When I was taught to sharpen in 1992, the flat back of the iron was holy ground. We were taught to flatten it completely and polish it like a mirror. Never mind that none of the old tools we were buying at flea markets looked like that.
With the old tools, there was rarely much work on the back, and forget a mirror polish. If there was any significant work on the back, it was up at the tip only.
Thanks to David Charlesworth and his “ruler trick,” many of us have left the dogma of a flat and polished reference surface behind. What matters is a sharp edge and the cutting geometry of that particular tool. Some should have a flat back. Most don’t need it.
Earlier this year, woodworker Carl Bilderback sent me a great little book called “Carpenters’ Tools” (1950, Frederick J. Drake & Co.) by H.H. Siegele (pronounced like “seagull” more or less). Siegele was a crusty carpenter from the old school who lived in Emporia, Kan. His book is a treatise on all the tools in the carpenter’s kit, including how to maintain them.
There is a lot of great information in the book that doesn’t get much coverage, such as how to use a scrub plane. This weekend I spent some time with his section on sharpening, just for kicks. Here’s what he says about sharpening a block plane.
“Fig. 84 shows two ways to finish the sharpening of a block plane bit on the oilstone. At the top is shown the bit in a 20-degree angle for finishing the bevel, while the back is finished flat against the stone. At the bottom the bevel is also finished on a 20-degree angle, but the back is finished at about a 5-degree angle. Both of these methods are good, but the bottom method is mostly used. It is a little better than the method shown at top, especially when the bit is hollow-ground, as most bits are when a small grinder is used.
“Fig. 85 gives two enlarged details of block plane bits in part, showing the two methods of sharpening such bits. The upper one shows a 20-degree angle for the bevel sharpening, and a 5-degree angle for the back, while the bottom shows a 23-degree bevel sharpening and a flat sharpening for the back.”
Note that Siegele doesn’t recommend this procedure for bench planes, only for block planes. Why? He doesn’t say, but I suspect that lifting the iron as you finish the back helps remove some of the wear that the back receives in a block plane. Some people call it a “wear bevel.” Please don’t make me talk about the “wear bevel.” Let’s just say that the bevel that faces the wood gets more wear than the part of the blade that faces the user.
In any case, Siegele wasn’t using a ruler to produce the back bevel, but the idea is the same: Polish the tip.
— Christopher Schwarz
Want to know more about sharpening? I heartily recommend Ron Hock’s “The Perfect Edge” (Popular Woodworking Books). Hock, the owner of Hock Tools, tells you everything you need to know in an easily digested format. It’s good stuff and should be on everyone’s bookshelf.