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  for Metal Finishing since 1989
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TOLERANCING FOR SURFACE ROUGHNESS

CAN ANY HELP WITH THIS QUESTION? ON THE PRINTS WE USUALLY SEE CALL OUT FOR SURFACE ROUGHNESS IN INCHES: 32 RA, OR 16 RA, OR 8 RA OR ANY NUMBER. BUT WHAT DOES THAT REALLY MEAN? I MEAN HOW MUCH IS THE UPPER AND LOWER TOLERANCE? EXAMPLE: IF THE CALL OUT IS: 32 RA, DOES THAT MEAN FROM 0 TO 32? DOES THAT MEAN FROM 0 TO 48? SO, WHEN WE HAVE CALLOUT OF ANY NUMBER "X RA" HOW MUCH IS ALLOWABLE DEVIATION? X RA + OR - HOW MUCH? AND I NEED THE INFORMATION FOR METRIC SYSTEM TO.

THANKS EVERYONE,

George Baricevic
- Fremont, California
2002

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Unlike machine specs that give you an exact range or tolerance, Ra or RMS do not normally give you a range. Probably that is due to the fact that these measurements are already based upon averaging. That is, the measurement is an average over a given surface profile. If Rz was given, that reading is more exact in that it tells you how many readings to take over what surface area. By the way, never, ever consider a deviation starting with zero.

AF Kenton
retired business owner - Hatboro, Pennsylvania
2002