What Is Surface Finish in CNC Machining?
Surface finish is the texture left on a part after the tool passes. It decides friction, sealing, coating adhesion and fatigue life. This page is for engineers and buyers who need to pick a number and defend it.
What surface finish actually measures
Surface finish describes the small-scale geometry of a cut face. It is not one number. A profile trace gives you two families of values: amplitude, which is how tall the peaks and valleys are, and spacing, which is how far apart they sit. Ra is the arithmetic mean of that profile over the sampling length. Rz is the average of the five highest peaks to the five deepest valleys in the same stretch.
Ra is popular because it is easy to compute and easy to put on a drawing. It is also blunt. Two surfaces can share the same Ra while behaving nothing alike. One may be a gentle wave, the other a field of sharp spikes. A seal does not care about the average. It cares about the tallest peak that can tear the elastomer.
That is why a callout of Ra alone is risky on sealing faces, bearing journals and fatigue-critical fillets. Add Rz or Rmax when peak height matters. Add a bearing-area curve when the part must hold oil. The measurement costs one extra trace and saves a rejected batch.
Which cutting parameters move the number
Feed per tooth sets the theoretical cusp height. A 12 mm two-flute cutter at 0.05 mm per tooth leaves a scallop you can feel with a fingernail. Drop to 0.02 mm per tooth and the same cutter leaves a face that reads near Ra 0.8 μm. Feed is the strongest lever you have, and it is the one that costs cycle time.
Tool nose radius and edge condition come second. A worn edge rubs instead of shears, and the surface smears. On aluminium this shows as built-up edge and torn patches. On 316L stainless it shows as work hardening and a dull grey face. Change the insert before the finish drifts, not after the parts fail inspection.
Spindle speed and depth of cut interact with the material. Higher speed usually lowers Ra in aluminium and mild steel, up to the point where chatter appears. Light finishing passes at 0.1–0.3 mm depth clean up what a heavy roughing pass left behind. On thin walls, lower the radial engagement instead of slowing the spindle.
- 1Feed per toothThe main driver of cusp height on milled faces.
- 2Tool nose radiusLarger radius spreads the cut and lowers Ra.
- 3Edge wearRubbing replaces shearing; finish falls off fast.
- 4ChatterShows as regular pitch marks; fix rigidity first.
Why the same cutter gives different results
Ductile materials such as 6061 aluminium, C36000 brass and low-carbon steel shear cleanly and polish well. They also gum up a sharp edge if the speed is too low. Brittle materials behave in the opposite way. Magnesium AZ31B cuts fast and dry but chips can ignite, so the finish plan has to include chip control.
Titanium TC4 and Inconel 718 sit in a harder class. They conduct heat poorly, so the cutting edge runs hot and the surface work hardens under a dull tool. A second pass over a work-hardened layer peels instead of cuts. Keep the finishing pass in the same setup and never let the tool dwell.
Plastics need their own rules. POM and PEEK machine to a fine matte finish with sharp, polished tooling and high rake. ABS and PC soften with heat and tear. Air blast beats coolant here, because a wet chip sticks to the face and drags across it.
How the machining process sets the floor
Three-axis milling reaches Ra 1.6–3.2 μm on flat faces without much effort. Ball-nose finishing on curved surfaces gets to Ra 0.8–1.6 μm when the stepover is kept small. Five-axis continuous contact keeps the tool normal to the surface and avoids the direction change marks you get from indexed passes.
Turning produces a helical lay that Ra captures well on cylindrical faces. Mill-turn centers cut both in one setup, which removes the mismatch at the joint between a turned diameter and a milled flat. That mismatch is a common source of leaks on hydraulic manifolds.
Grinding is the step after machining when the callout goes below Ra 0.4 μm or when flatness matters more than texture. Bead blasting, tumbling and brushing change the look and the texture together, so specify them as a separate operation after the dimensional cuts, and mask anything that must stay sharp.
Matching surface finish to the job
Pick the finish the function needs, not the tightest number available.
| Finish band | Typical use | Watch out for |
|---|---|---|
| Ra 3.2 μm as-machined | Brackets, covers, weld prep | Coating may show tool marks |
| Ra 1.6 μm standard | General machined faces | Seals still need Rz control |
| Ra 0.8 μm fine | Bearing seats, sliding faces | Cost rises with inspection time |
| Ra 0.4 μm or better | Hydraulic seals, optics mounts | Needs grinding or lapping |
| Anodized as-machined | Housings, EV parts | Anodizing amplifies scratches |
| Bead blasted | Cosmetic covers, handles | Masks edge break and radii |
The trade-off in one line
If the face seals or slides, specify Ra with Rz and accept the cost; if it only covers or mounts, Ra 3.2 μm as-machined is enough and chasing lower wastes cycle time.
Common questions
Is Ra the same as RMS?
No. Ra is the arithmetic mean deviation; RMS (or Rq) is the root mean square of the same profile. RMS weights tall peaks more heavily, so it always reads slightly higher than Ra on the same surface.
For a near-sinusoidal profile the ratio is about 1.11. On a spiky surface it can exceed 1.3. If a drawing says RMS, do not substitute Ra without checking.
Can I get Ra 0.2 μm from a milling cutter?
Only in narrow cases. A fine finishing pass with a sharp, balanced tool on aluminium or brass can approach Ra 0.2–0.8 μm on a flat face. Deep pockets, long reach tools and hard alloys will not get there.
When the callout is below Ra 0.4 μm across a whole part, plan for grinding, lapping or polishing after machining.
Does anodizing change the surface finish reading?
Yes. Anodizing grows an oxide layer that follows the existing texture and usually raises Ra by a small amount. Hardcoat builds a thicker layer and amplifies tool marks and scratches rather than hiding them.
If the final finish is anodized, inspect the machined surface with that in mind. A face that looks fine bare can look scratched after coating.
Why did my parts pass Ra but leak at the seal?
Because Ra averages the profile. A face with one tall peak and a deep valley can still hit the Ra target while the peak cuts through the seal lip.
Add Rz or Rmax to the callout and check the bearing-area curve. On sealing faces the peak height matters more than the mean.
How do I write a surface finish callout on a drawing?
Put the parameter, the value and the unit together, for example Ra 1.6 μm, and state whether it applies to the whole part or to marked faces. Add a note for the process only when it is required, such as ground or bead blasted.
Keep the number realistic. A blanket Ra 0.4 μm on every face raises cost across the part for no functional gain.
Does surface finish affect corrosion resistance?
It can. A rougher face holds moisture and chloride longer, so pitting starts sooner on stainless. Polished and passivated surfaces drain and dry faster.
For 316L parts in wet service, a finer finish plus passivation is worth the extra step.
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