How to Measure Surface Roughness (Step-by-Step)
A practical engineering guide to using a stylus profilometer for precise surface texture measurements. Focusing on standard procedures, common errors, and best practices.
Step 1: Clean the Surface Thoroughly
Before measurement, the surface must be free from oil, grease, dirt, and coolant. Even a tiny speck of dust (0.05 mm) is 50 times larger than a typical Ra measurement (0.001 mm or 1 µm).Use isopropyl alcohol and a lint-free cloth.
Step 2: Choose the Measurement Direction
Standard measurement direction is perpendicular to the lay (the direction of the machining marks). Measuring parallel to the marks will give an artificially low and incorrect roughness value.
Step 3: Select the Sampling Length (Cutoff λc)
The cutoff filter is used to separate roughness from waviness. Choosing the wrong cutoff is the #1 cause of measurement errors. Use the ISO 4288 standard table for selection:
| Expected Ra (µm) | Sampling Length (λc) [mm] | Evaluation Length (Ln) [mm] |
|---|---|---|
| 0.006 - 0.02 | 0.08 | 0.4 |
| 0.02 - 0.1 | 0.25 | 1.25 |
| 0.1 - 2.0 | 0.8 | 4.0 |
| 2.0 - 10.0 | 2.5 | 12.5 |
| 10.0 - 80.0 | 8.0 | 40.0 |
Step 4: Level the Workpiece
Ensure the surface is parallel to the profilometer's drive unit. While most modern software can "auto-level" a tilted profile, extreme tilt can introduce errors in the measurement of peak-to-valley parameters like Rz and Rt.
Step 5: Perform the Scan and Verify
Start the measurement and observe the profile trace on the screen. Check for "spikes" or "gaps" that might indicate a piece of dust or the stylus losing contact.The "Rule of 16%": According to ISO 4288, if more than 16% of the measured values exceed the specified limit, the surface is rejected.
Common Mistakes to Avoid
- Measuring on a Curved Surface: Without proper curvature compensation (R-surface compensation), the results will be completely wrong.
- Ignoring the Stylus Radius: A 5 µm stylus tip cannot accurately measure a 0.1 µm Rz value because it cannot reach the bottom of the narrow valleys.
- Too Fast Measurement Speed: Moving the stylus too fast can cause it to "jump" over peaks, especially on rough surfaces.