Making Surface Differences Clear With ISO 25178

When it comes to machined parts in our daily lives, there are a wide variety of surfaces: from smooth and shiny surfaces to rough and matte surfaces. These differences in the appearance of parts arise from differences in roughness between them. The surface of any machined part is a complex shape made of a series of peaks and troughs of varying heights, depths, and spacing. Surface roughness is defined as the shorter frequency of real surfaces relative to the troughs. The larger the asperity, the rougher the feel to the touch. These surfaces also generate diffuse reflections of light, so they have a matte finish. On the other hand, if the asperity is extremely small, the surface feels smooth to the touch and appears shiny. Surface roughness measurements can quantify this asperity. It is possible to perform numeric management of factors such as a product’s feel and functionality, which leads to stable product quality.

ISO 4287 is a standard for line roughness measurements established by the International Organization for Standardization. This standard assumes the use of contact measuring instruments, so sample data is captured as a line. This standard was established earlier than ISO 25178, explained below, so the use of ISO 4287 is widespread. Most commonly, when people speak about roughness, they are talking about roughness as described in ISO 4287. ISO 25178 Surface Texture is a collection of international standards relating to the analysis of surface roughness. While ISO 4287/ISO 13565-1 (Surface Texture: Profile Method) are based on analysis using the stylus method, ISO 25178 Surface Texture standards support two evaluation methods: contact type (stylus method) and non-contact type (optical probe). Because these evaluations target the entire surface, the dual-method approach resolves existing problems in the profile method: variations in measurement results depending on the measurement location and variations due to the scanning direction.

Sa (Arithmetical Mean Height)

Sa (Arithmetical Mean Height)

The Sa parameter is the extension of Ra (arithmetical mean height of a line) to a surface. It expresses, as an absolute value, the difference in height of each point compared to the arithmetical mean of the surface. This parameter is used generally to evaluate surface roughness.

Str (Texture Aspect Ratio)

Str (Texture Aspect Ratio)

The Str parameter expresses the isotropy and anisotropy of the texture. Str is a value from 0 to 1. A value close to 0 indicates that there is directionality present, and a value close to 1 indicates that the surface does not depend on directions.

Sdr (Developed Interfacial Area Ratio)

Sdr (Developed Interfacial Area Ratio)

The Sdr parameter is expressed as the percentage of the additional surface area created by asperities on the sample compared to the surface area of a perfectly flat surface over the same field-of-view.

Svk (Reduced Valley Depth)

Svk (Reduced Valley Depth)

The Svk parameter expresses the arithmetical mean of the reduced valley depth of the areal material ratio curve. Essentially, this is a measure of the valley depth below the core roughness. This indicates the depth of the area in which fluid applied to the surface accumulates, which is information that can be used to improve the lubricating properties of the surface.

The above is an extract from a guide published by Keyence. The full publication can be downloaded here.

For more information: www,keyence.com

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