What is the Significance of a Rockwell Hardness Rating?
 

In many tool descriptions in our catalogs, you will see the symbol "Rc" with an accompanying number. This indicates the hardness of the tool on the Rockwell C hardness scale. There have been many hardness testing systems used over the years, but the two primary surviving ones are Brinell, which measures the indent that a small steel ball will make in material when pressed against it with a predetermined load, and the Rockwell system, which similarly measures the penetration of a specially shaped diamond.

The Rockwell system has many scales, of which the A, B and C are most commonly used, to measure the relative hardness of different materials, primarily metals. The only scale used for hardened steel is the Rockwell C scale. At the lower end of the scale would be a handsaw, commonly Rc38-42. Cabinet scrapers were traditionally in this same range since they were frequently made from old handsaw blades. When the industry started to provide rectangles of steel plates specifically for use as cabinet scrapers, they hardened them to the same range. Only years later did Sandvik® and Veritas® provide scrapers about 10 points harder, in the Rc50 range. These scrapers hold an edge substantially longer than softer scrapers, although burnishing the edge takes a bit more work.

Chisels are typically in the Rc58-62 range, with the majority of Western chisels being in the Rc58-60 range and Japanese chisels Rc60 and above.

We emphasize Rockwell hardness in our catalogs, but it is only one factor in the ability of a tool to hold a good edge. Equally important is the alloy being used and even the hardening process itself; we note Rockwell hardness to indicate the trade-off between toughness and brittleness. A Rc63 chisel will wear more slowly, but will chip more easily than a Rc58 chisel.

Hardness is the easiest quality factor to verify. Maybe that's why its use has become so popular.

L.L.
02/02

Hardness Testing

With a bit of practice, anyone can learn how to test for tool hardness by using a file. As a general rule, the easier a tool is to file, the softer it is. If a tool has a high manganese content, it will file more easily, since manganese is added to tool steel to increase machinability. For ordinary carbon steels, relative hardness can be determined by filing. At one time, a file called a pillar testing file was commonly used for that purpose. Any material to be filed was first checked with the testing file. Pillar testing files have a very carefully controlled hardness, usually about Rc63 or Rc64. They should be used only for testing, in order to preserve their quality for an extended time. A regular file can be used for the same purpose, but a minor change in sharpness will immediately cause "skating" on a harder tool.

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