Surface roughness testers (profilometers) measure the texture of machined surfaces, reporting parameters such as Ra, Rz, and Rq. Calibration verifies the stylus condition, traversing mechanism, and measurement accuracy using certified roughness reference specimens. Surface finish control is essential for functional performance of precision parts.
Inspect the diamond stylus under magnification for wear, chipping, or contamination. A worn stylus tip (radius exceeding specification) will cause systematic measurement errors. Replace the stylus if damaged.
Power on the instrument and allow warm-up per manufacturer instructions. Set the measurement parameters (cutoff length, evaluation length, and filter type) to match the reference specimen certification conditions.
Measure each certified roughness reference specimen a minimum of three times, repositioning the stylus for each traverse. Use specimens that span the measurement range of the instrument (e.g., Ra 0.2 µm, Ra 1.6 µm, Ra 6.3 µm).
Compare the average measured Ra (or other certified parameter) to the certified value for each reference specimen. Calculate the error and assess whether it falls within the allowable tolerance.
Verify the traversing length using a calibrated scale or by measuring a known-length feature. Check that the traversing speed is within the specification for the selected cutoff length.
Record all measurements, reference specimen IDs and certified values, stylus condition, and instrument settings. Issue the calibration certificate and apply the calibration label.
The measured Ra value for each reference specimen must agree with the certified value within ±10% or ±0.02 µm (whichever is larger), per ISO 5436. Repeatability (range of three measurements on the same specimen) must be within 10% of the mean.
12 months, with periodic checks using reference specimens
A diamond stylus typically lasts for several thousand traverses on smooth surfaces, but can wear rapidly on hard or abrasive materials. Regular inspection under magnification is the best way to assess stylus condition. Most labs replace the stylus annually or when calibration results indicate degradation.
Use the same cutoff length specified on the reference specimen's calibration certificate. The standard cutoff for general machined surfaces is 0.8 mm (per ISO 4288). Using the wrong cutoff length will produce incorrect Ra values even if the instrument is functioning properly.
Gage blocks have very smooth surfaces (Ra < 0.01 µm) and can be used to verify the instrument reads near zero, but they are not suitable as the primary calibration standard. Certified roughness reference specimens with known Ra values across the measurement range are required.
CalibrationOS tracks due dates, stores certificates, and generates audit-ready reports.
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