Wednesday, May 2, 2007
I've decided that it is time to try to tackle the topic of shaving wood and grain direction. For handtools to be effective, they must be sharp and applied properly. The name of the game is knowing where to push or pull the tool in relation to the grain to get the desired result. While it sounds simple, it is a difficult concept to put into action. I have explained this countless times and had my student nod knowingly and then proceed to cut the wrong way! Of course, I take the blame as the instructor. But in discussions with other instructors, I've come to see that imparting a working knowledge of how to cut the fibers properly is somewhat of a holy grail of teaching woodworking. It seems to me that no single concept is too difficult to convey, but it is the adding up of the concepts on the fly that takes work. As you take each cut on a piece of wood, the rules can change. Hopefully, with a careful approach and a few postings, I'll be able to simplify it a bit.
Let's start by talking about working with split green wood. When the wood has been split, as opposed to sawn, the fibers will run roughly parallel to the outer surfaces of the piece. Shaving the wood can be viewed as almost a peeling process. Think of a banana. When we peel a banana, we are exploiting a weak bond between the skin and the fruit. With green wood, it is very similar. A drawknife will slide inbetween the fibers and prefer to follow along them because it is more difficult to cut across them. This "peeling" leaves a clean surface that is perfectly parallel to the fibers. An interesting difference between a sawn piece and a piece shaved to follow the fibers is that the only exposure of end grain of a shaved piece is at the ends. Any time that you cut across a fiber, be it in the middle of a board or in a carving, you are exposing endgrain. This means that in sawn or carved pieces there will be endgrain exposed all along the piece. The weakness between layers becomes exposed in this "all over" endgrain and the wood can break below the surface as a result. It is this subsurface damage that is often the first clue that we are going about cutting incorrectly. Luckily, preventing the weak bond between the fibers from breaking is as simple as offering them proper support. I'll go further into that soon.
Here is an experiment that may help clarify what I have covered here. Take a piece of sawn stock, perhaps a piece of 1X pine from the hardware store. Put it on edge in the shavehorse or vise so that you can shave the 3/4" dimension. Take a sharp drawknife (you have plenty of them right?!) and run it down the surface. Now the simple adage of "go the other way" if you get tear out, doesn't apply to the experiment. Go the wrong way. It will split and tear out etc...that is alright. Keep trying to get under the tear outs and soon the drawknife will find the fiber line. Most likely, it will not be parallel to the original sawn edge. As soon as the wood shaves smoothly, stop. Any further and you are likely to start cutting across the fibers in the opposite direction. It is vital to start seeing any board as a bundle of fibers. Once you can see the fibers and the way that they run in the board, you can learn to work with them, and that is when things start getting fun.
In the photo below, you can see that the shaving is very crude and fractured from the heavy splitting action of cutting the "wrong" way. But take note of the clean surface that it has left behind running parallel to the fibers.