How to select the right component surface – basic knowledge of surface roughness

Surface roughness and surface hardness are important properties that must be taken into account when selecting components. Although there are different methods for measuring the roughness or coarseness of a component surface, these measurements often have to be converted to a standard measure. This article explains how roughness properties can be determined in accordance with the standards JIS B 0031, JIS B 0601, DIN EN or ISO.

What is surface roughness?

In most cases, the surfaces of materials do not have a regular structure, but have deviations that can be divided into roughness, waviness and shape.

The term surface roughness describes the unevenness in the surface height. To quantify roughness, there are different calculation methods that take into account different surface properties.

Surface roughness can be influenced by various methods such as polishing, grinding or other surface treatment. Corrosion and wear also play a role.

How is surface roughness typically calculated?

The following parameters are important for the component calculations:

• Roughness curve = Measured surface finish (2D)
• R_a = Arithmetic roughness average
• R_z = Ten point roughness average
• R_y = Maximum height
• R_mr = Proportion of material or contact ratio

1. Measure surface finish and determine roughness curve

In order to check the surface quality of materials and components comprehensively and qualitatively, the surface must be assessed and analysed as a whole. Scanning measuring tips or probe cutting devices are used for this purpose to record the vertical deflection of the probe tip over the component surface. This provides information on the surface along a profile (2D), which is specified as a roughness curve. To guarantee comprehensive and high-quality inspection of the surface quality, the surface as a whole must be considered and analysed.

2. Determine average roughness R_a

After determining the roughness curve, "cut out" the part that extends over a reference length  in the direction of the average line.

This cut-out part is shown in a new diagram with X-axis in the same direction as the average line and Y-axis as the size.  calculated with using the following equation:

The average roughness R_a is typically shown in micrometres [µm].

3. Determine maximum height R_y

After determining the roughness curve, "cut out" the part that extends over a reference length  in the direction of the average line.

This cut-out part is shown in a new diagram with X-axis in the same direction as the average line and Y-axis as the size.  calculated with using the following equation:

The maximum height could be described as the distance from the global minimum to the global maximum of the curve.

The maximum height R_a is typically specified in micrometres [µm].

4. Determine ten-point roughness average R_z

After determining the roughness curve, "cut out" the part that extends over a reference length  in the direction of the average line.

The average values (Y_P) from the highest to the "fifth highest" peak value and the average values (Y_v) from the lowest to the "fifth lowest" value are added:

The ten-point roughness average R_z is typically shown in micrometres [µm].

5. Determine proportion of material Rmr

After determining the roughness curve, "cut out" the part that extends over a reference length  in the direction of the average line.

Proportion of material is the proportion of the contact surface (specific intersection) to the reference length under consideration . The proportion of material is determined using the following formula:

The proportion of material R_mr is typically specified in percent [%].

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