help me understand - tangenital shrinkage

[Mike Minto]

as green wood dries, i understand that it shrinks in more than one direction. i'm just having trouble understanding the concept of 'tangential' shrinkage - as it relates to shrinkage in a round piece of wood - can anyone help put it into perspective for me? thanks, mike

[Toney Robertson]

Well, I will give it a shot and hope I don't screw it up.

There is three axis in space. Cut a log and look at the end and put cross hairs on the end centered on the pith. This will define the x and y axis. The third axis (z) runs with the grain into the log. Wood shrinks significantly in two of these axis - x and y - very little in the z. As x and y shrink the circumference would have to shrink at a compatible rate for the log not to crack. No wood I know does that. So x and y shrink, the circumference can not keep up so something has to give and the wood tears itself apart - it cracks. That is also why logs crack from the pith.

To put it another way. The circumference (tangent) shrinks at an incompatible rate as compared to the radial (straight out from the center) shrinkage.

I really don't believe using the term tangent in this context is a correct usage since a tangent is a straight line that touches a curve but I have seen it used more than once so I am probably wrong.

I don't know if this helps you but I gave it a shot.

Toney

[Mike Minto]

Toney, thanks for the info. I have a book, 'understanding wood', and it goes in to great detail trying to explain the concept, but i'm still working on grasping it well. i suppose it's really not all that important for me to understand it, i just want to, haha. mike

[Roger Jensen]

Tangenital shrinking is usually associated with swimming in cold water.

I believe you're talking about tangential shrinkage.

Sorry, I couldn't resist...

[Mike Minto]

ok, thank you - tangential.

[David Walser]

Years ago Dale Nish explained the concept in a class I attended. IIRC, this is what's going on: Heart wood is denser and has less water than does the surrounding sap wood. Because of this, as the wood dries, the sap wood will shrink more than will the heart wood. It's this fact that causes the greater "tangental" shrinkage compared to the "radial" shrinkage.

To see how this works, consider a log that's 20" in diameter. Imagine that half the wood is sap wood. (That, is, 5" of the 10" radius is sap wood and 5" is heart wood. The heart wood would make up less than half of the total.) Now, assume two points, "A" and "B", are on the circumferance of the log, 10" apart. Each point would also be 10" from the center, "C", of the log. If you cut down from each point to the center of the log, you would end up with a pie slice with 10" on each side. The amount of shrinkage we'd expect along the line from A to C (and from B to C) would be less than the shrinkage along the line from A to B. Why? The distance from A to B is 100% sap wood while the distance from A to C is only 50% sap wood. Since sap wood shrinks more than heart wood, we get more shrinkage along the line of A to B.

In general, there's about 10% more shrinkage around the circumference than towards the center of the log. Also, in general, most cracking from shrinking can be avoided by splitting the log along it's length.

Anyway, that's what I remember of Dale Nish's explanation. If I got it wrong, it's my memory, and not Dale, that's at fault.

[Roger Jensen]

Mike - please don't take offense - the title just looked funny and I'm sure it was just a mis-key of the word.

[Curt Fuller]

as it relates to shrinkage in a round piece of wood - can anyone help put it into perspective for me? thanks, mike

Round becomes oval!

[Mike Minto]

thanks again; that's what i come here for. mike

[Jarrod McGehee]

Now this is going to be bad but here we go, well as you first wrote, tan"genital" shrinkage, that occurs down south on a man's body and you don't want that! I didn't do amazing in anatomy but I think I grasped it somewhat.

[Dick Sowa]

The scientific description of what happens is interesting, but the best way to see it, is to turn a green bowl. Leave the wall and bottom thickness about 1/10 the diameter. Then put it on a shelf. Then come back in 6 months to a year, and look at it and you will see right away how it warps and how each part of the wood shrinks at different rates. The first time I did that, I was blown away with how much the shape changed.

[Reed Gray]

This is the way I figure it. A tree trunk expands and contracts to allow water and nutrients to travel up and down the tree according to seasons. The outside of the tree is the live wood where most of the water moves, and the inside isn't exactly dead, but isn't too active. The diameter of the tree is what changes. The vertical grain of the wood is in a compression mode, and there is little movement that way between wet and dry seasons. So, as a bowl drys, usually, there is very little movement up and down the tree, as in parallel to the pith, but there can be a lot around the girth of the tree. In trees harvested in the spring, there is more water, and more shrinkage. In trees harvested in the fall and winter, there is less water in the tree, and less shrinkage. So, usually, the bowl will end up with the sides (flat grain) dipping down, and narrowing towards the center, and the ends (end grain) moving very little. I have seen several bowls that have moved just the opposite way. Not sure why, but there seem to be exceptions to every rule.
robo hippy

[Jeff Nicol]

Just like a lot of things that happen in the world they can be over thought and studies done to find the best and greatest answer. But in the end does it really change anything? It is sort of like spending billions of dollars on weight loss and gain to find "The Answer" when proper diet and exercise is the only answer no matter what. So this all relates to the state of our country and world, the more time and money you spend in the end we are still going broke!

Just wanted to relate that simple explanations are most often best and no matter what, the wood from a tree will move and dance around as it drys no matter what we do or think.

Not putting any one down but just my thoughts,

Jeff

[charlie knighton]

some very good advise so far

different pieces of the same tree wood react different,

consider mass, where the pith is in relation to rest of wood

this cracked sightly, but easily glued, use yellow glue insead of ca glue around pith cracks, good to clamp and leave overnight

Eve's from the bear clawed series turned on two axis

[Steve Schlumpf]

Mike - I've had this resource on my computer for reference for quite some time. I think they do a good job of answering your question.

http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-2535/NREM-5009web.pdf

[Steve Busey]

can anyone help put it into perspective for me?

Here's how I visualize it: Think of the grain of the bowl as a bundle of straws from MickeyD's. They'll squish (shrink) when pressured from the sides and the top, but no squishing (shrinking) from end-to-end of the grain. So, the grain is pointing to the "long ends" of your bowl. HTH.

[charlie knighton]

Steve,

as expected from the tangential and radial shrinkage values for cherry and walnut are low, but i was very surprized to see that sugar maple has a higher value than red oak. :confused: