Surface roughness

The roughness (or outdated and no longer conforming to standards roughness or roughness) is a term used in surface physics, refers to the unevenness of the surface height. For a quantitative characterization of the roughness there are different calculation methods that each take on different characteristics of the surface into account. The surface roughness can be influenced among other things by polishing, grinding, lapping, honing, etching, sandblasting, bristle blasting, etching, vapor deposition or corrosion.

The term refers to roughness continues to shape deviation second to fifth order with technical surfaces to DIN 4760th The roughness of a surface is technical specified in the surface dimensions of the technical drawing.

Especially in the art, the roughness is very important, for example in technical sliding or visible surfaces. The available instruments can be divided into three categories:

  • Manual methods. This includes the Rugotest. But this is not covered by the GPS standard chain.
  • Profile-based methods. These include stylus method
  • Field-based methods. These include optically -dimensionally measuring methods.

In optical profile and surface -based methods can be chosen among numerous methods of measurement. These are, among others, confocal microscopy, holography Conoscopic, focus variation or white light interferometry.

Roughness parameters on the profile

There are basically three Rauheitsangaben used in everyday life, which are given in units of microns.

  • The mean roughness is represented by the symbol is the mean distance of a measuring point - at the center line - on the surface. The center line intersects the reference line in the actual profile, so that the sum of the profile deviations (relative to the center line ) is minimum.
  • The so-called square roughness (English rms roughness root-mean -squared roughness: the square root of the mean ) is calculated from the mean square and corresponds to the " root mean square "
  • The so-called average roughness (also ten point height ), represented by the symbol, is determined as follows. A defined measurement path on the surface of the workpiece is divided into seven equal sampling lengths. The evaluation is performed but only about five of these routes, as the Gaussian filter to wendene half a sampling length before or follow-up needs or has a folding a non-negligible entry and exit behavior.
  • From the thus obtained five individual roughness depths of the average is calculated.

This characteristic value is not to be confused with the roughness or. is defined as the difference between maximum and minimum values ​​of the profile (-) based on the total measuring section, in the normal case, the five individual test sections. is the largest of the five individual roughness depths. However, the GPS standard chain provides for other measurement configurations.

Roughness parameters on the surface

The roughness on the surface is standardized in ISO 25178. In the meantime ( as of 2009) there are optical instruments that measure the roughness surface.

Discussion of roughness parameters

Is simplified as shown in Figure 1 for one dimension can be seen, the average roughness, and the square roughness only on the absolute deviation from the mean value of the height are dependent, but not. Of the distribution of the height values ​​of the surface This results, for example, the average roughness in the images A and C, while the values ​​for image B to calculate and

In mechanical engineering exist for the above problem solutions in the GPS standard chain. Here are to lead the characteristics of the Abbott curve and the amplitude density curve, and the difference between waviness and roughness. So, however, says the ripple roughness of a workpiece, for example, something about the quality of the tool, something about the quality of the machine. So it occurs frequently, such as quality problems, the roughness requirements are extremely increased, the ripple is causing the problem but " filtered " in the roughness. The GPS standard chain defines all Raukennwerte as Welligkeitskennwerte. Only difference is the respective " cut-off " frequency. Welligkeitskennwerte be prefixed with "W".

And are therefore inadequate to draw conclusions about the spatial frequency of the bumps. It is therefore only to determine the wavelength of the critical structures. As a result, if " P", " R" or " W" values ​​have to be specified. Hereafter is to decide whether the critical vertices are more likely to see from the profile, the Abbott curve or amplitude density curve. Only after a determination on a measured value for quality assurance is useful.

Method for paper

To determine the roughness or smoothness of the paper, there are different test methods. The majority of the test methods used today tries to characterize the smoothness of pressure at a defined pressure. For this purpose, air is often used as an aid, which flows between a reference surface and the paper surface under defined conditions. Bekk in the reference surface is a ground glass plate, in Bendtsen and Parker Print Surf a flat metal ring end face.

Optical test methods

Newer methods of measurement work with optical methods. The advantages of this method are the non-destructive measurement and the evaluation parameter of the complex surface and in the volume, as defined for example in ISO 25178. Optical methods are limited to the properties of the surface. When air flow method, a flow of the paper lead to distortion of roughness.

  • Confocal
  • Conoscopic holography
  • White light interferometry
  • Focus variation
  • Structured Illumination

Air flow method

  • The Bekk smoothness
  • Parker Print Surf smoothness
  • The smoothness according to Bendtsen

Bekk smoothness

  • Operating range: 2-5 s
  • Measuring surface: 10 cm ²
  • Pressure on the sample: 100 kPa

There are three different measurement ranges are possible:

  • A: 10 to 600 s with a large vacuum vessel pressure drop of 507 to 480 mbar, measured time = GL ( Bekk ) s
  • B :> 300 s ( to A ) with a small vacuum vessel pressure drop of 507 to 480 mbar, measured time x 10 = GL ( Bekk ) s
  • C: < 20 s ( to A ) with a large vacuum vessel pressure drop of 507 to 293 mbar, measured time: 10 = GL ( Bekk ) s

Roughness / smoothness according to Bendtsen

The roughness according to Bendtsen is the flow of air flow that passes between the measured ring of the measuring head of the Bendtsen apparatus and the sample surface and the set at a positive pressure is created.

  • Measuring range: 10 to 3000 ml / min
  • Measuring area: 100 × 0.15 mm ring
  • Pressure on sample: 10 N / cm ²
  • Differential pressure: ( 15 ± 0.2 ) mbar

Roughness / smoothness Parker Print Surf ( PPS)

The PPS roughness is also one of the air flow measurement method and is very widely used in the paper industry.

  • Measuring area: 98 mm × 51 microns ( annular)
  • Printing on sample ( 6.2 ± 0.1 ) kPa
  • Sample pad: hard rubber plate

The devices are calibrated by appropriate accredited experts. Are carried out within the paper industry robin tests in order to compare products and to determine the appropriate calibration intervals.

Harshness Grinding (abrasive cutting) Honing (metalworking) Structured-light 3D scanner Pulp and paper industry
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