Reaction wood/internal stresses - what's really going on?

[Kicked Back]

I wanted to batch make a bunch of table saw runners for various jigs out of some maple. I thicknessed some maple down to the same width as the mitre slots using a drum sander, everything was perfect, the wood stayed completely true. I start ripping a bunch of strips off on the table saw, about 5mm at a time. Banana. Banana. Banana. Meanwhile the original/non-offcut stayed true...

I all too familiar with internal stresses within the wood getting released upon machining, but what I'm less clear on is what actually causes it and whether some movement shouldn't be unexpected or if I'm just unlucky.

Googling anything to do with reaction wood/case hardening etc. always leads to a bunch of forum posts with folks saying the wood isn't dry/wasn't dried properly. I'm lucky enough to have a Wagner moisture meter and always confirm everything is dry upon purchase (~12-15% MC) and everything is stored in a fully insulated, dry workshop with plenty of time to acclimatise. Besides, the movement is instant rather than something that happens over days. Not like it's moisture redistributing in that timeframe.

Timber usually bought from (what I hope is) a reputable supplier (Timbersource).

Although I'm writing this post now, I've had this happen with maple, beach, ash and sapele (with its seemingly nice straight grain).

What I really want to know is: are all the various forum posts acting like this is a rare event just typical internet nonsense? How often does this happen in your work?

 

[Adam W.]

Reaction wood is found in branches, trees grown on a steep slope or leaning trees and is caused by the build up of compressive and tensile stresses on opposite sides of the stem or branch.

 

[Kicked Back]

Reaction wood is found in branches, trees grown on a steep slope or leaning trees and is caused by the build up of compressive and tensile stresses on opposite sides of the stem or branch.

How common is it that kind of stuff makes it to a timber supplier? Do the suppliers care when they source it?

 

[Kicked Back]

Tension wood fibers tend to pull out during sawing and planing operations, producing fuzzy or woolly grain.

Aha! I actually had weird woolly chips thrown at me by the table saw and thought "that's weird for maple"...

 

[clogs]

adam w is quite right.....
when I went to live in France, in the recent past there had been a severe hurricane....
even after 20 years u could see that all the tops of the pines were all cut at the same height....
anyway....
my good french freind warned me about cutting Oaks n H/Chestnuts as quite a few have very bad internal stresses, partiq twists....even someone locally was killed by such a reaction tree....
I did have a few with reaction wood when cutting them down....often the trunk will tear for no apparent reason or when near the last bit it'll twist like crazy....
and for those interested I have been sucessfully cutting trees down all my life....now 73.....
I had never seen twisters like that before...

as for suppliers they have no idea what they are buying....
the trees get chopped down and then run thru some huge bandsaw within hours.....
reaction wood is often hidden until it starts drying......
ur looking for old slow growth timber but thats all gone now....and were left with the standing rubbish thats around....
perhaps if u want to see twisted stock have five mins in B+Q or the skip at a decent timber merchant.....
try n use what u have on another project and buy some more....
sorry for the bad news....

 

[Adam W.]

How common is it that kind of stuff makes it to a timber supplier? Do the suppliers care when they source it?

They won't take branches and the agent who selects the timber at the road head in the forest should sort it all out and discard it, but the timber trade being a huge commercial operation based on profit, generally sources the cheapest available acceptable product, so it's quite common to get it at the merchant.

If you want to know read Desch and Dinwoodie, as I don't have the time to teach you all you need to know to be able to spot it at the yard.

https://www.abebooks.co.uk/book-search/title/timber-structure-properties-conversion/author/desch/

 

[TheTiddles]

A moisture meter tells you the moisture (approximately) where the meter is, it doesn’t say it’s uniform throughout. Rather like a thermometer tells you the temperature of a thermometer, not necessarily the thing you want to know.

As it seems to happen to you a lot, maybe some of the assumptions you are making about the conditions need to be considered.

 

[thetyreman]

it sounds like the wood may have needed to acclimatise for a bit longer in your environment before cutting into it, I did have it once but it's because I didn't let it season for long enough.

 

[--Tom--]

Once you’d thicknessed it did you let it stabilise again before continuing to process it?

