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Donald K. Cohen, Ph.D., is founder of Michigan Metrology (Livonia, MI). For more information, call (734) 953-5030, e-mail doncohen@michmet.com or visit www.michmet.com.
Specifications for surface texture frequently focus on surface “roughness”—the finer structures in the texture—often to the exclusion of the “waviness”—the larger structure of the texture. Unfortunately, problems related to sealing, vibration, noise, wear, etc., are regularly caused by issues hidden in the waviness domain, which cannot be captured by common roughness specifications.
How does the surface texture affect the friction between two dry parallel sliding surfaces? It turns out that this problem has challenged some of the greatest minds in science and engineering.
When measuring surface roughness, typically the average roughness, Ra, is specified. Ra is a measure of the average of the absolute value of the variation of the surface texture profile about a best-fit mean line. A typical specification for Ra may be 16 microinches, or 0.4 micrometers.