What Is Surface Roughness? (Ra vs Rz and Reading the Symbols)

Surface roughness measures the fine irregularities on a machined face. The two representative values on a drawing are Ra and Rz. Because they directly affect function (sliding, sealing, appearance, fatigue strength), understand them correctly.

Ra vs Rz

• Ra (arithmetic mean roughness): the average of irregularities over the sampling length. The most common; shows the overall trend of the surface
• Rz (maximum height roughness): the difference between the highest peak and deepest valley within the sampling length. Sensitive to local scratches or burrs

The same surface gives different Ra and Rz (roughly Rz ≒ 4× Ra, but it varies by surface). Always check which the drawing specifies.

Triangle marks (old JIS) vs Ra

Reference values for the old marks (▽) on legacy drawings.

Mark	Meaning	Approx. Ra
〜 (wave)	As-cast/as-received	No machining
▽	Standard	around Ra 6.3
▽▽	Medium	around Ra 1.6
▽▽▽	Fine	around Ra 0.8
▽▽▽▽	Precision	around Ra 0.2

Reading the surface texture symbol

A "√"-shaped symbol, with a roughness value or machining method.

• Value position: Ra1.6, etc. written above or beside the symbol
• Method specification: sometimes "turning," "grinding" is written on the horizontal bar
• Lay direction: a symbol may specify the direction of machining marks

Typical Ra by machining process (reference)

Process	Approx. Ra
Turning / milling	Ra 1.6 – 6.3
Grinding	Ra 0.2 – 0.8
Lapping / superfinishing	Ra 0.05 – 0.1

Tips & common mistakes

• Over-finishing raises cost: match roughness to what function needs
• Confusing Ra and Rz when reading (values differ)
• "Smaller number = smoother": Ra0.2 is far smoother than Ra6.3
• Sliding/sealing faces need fine finish; non-functional faces can be coarse

FAQ

Q. Should I specify Ra or Rz? A. Ra is mainstream for general faces. To control local scratches, add Rz. Follow the drawing.

Q. How smooth is Ra1.6? A. It's a typical turning/milling finish. For smoother, you need grinding (Ra0.8 or below).

Q. Can roughness be measured? A. Yes, with a roughness tester (stylus type, etc.). Eyeballing or nail feel is only a rough guide.

Conclusion

Confirm "Ra or Rz" together with "which value." Over-finishing raises cost, so match the roughness to what the function actually requires.

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