Surface Roughness and Surface Finish Engineering Comparison: Standard Conversion, Differences and Application Guidelines

In the fields of mechanical manufacturing and precision engineering, the microscopic geometric characteristics of part surfaces have a critical impact on their performance.

Surface roughness and surface finish, as two evaluation systems used to describe these characteristics, share a historical background but also exhibit a clear relationship of substitution in modern engineering applications.

This paper aims to systematically review their definitions, evaluation parameters, grade correspondences, and differences in engineering applications, providing technical personnel with a clear reference framework.

Analysis of Basic Concepts

• Surface Finish

Surface finish is a surface quality evaluation standard adopted early on in China, primarily based on GB/T 1031-1968, “Surface Finish.”

It classifies surface quality into 14 grades, ranging from ▽1 to ▽14, with higher numbers indicating a smoother surface.

This system relies on subjective visual and tactile judgments and lacks precise numerical quantification metrics.

• Surface Roughness

Surface roughness is an internationally recognized standard for evaluating surface quality.

The current standard in China is GB/T 3505-2009, “Geometric Product Specifications (GPS)—Surface Texture—Profile Method—Terminology, Definitions, and Parameters.”

It enables an objective, quantitative assessment of surface quality through the parametric description of microscopic geometric shape errors.

Common parameters include:

Ra: Arithmetic Mean Deviation (μm)

Rz: Ten-Point Height of Micro-Roughness (μm)

Ry/Rmax: Maximum Height (μm)

Correspondence Between Old and New Standards

According to the Chinese Metrological Technical Specification JJF 1101-2003, “Conversion Relationships Between Surface Roughness and Surface Finish,” the following correspondences exist between the two:

Analysis of Key Differences

• Fundamental Differences in Evaluation Methods

Surface Finish: Relying on visual and tactile comparisons, it is highly susceptible to subjective factors and difficult to automate.

Roughness: Measured using instruments such as stylus profilometers and optical interferometers, it provides precise numerical values and generates contour curves.

• Differences in Parameter Systems

Surface finish is designated by a single grade number, whereas surface roughness encompasses multiple parameters (Ra, Rz, Rsm, etc.), allowing the most appropriate evaluation parameter to be selected based on functional requirements.

For example:

Sealing surfaces focus on the Rz value (which reflects peak height).

Sliding bearings focus on the Ra value and texture direction.

Coating substrates focus on Rz and Rmr (material ratio).

• The Need for Standard Updates

China abolished its surface finish standards in the 1990s and fully adopted the surface roughness system.

The main reasons include:

The need to align with the international standard ISO 1302.

The requirements for precise measurement in digital manufacturing.

The need for traceability in quality control.

Engineering Application Recommendations

• Conversion of Drawing Annotations

For surface finish annotations in older drawings, convert them to roughness values according to the comparison table above, and specify the basis for the conversion in the technical documentation.

It is recommended to prioritize the use of the Ra parameter, as it offers superior statistical stability compared to Rz.

• Surface Requirements for Typical Parts

• Measurement Precautions

The sampling length must comply with the standards and is typically between 0.08 mm and 8 mm.

The evaluation length should be five times the sampling length.

When applying filtering, distinguish between roughness and waviness (select the λc cutoff wavelength).

Conclusion

Although surface roughness and surface finish describe the same physical property, there are fundamental differences between them in terms of assessment accuracy, standardization, and international applicability.

Modern manufacturing must fully transition from surface finish to surface roughness and establish a precise, parameter-based control system.

For special cases (such as the repair of antique equipment or the restoration of historical drawings), surface finish may be retained as an auxiliary reference;

However, all new designs must adopt roughness specifications.

With the development of nanomanufacturing technologies, three-dimensional evaluation parameters of surface topography (such as Sa and Sq) are becoming a new research focus, and this will be the future direction of surface quality control.