February 2020 Volume 2

MATERIALS

This informative article addresses surface roughness. It discusses in-house standards for surface roughness, explains the Ra scale for surface roughness and what that scale actually represents,and talks about why surface roughness is important. Finally, it talks about how perception can influence objectivity with respect to measurement of surface roughness. Basic industry standards of surface finish for some equipment manufacturers are 63 Ra for Blanchard ground plate and 32 Ra for surface ground plate. It must be understood, though, that many factors contribute to these values that can cause different readings. Those factors includes lubricants, oxidation, scratches, and condition of testing unit to name a few. Variable testing results are possible. How Rough is Smooth? By Steve Janiszewski

RMS surface roughness compared to the Ra calculation. Surface roughness can be important for different reasons.Themost common place for measuring surface roughness may be on ground plate. In that situation, the surface roughness is important mainly because it typically correlates to how well we were able to hold size and parallelism of the plate. A poor or inconsistent surface roughness can be an immediate visual clue that there may be a problem with size or parallelism. Aside fromthe surface roughness that can bemeasured onBlanchard ground or surface ground plates, there are other situations where surface roughness matters. For example, the surface roughness of milled pockets or machined holes can affect performance– specifically with regard to wear resistance and crack formation. Rough surfaces typically wear more quickly than smooth surfaces. Avoiding crack formation is also important in many of the stamping application tooling components, and bolster components and the bolster itself in forging applications.

Measuring Roughness Surface roughness can be measured several different ways but by far the most common method is the calculation for Ra, which corresponds with the average of the absolute roughness value compared to the mean. Imagine that you are looking at the surface of a plate, visualized as such (see picture for reference). We are concerned with the height of the peaks and valleys of a surface compared to the average. If the peaks and valleys are relatively large compared to the average roughness value, then the surface is rough and Ra is high. If the peaks and valleys are relatively small compared to the average roughness value, then the surface is smooth and Ra is low. Another surface roughness scale that may be used is the root-mean square (RMS) scale. The formula used to calculate RMS surface roughness is different than Ra but uses the same individual height measurements of a surface’s peaks and valleys. The RMS scale is typically not specified as commonly as the Ra scale because a single large peak can raise and distort the overall measurement of

FIA MAGAZINE | FEBRUARY 2020 42