Did you reduce the thickness from both sides or just one?


If it’s happening each time you try, and with different timbers it sounds like part of the process is off

 

[Kicked Back]

Once you’d thicknessed it did you let it stabilise again before continuing to process it?

Did you reduce the thickness from both sides or just one?


If it’s happening each time you try, and with different timbers it sounds like part of the process is off

Alternating sides, no time to stabilise, but it wasn't a massive change in thickness. The thicknessed piece, which I stopped ripping, is still completely true 24 hours later. It's only the thin-ish offcuts that are springing about.

It's not happening each time I try.

 

[Adam W.]

It's quite common to get boards with internal stresses which causes them to spring open when being milled or re-sawn. If all the trees that were bent and had offset piths were sorted out and discarded, the price of timber would rocket and the price of firewood would plummet.

Fron Desch and Dinwoodie.

"In sawing, planing and turning, the surface of tension wood tends to be woolly since the fibres tend to be pulled out rather than cut, a manifestation of its higher than normal cellulose content"

 

[sawtooth-9]

Sometimes it's just a matter of patience.
It's a smart move to cut well oversize before dressing
Its even smarter to dress well oversize and store ( a few years ) before final dressing
Warping, twisting, shrinkage and cupping are all time and moisture related.As we all know, it takes time to really relax - and it's the same for wood.
And, yes ,you must store flat with good ventilation - for good seasoning.

 

[peter-harrison]

I've found that timber from the USA is very bad for this. I've had a couple of conversations with sawmill or timber yard workers where they have told me that the way they season timber there is pretty savage. It's cooked hot and fast. I've also had log sawn North American timber- cherry and walnut- which is produced from imported logs which are sawn and seasoned here- and it's like a different species. Flatter and without that harshness you get with standard USA timber. Thorogoods sell it- not sure who else.

 

[Astrobits]

There is a little device that has been invented for just this phenomenon. It's called a Riving Knife. Known and used at least since the 19th century.
Nigel

 

[Sgian Dubh]

I've found that timber from the USA is very bad for this. I've had a couple of conversations with sawmill or timber yard workers where they have told me that the way they season timber there is pretty savage. It's cooked hot and fast. I've also had log sawn North American timber- cherry and walnut- which is produced from imported logs which are sawn and seasoned here- and it's like a different species. Flatter and without that harshness you get with standard USA timber. Thorogoods sell it- not sure who else.

I think the primary reason for the way American processed and dried material tends to behave somewhat differently to the wood from, for example, Europe is down to two factors. One, obviously, is species differentiation, e.g., American white oaks (there are a few that get that name) are not exactly the same as European oaks. The properties of both are similar, but there are sure to be some small variations in character and behavior.

The second, as you suggest, is to do with seasoning in a kiln. The North American target for furniture grade wood is 7% ±1% MC. In Europe the target furniture grade standard is 12% ±3%MC. 7 percent MC also happens to be at about the lowest MC that hardwoods machine well because poorer machining tends to be a characteristic of hardwoods dried to 6 percent MC or lower. The North American lumber driers have historically catered primarily for the large home based users of their product, e.g., furniture and flooring manufacturers, and so on. Their intention, through drying to 7% MC was therefore to allow for some moisture regain between the kiln and the end user caused by things such as storage, packaging up, transportation, etc.

However, due to a form of hysteresis, drying to this relatively low MC does have a slight causal effect resulting in poorer machining and working capability even when the wood has regained moisture to say, 10 - 12% MC, a figure closely matching the European kiln drying target. The reason being that wood kilned to 7% MC is made stiffer as a result of the drying (stiffer than drying to 12% MC as in Europe) and, due to the aforementioned hysteresis, it is quite difficult to make wood softer and more pliable again through moisture regain once it has been significantly drier and stiffer: this can have an effect on things like internal stress, wooliness of the grain, and so on. Slainte.

 

[Adam W.]

Richard, in your last paragraph, you've put 2%MC, but I think it should read 12%MC

 

[Sgian Dubh]

Richard, in your last paragraph, you've put 2%MC, but I think it should read 12%MC

Good spot. Brain fart on my part. Thanks, and corrected. Slainte.