What does surface roughness Ra indicate, and what does a value of 63 microinches (1.6 μm) imply?

What does surface roughness Ra indicate, and what does a value of 63 microinches (1.6 μm) imply?

• A. Ra measures the maximum height of peaks on a surface.
• B. Ra is the total roughness average over length.
• C. Ra measures the average distance of peaks from a reference line.
• D. Ra measures the average surface deviation; 63 μin (~1.6 μm) indicates a relatively smooth surface, suitable for many mechanical fits.

Answer: (D)

Ra is the arithmetic average of the deviations of a surface profile from a reference mean line over a length of measurement. In other words, it captures how far the surface profile wanders away from an ideal flat line, averaged across the measured area. It does not describe the tallest peaks alone, nor is it simply a “total” roughness over length, and it’s not just the distance of peaks from a base line without accounting for valleys as well. The value you’re given, 63 microinches (about 1.6 micrometers), is a fairly smooth finish by many engineering standards. Such a roughness level is typically suitable for many mechanical fits and sliding contacts, where lower roughness reduces friction and wear.

Ra is the arithmetic average of the deviations of a surface profile from a reference mean line over a length of measurement. In other words, it captures how far the surface profile wanders away from an ideal flat line, averaged across the measured area. It does not describe the tallest peaks alone, nor is it simply a “total” roughness over length, and it’s not just the distance of peaks from a base line without accounting for valleys as well. The value you’re given, 63 microinches (about 1.6 micrometers), is a fairly smooth finish by many engineering standards. Such a roughness level is typically suitable for many mechanical fits and sliding contacts, where lower roughness reduces friction and wear.