We all work to cutting lists when making stuff….
but what do the readers do when they have stock they thickness down…say 8 inches wide, by 5 feet long and it machines as flat as a pancake…no bow even when left for some period of time..but when you come to cut out the sub components from the same stock thickness, the stresses in that wood are released and bow like a dogs back leg??
It doesn’t happen all the time, I know..I would just like a few comments?
Has anyone got an answer???
Replies
Strongbo1
What I have done to minimize this situation is to cut the components from rough stock before jointing and planing, ie 4" to 6" longer than needed and 1/2" to 3/4" wider. This way the stresses are released before milling. If I have the time I will also let these "components in the rough" sit for a day.
Hope this helps.
Bill
Don't buy any more lumber from that source.
Right away I thought of what we used to call swamp Oak or So. Oak from south of Appalachia . The tension was common and now that I use Northern Oak those problems are fewer . Maybe the growing conditions and the change of humidity that affects the So. Oak . Yes this happens and with different species from time to time . I was always under the impression the drying had much to do with this problem . Things like cell collapse , case hardening , and springy boards are a few of the unforgiving characteristics of the materials available to us . Try and use the right board in the right place .
good luck dusty
My wood ages a couple years before I cut it. It never moves.
I throw it out and start over.
Generally I cut the pieces to rough size, say 6" longer (to allow for planer snipe) and ½" wider or more, depending on how expensive it is, then mill it to final dimensions. If the stock is quite distorted and checked then I cut it rougher, let it sit a day or more, then cut it closer. I reserve the most distorted stock for the smallest components.
Sounds like surface dry. Somebody rushing a kiln?
I agree with George. The stuff I've been splitting lately was left to dry indoors for about 100 years. Old beams. Very light, very dry, very nice wood. Should make a beautiful floor.
I used to stack freshly ripped wood with stickers and lots of weight on top, with only limited success. Sounds like your board wasn't ready yet.
Strongbo1, you're describing a classic case of case hardening. The wood wasn't kiln dried correctly to relax internal stresses. If you use boards like this in essentially the same dimensions as they were kilned (or remove equal amounts from both faces of the board...or don't have to rip it) case hardening doesn't cause much of a problem...because the internal stresses are able to counter balance each other...but when you disrupt this balance by redimensioning the stock, you open Pandora's box.
I agree with Dunc. What you need is a supplier who has a kiln operator who knows what he's doing. The major mills that produce construction lumber, mostly softwood species...which are typically used in the dimensions they are kilned at...can get away with much faster kiln schedules and less attention paid to relaxing internal stresses toward the end of the kiln process. But drying hardwoods for the furniture market is a totally different challenge and it takes a real pro to do it right.
Jonforgive the rookie question; I've had this same thing happen a few times inspite of buying only quarter sawn stock. If the stock was air dried, would the same prob happen??Mike Wallace
Stay safe....Have fun
It's not a rookie question, Mike. Kiln drying lumber correctly can get to be pretty tricky.
Correctly kiln dried stock will contain less drying stress than air dried stock at the lower Equilibrium Moisture Content levels that most furniture is exposed to in modern, centrally heated homes during the winter months. However, lumber kilns place too much power in the hands of mankind and improperly kiln dried lumber can contain drying stresses that exceed anything Mother Nature would ever allow to happen as she dries wood under ambient conditions.
The explanation for this gets a little lengthy...and the following is an over simplification...but basically the stresses that develop in wood as it dries out result from the fact that the wood cells want to shrink when they begin to give up the moisture that is in the cellulose tissue of their walls. However, the natural adhesive in wood (lignin) that bonds the cells together remains rigid and tries to hold the cells in the position they were in while bloated with water.
The resulting stresses begin to develop as the wood drops below what is called the Fiber Saturation Point...which is when the wood tissue itself starts to give up moisture (desiccate). With most species, this occurs when the wood's MC is at or around about 28%. The wood then begins to shrink and it shrinks pretty much in proportion to continuing loss in moisture. In other words, if the average volumetric shrinkage for that particular species of wood is 14%, green to ovendry, it will lose about 1/2% in volume for each additional 1% loss in moisture content as the MC drops below 28%. Basically, with air dried wood, the stesses become steadily greater as the wood dries out...and therefore air dried stock is more prone to stress related problems, such as warping and checking, as it's moisture content approaches 0%.
The kiln process alters this relationship in that it dries the wood at temperatures that are high enough to soften the wood's natural adhesive (the lignin). As a result, the wood cells are able to "slip and slide" relative to each other as they shrink and reposition themselves while the hot lignin is somewhat plastic. However, when the kiln is turned off and the lignin cools, the cells are once again locked into position in their new locations. Therefore, if the entire board was at a uniform moisture content of, lets say, 8% at the time the lignin hardened again, that wood would be essentially stress free as long as it maintained an Equilibrium Moisture Content of 8%...So, basically, what kiln drying does is that it reestablishes the stress starting point. While air dried stock has a stress starting point based on its Fiber Saturation Point of about 28%MC, our hypothetical example of kiln dried stock would have a stress starting point of only 8%. It would therefore perform better (be less vulnerable to stress related warping and checking) at lower humidity levels than would the air dried stock.
