CNC Surface Finishing Guide

• 1
  What actually creates a machined surface
• 2
  How Ra is measured and where the number comes from
• 3
  Machined, blasted, brushed, polished, coated
• 4
  Why the same finish behaves differently per alloy
• 5
  Specifying finish without over-constraining the shop
• 6
  Finish options, typical Ra and where they fit

Every machined surface is a set of tool marks. The cutting edge travels across the workpiece and leaves a repeating pattern whose pitch and depth depend on feed per tooth, tool nose radius, spindle speed and how much the tool deflects under load. Change any one of those and the surface changes.

The theoretical peak-to-valley height for a turning or milling pass follows the tool geometry. A larger nose radius at the same feed flattens the scallops and lowers Ra. Pushing feed up at a fixed radius deepens them. That is why two shops can hit the same ±0.005 mm dimension and still hand back visibly different surfaces.

Real surfaces are never purely geometric. Built-up edge, chatter, chip recutting and tool wear all add randomness on top of the ideal profile. A worn insert may still hold size while producing torn aluminium and a Ra reading twice what the drawing allows.

• 1
  Feed per toothThe single largest lever on Ra in milling and turning.
• 2
  Nose radiusLarger radius at the same feed gives a shallower scallop.
• 3
  Tool wearA dull edge rubs instead of shearing and raises Ra fast.
• 4
  RigidityLong overhangs and thin walls convert cutting force into chatter.

Ra is the arithmetic mean deviation of the roughness profile from its mean line, sampled over a cutoff length. It says nothing about the shape of the peaks. A surface with sharp, closely spaced peaks and one with rounded, widely spaced peaks can report the same Ra and wear very differently in service.

The cutoff length matters. A 0.8 mm cutoff filters out longer waviness; a 2.5 mm cutoff lets it back in. If a drawing calls out Ra 0.8 μm without a cutoff, the inspection result depends on the instrument setting. State the standard and the cutoff on the drawing.

Typical as-machined output from our 127 high-precision CNC machines sits at Ra 1.6-3.2 μm. Careful finishing passes reach Ra 0.8-1.6 μm. Below Ra 0.8 μm we change the process, not just the parameters, because the surface becomes the limiting factor rather than the geometry.

• 1
  State the cutoff0.8 mm and 2.5 mm cutoffs give different Ra values.
• 2
  Direction mattersRa across the lay is usually higher than along it.
• 3
  Sample the right areaMeasure where the part functions, not on a free face.

Machined finishes are the cheapest because they need no extra setup. Bead blasting removes tool marks and gives a uniform matte surface, and it hides small scratches well. It also rounds edges slightly, so do not specify it on a sealing face or a press fit where edge geometry is functional.

Brushing produces a directional grain, usually on stainless and aluminium for appearance. Polishing steps through progressively finer abrasives to reach a reflective surface, and it is labor intensive. On aluminium, polishing is often followed by clear anodizing to lock in the look.

Coatings change the surface rather than removing material. Anodizing grows an oxide layer that adds roughly half its thickness to each dimension, which matters on tight-tolerance features. Electroless nickel deposits evenly on complex geometry and holds tolerance well. Powder coating adds 50-100 μm per side and should never be specified on a mating bore.

• 1
  As-machinedRa 1.6-3.2 μm, no extra cost, keep tool marks visible.
• 2
  Bead blastingUniform matte, masks scratches, slightly breaks edges.
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  AnodizingClear, colour, hardcoat or conductive; dimension grows.
• 4
  Electroless nickelEven build on complex parts, good for wear surfaces.

Aluminium 6061 machines cleanly and takes a fine finish without much effort, but it is soft and galls onto the tool if the cutting edge is dull or the coolant is weak. High-silicon grades such as ADC12 are abrasive and wear tools quickly, so surface quality drifts unless inserts are changed on schedule.

Austenitic stainless 304 and 316 work harden. If the tool rubs instead of cutting, the surface hardens and the next pass cuts worse material. Light passes with a sharp edge and enough feed to stay under the hardened layer are better than a slow, gentle pass. Free-machining 303 avoids most of this.

Titanium Ti-6Al-4V has low thermal conductivity, so heat stays at the cutting edge. It also tends to smear. Finishes on titanium are usually rougher than the same operation on steel, and polishing is harder because the material loads the abrasive. Inconel behaves similarly and is slower still.

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  Aluminium 6061Easy to finish, galls if the edge is dull.
• 2
  Stainless 304/316Work hardens; avoid rubbing passes.
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  Ti-6Al-4VHeat stays at the edge and the surface smears.
• 4
  Plastics and PEEKFuzz, melt or chip depending on feed and cooling.

Call out finish only where it does work. A housing bore that locates a bearing needs a defined Ra; the outside of the same housing usually does not. Blanket-calling a fine finish on every face adds cost and forces slow passes on geometry that never touches anything.

Use the standard symbol and add the process when it matters. If the surface must be bead blasted after machining, say so, because that changes the order of operations and may require masking. If a face must stay as-machined for appearance, say that too, otherwise the shop may blast it for uniformity.

For functional surfaces, pair Ra with a geometric tolerance. A sealing face with good Ra but poor flatness still leaks. On hydraulic and pneumatic parts, flatness or cylindricity usually controls performance more than the roughness average does.

• 1
  Finish where it functionsNot on every face of the part.
• 2
  Name the processBead blast, brush or polish changes the routing.
• 3
  Pair Ra with formFlatness and cylindricity often matter more.