Making Mortise Chisels
Copyright © 2001 by Jim Wilson
(Click images to enlarge)
Mortise chisels are designed for heavy pounding and prying. Their handles have metal hoops, or ferrules, around the top and bottom to deter splitting.
Fitted onto the tang at the base of the handle is a bolster. When the handle is struck, the force is transmitted through the handle to the bolster, which bears on the chisel body. Some mortise chisels have a leather washer between the handle and bolster to soften the acceleration of the blade into the work.
|The chisel blade is precision ground high carbon tool steel with the cutting edge beveled at 35°. It is about 7" long, not including the 2-1/8" tang. The sides of the blade are square to the back of the chisel, not beveled.|
Precisely controlled heat treatment hardens and tempers the blade to HRC 62 (a value of 62 on the Rockwell hardness "C" scale). Hardening is done in a vacuum to prevent decarburization, and a high-pressure inert gas quench is used to minimize distortion and maximize hardness. Two tempering cycles stress-relieve the steel and reduce brittleness, while minimizing loss of hardness.
Next, a highly refined deep cryogenic process significantly improves wear resistance and ultimately edge retention, while avoiding the typically associated increase in brittleness. The molecular changes in the steel are permanent, irreversible, and uniform throughout, and will last the life of steel regardless of subsequent finishing operations and regrinds. The end result is a blade which resists chipping, takes a better edge and holds it longer.
The tang is annealed to eliminate the possibility of a fracture from an errant blow to the handle, and then the blade is finish ground and honed by hand.
The bolsters are made from 3/16" mild steel. The holes are rectangular with slightly concave sides, and sized to a drive fit on the tang. If you are making your own, it is easier to simply drill a round hole 0.010" to 0.030" smaller than the tang's widest diagonal, and file to fit.
I clamp the chisel in the vise, drop the bolster onto the tang, and drive it on with a short length of pipe, forcing it flush with the tool shoulder. In the words of Alexander Weygers, the mild steel bolster yields to the harder steel of the tang. This cuts and compresses the bolster steel, locking it tightly onto the tang at the shoulder.
Finally, the handle is mounted on the blade. The tang is heated until the tip just begins to glow dark red in dim light. Quickly, the handle is driven onto the hot tang with rapid, light mallet taps, until it is about 1/8" above the bolster (or leather washer, if one is used). After allowing the tang and handle to cool for a minute or two, the handle is driven the rest of the way to the bolster, which clinches it permanently in place, and then it is allowed to cool completely.
|A finished chisel with a cherry handle.|
|The massive 1" mortise chisel with a bloodwood handle.|
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