But here's the rub: As lumber is being kiln dried, the moisture content of the heated board is not uniform. It's outer surfaces that are giving up moisture are drier than the wood tissue closer to the center. If the lignin is allowed to cool while these moisture differentials exist, it sets up stress gradients in the board. The outer wood tissue is locked down in a more shrunken state than is the deeper tissue (hence the term "case hardened"). Everything is hunky dory as long as these counter balancing stresses hold the board together...but when you rip a case hardened board, you essentially have two boards with one edge that wants to expand and the new, freshly cut edge that wants to contract...so, the two pieces begin to torque...usually they splay out like a banana peel...or they may pinch together (that's another story)...but what they won't do is stay straight.
The procedure for preventing case hardening involves injecting some steam into the kiln late in the drying process so that the outer surfaces are kept a little more moist as the temperature is brought down...which sounds simple, but in practice it's a pretty arty thing to accomplish...and it's why experienced kiln operators are worth their weight in gold.
Jon,
So, at what temp does the lignin start to soften? Wondering if the wood in my shop attic gets hot enough to slip-n-slide as it's curing. I've seen the thermometer that's hanging up there pegged out at 128*F... It seems to me that the hot-cool/day-nite cycle that the wood undergoes in the attic tends to allow drying stresses to equalize over time, izzat so?
Regards,
Ray
Ray, lignin is a very complex compound and its composition varies slightly from species to species...but if you were to write out the chemical formula for a typical lignin, it would fill half a type written page.
Technically, the melting point of lignin (that point at which it totally liquifies) ranges between 250 degrees C and 275 degrees C. (i.e., upwards of 400 degrees F). However, lignin contains some fatty acids in its composition and these acids liquify at much lower temperatures...actually they melt substantially bellow the boiling point of water...in the range of about 180 degrees F. As these acids melt, they give lignin a certain degree of plasticity that allows the wood cells to get up and move around a little. In fact, this is basically how steam bending works...the moisture softens the cellulose, making the wood cells more pliable, while the heat makes the lignin plastic enough to let the cells reposition.
I can't explain it chemically, but it's interesting that once the lignin has been heated to where the fatty acids melt, when it cools down it seems to be more brittle. Could be that some of the fatty acids escape the compound (?)...But the practical effect appears to be that kiln dried wood tends to be a little more rigid. It seems to lose some of its "waxiness" and doesn't shape or carve quite as nicely. Also, once wood has been kiln dried, it won't steam bend as well, even if you thoroughly soak it to where it is completely saturated...So, kiln drying is a mixed blessing in many ways.
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If you're only achieving temperatures in the 120F to 140F range in your attic, it's doubtful it's affecting the lignin much. It might be affecting some of the other wood extractives that are more volatile; turpenes, etc...possibly even gums and resins to some noticeable degree (resins bleeding to the surface or color changes) that would alter a few of the wood's working characteristics. I guess constant flexing of the wood tissue caused by heating and cooling over long periods of time might relieve some of the stresses by breaking down bonds. At least in my experience, very old wood (like some 19th century walnut and chestnut that I've worked with) does seem to be both a little more stable, less elastic, and even sort of powdery to work with..but, Ray, the lignin up in that bearly cozy warm attic of yours is probably laughing at you.
Edited 5/29/2004 11:28 pm ET by Jon Arno
Edited 5/29/2004 11:39 pm ET by Jon Arno
Thanks, Jon,
I suspected you'd tell me that. I've bent wood by steaming, and after immersion in hot water, I just didn't know how low a temp might soften up the lignin. Many thanks for your willingness to share your knowledge.
Regards,
Ray
Jon,
Your response wasn't directed to me, but I must thank you. That was a great explanation of a complicated thing. Have you read Understand Wood? Great book.
Mike
>>"Have you read Understanding Wood? Great book."<<
Yes, and it's an outstanding basic reference, especially for beginning woodworkers. Bruce Hoadley's other book; Identifying Wood, is also an excellent place to start, if you're interested in wood anatomy and how it's used to identify various wood species.
Bruce is an excellent writer and a very personable guy. He's an old codger like me and either retired, or wishing he could...but he's still one of the best minds in the field. He's been a professor at UMass since shortly after we drove the British out of New England...and we've communicated back and forth a few times over the past 25 years.
Jon,
Thanks for the great explanation, your a real resourse in this part of the forum!
I got a question about kiln drying as a buddy and I are playing with doing it to some walnut logs we found.
From what your saying, I would gather that inexperienced operators would have much better luck kiln drying thinner wood than thicker. Is that correct?
Reason I ask is gunstock blanks are usually 2-3 inches but need to be REALLY stable. Most that I buy are air dryed about 5-6 years. Internal stress is a BAD thing in a gunstock so I think I will stick to using air dryed for stocks but the kiln stuff for boxes and trinkets.
Thinner stock is both quicker and easier to kiln dry. If you'll be using mostly 4/4 anyway, that's probably the way to go. The fact that you're dealing with walnut also helps reduce the risks. Walnut is quite stable and it has a very low T/R ratio, so it's not particularly prone to distortion.
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