Last week we took a look at how pointy a plane iron needs to be in order to take a shaving and saw that even a very slightly blunted, worn, or rounded off plane iron can have a tendency to want to roll out of the cut rather than make a cut. I hinted at the issue of tear-out, where the shaving of wood - instead of being cleanly sliced out of the wood - splits off ahead of the cutting iron and lifts up wood below the surface being planed. Next week I want to address how to prevent tear-out, but this week, let's look at what happens as you take a deeper cut and your shaving gets thicker and stronger and the conditions become ripe for tear-out.
Just for the sake of argument, let's assume your plane iron is sticking out deep enough from the plane body so that it will cut and produce a shaving of some thickness. The thinner the shaving, the less force it will take to create the shaving. The reason for that is that a shaving is very similar to a leaf spring. We are taking this pointy, "V" shaped bit of steel and forcing it into the wood, trying to cut the fibers and cells of the wood apart to separate the shaving from the body of the wood, and then bending the shaving out of the way so that the plane blade can continue cutting forward. And the wood fights back. The thicker the shaving, the stronger the shaving, allowing it to fight back so the plane iron is harder to push. And the stronger the shaving, the greater its tendency to lift up wood in front of the blade that's so wood is not being cut - it's being split out.
You can also think of a thin shaving as a thin stick. When you try to pry something up with a thin stick, the thin stick will bend, whereas a thick stick will stay straight and is strong enough to be used to pry something up. And this is what happens. In order to move forward, the blade pushes the shaving up and away. If the shaving is thin, the shaving bends; if the shaving is thick, the shaving will have strength: enough strength to make it harder to push the plane forward, and also enough strength to act as a pry bar on the wood that is still attached ahead of the blade edge.
How much force it takes to actually cut the wood depends on the material. Oak, for example, is a very hard material that fights back more than Walnut. (The fact that Oak splits more easily is another matter to be considered.) The actual force needed to move the plane through the wood is a combination of the force it takes to cut the wood and the force needed to move the shaving blocking the way.
Wood isn't a homogeneous material. It has layers of cells and it wants to come apart along the weaker lines of grain. Some species of wood are flexible and bendy, some brittle and crack easily. Some are hard and some are soft. But within a specific piece of wood, there are generally four scenarios: no grain to the wood; the grain is parallel to the cut; the grain is going uphill or rising ahead of the iron; the grain is going downhill or falling away from the iron.
The first two cases we can dismiss. Wood is rarely a homogeneous material and rarely would you encounter no grain or the grain running parallel to the cut. Most woods have noticeable grain - and the grain lines are weaker than the body of the material. A strong shaving without weaker grain lines running at an angle might want to split out (hopefully just straight ahead of the blade) but you will end up planing faster. Planing uphill, with the grain rising in front of the iron
But it's a little different if you're planing uphill, or with the grain rising ahead of the iron. If a thicker shaving starts causing splitting out ahead of the blade, the splitting will go up in the wood above the cut. The plane will be harder to push because of the thicker shaving. But you will still get a nice surface because you are planing away the split-out surface. Planing downhill with the grain falling away in front of the iron
It's when the grain goes the other way, and falls ahead of the iron, and you are planing downhill, that you will get tear-out. A split will end up below the surface of the cut you are making and won't get instantly planed away. Result: tear-out.
So there are three rules to get a good surface when planing: (1) have a very sharp iron that can slice wood with less force than a thick shaving can exert so the wood is cut before it can split out. (2) take a thin enough shaving so the shaving doesn't have too much strength to pull out in front of the cut. You have to cut the material, cut the cells, and not split out ahead of the cut. (3) plane uphill, with the grain rising ahead of the iron, so if there is some splitting ahead of the iron, it gets immediately planed away.
If only it would be that simple. Wood itself isn't consistent, and as we all know, grain can go up and down, in and out, and vary direction all the time. So these very simple rules are very hard to follow all the time, even in the length of a single board.
We immediately have three potential problems. The first potential problem is, if you're planing wood for dimension you kind of want to get rid of as much material as possible and to work fast and plane as thick of a shaving as possible. The second potential problem with planing with the grain is that it is not always possible in decorative or figured wood. Even regular wood can have grain that constantly changes direction. And this second problem implies problems even with a very, very thin shaving on certain woods, if you're not planing with the grain, you will tear out. The third problem is that the difference between a very very sharp iron that can cut before tearing out and one that is sharp enough to use but can still tear-out is just a matter of very little wear and use on an iron.
What can you do to solve this problem? The theory is really simple. The strength that tears out the wood is in the thickness of the shaving and the length of the shaving, which act as a lever. If you can't make your shaving thin enough (or if the shaving is super thin and still tears out), the goal should be to have as short a lever as possible so the shaving can't get the leverage to pry wood from ahead of the blade.
The three solutions (really five) will be the subject for next time - along with samples of planes that implement the various solutions.
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