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How to Use a Surface Roughness Tester

Guide · Updated October 28, 2026

A profilometer drags a fine diamond stylus across a surface and reports roughness numbers like Ra. Calibrate it on the reference specimen, choose the right cutoff and measure across the machining lay.

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Step by step

Calibrate: Measure the reference specimen and adjust.
Choose cutoff: 0.8 mm (0.030 in) for most machined surfaces.
Position: Stylus perpendicular to the lay.
Level: Set the part level with the drive.
Measure: Run several traces.
Read: Ra or Rz as called out.
Record: Average and note cutoff.

Worked example

The drawing calls 32 µin Ra max. Readings: 28, 31 and 30 µin Ra; average 29.7 µin, passes. In metric, 32 µin ≈ 0.8 µm (1 µm = 39.37 µin).

Ra vs Rz

Ra is average roughness; Rz is average peak-to-valley and is usually 4–7 times Ra.

Common problems

Wrong cutoff changes readings. Measuring along the lay reads low.

Tips

Worked example: reading Ra and converting units

Set the cutoff (lambda c) to match the finish: 0.8 mm is standard for most turned and ground surfaces with Ra between 0.1 and 2 µm, and 2.5 mm for rougher milled surfaces. The usual evaluation length is five cutoffs, so 4.0 mm of traverse at 0.8 mm. Before measuring, check the tester on its reference specimen; if the specimen is marked 2.97 µm Ra and the tester reads 3.05 µm, it is within about 3% and fine for shop use.

To convert units, 1 µm = 39.37 µin. A drawing callout of 32 µin Ra is about 0.8 µm, 63 µin is about 1.6 µm and 125 µin is about 3.2 µm. Always measure across the lay (at right angles to the tool marks) unless the drawing says otherwise, because measuring along the lay of a turned part can read far lower than the true value. Take three readings at different spots and report the average and the highest.

Keep the stylus tip clean and protected; a chipped 2 µm or 5 µm radius tip reads rougher than the surface really is. Store the reference specimen in its case and never measure it on the same spot twice in a row.

Worked example: Ra cutoff selection

Per ISO 4288, Ra between 0.1 and 2 µm uses a 0.8 mm cutoff and 4 mm evaluation length; Ra between 2 and 10 µm uses 2.5 mm cutoff. A turned finish expected around 63 µin (1.6 µm) uses 0.8 mm cutoff.

Measure across the lay, not along it, or the reading will be low.

Fuller worked example: 32 µin Ra and cutoff

A drawing calls 32 µin Ra. Since 1 µm = 39.37 µin, that is 32 ÷ 39.37 ≈ 0.81 µm Ra. For non-periodic profiles with Ra between 0.1 and 2 µm, ISO 4288 recommends a 0.8 mm cutoff and a 4 mm evaluation length (five cutoffs).

Check the tester on its reference specimen first; if the specimen is marked 2.97 µm Ra and the tester reads 3.05 µm, the 2.7% difference is usually within the instrument tolerance. Measure across the lay, not along it, or the reading will be optimistic.

FAQ

What is Ra?

Arithmetic average roughness.

Which cutoff?

0.8 mm for most.

Which direction?

Across the lay.

How to convert µin to µm?

Divide by 39.37.

How often calibrate?

Each use or shift.

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