Chisel Therminology

Posted by Easy WoodCraft on

Mar 9, 2017

Chisels can be broadly classified into two groups: those designed for use with hand pressure only, and those intended to be driven by a mallet. Chisels made for hand use only may be either firmer chisels (originally called "former" chisels because they were used to form rough shapes) or paring chisels. Mallet-driven chisels are mostly thick, stout chisels used to chop mortises. Over time, any heavy-duty chisel became known as a mortise chisel, even though they could be used for carving or other shaping work.

Most chisels are similar in appearance. They each have a wood or plastic handle and a blade with a tang or socket (bottom drawing). What sets them apart primarily is the profile of the blade (top drawing) and their respective grinds or cutting angles (middle drawing).

Mortise chisels are easily identified by their thick, almost square blades. The bevel is ground to a steep angle to hold up under the constant blows of a mallet. Firmer chisels can have either square or beveled edges. The bevel for squareedged firmer chisels is usually a bit steeper than for bevel-edged chisels, which are most often used for paring. Note that in the middle drawing two sets of angles are given. As a general rule of thumb, chisels used on hardwoods require a steeper bevel to cut effectively than those used on softwoods.

METAL HARDNESS

I One of the most important characteristics of a chisel that affects its ability to hold an edge—and its overall performance—is its hardness. The hardness of metal is tested using two common methods: the Brinell test and the Rockwell test.

The Brinell test

In this test, a hardened steel ball is pressed into the smooth surface of the metal to make an indentation that's measured under a microscope. Once the spherical area of the indentation is calculated (taking into account the pressure applied), the stress per unit of area when the ball comes to rest is calculated, and the hardness number obtained.

The Rockwell test

With the Rockwell test, a metal ball or cone with a known hardness is pressed into the metal being tested. The Rockwell hardness is defined at the point of deformation of or penetration into, the metal being tested. The Rockwell Scale, developed in conjunction with the ASTM (American Society for Testing and Materials), is the mostoften-used measure of metal hardness in the world today. In general, the higher the alphanumeric designation, the harder the metal. So, a "C" is harder than a "B" a "30" is harder than a "20," etc. The C scale is the most-used Rockwell scale and is often denoted Rc. Chisels made with steel tested at Rc 58 to 62 have been proven over time to stand up well.

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