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When is Smooth Too Rough?

When you think of roughness, you might envision a wide assortment of sandpaper grits, whereas the abrasive material size decreases, the grit number increases. Coarse 60 grit sandpaper is almost pebble-like.  Medium sandpaper-like that is used on wood is 120 or 150 grit.  A super smooth 3200 grit wet and dry sandpaper is so fine it could practically pass for paper. As you probably already know, using the proper consecutive sandpaper grits is essential to obtaining your desired outcome when finishing (or refinishing) a workpiece.

Similarly, understanding surface roughness as it relates to a machined part, polished item, or manufactured part is crucial in determining how the part will perform, wear, appear, or function. When evaluating components, it is often necessary to know not only the surface roughness but other characteristics like waviness or form. Surface roughness is defined as the finer height deviations of the component’s surface. The term surface texture includes all component features, and it covers both waviness and the finer roughness details.

Defining a Surface

The illustration shows some of the characteristics included when measuring and quantifying a surface.

Surface Texture The topography of a surface composed of certain deviations that are typical of the real surface. It includes roughness and waviness.

Lay direction of finish pattern.

Profile  a sectional view or slice through the part structure.

Form – general shape of the surface; can indicate inaccurate machining or a part under stress.

Waviness widely spaced irregularities; this generally relates to machine vibration or chatter.

Roughness closely spaced irregularities; usually caused by cutting tools, sanding, or grinding.

As you can imagine, accurate and repeatable measurements of a surface make it possible to control performance and function on mechanical and manufactured components. For example, metrology can help ensure a surface is within specifications to reduce the wear of a moving part and increase its lifespan. It can ensure that the designed fit between a shaft and a seal will be achieved. And it can help decrease fuel consumption and reduce emissions of an engine by providing good fuel seals to minimize leakage. In other industries, it helps to optimize the efficiency of solar cells, obtain an excellent specular finish on a metal surface, and reduce an electronic component’s heat output. All these design and manufacturing elements can be obtained by measuring and controlling specific surface texture parameters.

Confidence in the measurement is critical to making good use of roughness data. One way to build up the level of confidence is to measure a known ‘standard’ that should have a particular level of roughness and compare that value to the results from the measurement tool.  Doing this in the right area with the proper alignment and analysis techniques is crucial to gaining confidence. One tool that can be used with high confidence is a 3D non-contact optical profiler such as the ZYGO Nexview, NewView, or ZeGage. These instruments utilize coherence scanning interferometry (CSI), which is ideal for quickly evaluating a three-dimensional region without damaging the surface.

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