Steel Surface Roughness: How to Interpret Ra and Rz

Steel surface roughness is evaluated by measuring small-scale peaks and valleys on the surface. Ra and Rz are different parameters used in this evaluation. A bar having a visually bright or smooth appearance does not prove that it meets a specified Ra or Rz value; appropriate measuring equipment and clearly defined measurement conditions are required. Surface requirements should not be determined based on appearance alone. Assembly, friction, sealing, and subsequent coating processes may require different surface characteristics. The required value should therefore be selected according to the function of the part and its technical drawing.

What Is the Main Difference Between Ra and Rz?

Ra expresses the arithmetic mean of the deviations of the evaluated roughness profile from the mean line. Rz is a parameter related to the peak and valley heights of the profile. The exact method of evaluation depends on the definitions in the applicable standard and the measurement settings. Mitutoyo’s surface measurement guide demonstrates that different profile parameters represent different characteristics of a surface. For technical drawings that use both older and newer standard designations, it is useful to check the version of the referenced standard in addition to the parameter name.

Can Rz Be Calculated from an Ra Value?

There is no fixed conversion factor that applies to every surface. Two surfaces with the same Ra value can have different distributions of peaks and valleys. Therefore, verifying an Rz requirement simply by multiplying a measured Ra value by a fixed number is not a reliable acceptance method.

If both parameters are specified, the measurement report should show each parameter using the relevant measurement settings. If the meaning of a symbol or parameter used in an older technical drawing is unclear, technical clarification should be obtained before production begins.

What Conditions Affect Surface Roughness Measurements?

The main conditions that can affect surface roughness measurement results include:

• Measurement direction: The direction of measurement relative to the surface pattern or lay
• Filtering: Filtering settings and evaluation length
• Instrument: Stylus characteristics, measurement capability, and verification status of the instrument
• Surface cleanliness: Oil, dirt, scratches, or local damage on the surface
• Positioning: How a cylindrical component is positioned under the measuring instrument

When these conditions are not defined, different operators may report different results for the same part. The measurement plan should specify which surface will be inspected, how many locations will be measured, and which settings will be used. The method for evaluating results close to the acceptance limit should also be determined in advance.

Is Lower Surface Roughness Always Better?

The suitability of a surface depends on the application. In sealing systems, for example, surface texture should be evaluated together with its relationship to lubrication. SKF’s guidance on shaft surfaces and seal performance highlights the need to consider both contact conditions and lubricant retention.

Therefore, specifying a roughness value lower than the design actually requires may create unnecessary additional processing costs. Once the functional limits have been established, an appropriate manufacturing method should be selected. The surface condition of the supplied steel bar and the final surface requirement of the finished component should also be considered separately.

How Should Surface Roughness Be Specified When Ordering Steel Bars?

Descriptions such as “bright surface” or “good surface quality” should be supplemented with measurable requirements. The purchase specification should clearly state:

• The parameter (Ra, Rz, etc.)
• The unit and limit value
• The applicable standard
• The measurement direction, where required

If there are specific limits for surface defects or scratches, these should be defined separately from the roughness value. An Ra value does not demonstrate that a surface is completely free from scratches or that the product complies with its geometric tolerances. Dimensional, geometric, and surface inspections complement one another. For more information about this distinction, you can also review our steel tolerances guide.

Conclusion

Ra and Rz are parameters that describe different aspects of surface roughness and cannot be reliably converted into one another using a fixed factor. An appropriate surface requirement should be selected according to the function of the component and clearly specified in the purchase documentation, including the parameter, limit value, applicable standard, and measurement conditions.

When evaluating cold-drawn steel products, you can share your required surface specifications with Uyar Çelik through our contact page.