Milling. What happened?

[Ron Swanson Jr.]

After 15 years of woodworking, I know absolutely nothing about milling. (some might say I know nothing about woodworking too, but that's a whole different kettle of fish.) anyways,  I know how to purchase lumber that's already been cut and dried, but I'm lacking in knowledge of the things that happened before that point.

So when my neighbor took down a maple tree, I grabbed a couple logs and ran them through my band saw more as an experiment than anything else. After a week or so they split as you can see in the pictures. Massive checks appeared.

What happened here? How do I prevent it from happening again?

I'm not going to be milling lumber on a regular basis that I can see, but it would be nice to be able to make bowl bblanks and things like that, assuming I can dry them, etc.

But this is strictly to satisfy my curiosity and learn a little.

 

Thanks in advance.

[Mark J]

Each piece that you have there contains the pith, i.e. the very center of the tree.  This part of the tree experiences the greatest (or is it the least?) dimensional change with drying.  This results in a big stress and hence the big cracks. 

[Bmac]

Mark J said it, the pith is the problem.

Not only is the pith very unstable, you often get radial cracking, very similar to what you are showing. As the wood dries it shrinks, as it shrinks the circumference shrinks more than the radius and cracks can form because of the stress this induces. Another way to look at it is the crack decreases the stress of the shrinking circumference, you could say the crack shortens the circumference to deal with the stress. This is why cookies/discs of wood crack.

You can avoid alot of these problems with piths if you cut in quarters or halves the log you are milling.

 

[Chestnut]

When i cut firewood logs on my bandsaw i draw a line on the log strait from pith to pith and the first cut i do basically removes the pith. This is wasteful on larger logs but for small firewood stuff i get the best boards this way.

There is some usable material salvageable on those beams you cut but it's not as much as if you removed the pith.

[Ron Swanson Jr.]

Thanks gents. @Bmac it would've been a L O N G time before I arrived at the conclusion that the pith moves at a different speed than the rest of the log. It looks like it expanded well the sapwood did not? And that's why it's split?

 

Thanks, @Chestnut that makes perfect sense. Like I said, this was more of an Experiment then a serious attempt at producing usable stock. And I've learned more in 5 minutes from you guys then I had in 15 years on my own. So that's why I love this forum. Take care guys.

[Bmac]

1 hour ago, applejackson said:

Thanks gents. @Bmac it would've been a L O N G time before I arrived at the conclusion that the pith moves at a different speed than the rest of the log. It looks like it expanded well the sapwood did not? And that's why it's split?

.

Well I don't think your splits had anything to do with expansion, it's all contraction related. Wood shrinks the most across the grain, when the pith is included it shrinks toward or around the pith. This is what caused the stresses on the pieces and hence the splitting. It's easier to visualize in a circular piece that I showed, but the same principle holds true in square pieces. In the picture I showed you can easily see the pith, it's the darker center made up of the first growth rings.

As I stated and Nut stated, cutting the log in quarters or halves down the pith corrects the problem, it allows shrinking with out building up the stresses that cause the splits you experienced. Pith contained in boards reacts a little differently, or rather has different problems. The wood that makes up the pith is the juvenile wood, the young immature wood that used to be the sapling. It's the sapling and a few rings of growth laid down around the center of the tree.This wood is weaker, moves more, and more prone to cracking and warping. I've milled a lot of lumber containing the pith and it is almost always defective and needs to be cut out when prepping boards. But you can still get very usable lumber from boards containing the pith with a little work. When you have a board with a pith running right down the middle you have two nice quartersawn boards on either side of the pith. 

Finally, because I realize this can be alittle difficult to explain, the problem you had was because the pith was contained inside a relatively thick beam, it looks like you cut the sapwood off of all 4 sides. The stresses built up around the pith from shrinkage causing the split. If you had milled 5/4 or 8/4 or 4/4 or any /4 you wouldn't have had the big split, just issues explained above with the boards containing the pith. 

Hope this makes sense. The pith concept took me a while to understand and appreciate, but it is a principle that is not only important for guys milling lumber to know, but also for guys buying lumber.

[Ron Swanson Jr.]

Thank you @Bmac for the very informative and interesting answer. Much appreciated!