Ra vs Rz roughness: difference, measurement and limits
On the lip seal seat of a hydraulic drive shaft, the surface finish symbol calls for Ra 0.8 together with Rz 3.2 under ISO 21920. The metrologist sets the stylus roughness tester against the edge of the part, finished by turning. If they pick the wrong cut-off wavelength filter (lambda-c), the displayed reading can double, and a scored seat that will leak in service gets approved.
Ra and Rz: definitions and difference
ISO 21920, which replaces the former ISO 4287 and ISO 4288, defines Ra and Rz from the digital filtering of the primary profile. Ra measures the arithmetic mean deviation of the peaks and valleys from the mean line. Rz measures the maximum height of the profile, from the highest peak to the deepest valley.
| Parameter | ISO 21920 definition | Sensitivity to isolated peaks | Main use |
|---|---|---|---|
| Ra (arithmetic mean) | Arithmetic mean deviation of the profile | Low (smoothing effect) | General machining control |
| Rz (maximum height) | Maximum height of the profile | Very high (catches scratches) | Sealing and seal seats |
| Rt (total height) | Total height over the full traverse | Highest | Assessing tool marks |
Ra smooths out the extremes. A surface with broad, gentle waves and a surface with one fine but very deep scratch can both show the same Ra of 0.8 µm.
Rz captures how severe the highest peak and the deepest valley are. For elastomer seals, a single deep scratch (high Rz) is enough to cause a leak, even when the overall Ra stays perfect. That’s why specifying Ra alone doesn’t guarantee a seal seat.
Aerospace drawings therefore often pair a maximum limit on Ra with a maximum limit on Rz: Ra keeps the average friction in check, while Rz prevents fatigue cracks from starting in machining micro-valleys.
Choosing the λc cut-off and the evaluation length
| Ra roughness range (µm) | Cut-off lambda-c (mm) | Sampling length lp (mm) | Evaluation length ln (mm) |
|---|---|---|---|
| Over 0.02 up to 0.1 | 0.25 | 0.25 | 1.25 |
| Over 0.1 up to 2.0 | 0.8 | 0.8 | 4.0 |
| Over 2.0 up to 10.0 | 2.5 | 2.5 | 12.5 |
| Over 10.0 up to 80.0 | 8.0 | 8.0 | 40.0 |
The choice of cut-off sets the exact boundary between roughness (micro-geometry) and waviness (macro-geometry) of the machined surface. The lambda-c filter extracts the profile frequencies that make up the surface finish: for a standard requirement of Ra 0.8 µm, the standard calls for a 0.8 mm cut-off.
The standard evaluation length ln is made of 5 successive sampling lengths. With a 0.8 mm cut-off, the stylus must travel over at least 4.0 mm of usable profile.
Cutting the stylus travel down to 1.5 mm to save time destroys the statistical repeatability of the measurement and invalidates the inspection record.
The specifications behind surface finish symbols are on our page about ISO geometric symbols.
Full example: measuring roughness on a bearing seat
A technician inspects the surface finish of the bearing seat on a 15-5PH stainless steel propeller shaft. The drawing specifies ISO 21920 - Ra max 0.8 µm / Rz max 3.2 µm.
The check is done with a diamond stylus roughness tester with a 2 µm tip radius, calibrated on a certified glass standard before the measuring session. The part is held at 20 °C.
- Cut-off lambda-c: 0.8 mm.
- Evaluation length programmed: 5 × 0.8 = 4.0 mm.
- Trace lengthways, perpendicular to the turning marks.
- Ra value given by the instrument: 0.92 µm.
- Rz value given by the instrument: 3.85 µm.
Verdict: the Ra value (0.92 µm) exceeds the 0.8 µm limit by 0.12 µm. The Rz value (3.85 µm) exceeds the 3.2 µm limit by 0.65 µm. The seat is declared non-conforming. Polishing with fine emery cloth or a superfinishing grinding pass is needed to bring the roughness peaks down.
Common pitfalls when measuring roughness on the shop floor
The first pitfall concerns the tracing direction. On a turned part, the tool marks run around the circumference. The trace must run along the shaft axis (perpendicular to the marks). Tracing along a circumferential mark gives an Ra three times too low.
The second comes from the radius of the diamond stylus tip. ISO 21920 recommends a 2 µm tip radius for low roughness values. A worn stylus whose tip radius has reached 10 µm can’t get to the bottom of the machining micro-valleys, which hides Rz defects.
The third is applying the 16% rule instead of the max rule. Under ISO 21920, if the word “max” doesn’t appear next to the symbol (for example Ra 0.8), the part is accepted when fewer than 16% of the readings exceed the value. If “max” is written (Ra max 0.8), no reading may exceed it anywhere on the profile.
To bring these roughness parameters into your inspection grids with FAI traceability, CoteCote extracts the surface finish requirements from the drawing and fills your dimensional quality inspection tables.