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Choosing Timbers Wisely: Density, Strength, and More

Choosing Timbers Wisely: Density, Strength, and More

Wood Strength 101: Lessons Learned the Hard Way

Last night, my trusty 18-inch bowsaw finally met its demise. It was a custom-made beauty I had purchased from CME Handworks a few years back, and it had served me well. But alas, the arm just couldn’t take the strain anymore and snapped right off.

As I posted the unfortunate news on Keek, I was surprised by the shocked reactions from my social media followers. Apparently, my choice of Maple for the saw’s construction was not the wisest. Within minutes, I had comments from wood enthusiasts far and wide, all eager to educate me on the finer points of wood selection.

Well, consider this my opportunity to share that wood knowledge with you. I’m going to try to keep things simple, but you woodworking nerds out there are free to come at me with your lumber rulers and moisture meters if I miss anything.

The Trouble with Hardness

The first thing to understand is that hardness does not equal strength. Just because a wood is hard, like Maple, doesn’t automatically make it the best choice for a tool that needs to withstand dynamic forces. In fact, the very qualities that give Maple its hardness can also make it more brittle and prone to sudden failure.

You see, Maple is a diffuse-porous wood, meaning its tiny pores are evenly distributed throughout, with no distinct plane of strength. This tight, dense structure gives Maple its impressive Janka hardness rating, but it also means the wood has less flexibility and “give” when under stress.

Compare that to a ring-porous wood like Ash, which has larger, more organized pores arranged in distinct annual growth rings. This creates long, uninterrupted fibers that can bend and flex, rather than just snapping. It’s a bit like the difference between a tightly-packed bundle of spaghetti and a looser bundle – the tightly-packed one is more likely to break when you try to bend it.

The Impact of Drying

Another key factor is how the wood is dried. Kiln-dried lumber, while more dimensionally stable, actually has its fibers hardened and brittled by the intense heat and steam used in the process. It’s kind of like taking a soft, pliable flour tortilla and tossing it in the oven – you end up with a crispy, breakable shell.

On the other hand, air-dried wood retains more of its natural flexibility, as the moisture is allowed to evaporate gradually without the harsh environmental conditions of a kiln. It’s like wrapping that tortilla in a damp paper towel before baking – you get a warmer, more pliable end product.

So when it comes to wood that needs to withstand dynamic forces, air-dried material is generally your best bet, regardless of species. The long, continuous fibers and natural pliability provide a better defense against the stresses of tools like my ill-fated bowsaw.

Cutting Across the Grain

Of course, the way the wood is processed also plays a big role in its strength. Sawing, by its very nature, cuts across the grain of the wood, which significantly weakens the material. Riving, on the other hand, splits the wood along the grain, leaving those long, strong fibers intact.

Imagine taking a 1-inch piece of a board and trying to break it in half with your bare hands. It’s pretty easy, right? That’s because when you cut across the grain, you’re left with a bunch of short, disconnected fibers held together only by the “glue” of the lignin and other natural binders. But try breaking that same piece of wood lengthwise, and you’ll find it’s much, much stronger.

Applying the Principles

So how do these principles translate to real-world woodworking projects? Well, it really depends on the specific application and the stresses the wood will be subjected to.

For something like a keepsake box, which sees very little dynamic force, the wood choice isn’t that critical. But for things like chair frames, roof trusses, and yes, bowsaws, the selection and orientation of the lumber becomes much more important.

Ideally, you want to be working with air-dried, ring-porous woods that can flex and bend without breaking. Species like Ash, Hickory, Oak, and even Walnut (which is semi-ring-porous) are excellent choices. And if possible, you’ll want to rive or split your parts to take advantage of those long, continuous grain fibers.

Of course, not all of us have the luxury of a endless supply of perfectly air-dried, rived material. In those cases, we have to do the best we can with what we have access to. That’s why I ended up making my little turning saw out of Maple – it was just what I had on hand, and the shorter, narrower blade meant it didn’t need as much tension and brute strength to operate.

But for a full-size bowsaw like the one that finally met its demise, the wood choice is critical. That’s why the model available from Gramercy uses the more appropriate Hickory – a naturally bendy, ring-porous wood that can handle the dynamic stresses much better than my Maple arms.

Striking the Right Balance

The key takeaway here is that wood selection is all about striking the right balance between strength, flexibility, and availability. The more heavy-duty the application, the more thought you need to put into your lumber choices.

Even my trusty Maple bowsaw managed to hold up for four years of hard use before finally succumbing to the constant strain. But when it did snap, I wasn’t the least bit surprised. I knew the day would come, and I was prepared for it.

So the next time I remake those bowsaw arms, I’ll be sure to choose a more appropriate wood species – maybe Ash or Hickory. And I’ll probably even make a spare, just in case. Because when it comes to the tools and structures that have to withstand serious dynamic forces, you can never be too careful in Choosing Timbers Wisely.

Now, if you’ll excuse me, I’m off to make a spaghetti burrito. Or maybe a tortilla wrap – I’m feeling a bit peckish.

Additional Resources

If you’re interested in learning more about working with wood grain for maximum strength, be sure to check out the latest episode of the Wood Talk Online Radio podcast. And for the latest updates from the lumber industry, don’t miss Shannon’s Lumber Industry Update.

Happy woodworking, and remember – difficult is just easy work done slowly!

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