Measuring surface roughness: Ra and Rz explained

What do Ra and Rz actually mean?

Ra and Rz describe the same profile from two angles. Ra is the arithmetic mean roughness, the average of all profile deviations from the mean line over the evaluation length. Rz is the mean roughness depth taken from five sampling lengths and reacts far more strongly to individual scratches and peaks.

Because Ra averages out extremes, a surface with one deep groove can still show a low Ra. Rz makes such outliers visible, which is why drawings often specify both parameters together.

• Ra: arithmetic mean roughness, robust and widely used.
• Rz: mean roughness depth, sensitive to single peaks and grooves.
• Rmax or Rz1max: greatest single roughness depth within the length.
• RSm: mean groove spacing, describing waviness along the surface.

How does the stylus method work?

In the stylus method a fine diamond tip traverses the surface at constant feed. The vertical deflection of the tip is converted into a height profile, from which the instrument calculates the parameters.

The stylus tip radius is typically 2 µm, or 5 µm on finer surfaces. Two settings are decisive: the cut-off filter (λc) and the evaluation length. Under ISO, the total evaluation length ln comprises five sampling lengths lr plus pre- and post-travel.

Which roughness tester suits my task?

For shop floor and production use a mobile handheld unit with an inductive pickup is usually enough, showing Ra, Rz and further parameters directly. For labs, mould making and contour inspection, stationary stylus instruments with a reference plane and software are the better choice.

• Match measuring range and resolution to the expected roughness depth.
• Check the parameter set: at least Ra, Rz, Rmax and RSm.
• Switchable cut-off values 0.25 / 0.8 / 2.5 mm for different surfaces.
• Probe arm and drive unit for hard-to-reach features (bores, grooves).
• Order a calibration standard so you can re-adjust regularly.

How do I measure correctly in practice?

Reproducible results depend on clean preparation. Part and stylus must be free of grease and dust, the instrument stands vibration-free, and the measuring direction runs across the machining grooves.

• Clean the surface with isopropanol and let it dry.
• Trace across the groove structure, not in the feed direction.
• Measure at several points and form an average.
• Record cut-off and evaluation length in the test report.
• Inspect the stylus tip for wear and damage before each shift.

Frequently asked questions