Friday, May 4, 2007
It's all about Support
Time for another swing at grain direction. The photo above is of a quartersawn piece of cherry. I used it because of the easy to see growth ring lines that can be used to represent fibers. As you see, I've accentuated the lines with a marker and cut the top of the board at an angle. The basic concept that I am trying to convey is that with support, fibers will shear cleanly and without it they will deflect and break below the surface causing a rough uncontrolled result.
Before I go into the photo, I'd like to introduce an idea that we all already practice. Imagine cutting a celery stalk (across the stalk). Do you automatically imagine a cutting board as well? The cutting board supplies the backing that will allow us to cleanly shear the celery. It is the same idea when we shear the fibers in a piece of wood, they need support.
Now back to the photo. To cleanly shear the fibers, which way would you cut? Forgive me if this seems too simple, it can very quickly become perplexing. By cutting the direction of the arrow on the right, each fiber will be fully supported by the adjacent fiber. If you were to cut the direction indicated by the arrow on the left, there is an area, albeit small, that has no support. This causes the fiber to deflect and break rather than cleanly shear.
I have been wracking my brain for years to come up with a simple truth to apply to all situations to explain which way to cut. Here's what I've come up with. In any cut that is not parallel or perpendicular to the fibers, the direction to cut will have adjacent supporting fibers that extend beyond the fiber being cut. Looking at the photo above, you'll see that it is a simple and clear case of this. The only problem would come at the top point where there would be no adjacent fibers beyond it risking a blowout. I'll address this soon.
Most of the shapes that we wish to make in wood (especially in chairs) aren't always so simple, but the need to have supporting adjacent fibers remains critical. I invite feedback on this topic because as much as I have a desire to help my fellow woodworkers, I could use your help in honing my ability to convey this difficult concept. I will cover some more complex shapes soon.