How to select the right component surface – Basic knowledge about surface roughness

Surface roughness and surface hardness are important properties that must be considered when selecting components. Although there are various methods to measure roughness or surface roughness of the surface of a component. However, these measurements often have to be converted to a standard dimension. This article explains how roughness properties can be determined according to 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. They contain deviations that are divided into roughness, ripples and shape.

The term surface roughness or roughness describes the unevenness of the surface height. In order to quantify the roughness, there are different calculation methods that consider the different properties of the surface.

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

How is the surface roughness typically calculated?

The following parameters are important for calculating the components:

• Roughness curve = measured surface condition (2D)
• R_a = arithmetic roughness average
• R_z = ten-point roughness average
• R_y = maximum height
• R_mr = material portion or support portion

1. Measuring of the surface quality and determination of the roughness curve

In order to comprehensively and qualitatively check the surface quality of materials and components, it is necessary to check and analyze the surface in its entirety. For this purpose, scanning measuring tips or scanning devices are used to detect the vertical deflection of the scanning tip over the component surface. The information about the surface along one profile (2D) can thus be obtained, which is specified as a roughness curve. In order to ensure a comprehensive and high-quality examination of the surface quality, the surface must be considered and analyzed in its entirety.

2. Determine average roughness R_a

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

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

The average roughness R_a is typically indicated in micrometers [μm].

3. Determine the maximum height R_y

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

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

The maximum height is, so to speak, the distance from the global minimum to the global maximum of the curve.

The maximum height R_y is typically specified in micrometers [μm].

4. Determine ten-point roughness average R_z

After determining the roughness curve, you "cut out" the part that extends over a reference length ℓ 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 specified in micrometers [μm].

5. Determination of the material portion R_mr

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

The material portion is the portion of the positioning surface (a specific cutting line) relative to the observed reference length ℓ. The material portion is determined with the following formula:

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

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