Posted earlier "Pith - The Pits!? and learned some more about "pith in" cracking. With some of the feedback from those responses, some reading in Hoadley's "Understanding Wood" and some playing with tables of radial and tangential shrinkage values for a bunch of woods and some work in a spreadsheet to calculate T/R Ratios I'm starting to get a feel for what's going on and why the pith propogating cracks might be aviodable.
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Would appreciate feedback - illustrations and text make sense and are correct? Still need to do the "what you can do to reduce or eliminate pith generated cracking" illustrations and text but want to get thisfirst set of info right first.
Well Charlie I think you nailed it first time. Here's a visual that might help some more. Wood shrinks the most around the annual rings. A log looses about 10% of it's circumference as it dries. A 10" diameter log has an approx. circumference of 30" and 10% of 30 = 3. So as it dries it needs to open a 3" wide crack. In reality it is not quite this bad but you get the picture. If the log is split down the pith it can shrink from both ends of the diameter chord so the chances of it cracking are greatly reduced.
Leave it to the folks at FPL. The've got the data. They also know how to spell lignin
Wood doesn't shrink 10% in circumference. The figures cited by FPL (fig
3-5) are from FSP to 0% MC, something unobtainable save in an oven. At a human-comfortable Temp/RH of ~50%, that's 9% moisture content, which means the wood will have lost 2/3 of the full shrinkage figure. Not to mention that the critical figure in circumference, as always, is the radial (2 pi r) shrink.
Read the data and get enough of a handle not to make obvious mistakes before you try to interpret it.
What is it about the words "most", "about", "aproximate", "in reality it's not quite this bad", "but you get the picture" that you can't seem to grasp. It seems to me that Canchippy put in all the discalimers and was only speaking "in general".
What is it about the words "most", "about", "aproximate", "in reality it's not quite this bad", "but you get the picture" that you can't seem to grasp. It seems to me that Canchippy put in all the discalimers and was only speaking "in general".
Not everyone wants to be as pedantic as you.
Not everyone wants to learn enough to understand. Think hard on this. Decrease in _radius_ decreases the circumferance of a circle, not the shrinking of a chord. That's effect (radial check), not cause. Simple, IF you think.
What started as an serious request is beginning to crack me up. I thought you and canchippy _were thinking, ebd.
Maybe, almost, could be, might, on occasion, possibly, whoever, whyever, whenever, wherever and your ever loving "whatever" all seem like thinking outside the book to most (well some) of us here.
"I thought, therefore I presumed that I would always be totally right" said Newton to Einstein. Is it possible that eventually even today's FPL physicists might have to change their thinking at least a little bit? If so, you two could, even might, be thought of as thinking outside the pith.
There's no need for me to lisp. ....it's PISS, not PITH that diverted the query and began the present tempest in a growth ring. Well, maybe. :)
I think the idea of trying to illustrate how much the perimeter length changes due to the Radial % change. While a simplified illustration may work I won't know 'til I've got some ideas on "paper" (ok so it'll be digital).
I've have this idea that the Tangential/Radial Ratio is the key to success - or - a way to avoid almost guaranteed failure. I did a chart of the T/R Ratio sorted in ascending order for a bunch of hardwoods I have T & R values for. The two woods I turned green and with "pith in" that haven't cracked in over a year, and were turned thicker than I can turn now, were dried probably way faster than reasonable, let alone properly, and went right into the house.
Please have a look at the following, pick out the woods that you've found survive "pith in" and the ones that haven't and see where they are on the chart. Is the T/R Ratio a good "predictor" for you?
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Thanks for having a look at all this stuff and for your observations and suggestions. charlie b
> > Thanks for having a look at all this stuff and > for your observations and suggestions. >
All of the family salicaceae are virtually bulletproof in drying. That's the right end of your chart. Cottonwoods, poplars and willows. Short of throwing them in the afternoon sun, they survive.
Not to beat the horse too much, but there are lists of working and drying properties already compiled by the kind but misguided, group of pseudoscientific pedantics over in Madison.
