Ra vs Rz vs Rq.
Arithmetic mean
Average deviation from mean line. Statistically smooths out peaks — a surface with few deep pits and one extreme peak can have same Ra as uniform surface. Easy to measure, widely used, sometimes misleading.
Max peak to valley
Average of five largest peak-to-valley heights in sample. Better captures extreme peaks — more relevant for fatigue life, sealing applications.
Root mean square
RMS deviation — similar to Ra but weights larger deviations more. Used in optical applications. Typical Rq ≈ 1.25 × Ra.
Max peak
Maximum peak height above mean line. Critical when peaks cause problems (sealing, biological applications).
Max valley depth
Maximum valley depth below mean line. Important for corrosion initiation sites, crack initiation under fatigue loading.
Peak to valley
Rp + Rv — total peak-to-valley range over sample length. Captures single extreme features that Ra/Rz smooth out.
Different parameters measure different aspects of surface texture. Ra is most common but not always appropriate.
What Ra each process delivers.
| Process | Typical Ra (µm) | Best case Ra | Rz equivalent |
|---|---|---|---|
| Rough milling | 6.3-12.5 | 3.2 | 50-100 µm |
| Finish milling | 1.6-3.2 | 0.8 | 10-20 µm |
| Fine milling (CBN/diamond) | 0.4-0.8 | 0.2 | 3-5 µm |
| Turning (rough) | 3.2-6.3 | 1.6 | 20-40 µm |
| Turning (finish) | 0.8-1.6 | 0.4 | 5-10 µm |
| Drilling | 1.6-3.2 | — | 10-20 µm |
| Reaming | 0.8-1.6 | 0.4 | 5-10 µm |
| Grinding | 0.2-0.8 | 0.05 | 2-5 µm |
| Honing | 0.1-0.4 | 0.05 | 1-3 µm |
| Lapping | 0.05-0.2 | 0.01 | 0.5-1 µm |
| Polishing (manual) | 0.05-0.2 | 0.01 | 0.5-1 µm |
| Electropolishing | 0.1-0.4 | 0.05 | 1-3 µm |
| Bead blasting | 1.6-6.3 | — | 10-50 µm |
| Anodizing | Matches substrate +10% | — | — |
| Plating | Matches substrate | — | — |
Typical surface finish by manufacturing process. Specify Ra within process capability or process is wrong.
Drawing symbols.
Basic symbols
Parameter callouts
Applications requiring specific Ra.
Sealing surfaces: O-ring glands: Ra 0.4-0.8 µm. Smoother than this, O-ring extrudes. Rougher, O-ring doesn't seal. Gaskets against soft seal: Ra 0.8-1.6 µm. Metal-to-metal sealing: Ra 0.1 µm required. Specify Ra matched to seal type.
Bearing surfaces: Rolling bearings (outer/inner races): Ra 0.2 µm. Plain sliding bearings: Ra 0.4-0.8 µm (grooved for lubricant retention). Hydrodynamic bearings: Ra 0.1-0.2 µm with crosshatch pattern. Improper Ra causes premature wear.
Fatigue-loaded parts: Surface roughness directly affects fatigue life — deeper valleys initiate cracks. Aerospace fatigue parts: Ra 0.4-0.8 µm typical. For extreme fatigue (spring, high-cycle): polished to Ra 0.1 µm or shot peened.
Cosmetic surfaces: Consumer product surfaces: Ra 0.4-1.6 µm depending on finish type. Polished appearance: Ra 0.2 µm or better. Matte appearance (bead blast): Ra 1.6-3.2 µm for even texture.
Optical surfaces: Reflective optics: Rq 0.05 µm or better for visible light. Infrared optics: Rq 0.2 µm adequate. UV optics: Rq 0.025 µm or better. Requires specialized polishing processes beyond standard machining.
Medical and food contact: Food contact: Ra 0.8 µm (3-A Sanitary). Pharmaceutical (BPE): Ra 0.38 µm electropolished. Medical implant articulating surfaces: Ra 0.025-0.05 µm (joint replacement). Specify per regulatory requirement.
FAQ
Ra 0.4 costs roughly 2× Ra 0.8 to produce. Specify Ra 0.4 only when function requires. Over-specifying Ra is a common cost increase. Review each surface: is this a mating surface? Seal? Cosmetic? Apply appropriate Ra per function. Non-critical surfaces can often be Ra 1.6 or 3.2 (standard milled finish).
For most applications, Ra is adequate. Specify Rz when extreme peaks matter: sealing surfaces (peaks poke through gasket), fatigue loading (peaks become stress concentrators), optical applications. For "normal" mating or general surfaces, Ra is the standard spec. Check application — if peaks matter, specify Rz in addition to Ra.
Sometimes. For mating surfaces, perpendicular texture (90° to sliding) seals better than parallel texture. For aesthetic surfaces, lay direction affects visual appearance. For fatigue, texture perpendicular to load is more problematic than parallel. Specify lay direction via arrow on surface finish symbol when it matters. For general surfaces, unspecified lay acceptable.
Standard measurement: contact profilometer — stylus traverses surface measuring height variations. Non-contact: optical profilometer (white light interferometry), AFM (atomic force microscopy for sub-micron). For production, contact profilometers are standard and fast. ISO 4287 defines standard measurement procedure. Our shop uses Mitutoyo SJ-410 profilometer — traceable to NIST.
Anodizing: follows substrate finish, adds minor texture. Ra typically 10-20% higher than substrate. Hardcoat anodize rougher than Type II. Plating: generally follows substrate finish. Electroless nickel plating smoother than substrate. Hard chrome plating smoother still. For critical post-finish Ra, specify substrate Ra + post-finish Ra separately.
Common cost problem. Ra 0.025 µm is lapping — 20× cost of standard CNC. Specifying this on nothing-critical surface wastes money. DFM review: if Ra below 0.4 µm, question whether needed. Often the drawing was copied from similar part where it actually was needed. Ask manufacturer for actual Ra delivered — may be adequate without explicit spec.
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