Question about mixing dried and greenwoods.

[PhilCook]

Hi,

Very new to this so sorry if its an obvious question. M son and I are making a large rustic storytellers chair out of logs and unfinished wood. If i were to cut out rounded mortise in the greenwood (its a log about 6" in diameter) and use dried out timber for my tenons would the joint further secure as all the wood dried or would it just create too much pressure and split the wood?

Thanks

 

[Sheffield Tony]

It is common green woodworking practice to dry the spindles, then assemble with the legs green to allow the joints to tighten with drying. Whether there is a risk of splitting depends on how tightly you make them fit. The only times I have had splitting is immediately during assembly, not during the drying. It is quite normal to make them a tight enough fit that a big mallet is required for assembly.

Green woodworkers rule of thumb, popularised by Mike Abbott, is to allow 10% for drying shrinkage. So a 5/8 tenon cutter (he plugs the Veritas ones) will make a tenon on green wood that will, when dry, fit a hole bored with a 9/16 auger. For this reason 9/16 is an increasingly difficult size to find ! The advantage of cutting the tenon when the wood is green is that it will dry oval - assembling with the longer axis of the oval along the grain of the leg reduces splitting risk.

If you are using wood seasoned already, it will probably be drier than green wood left overnight somewhere warm, so maybe don't make the joints too snug. Or near to an end ! You need to experiment. And report back with a WIP please

 

[PhilCook]

That's fantastic thanks for this is very helpful and means that we can make a start straight away(as soon as the wife is out).

 

[Sgian Dubh]

Green woodworkers rule of thumb, popularised by Mike Abbott, is to allow 10% for drying shrinkage.

That's interesting. I wonder if he meant to up to 10% rather than actually allowing 10%? There aren't really very many species of wood that, as they dry from fibre saturation point (FSP, ~30% MC and above), down to 0% MC that might shrink tangentially by 10% or more. In addition, radial shrinkage tends to be in the region of approximately half the shrinkage of tangential shrinkage. Further, most wood in use in artifacts never gets down to 0% MC, something that only normally occurs during an oven drying test used to determine moisture content of the wood prior to the testing. In use, wood used in furniture located in habitable buildings tends to range in MC from perhaps 7% - ~13% MC.

I'm not disputing the utility of allowing for shrinkage and expansion in the forming of the joints where a wetter seat, air dried for example to about 20% MC has a hole bored in it to accept a drier leg spigot starting at, say, 7% MC, and other joints utilising this technique. The technique is well known, I'm just curious about the amount of allowance of 10% shrinkage you mention.

Here's a selection of wood species that typically shrink tangentially by ±10% as they dry from FSP to 0% MC. Of all the wood species for which I have figures, the following list encompasses approximately 25% of their number. I excluded from this list pretty anything with a shrinkage factor of less than 9%, with the exception European oak at 8.9%, as can be seen. Slainte.

American basswood: 9.3%
American beech: 11.9%
European beech: 10.7%
Yellow birch: 9.5%
African ebony: 10.8%
American elm: 9.5%
Shagbark hickory: 10.5%
Hop-hornbeam: 10%
American hornbeam: 11.4%
Jarrah: 11%
Keruing: 10.9%
Lime: 9.3%
Pacific madrone: 12.4%
Hard maple: 9.9%
American white oak: 10.4%
European oak: 8.9%
European sycamore: 9.9%

 

[Sgian Dubh]

That's interesting. I wonder if he meant to up to 10% rather than actually allowing 10%?

Here's an edit of sorts, based on a thought I had some time after posting the above. I realised that perhaps Mike Abbot had a fair point.

He is/was, I think, although I could be wrong, talking about inserting a dried spigot such as a leg at, say 7% MC into a hole bored into a piece of wood, a seat for example, that's perhaps 20 - 22% MC. So, add together something like 3 - 5% volumetric expansion of the dried part, and similar 3 - 5% volumetric shrinkage of the wetter part, and the numbers are getting close to the 10% allowance described.

It looks like I've answered my own question. I got sidelined or distracted by Sheffield Tony's mention only of "drying shrinkage", and didn't really think about the wetting expansion. I know, for example, that in traditional Windsor chair making in and around High Wycombe a century or more ago, they would keep stacks of worked legs and other parts above the workshop fireplace to dry out ready to insert into the other parts such as the wetter and freshly worked seats.

It's funny how a bit of reflection can sometimes result in finding an alternative interpretation to the one that first comes to mind. Slainte.

 

[Woody2Shoes]

That's interesting. I wonder if he meant to up to 10% rather than actually allowing 10%?

...... I know, for example, that in traditional Windsor chair making in and around High Wycombe a century or more ago, they would keep stacks of worked legs and other parts above the workshop fireplace to dry out ready to insert into the other parts such as the wetter and freshly worked seats.
.......

Here's a modern equivalent - I could listen to this guy all day......

https://www.youtube.com/watch?v=ORykaBgTc98