The picture of the slice of a tree with the slit to the pith that is on the front cover of the Hoadley book is (IMHO) an excellent way to make the point.
You have a lot of technical details, but you need something that makes it clear what will happen - and do it visually.
Just think - without that slit to the center of the tree, the stresses will build up as the wood dries. They have to go somewhere.
Thanks for the info. Went "Alltheweb.com" searching on "salicaceae" and have begun wading through the latin - then the english. Linking "science" to "layperson" is the fun part since most woodworkers are not scientists.
Have been through something similar before trying to find info on "English Sycamore", which is actually an Acer (maple). Narrowed it down to one of three possibilities but never could determine which of the three is what I have 200 bf of. Problem was/is that I need the specific gravity of the wood I have in order to get its current MC as the Wagner moisture meter requires SG and the range amongst the three possibilities is from 0.49 to about 0.69.
The fun and games is the gap between scientific name and common name, and it's normally the common name that your given when you ask "What wood is this?".
Well there's "data" and then there's useful information in a form you can actually USE / Understand - without having to play with the data a bit first. I found the Tangential and Radial shrinkage percentages but not the T/R Ratios. Getting the T & R values for the woods in the chart into a spreadsheet were it could be manipulated in order to look for patterns and then putting that info in a form a layman woodworker could quickly undertand - and use took some work. It's a simple easy to understand and use "guide" I'm trying to find or come up with.
The Lee Valley "Wood Movement Reference Guide" is a great example of the type of thing I'm trying to put together for myself and other woodturners. If you know what wood you have, this handy little "wheel" will give you a pretty good idea of how much movement you need to accomodate for your board width - say in a frame and panel door, or a table top.
I'm looking into whether there might be something similar for "Pith In" branch turning. Might be a fools errand but we'll see.
(Don't know where the Forest Practices Lab (FPL) is but are they your referenced Madison folks?
That image will show you WHAT. I'm also looking for WHY - AND - what you can reasonably do to a) not waste time of woods that really really really want to self destruct as it dries. b) select a wood that may not or probably won't try and self destruct as it dries
Agree absolutely - sort of. Showing what MIGHT happen - IF you don't take steps to not have them happen - in a single simple easy to understand illustration is one of the things that I'm after. But I'm also after "predictors" / "indicators" of which woods are proned to cracking no matter what YOU do and those that are less proned to cracking DESPITE what YOU do, or don't do.
Don't know if you watched the "animation" on this page or not. If you have, why doesn't it show what's probably happening? Suggestins please.
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> Just think - without that slit to the center of the tree, the > stresses will build up as the wood dries. They have to go somewhere. Actually, they don't HAVE TO go anywhere. They can stay in the wood and increase the wood's ability to resist various types of loads. "Pith In" cracking doesn't ALWAYS happen. I know because I've found two woods that don't crack when dried - english walnut and some kind of magnolia. And I have turned and dried some "pith in" fruitwood (apricot and plum) that some do and some don't crack, even given similar wall thicknesses, shapes and drying method and time. Rather than spend a lot of Trial And Error time turning things that may or may not self destruct I'm invstigating whether there are characteristic of a wood that a) are known and b) can be used to help indicate if that wood is a good or bad candidate for a wet turned "pith in" piece.
Acer pseudoplatanus is generally what passes as "sycamore" over there.
Take a cube or two of your wood, weigh to the nearest tenth of a gram, then warm in a 200 degree oven for an hour or so. Weigh again, return until there's no weight lost. That's your zero. The percentage loss is your percentage MC. That's how they do it in Madison. "As a percentage of oven-dry weight." as the charts say. How we calibrated unknowns at school, then applied our developed correction to compensate for the meter differences. More samples, more thoretical accuracy.
If you have a hygrometer, and every woodworker should have one, you can take a reading after a couple three days of steady RH and find the corresponding MC against the RH/temp chart. The procedure for obtaining SG is outlined beginning on 3-11 if you want to use it as a cross-check.
