In CNC machining, achieving the right surface finish is crucial for part functionality, aesthetics, and cost. Two key parameters, Ra (Average Roughness) and Rz (Mean Roughness Depth), form the cornerstone of surface finish specifications. Understanding their differences is essential for effective communication between designers and machinists.

Ra: The Arithmetic Average

Ra is the most commonly used parameter. It measures the average of the absolute heights and depths of the surface profile from its mean line over a sampling length. Think of it as the overall "texture" of the surface. A lower Ra value indicates a smoother finish.

Typical Applications: General part specifications, visible surfaces, parts requiring good cosmetic appearance, and surfaces where friction needs to be controlled. For example, an aluminum CNC machined enclosure might call for an Ra ≤ 3.2 µm for a uniform, smooth look.

Rz: Assessing Peak-to-Valley

Rz measures the average maximum height of the profile by calculating the distance between the highest peak and the deepest valley over five sampling lengths. It is more sensitive to extreme surface variations that Ra might average out. This makes Rz critical for applications where peak integrity or sealing is vital.

Typical Applications: Sealing surfaces, bearing contact areas, and parts subject to fatigue stress. For instance, a hydraulic manifold block sealing face would often be specified with a strict Rz value to ensure leak-proof performance.Refer to  ISO 21920-2:2021, the standard for surface texture parameters.

Ra vs. Rz: Choosing the Right Callout

While related, Ra and Rz are not interchangeable. A surface can have a decent Ra but a poor Rz if it has deep, isolated scratches. Rz often provides a better picture of surface extremes.

Use Ra for: General smoothness requirements, cosmetic finishes, and controlling friction/wear in most dynamic applications.

Use Rz for: Functional surfaces where the deepest valleys or highest peaks are critical, such as for sealing, coating adhesion, or load-bearing fatigue resistance.

 common Ra/Rz values and their corresponding machining processes

Specifying Surface Finish: Best Practices

1.Specify the Parameter: Always state whether you require Ra or Rz.

2.Define the Value: Use standard values (e.g., Ra 1.6. Rz 10). Unnecessarily tight tolerances increase cost.

3.Indicate the Sampling Length: This is often implied by the value but can be specified for clarity.

4.Consider the Process: Different CNC machining operations produce characteristic finishes. A finish of Ra 0.4 µm will require grinding or polishing after milling, significantly impacting lead time and price.

Conclusion

Specifying the correct surface finish standard is a balance between function and cost. By understanding the distinct roles of Ra and Rz, engineers can create more precise drawings, and manufacturers can deliver parts that perform reliably. Always consult with your CNC machining partner early to determine the most efficient and cost-effective way to achieve your desired surface texture.