Steel and Sharpness

A comprehensive description of the alloys of steel and their properties would be beyond the scope of this site even if I were qualified to write one. I will however attempt to define some of the basic concepts that are important to understanding the qualities needed in plane blades.

Hardness describes resistance to deformation. It is measured using a diamond stylus that is dropped with a known amount of energy onto the surface of the test piece. The depth of the resulting dent is used to calculate the hardness of the material tested. For hardened steel, the Rockwell C scale is used. Typical values for plane blades range from Rc56 to Rc64. A blade that is too soft will deform in use, leading to early edge failure. A blade that is too hard will tend to chip. Some alloys at a given hardness are more resistant to chipping than other alloys at the same hardness. W1 water-hardening carbon steel has good resistance to chipping, A2 has less, and CPM 3V falls between the two. Resistance to chipping is closely related to toughness.

Toughness is the opposite of brittleness, and describes resistance to breakage. It is more difficult to measure toughness than hardness. The standard tests involve striking a test piece of steel with a swinging weight to see how much energy is needed to break the steel. This property is notch-sensitive, meaning that a scratch or groove will make breakage more likely. In plane blades a lack of toughness will result in chipping if the bevel angle is too acute.

Abrasion Resistance is what accounts for long blade wear. While harder blades generally have better abrasion resistance than softer ones, blades of different alloys at the same hardness may differ significantly in abrasion resistance. High alloy steels that contain finely distributed carbides can have extremely high abrasion resistance even though they are no harder than other blades. Abrasion resistance can be measured, but the results depend on what the steel is abraded by. Most tests are done with steel on steel, since this is the type of abrasion most common in machinery. Abrasion by wood hasnít been studied, to my knowledge. There are so many species of wood with such differing properties that it would be difficult to make generalizations.

Other properties can be important in plane blades even though they donít directly affect cutting performance. One of my older cast steel blades has an alarming tendency to rust, even when kept in a heated shop. In spite of its good edge qualities, it takes too much work to maintain it. Most of my other blades are much more resistant to discoloration and rusting, with the Clifton being the most prone to problems in this area.

As a practical matter, I consider a blade that requires unusual equipment to sharpen to be a poor choice for general use. My CPM 3V blade borders on being too hard to sharpen. It grinds well, though slowly, on my Tormek wet grinder and hones very well with diamond paste. The King 800x waterstone works well on the 3V, but the Takenoko finishing stone and the Norton 8000x stone cut it only very slowly. Iíve put together a honing system based on a Millers Falls jig that allows infinite adjustment of the bevel angle within a range of about nine degrees, and using this system I can put a very fine edge on the 3V quite quickly. Without the jig it would require good technique and considerable patience.

Even more difficult to sharpen is the Holtey S53 blade. Grinding is extremely slow, and the grinding wheel tends to glaze quickly. Honing with diamond is effective.


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