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Difference between Ra and Rc - surface roughness

Q. In surface roughness measurement, we basically use two units Ra and Rc. Basically what is the difference between the two?

DIPAK NANDI
machine shop engineer - Ahmedabad, Gujarat, India

March 16, 2011

Hi, Dipak.

I've never heard of an Rc scale for surface roughness. The only Rc that I am familiar with means "Rockwell C scale" -- a measure of hardness.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 16, 2011

A. In the thin air of the Rocky Mountains, Rz sounds a lot like Rc.

Some very good info here: https://www.taylor-hobson.com/faq.php

(Reposted from CR4)

Willie Alexander
- Colorado Springs, Colorado

A. Ra and Rc are similar measurements with different scopes. Ra is the arithmetic mean of the deviations of crests/troughs in a surface height, whereas Rc is more isolated, measuring the mean deviation of a given band of measured surface. To simplify, Rc is a shorter measure distance.

Charles Lambert
Pool Designer - KC, Missouri
January 26, 2023

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