Rockwell [HRC/HRB/HRA] is one of the most common units used for listing the hardness of machined materials. the test is done by measuring the depth of penetration of a sphere under a large load compared to the penetration made by a reference preload. The Rockwell scale is divided into 9 sub-scales marked by letters A-K. Each scale uses a different reference load and different sphere sizes. In machining, the most common is the C scale (HRc]. Machined metal is usually between 10-65 HRC.
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