What is Ra?
Ra (Roughness Average) is the arithmetic average of the absolute values of the roughness profile deviations from the mean line. It is the most commonly used surface roughness parameter.
- Definition — average deviation from the mean line
- Units — micrometers (μm) or microinches (μin)
- Standard — ISO 4287
- Common values — Ra 0.4, 0.8, 1.6, 3.2
Ra is the most widely specified surface roughness parameter and is suitable for most applications.
What is Rz?
Rz (Average Maximum Height) is the average of the five highest peaks and five lowest valleys within the sampling length. It is more sensitive to extreme surface features.
- Definition — average of five highest peaks and lowest valleys
- Units — micrometers (μm)
- Standard — ISO 4287
- Typical relationship — Rz is approximately 4-6 times Ra
Rz is more sensitive to outliers. If surface finish consistency is critical, consider specifying both Ra and Rz.
Key differences
| Factor | Ra | Rz |
|---|---|---|
| Definition | Average deviation from mean line | Average of 5 highest peaks and 5 lowest valleys |
| Sensitivity to outliers | Low | High |
| Typical ratio to Ra | 1x | 4-6x |
| Application | General surface finish | Critical surfaces, sealing applications |
Assuming Ra and Rz are interchangeable. They measure different characteristics and are not directly comparable.
When to use each parameter
- Use Ra for — general surface finish requirements, most engineering drawings
- Use Rz for — sealing surfaces, applications sensitive to extreme peaks or valleys
- Use both — when surface consistency is critical
Frequently Asked Questions
What is the difference between Ra and Rz?
Ra measures the average deviation from the mean line, while Rz measures the average of the five highest peaks and five lowest valleys.
Which parameter is more commonly used?
Ra is more commonly used and is the standard for most engineering drawings.
How do I convert Ra to Rz?
There is no exact conversion. However, Rz is typically 4-6 times Ra for most surfaces.
What is a typical Ra value for CNC machining?
Standard as-machined finish is typically Ra 1.6–3.2 μm.