Hand Tools Archive


Bill Tindall, E.Tn.
For those that remember their physics history , Thompson in the canon boring experiment and later Joule, demonstrated that work and heat are equivalent. The more work you put into planing, force times distance, the more heat will be generated. The heat generated by the friction of the shaving rubbing on the cap iron and blade will equal force times distance. This heat divided by the time to generate it will be the heat flux into the blade, and cap iron, if there is one. That's the easy part.

Heat flux out of the blade tip will be a complicated function of wood heat capacity, temperature rise of the shaving and mass of the shaving, plus a complicated function of the mass of the blade and cap iron and steel conductivity and heat capacity and a few other variables.

The temperature of the blade tip will be the temperature where the heat out equals heat in. Heat is generated by friction so this heat input is being generated all along the front and back of the blade that is rubbing on wood plus the shaving rubbing on the cap iron. This heat input is not concentrated at the tip. In fact the frictional forces at the tip may be substantially less than along the blade surfaces because of the splitting action at the tip.

Bottom line-

Scrapers get hot because the amount of shavings carrying heat away is puny, the work input is high and the mass of the metal is low compared to a plane blade.

Speculation- in normal planing I don't see the opportunity for a bade tip to reach a temperature where it will loose hardness. There is too much heat capacity of the metal and it is a good conductor to carry the heat away from the tip. Of greater interest is whether the blade will reach a temperature where corrosive wear becomes important, as it is in milling wood with a high speed router bit. If it does the chromium content of A2 and PMV11 could contribute to their edge life.

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