Ecoroll: Beschreibung von Oberflächen

In purely scientific terms, a surface is an infinitely thin layer that separates an object from its surroundings. You could also say that it is what is visible of an object. Although the surface is actually infinitely thin, it has a major influence on the behaviour of a component. Let's look at friction or adhesive surfaces, for example. Depending on the shape of the surface, it is either better suited for rubbing or better suited for adhesion. It is therefore necessary to adapt the quality of a surface to the respective application.

However, this raises the question of how surfaces can be described and compared with each other. There are various standards that deal with the measurement and evaluation of surfaces. A basic distinction must first be made between three different types of surface. Firstly, there is the real surface of a component. This represents the exact image of the surface, but cannot be recorded using a characteristic value and cannot be measured precisely. At the other end of the scale is the ideal surface. It represents the absolute desired state of the designer. For example, a simple shaft is visualised in the CAD system as an ideal cylinder. However, when this component is manufactured, it deviates from the ideal state. Between the ideal and the real surface there is the technical surface. It is characterised by the fact that it simplifies reality as much as possible and as little as necessary. It can be measured and evaluated using characteristic values.

A real surface is always characterised by errors in relation to the ideal surface. In principle, these defects can be divided into six categories. These six shape deviations are subdivided and described in orders. The shape deviation of the 1st order describes the shape error, the 6th order is so detailed that it considers the atomic or lattice level of the surface. The following figure shows the six orders of shape deviation and their degree of influence.

A surface is measured using suitable measuring systems. A basic distinction can be made here between 2D and 3D measuring systems and between tactile and optical measuring systems. 2D measurements are usually tactile. Here, a wand with a defined geometry is guided over the surface. 3D measurements are usually optical and, in simplified terms, record a surface area using a photo with depth information. Due to their widespread use, only 2D measurements will be discussed below.

If a surface is recorded tactilely, the irrelevant shape deviations must first be filtered out. The shape and waviness are usually removed from the measured primary profile. For this purpose, a Gaussian filter with a cut-off wavelength λc is applied in accordance with DIN ISO 4288. The cut-off wavelength must be selected in accordance with the standard to match the expected roughness. A cut-off wavelength of λc = 0.8 mm is selected in the usual areas for production technology.

The measuring section then results from 7 x λc. These seven individual measuring sections li are then used partly for filtering and partly for measuring. The first and last individual measuring sections align the profile, the individual measuring sections 2 to 6 are used for measurement.

The filtered signal can then be used to determine characteristic values. The characteristic values Ra, Rz, Rmax or Rt are frequently used. These are characteristic values that describe the height of the profile, but say little about the shape of the surface. So-called functional parameters are suitable for this purpose. They can be determined using the material contact ratio curve or Abbott curve, for example.