Key takeaways
- Ra is an average height over the sampling length, so a few deep scratches barely move it.
- Rz is a peak to valley height, so it reacts to the deepest valley and the highest peak.
- For the same surface, Rz is typically four to six times Ra, which is why a single number is not a specification.
- Name the parameter, the value, the cutoff and the measurement direction, or two inspectors will disagree.
What Ra actually measures
Ra is the arithmetical mean deviation of the assessed profile. The instrument traces the surface, filters the profile, and averages the absolute distance of every point from the mean line over the sampling length. Because it is an average, it behaves like an average of anything: a large area of small variation and a small area of large variation can produce the same figure.
That is useful for a general finish call and misleading for a functional one. A turned face with a consistent helical lay and a face with one deep score can share an Ra value, even though a seal or a contact would behave differently on each. Ra describes the general level of the surface, not the worst point on it.
Ra is also direction sensitive. A turned surface has a strong lay around the circumference, and the reading across the lay differs from the reading along it. The drawing should say which direction the value applies to, or the inspector should record it.
What Rz measures
Rz is the maximum height of the profile within a sampling length, measured as the sum of the highest peak and the deepest valley from the mean line. In the current ISO 4287 definition it is a single peak to valley figure. The older German definition, sometimes written Rz(DIN), averaged the five highest peaks and five deepest valleys, so a drawing that cites Rz without a standard can mean two different things.
Because Rz looks at extremes, it is the better call where a peak can do damage. A sealing face, a bearing surface or an electrical contact can fail on one high spot, even when the average is acceptable. Rz also correlates better with how a surface feels and how it wears in, because the peaks carry the load first.
The trade-off is repeatability. Rz depends on the single worst feature in the traced length, so it scatters more between measurements and between instruments. A single scratch from handling can change Rz without changing the process.
The two parameters side by side
The table gathers the practical differences. It is a reading guide, not a replacement for the standard.
| Point | Ra | Rz |
|---|---|---|
| Definition | Mean of absolute profile deviation | Maximum peak to valley height |
| Reacts to one deep scratch | Barely | Strongly |
| Repeatability | Good, stable across traces | Lower, depends on the worst feature |
| Typical ratio | Reference value | About 4 to 6 times Ra on the same surface |
| Good for | General finish, process control | Sealing, contact and wear faces |
| Risk if used alone | Hides isolated peaks and valleys | One defect fails an otherwise good part |
The ratio depends on the process, the material and the filter. It is not a conversion factor.
Why one surface gives two numbers
A milled face with a scalloped path has a regular peak and valley pattern. Its Ra sits at roughly a quarter to a sixth of its Rz, because the average is dominated by the many points on the flanks rather than by the extremes. Grinding produces a finer, denser pattern, and the ratio usually stays in a similar band, though the absolute values are much lower.
The ratio breaks down when the surface has two populations: a fine background and a few deep marks. A casting that has been faced and then scratched in handling can have an acceptable Ra and a poor Rz. That is exactly the case where a single Ra call passes a part that will leak or arc.
The ratio also depends on the cutoff and the filter. A short cutoff follows the fine detail and ignores the long wave. A long cutoff includes more of the waviness, which is a shape error rather than roughness. Two labs can measure the same part and report different numbers without either being wrong, simply because the settings differ.
How to write the callout
A complete callout has four parts: the parameter, the value, the cutoff and the process or direction where it matters. On a drawing to ISO 1302, the surface texture symbol carries the parameter and the value above the line, and the machining method or note beside it. A typical call is Ra 1.6 with a 0.8 mm cutoff, or Rz 6.3 where a peak limit matters.
Do not put a finish call on every face. Put it on the faces that seal, slide, carry current or locate, and leave the rest to the general note. A drawing with Ra 0.8 on every surface tells the supplier that the finish is decorative, and the price will reflect the caution rather than the function.
Where a surface is anisotropic, add the lay symbol. A turned seal journal and a milled sealing face do not behave the same way at the same Ra, because the lay controls how a seal or a gasket beds in. If the direction is not controlled, the number is only half a specification.
One number is not a finish specification
Parameter, value, cutoff and direction together describe a surface. Remove any one of them and the inspection result becomes negotiable.
What each process can hold
The bands below are typical for a capable shop and a stable process. They are illustrative, and the real limit depends on the material, the tool and the rigidity of the setup.
- Rough turning and milling: Ra 3.2 to 6.3 µm, suitable for clearance faces and non-functional surfaces.
- Standard turning and milling: Ra 1.6 to 3.2 µm, the usual band for a general machined face.
- Fine turning and finish milling: Ra 0.8 to 1.6 µm, reached with a light finish pass and a sharp edge.
- Grinding: Ra 0.2 to 1.6 µm, with the lower end needing a dressed wheel and a rigid machine.
- Lapping and honing: Ra 0.05 to 0.4 µm, used for sealing faces and precision bores.
Measurement habits that change the result
The stylus, the skid and the cutoff all affect the reading. A skidless instrument follows the true profile and is needed for waviness and for form. A skid-based instrument references the surface itself and is fine for roughness on a nominally flat face, but it can read the curvature of a small diameter as roughness.
The part temperature and cleanliness matter as much as the instrument. A film of oil fills the valleys and lowers the reading. A burr or a handling mark raises it. A curved surface needs an orientation that keeps the stylus normal to the surface, or the geometry is recorded as roughness.
Record the settings with the result. Parameter, cutoff, evaluation length, filter and direction should appear on the inspection report, because a number without them cannot be reproduced. If a supplier and a customer disagree, the settings are usually the first thing to compare.
What to put on the drawing
Start from the function. A sealing face needs a peak limit, so specify Rz as well as Ra, or specify Rz alone. A sliding bearing needs a plateau surface, so a bearing ratio curve or a material ratio call may be more useful than either height parameter. A contact pad needs a clean, oxide-free surface, so the finish call should sit beside a cleaning and plating note.
For everything else, a general note such as Ra 3.2 with a stated cutoff is enough. Add the standard, ISO 1302 for the symbol and ISO 4287 for the terms, so there is no argument about definitions. If the part is measured on a curved surface, say so, because the instrument setup is part of the specification.
Frequently asked questions
Is Rz always about four times Ra?
Often, on a regular machined surface, but it is not a conversion. The ratio depends on the process, the filter and the cutoff. Use a measured value on the actual surface rather than a multiplier from a chart.
Which is better for a sealing face, Ra or Rz?
Rz, or Rz together with Ra. A seal fails on the highest peak or the deepest valley, and Ra averages those away. A peak height call catches the feature that matters.
Why do two labs report different roughness on the same part?
They may use different cutoffs, filters, evaluation lengths or trace directions, or the part may have been measured at a different temperature or with a different stylus. Compare the recorded settings before comparing the numbers.
Can I call Ra 0.4 on a milled face?
Yes, but expect a finishing pass, a smaller stepover and a longer cycle. A very fine call on a large face is a cost decision as much as a technical one, so check that the function needs it.
Sources and further reading
- ISO 4287:1997, Geometrical product specifications, surface texture, profile method
- ISO 1302:2002, Indication of surface texture in technical product documentation
- ISO 4288:1996, Rules and procedures for the assessment of surface texture
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
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