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Machining terminology

Looking at a pressed part what is the difference between Ra and Rms measurement specification. Is there a difference? If so, what is the formula, measuring technique?

Mark Farrelly
- Adelaide, South Australia

Mark,

Ra is the average height of the peaks and valleys in a surface. Obviously average not always tells you the real story (a minus 3 for instance would cancel a plus three in a normal average method). RMS is a mathematical or statistical way to express what you really want to now. It means Root Mean Square and by taking the square of all the numbers and then by taking the square roots, absolute values are obtained. Its closer to what you need to know about a surface.

Good luck,

Guillermo Marrufo
Monterrey, NL, Mexico

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This question comes up a lot and is not a easy question to answer. RMS stands for root means square. It is a mathematical term and measurement of the surface profile of the material in question. It is also know as Rq. Now RA means roughness average. Both measurement scales can be taken with a profilometer, but the programmed info is different. The difference of the 2 scales is that an RA is a measurement of ONLY the peaks of the surface profile; whereas, an RMS or Rq is both the peaks and valleys. There is NO direct correlation; however, most people use a 4:1 scale of adding or subtracting to get one from the other. The actual scale is a changeable ratio that can be anywhere from 1:1.11 to 1:1.45 depending upon the previous machining process.

AF Kenton
retired business owner - Hatboro, Pennsylvania