Sharpening chisels consistently, part 2

2 04 2010

After my previous post about sharpening chisels, I received a helpful comment which informed me that another way to set angles consistently for sharpening chisels is to make a blade projection board. This takes just a little bit of work and has many advantages over the marking method I wrote about before.

The angle of the blade in the honing guide is determined by how far it projects from the front, and so the key to getting the same angle every time is to make sure that the blade projection length is the same every time. Having a physical guide is faster and more precise than visually lining up a mark.

Making the jig is simple. First, decide which angles you want to use for the primary and secondary bevel. Next, put the blade in the honing guide and adjust the blade projection until you get the desired angles. For each angle, measure how far the blade sticks out. Then take a board and attach stops that are those distances from the edge.

To gauge blade angles, I used an application for my iPod Touch called Clinometer. Even though it’s not particularly important to set it at a specific angle (27 vs. 28 degrees, for example), it’s still nice to know what the angle is.

Measuring the angle of the chisel in the guide, on a glass plate (the other side happens to have sandpaper glued to it).

I made the jig with stops for 25 degrees for the primary bevel, and 28 degrees for the secondary bevel. The stops were placed at 41 and 34 millimeters.

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Sharpening and resharpening chisels at a consistent angle

1 04 2010

According to the Internet, a sharp knife is a safe knife, and the sharper, the better. Some people like them to be literally razor sharp — sharp enough to shave with. (For reference, good knives, when new, are usually sharp but not shaving-sharp.) For me, as long as a knife is just plain sharp and can cut easily, I’m happy. Going the extra distance to make it razor sharp doesn’t make it perform that much better, at least for my purposes.

With woodworking tools, it’s a different story. A razor sharp chisel or hand plane lets you do things that simply aren’t possible with one that is merely sharp. Since wood fibers are much tougher than (most) food, having a really sharp chisel matters more than having a really sharp knife. With a chisel, it can be the difference between using a hammer and just cutting by hand, or between leaving a rough edge and a glassy smooth one. A very sharp plane can leave a surface that’s smoother and shinier than is possible with sandpaper.

When sharpening a chisel or plane blade, it’s important to hold it at a consistent angle so that the very edge of the blade doesn’t get ground down at a more obtuse angle, and so that you can assess sharpness by looking at how it reflects the light. If the surface is rounded instead of flat, the light doesn’t reflect off it all at once, making it difficult to tell if you’ve ground away all imperfections in the cutting edge.

Many expert woodworkers sharpen their tools by hand and have trained themselves to hold the blade at a steady angle. I’ve tried doing it this way, and I always end up with a blade that has a rounded profile. So I bought a cheap (about $10) honing guide. It holds the blade at a steady angle and has a little wheel that lets you roll it along a sharpening stone.

A chisel in the honing guide

The guide makes it trivial to grind the chisel at a steady angle, but it’s still a bit of work to set the angle of the chisel the consistently, time after time. What I did previously was this: put the chisel in, tighten the holding screw, then place it on a flat surface and check if any light is visible between the blade and the surface. If so, loosen the screw, move the blade, and do it again. Usually it took around three or four tries before I got it just right. And then there’s the issue of grinding the microbevel, which has an angle a few degrees steeper than the main part of the blade.

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