A curved cutting edge is critical to most operations with your bench planes. The curve prevents the corners of the iron from digging into your work, and it allows you to correct the flatness of the face or edge of a board.
But how do you create this curve, sometimes called a “camber?” And how do you create it with a honing guide, which seems to encourage a cutter that is sharpened straight across?
There are lots of valid ways to create the curve. Here’s how I do it. My method has the provenance of a stray dog , a little David Charlesworth, a little Robert Wearing, and a little bit from everyone else who has ever taught me sharpening.
I start with the #1,000-grit waterstone. This stone cuts quickly enough to shape an edge or remove small nicks or chips. Clamp your cutter in your honing guide then (mentally) divide its edge into five “positions” (see the photo above for details).
The trick to creating a curve is to put finger pressure at each position. At position “1,” put your fingers firmly against the corner and sharpen the corner for 10 strokes.
Then move your fingers to the other corner (position “2”) and go for another 10 strokes. Then, at positions “3” and “4,” go for seven strokes. Then do a few strokes in the center at position “5.” Now check your work with a square.
You need to learn what the curve should look like for each of your planes. Here are the basic principles: If the iron is bedded at a high angle greater than 45Ã?Â°, you need less curve. If the iron is bedded at a lower angle such as 12Ã?Â° or 20Ã?Â°, you need more curvature to get the same effect.
And what is the desired effect? You want to take the widest shaving possible without the corners of the cutter digging in. There is math here. Having a .005″ arc-to-chord curve at 45Ã?Â° results in a curve of .0035″ being exposed out of the mouth. (If you have a bevel-up plane bedded at 12Ã?Â°, the same .005″ arc-to-chord curve will result in .001″ curve being exposed in the mouth , thanks to woodworker Rob Porcaro for the formula.)
The honest truth is you just need to learn what the right curve looks like when you show the cutting edge to a straight edge. If there is too much curve, sharpen some more in the middle position (5) to flatten the curve. If the curve is too flat, add more finger pressure or strokes at the corners to increase the curvature.
When you have a satisfactory curve, advance to the polishing grits (#4,000 then #8,000) and repeat the same regimen. The polishing grits will remove less metal, but you definitely can increase or decrease the curvature while polishing.
It takes a little practice to find the right curvature for your plane, but the rewards are enormous: Shimmering handplaned surfaces with a sensuous, scalloped and touchable texture. It’s worth the effort.
– Christopher Schwarz
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