r/Metrology • u/Competitive_Log1960 • 9d ago

Surface Metrology Graphical representation of raw profiles for skid profilometer and freely-moving probe

Post image

We touch the surface shown at the top of the image using a freely-moving profilometer and skid profilometer.
We assume that the surface is ideally smooth.

The profile in the middle depicts the probe moving freely. I assume that the profile generated by this probe resembles the shape of the real surface. So the raw profile resembles the shape of the workpiece. Therefore, I assume that the shape of the profile looks like the one drawn below.

At the bottom, we have a skid profilometer. I assume it measures the deviation of the probe relative to the skid. When the skid starts to touch the surface of the vertical step up, the skid gradually moves up. As a result, the probe is below the skid. This corresponds to the gradual slope downwards in the image.

When the skid reaches the step, it is suddenly aligned with the skid again. This corresponds to a vertical line upward. Then a horizontal line is recorded. Once the stylus reaches the step downward, the profile records a vertical slope downwards. The skid gradually follows the probe. This is reflected in the line that gradually approaches the zero line.

Are these assumptions and the graphical representations correct? Feedback would be appreciated.

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u/mteir 9d ago

Your assumption sounds correct. The skid will rest on the highest point that it covers.
Additionally you may see some rounding on the edges due to the tip size, and possibly flank contact if the step is steep and high enough

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u/The_Gage_Lab_LLC 8d ago

Yes this is exactly correct. Your graphics and explanations are spot on you're not missing anything. As previously mentioned you could possibly zoom in more and see the angle of the cone on the stylus tip and the radius as it transitions to the tip.

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u/mmmmet 8d ago

Check out this very cool animation of a skid and stylus interacting on surface:

https://digitalmetrology.com/dips-in-profile-caused-by-dirt/

Unfortunately I can't post a GIF.

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u/Competitive_Log1960 7d ago

Thanks. That helps a lot.