Precision CNC surface finish: what Ra really tells you

• 1
  Where the texture on a precision CNC surface finish comes from
• 2
  How Ra and Rz describe the same surface differently
• 3
  Which finishing process reaches which range
• 4
  Why the number matters to fatigue, friction and sealing
• 5
  How to put a precision CNC surface finish on a drawing
• 6
  Process, typical reachable Ra and where it fits

Every milling or turning pass leaves a groove pattern behind. The tool nose radius, the feed per tooth and the spindle speed set the spacing of those grooves; the depth depends on how much the tool deflects and how much the material springs back. Even on a stable machine, the surface you measure is the sum of these effects, not a single number the operator dials in.

Built-up edge is the usual culprit when roughness drifts. Aluminium and low-carbon steel tend to weld fine chips onto the cutting edge at low speeds. The edge grows, breaks off and smears material across the flank face, and Ra can jump two grades on the same program. Raising surface speed or switching to a coated insert with a sharper rake usually settles it.

Material matters as much as the machine. 6061-T6 cuts clean and holds Ra 1.6–3.2 μm as machined. 304 stainless work-hardens under a dull tool, and titanium Ti-6Al-4V conducts heat poorly, so both push the same cutter toward a rougher result unless feeds and coolant are changed. Plastics cut clean but leave fuzz at the exit edge.

Vibration and fixturing sit underneath all of it. A thin wall that rings at 2,000 rpm will show chatter marks spaced at the tooth frequency. Stiffening the setup or dropping radial engagement often does more for the finish than any downstream polish.

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  Feed dominatesHalving feed per tooth roughly halves the theoretical cusp height.
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  Sharp edge winsA worn insert raises cutting force and roughness together.
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  Setup is half the jobChatter marks cannot be polished out of a functional seat.

Ra is the arithmetic mean deviation of the profile from its center line over the sampling length. It averages peaks and valleys, so a surface with a few deep scratches and a mostly smooth field can still report a good Ra. That is the main reason a part can pass an Ra check and still leak at a seal face.

Rz is the average peak-to-valley height across the sampling length. It reacts to the deepest grooves, which is why seal faces, hydraulic bores and bearing journals are often specified in Rz. A turned shaft at Ra 0.8 μm can sit at Rz 4 μm if the feed marks are deep and regular.

Measurement method changes the number too. A skidded stylus with a 2 μm tip filters out fine detail; a 5 μm tip on a curved surface reads higher because it cannot reach into narrow valleys. Optical profilometers read area rather than a line, and they disagree with contact instruments on porous or translucent materials.

Cutoff length has to match the spec. ISO 4288 sets the default cutoff from the roughness value itself. If a drawing says Ra 0.8 μm and the inspector uses a 0.8 mm cutoff when the correct one is 2.5 mm, the reported value is not comparable. Put the standard and cutoff on the drawing, not just the number.

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  Ra for general controlGood for sliding fits and appearance specifications.
• 2
  Rz for sealingUse where a single deep valley can cause a leak path.
• 3
  State the standardISO 4288 cutoff and evaluation length belong on the drawing.

As-machined results sit between Ra 1.6 and 3.2 μm for most metals on a well-controlled 3-axis or 5-axis cut. That is enough for brackets, housings, mounting plates and most non-sealing interfaces. Going below it usually means changing the process, not just the parameters.

Bead blasting produces a uniform matte surface and masks tool marks rather than removing them. It adds no dimensional control and can round sharp edges if the operator is not careful. Glass bead at low pressure on aluminium gives a consistent cosmetic finish; the same setting on soft brass may embed media.

Tumbling and vibratory finishing deburr and edge-break efficiently, but they work the whole part. A Ø6 mm dowel hole that needs to stay sharp will lose its edge. Brushing gives directional grain that hides scratches on flat faces but is hard to control on complex 3D geometry.

Polishing and lapping reach Ra 0.2–0.8 μm and below, but they are hand-intensive and geometry-limited. A deep pocket or an internal channel cannot be reached with a wheel. Where a fine finish is needed inside a bore, it usually has to come from a reamer, a fine boring head or a burnishing tool during the cut.

• 1
  Blasting hides, not removesSurface texture is randomized, not reduced.
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  Mass finishing is blindIt touches every edge on the part, wanted or not.
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  Internal features need in-cut finishBores and channels are hard to reach with abrasives.

Fatigue cracks start at surface discontinuities. A turned or milled surface under cyclic load concentrates stress at the groove roots, and a rough surface can cut fatigue life noticeably compared with a polished one of the same material. For rotating or vibrating parts, the finish specification is a life specification.

Friction and wear follow the real contact area, not the nominal one. Two Ra 3.2 μm surfaces touch only at the peaks, so the local pressure is high and the peaks wear in quickly. A finer finish raises real contact area and lowers running-in wear, which matters for sliding guides and cam surfaces.

Sealing is where surface texture becomes binary. An O-ring needs a smooth, non-directional surface; a spiral groove from turning gives the seal a path to leak along. That is why seal counterfaces are usually specified in Rz with a plunge or axial grind orientation rather than a longitudinal turn.

Coating adhesion depends on the same texture. Anodizing, electroless nickel and powder coating all need a clean, slightly roughened surface to key into. A mirror polish before plating often produces a coating that chips at the first handling ding, because there is nothing for the coating to grip.

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  Fatigue starts at groovesCyclic parts deserve a finish callout, not a default.
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  Seals hate directionOrientation matters as much as the Ra value.
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  Too smooth can hurt coatingPlating needs some anchor profile to bond.

Specify finish only where it does something. A blanket note of Ra 0.8 μm over the whole part raises cost across every face, including the ones nobody touches. Put the symbol on functional faces: seal counterfaces, bearing seats, sliding bores, optical mounts.

Give the direction where it matters. The ISO 1302 symbol lets you state whether the lay is parallel, perpendicular, crossed, circular or non-directional. For a rotating seal, a non-directional or perpendicular lay is very different from a longitudinal one at the same Ra.

State the measuring standard and the cutoff. ISO 4288 and ASME B46.1 do not produce the same reported value on the same surface. If the part crosses borders, say which one the inspection report should follow, and ask for the profile trace, not just the number.

Leave room for the process. If a bore needs Ra 0.4 μm, the machining plan needs a fine boring or roller burnishing step, which adds a setup. Telling the shop the function lets them choose the cheapest route to the same result.

• 1
  Stop the blanket noteFinish the faces that do work; leave the rest as machined.
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  Lay direction is a specSame Ra, different sealing behavior.
• 3
  Ask for the traceA single Ra number hides the profile shape.