What is rz in surface roughness
Last updated: April 1, 2026
Key Facts
- Rz is defined by ISO 4287 international standard for surface roughness measurement
- The parameter calculates the average of the maximum heights and depths across five consecutive sampling segments
- Rz is measured in micrometers (µm) and helps determine surface texture suitability for mechanical assembly
- Higher Rz values indicate rougher surfaces, while lower values represent smoother finishes
- Rz is particularly useful in automotive, aerospace, and machinery industries for quality control
Overview
Rz, also known as maximum roughness height or ten-point mean roughness, is a critical parameter in surface roughness assessment used across manufacturing industries. Unlike simpler measurements like Ra (average roughness), Rz focuses specifically on the extreme variations in surface topology by measuring the maximum height variations.
How Rz is Calculated
The Rz value is determined by dividing a measured profile into five equal segments. For each segment, the distance between the highest peak and lowest valley is recorded. The Rz value then represents the average of these five maximum peak-to-valley distances. This calculation method makes Rz particularly sensitive to surface defects and anomalies.
Measurement Standards
Rz measurements are standardized under ISO 4287, the international standard for geometric product specifications and surface texture. This ensures consistency across different countries and industries. Different industries may also reference standards like DIN 4768 (Germany) or JIS B 0601 (Japan), which define similar parameters.
Applications and Importance
In manufacturing, Rz is essential for:
- Assessing bearing surfaces and friction-dependent components
- Evaluating sealing surfaces for hydraulic and pneumatic systems
- Quality control in precision machining operations
- Determining if surfaces meet specification requirements before assembly
- Predictive analysis of component wear and performance
Rz vs Ra Comparison
While Ra (arithmetic mean deviation) provides an overall smoothness average, Rz emphasizes extreme deviations. Ra is more commonly used for general surface characterization, while Rz is preferred when peak-to-valley variations are critical to function. A surface with the same Ra but different Rz values can perform very differently in practical applications.
Related Questions
What is the difference between Ra and Rz surface roughness?
Ra measures the average deviation from the mean surface profile, while Rz measures the average of maximum peak-to-valley heights. Rz is more sensitive to surface defects and extreme variations, making it better for applications where anomalies are critical.
What are typical Rz values for different surface finishes?
Rough machined surfaces typically have Rz values of 6-25 µm, ground surfaces 0.8-6 µm, and polished surfaces below 0.8 µm. The appropriate Rz specification depends on the component's functional requirements.
How is Rz measured in practice?
Rz is measured using a profilometer or roughness tester that traces across the surface and records the vertical deviations. The device electronically divides the measurement into segments and calculates the peak-to-valley distances automatically.