It is precisely the utility of your data that is under question. Once again, it's the rate of curvature and orientation of the annual rings that count, with a nod toward the thickness of the more shrinkable earlywood. How you use it is to look at the orientation in the piece you're trying to make, and apply the corrections for radial, say, in a traditional cross-grain bowl, assuming that 50% will be long-grain drop on the sides, while the widest continuous flat grain area will determine the shrink across. Once again, approximately 50% of the published tangential is a good figure for 1" wall thickness. Thinner walls, since all shrinkage is local, or more gradual tapers to the bottom will take a lesser percentage.
Useable data? You bet. Allows you to compromise between restyle room in greater thickness and drying time in thinner walls. Long grain is a mental
90 degree mental adjustment.
That is undoubtedly based on the averages compiled and put forth in the Wood Handbook. Other people hate to waste time and money reinventing the wheel when all they wanted was an interpretation.
That you already have. Thin, tapered, allow for anamolous annual ring orientation. You've also been reminded of the differential dry rates of flat/quarter/end grain, and been given good information as to the best way to exploit that. The 10-15 times faster rate of loss through endgrain leaves your long grain walls plastic even after you have lost your moisture through the end grain. Since it is difference that stresses, I even mentioned that allowing circulation underneath the end grain bottom will minimize the difference and the stress cracks.
Charlie, I certainly applaud your efforts to find a single parameter as an easy indicator for, how and why certain timbers crack or don't crack.
In your case, I'm arguing with the choir, but I suggest one possible fault or at least one weakness in strengthening your hypothesis as you take if from the particular to the general. Inductively, you hypothesize that T/R ratios might indicate a timber's cracking characteristics and then set out to make the inference statistically valid by using other's T/R data.
I just think you would better validate your hypothesis by using your own "trials and errors" (controlled experiments) to accumulate and test as much of your data as possible and to test a significant part of other's data that you have to use.
I may have missed the boat entirely and your purpose may be to speculate by association then validate by authority as so many 'studies' are currently being done in my field.
Please do not take this as pontifical criticism or as an attempt to lessen the value of both your fresh thinking and thoughtful review of your turning experiences. Your input is an asset to this ng. Thanks.
George I was simply passing on a visual picture that was passed on to me by Lyle Jamieson during a Demo. I never questioned his source or his accuracy because it doesn't warrant it. But I do still carry with me the image his words created in my lazy feeble old mind so i figure it was of some use. Oh by the way if you can be bothered go back and read the opening lines of my post. Wood shrinks the most around the annual rings. A log looses about 10% of it's circumference as it dries. A 10" diameter log has an approx. circumference of 30" and 10% of 30 = 3. Notice the word circumference?
I'm trying to get some feedback on whether this idea is worth pursuing further. The illustrations take a bit of time to come up with and actually make - the T/R Ratio graphic for example was a half day playing with the data in a spreadsheet and another hour or so to take a graph out of the spreadsheet and pretty it up in a primitive CAD program. While interesting in a strange way doing this stuff is not fun - but potentially enlightening. If nothing else, it's increasing my knowledge and understanding of wood and woods.
Given ALL the different woods out there to play with, if I started a controlled experiment now I'd probably be dead before I had enough data and that data analyzed and the conclusions written up and illustrated. A nun told me if you're going to write a story, take a small slice of the big pie and really get into it. So my small slice of the pie - at the moment - is wet "pith in" and no cracking. My hope is that some folks in the forum will help me see if this T/R Ratio thing agrees with their experiences trning wet "pith in" woods. The information will be anecdotal since info about the other major drying and dimensions specifics are seldom provided by volunteers.
Given all the characteristics of woods, wet and dry, I suspect only a few have significant affects/effects on "pith in" cracking
- annular rings concentric or not (predictors of tension and compression wood)
- the T/R Ratio
- the amount and size of medullary (sp?) rays
- the specific gravity (hypothesis is that resinous woods are less proned to cracking - pine perhaps being an exception?)
As is often the case, this may turn out to be a fool's errand but I am learning somethings about wood I didn't know, or didn't know much about - and that's always useful.
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