Question about sawing

Okrafarmer · December 19, 2012, 04:10:07 AM

Quote from: GeneWengert-WoodDoc on December 19, 2012, 02:16:01 AM
Regarding the pith, which is the exact center and about the same diameter as a pencil, it is not even wood. The next 15 rings around the pith are special wood so that the tree can easily bend in the wind, ice and snow, etc. there are microscopic differences, which lead to lengthwise shrinkage of 3% or so while more mature wood has zero shrinkage, is much weaker than mature wood, is more likely to have compression and tension wood, is often full of knots, has steep SOG, etc. in many products, we avoid this juvenile wood.

I think some of us on the Forum (and elsewhere) have gotten into the (bad?) habit of using the terms pith, heart, heartwood, and juvenile wood a little bit too interchangeably at times. We box the heart, but heartwood is a different thing. In some species, the heartwood is most of the wood, such as in black locust. On others, it is nonexistent or very small, such as in persimmon. In many species it seems we are trying to get under the sapwood to get the "good" heartwood, and yet we are trying to avoid the juvenile wood, which, like the sapwood, is poor wood. So in a log like cherry, for instance, we are trying to avoid both, which means it's hard to get a decent piece of lumber out of a 12" diameter log. Or even a 14" log. If we have to avoid the sapwood (often 20% or more of a cherry's diameter) and the juvenile wood (often 20-30% of a cherry's diameter), what do we have left, but narrow flooring boards? So I think what SCSmith is getting at in regards to the pith wood is-- in what species can you leave some of that inner juvenile wood in your lumber without harming anything? For example, walnut comes to mind as being a much more stable wood than, oh, persimmon, for instance. When milling a log, we are trying to include as much of the lumber volume as possible for sale. If we have to throw out large portions, such as the juvenile portion, it is harder to be profitable. So how do we incorporate the juvenile wood into our lumber sales program, when is it permissible to leave some of the juv wood in a board or slab, and what species are more forgiving about this than the others? Also, about the juvenile wood-- is it always exactly fifteen rings, is that more of an approximation, and are the outer rings within the juv wood stronger than the inner ones-- ie, is it a gradual strengthening the farther out you get, or is it more of a sharp demarcation?

Ianab · December 19, 2012, 04:45:23 AM

I'm pretty sure the 15 growth rings is a generalisation, probably about right for the common Nth American hardwood species? A 15 year old oak or cherry isn't going to yield any high quality lumber, no matter how you saw it.

But a 15 year old Radiata pine or Lyptus can be a decent saw log. Sure that area immediately around the pith is going to be unstable juvenile wood, and react as Dr Gene states. But with growth rings ~1" wide, you get out of that unstable wood pretty quick. Those fast growing trees also never actually get to form any sap wood, effectively it's all sapwood, so you don't have the sapwood issues.

Different (more stable) species will also let you sneak in closer to the pith before the wood becomes too unstable. Working with local Monterey cypress and Port Orford cedar, if a board is completely free of that centre pith, it's generally OK. Leaving the pith in a board is generally going to cause issues.

But the comments about taper sawing (following the grain) where possible are certainly true. Leave the taper waste in that low grade centre pith area, and taking the straight grain clear boards from the outside of the log makes a lot of sense.

Ian

Piston · December 19, 2012, 04:26:05 PM

The article in Sawmilll and Woodlot about taper sawing has been my favorite article since I read it. I've reread it numerous times and it is very well written.

GeneWengert-WoodDoc · December 19, 2012, 07:36:00 PM

Indeed, it is a gradual transition. Certainly, 15 years is a generalization, but it a good rule of thumb.

I would suggest, and maybe we should start a new thread, that the way we become more profitable is by improved marketing and through valued added manufacturing. The commodity called lumber needs more customization. I am giving a keynote talk on January 17 about this and will post my talk...I could go on forever, and I know it works and have examples in the talk.