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.

5 comments:

Anonymous said...

I was always taught to plane 'uphill'. The rule applies to both power and hand tools.

Herman

Peter Galbert said...

Herman,
I think that I understand what you mean, but could you offer a definition of "uphill"? Thanks

Harry said...

I think uphill applies to planing a sawn board where the grain runs out on the surface where you are planing. If you look at tha adjacent face the grain will slant. Planing uphill ensures that the fibers being cut are supported.

Peter Galbert said...

Harry, I see where the notion of uphill applies to planing a board. Sometimes a board cut from a curved or twisted tree can make following a simple adage difficult. For instance, on wide boards it is common to have the fibers ascend to the surface one direction on the left of the center and another on the right! What I am working towards is a simple way to convey cutting more complex shapes. I will be showing increasingly complex cuts in the coming posts and welcome your feedback to help me narrow it down. Often the best cuts confound a previously defined explanation. Thanks for writing, we're all in this one together.

Herman said...

Planing uphill means the grain runs out in the direction you are planing. But you are right, it can change in many places on any board. As well it depends on the growth ring orientation, there are times when the surface of the board will tell you one thing while the edge will tell you another. I wish I has a simple rule. One thing we can count on is that tear outs when they occur will be much worse with dull tools. A razor sharp plane will often forgive your mistakes.