.2 μm. That is fine for brackets, housings and most internal faces. It is not fine for a hydraulic spool bore or an optical mount. Those need a second operation: fine boring, grinding, lapping or polishing.

Stainless and titanium behave differently. They work-harden and they tend to smear rather than shear cleanly, so the same parameters that give Ra 1.6 μm in 6061 may only reach Ra 2.5 μm in 316L. Sharp tooling, generous coolant and a positive rake geometry close most of that gap.

Soft plastics are their own case. POM and ABS cut cleanly but scratch easily, and a polished surface can be ruined by a careless deburr. If appearance matters, we leave the cosmetic face to the end of the program and protect it during handling.

Hardened steel above 45 HRC usually needs a grinding or hard-milling route. Trying to hit Ra 0.4 μm with a standard end mill on hardened stock burns tools and rarely holds the number across a batch.

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  Ra 1.6–3.2 μmNormal as-machined result. Brackets, housings, internal faces.
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  Ra 0.8–1.6 μmReachable with a controlled finishing pass on most alloys.
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  Ra 0.2–0.8 μmNeeds fine boring, lapping or polishing. Expect extra cost.

A static O-ring groove wants a smooth but not mirror surface. Too rough and the elastomer cannot fill the valleys; too smooth and the seal can slip or trap no lubricant. In practice Ra 0.8–1.6 μm on the groove floor and flanks works well for most hydraulic and pneumatic joints.

Sliding contacts are the opposite case. A cylinder bore or a linear bearing surface needs valleys that hold oil and a plateau that carries load. That is why plateau honing exists. A pure mirror finish on a sliding steel surface often scuffs because there is nowhere for lubricant to sit.

Interference fits depend on the actual contact area, which drops as roughness rises. A press fit designed on nominal diameters can lose grip if the surfaces are rough enough that peaks flatten under load. For critical fits we check the surface spec alongside the tolerance band.

Coating and bonding follow the same rule. Anodize, electroless nickel and structural adhesive all need a mechanical key. A bead-blasted or lightly brushed surface at Ra 1.6–3.2 μm usually bonds better than a polished one.

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  Static sealsRa 0.8–1.6 μm. Enough texture for the elastomer to bite.
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  Sliding contactsPlateau finish. Valleys hold lubricant, peaks carry load.
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  Press fitsRough surfaces reduce real contact area and grip.
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  CoatingsNeeds a key. Polished faces bond poorly.

Put the roughness symbol only on the faces that need it. A blanket note of Ra 0.8 μm across a whole drawing multiplies cost with no functional gain. Most parts have two or three faces that actually matter, and the rest can run as-machined.

State the parameter and the cut-off. Ra 1.6 μm at 0.8 mm cut-off is a different requirement from Ra 1.6 μm at 2.5 mm. Adding the cut-off removes most disputes at incoming inspection.

Say whether the value applies before or after coating. Anodizing adds 5–15 μm of oxide that follows the substrate, so a rough surface stays rough and often gets slightly rougher. Plating can level small peaks but not deep scratches.

Finally, mark the lay direction when it matters. A turned finish with circumferential lay can pump fluid along a shaft. A cross-hatched or random lay holds lubricant better on a wear face.

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  Spot calloutsSpec roughness only where function requires it.
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  State the cut-offRa 1.6 μm at 0.8 mm is not the same as at 2.5 mm.
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  Before or after coatingAnodize follows the substrate; plating levels small peaks only.