Solutions of the Surface Metrology Quiz

Find here the solutions of the Surface Metrology Quiz with explanations and justifications. Do not read this page if you did not do the quiz. Try it on Surface Metrology Quiz

Question 1: Which of the following parameters are related to the height distribution?

Correct answers: B, C, D

ISO 4287 is the master standard for profile parameters in the ISO GPS system. It defines terms and provides definitions for common parameters.

Rz is the maximum height of the profile: defined on the sampling length, this parameter is frequently used to assess roughness height, excluding localized protruding peaks or deep valleys. It is not related to the height distribution.

Rq is the root mean square deviation of the assessed profile. This corresponds to the standard deviation of the height distribution, defined on the sampling length. Rq provides the same type of information as Ra.

Rsk is the skewness of the assessed profile or the asymmetry of the height distribution, defined on the sampling length. This parameter is important as it gives information on the morphology of the surface texture (see also next question). However, this parameter does not give any information on the absolute height of the profile, contrary to Ra, Rq or Rz.

Rmr is the material ratio at a given depth. This parameter gives the percentage of material cut at a given depth from the top of the profile. The reference may also be taken from the center line or another reference height (c0). It is calculated from the Abbott-Firestone curve which itself is the cumulative probability curve of the height distribution.

See the page: Profile parameters

Question 2: Which of the below height distribution graphs corresponds to a negative skewness?

Correct answer: B

The Rsk parameter corresponds to the skewness of the assessed profile: the asymmetry of the height distribution, defined on the sampling length. This parameter is important as it gives information on the morphology of the surface texture.

Positive values correspond to high peaks spread on a regular surface (distribution skewed towards bottom) while negative values are found on surfaces with pores and scratches (distribution skewed towards top). It is therefore interesting when contact or lubrication functions are required. However, this parameter does not give any information on the absolute height of the profile, contrary to Ra.

See the page Profile parameters

Question 3: Which of the following statements are true?

Correct answers: B, C

Roughness parameters are filtered by default with a Gaussian filter but specification may require another filter such as the Spline filter or the robust Gaussian filter.

P parameters are calculated on the primary profile, and therefore are not filtered. In some cases, when the workpiece is flat enough and there is no waviness, the filter is not required. It may be the case if the profile is very short such as with small mechanical components (e.g. watch mechanism). Without specific indications, default settings shall be used. In case of surface texture, the Gaussian filter is the default filter.

The Gaussian filter is not tolerant to form nor to local irregularities. In presence of high peaks, deep valleys or geometrical structures, the Robust Gaussian filter is the preferred choice such as with structured surfaces, stratified surfaces and in presence of form, grooves or pores. In particular, it is a good replacement for and improvement on the double Gaussian filter that is defined in [ISO 13565-1].

See the page Filtration techniques

Question 4: Which of the following instrument technologies have a dedicated document in the new areal standard?

Correct answers: A, C, D, F

Parts 600-701 of ISO 25178 deal with nominal characteristics of areal surface topography instruments.

Interferometers are described in part 604

Stylus profilometer is described in part 601

Chromatic confocal probe is described in part 602

Confocal microscopes are described in part 607

See the page Introduction to surface texture

Question 5: In which of the following cases it is necessary to evaluate surface texture on a surface instead of on a profile?

Correct answers: B, C, E

A surface is said to be isotropic when it presents identical characteristics regardless of the direction of measurement. In that case, a profile extracted anywhere or in any direction will be a good assessment of the surface. When an extracted profile is different depending on where it is extracted or on its direction, it means that the surface cannot be assessed correctly with a profile and requires an areal characterization (with a surface)

See the page Areal field parameters

Question 6: Feature parameters are areal parameters...

Correct answers: B, C, D

Feature parameters focus only on particular points or portions of the surface when quantifying topology. In particular it identifies peaks, pits, saddle points, hills, dales, course and ridge lines. They represent the surface topology with features that play a role in the surface function.

Contrary to Field parameters that use all surface points (height, spacing or slope), feature parameters use only selected points on the detected features.

In the ISO 25178-2 standard, feature detection is based on watershed segmentation of the surface followed by a pruning method aimed at selecting significant features on the surface. This pruning is based on a height discrimination between extrema points and saddle points.

See the page Areal feature parameters

Question 7: Wolf pruning is...

Correct answers: B, C, D

Wolf pruning is a method which allows the simplification of an over-segmented surface. It reduces the number of significant features. It is based by default on a percentage of Sz (i.e. maximum peak-to-valley height). As such, it is considered as a type of nesting index, as the cut-off value is for a standard filter.

Watch this video for explanations: (minute 10:53 onwards)

Question 8: Which of the following statements about volume parameters are true?

Correct answers: A, B, D

Volume parameters represent an evolution of the functional indices and the family of Sk parameters, and are also calculated with respect to the Abbott curve.

Volume parameters define material volume and void volume between height thresholds.

Vmp characterizes the material volume of peaks, which is a concept close to Spk which calculated peak height.

Depending on the surface, Vmc can be larger, smaller or equal to Vmp.

See the page Areal functional parameters

Question 9: Which surface characteristics are assessed with Str?

Correct answer: B

Str is an indicator of surface isotropy.

Complexity is assessed by Sdr or Sdq.

Flatness can be assessed by FLTx parameters or by Sz after levelling and low-pass filter.

Smoothness is a surface attribute sensed by human, not a surface texture parameter.

See how Str is evaluated from the autocorrelation map: Areal field parameters

Question 10: Which of the following statements about saddle points are true?

Correct answers: A, C

A saddle point is a local minimum on a ridge line and a local maximum on a course line. They are always at the intersection of a ridge line and a course line.

All downward paths converge towards the pit point, not the saddle point.

As any feature, a saddle is found after segmentation and pruning process.

While a saddle is between a pit and a peak, it is not necessarily exactly between in terms of height. It may be closer to the peak or to the pit.

See the page Areal feature parameters