Hand plane skew angles

7 06 2010

Usually hand planes are pushed straight ahead, but sometimes it’s useful to turn them a little bit relative to the direction of motion.

When you push a hand plane straight forward, the cutting edge is a line that is perpendicular to the direction of motion. We can call this “normal” planing (pardon the pun). When you rotate the plane a bit and still push it in the same direction, the cutting edge is now at an oblique angle to the direction of motion. This is planing at a skew.

Left: normal plane motion. Right: skewed plane motion. The red line represents the edge of the blade.

One thing that skewing does is reduce the effective angle. This is the angle between the planed surface along the axis of motion, and the upper side of the cutting edge. A lower effective angle should reduce resistance when making a cut. Here’s what it looks like when you plane at a skew. (In practice, there’s usually not a big arrow in front of the blade, nor is there tiny writing all over the place.)

The relationship between the pitch, skew, and effective angles. The pitch angle is the angle between the upper side of the bevel and the surface being planed. The angle of the lower bevel doesn't matter here, so for simplicity, it's shown as flat on the surface being planed.

How do you calculate the effective angle from the blade angle and skew angle? It might help to have some definitions here.

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A homemade straightedge

16 05 2010

If you go to the office store or hardware store and buy a ruler, you might think that you’ve gotten two for the price of one: a length-measuring tool, and a straightedge. Unfortunately, this isn’t always the case. A regular old ruler may or may not be straight. Try this: put the ruler it down on a sheet of paper, mark a line along the edge with a 0.5 mm mechanical pencil, then flip it over and mark another line on top of the previous one. If the lines are directly on top of each other, the edge is straight — at least, it’s straight enough that the error can’t be seen with half-millimeter pencil line. I have at least one ruler where the gap between the lines is over 1 mm.

Nothing is perfectly straight. The only question is how far from straight your straightedge is. For most drawing and measuring applications, if it passes the test above, it’s good enough. But sometimes you need it to be better. In woodworking, the surfaces of some tools need to be very flat — much flatter than 0.5 mm. A hand plane with a sole that’s flat to only 0.5 mm would be worse than useless; it would unpredictable and inconsistent, and would gouge the workpiece horribly. (Unfortunately, inexpensive metal hand planes are all like this out of the box — maybe not as bad as 0.5mm, but bad enough that they really can only be used to damage wood. Seriously. This is not an exaggeration.) I don’t know exactly how flat the sole of a hand plane needs to be, but it certainly needs to be better than 0.5 mm, or even 0.1 mm.

I was considering buying a good straightedge, but before I actually did it, I saw in Toshio Odate’s book Japanese Woodworking Tools a method for makingĀ  a straightedge. The purpose of the straightedge in his book is specifically for checking the sole of a plane, but it could be used for any purpose.

To make it: the short version of the story is that you take two pieces of wood, put next to each other and plane them, then “unfold” the two objects so that the planed edges are facing each other, then check for light between them. If there’s light, you shave away the high spots and check again. The are two reasons why you make a pair of objects: first, you don’t need a good reference straightedge to check to see if your new straightedge is actually straight, and second, any deviation will be doubled, making it possible to see errors that are half the size of what you would be able to see even if you had a perfect straightedge to compare it to. This paired-planing method is, in theory, twice as accurate as making a single object and comparing to perfect straightedge.

Planing the two parts of the straightedge. There are two thin pieces of wood, clamped together so they don't fall over.

Checking for straightness. The pieces are "unfolded" so that the planed sides are facing each other. In this photo, the ends here are bowed apart, indicating that I put too much pressure at the end of the planing stroke. I eventually got it much better, but not perfect.

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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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