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Lumber movement, when in the drying process does it happen

Started by hh76, January 07, 2025, 10:00:31 AM

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hh76

Hopefully a quick question.

At what point in the drying process would you expect most of the movement/shrinkage?  Does most of that occur early on, or does a lot happen in the last stages, kind of like laundry shrinking?

The reason for the question.  I've got a large stack of walnut that I'm planning on making into flooring.  It's been air drying for close to a year, and I was planning on bringing to a kiln to finish and sterilize.  My father, who is always looking for something to do, has offered to start cutting down closer to finished size before the kiln.  His thought was that it would be easier to handle, and cheaper to dry.  My fear is that it may move during the final stages and I'd have less material to overcome any cupping/twisting?

rusticretreater

Basically, it moves all the time.  Movement also is driven by exposure, air flow, imbalances in the wood, etc.  No two pieces of wood will behave the same.   As soon as moisture starts leaving the wood, it is moving.  The most movement is early on in the process as a drying outer surface pulls the wood in different directions.

I personally wouldn't cut anything until the entire kiln drying process is completed.  How much of the wood would be waste?  Are the waste pieces large enough to use for something else?  Then kiln it.

Did your air dried wood end up straight enough to trim a bit off?  Think about the good ol' 2 x 4 which is actually 1.5 x 3 1/2.  A quarter inch planed off each side to make it straight after kilning.
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hh76

Quote from: rusticretreater on January 07, 2025, 10:42:05 AMBasically, it moves all the time.  Movement also is driven by exposure, air flow, imbalances in the wood, etc.  No two pieces of wood will behave the same.  As soon as moisture starts leaving the wood, it is moving.  The most movement is early on in the process as a drying outer surface pulls the wood in different directions.

I personally wouldn't cut anything until the entire kiln drying process is completed.  How much of the wood would be waste?  Are the waste pieces large enough to use for something else?  Then kiln it.

Did your air dried wood end up straight enough to trim a bit off?  Think about the good ol' 2 x 4 which is actually 1.5 x 3 1/2.  A quarter inch planed off each side to make it straight after kilning.
Thanks for the reply.  I had a feeling it would be more complicated than the question asked.  These new hobbies have a way of teaching us how little we know about the things we never thought to question.

More than likely, there won't be a lot of useful waste.  I was thinking that I wouldn't mind a touch of sapwood on some of the boards to give the floor a little contrast.  Because of that it would leave mostly sapwood off cuts, which is what would be cut before the kiln.

 Most of it seems to be fairly straight.  Some slight cupping on wider boards, but those can be ripped into separate boards.  It was all cut to 1-1/4", with the thought that I would plane down to .75-1" depending on how it all looked when it was time to process.

beenthere

As I understand, shrinkage of wood only occurs after the free water is gone (fiber saturation point at or around 28% mc), leaving the water in the cell walls to evaporate.  When the water within the cell walls of the wood leaves, the cell walls shrink.
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Brad_bb

Making flooring you need the flattest lumber you can get for the least amount of wood loss from machining. 

To this end it's important to know how thick you cut it start with?  I cut 4/4  1-1/16" thick.  After drying and machining it's typically 15/16" thick.  How much sapwood did you leave on? That will affect how much it wants to warp/twist. How did you stack and sticker it.  Stacking and stickering is one of the most important things when air drying.  It needs to be flat when sticker stacked, and needs weight on it to keep it flat while air drying.  The amount it will shrink in face width for flat cut of any given species can roughly be calculated  suing the published shrinkage rates for that direction.

I would not want to just bring flat stacked air dried lumber to a kiln.  I've seen new sawyers and when was new I stacked with individual board stacks with short stickers and too little or no weight.  This leads to a lot of twist, cupping etc. 

It's going to cost you more for them to stack it if you bring loose boards, and you don't know what they will do exactly.  I want to control the flatness which means I make my own drying pallets and stack and sticker on the pallets for air drying, then after I strap the stacks with Kubenic strapping to haul the pallet to the kiln.  The kiln I work with just loads the pallet into the kiln.  No work needed.  Once I get there I check the pallet to make sure nothing has shifted and if so I fix it quickly.  All they have to do is load it into the kiln and stack weight on top of it.  YOU will put more effort into stacking well than anyone else will in my opinion.

Once it's done, I haul it directly to another shop to machine it all flat and straight line rip one side.  That's what I do when I'm not making flooring, but the same shop I take it to specializes in flooring and if you want will complete machine it for flooring.  Find a flooring maker that isn't too big, but is meticulous about making good flooring.

The kiln and the secondary processor will appreciate you making it easier for them, and bringing them flat lumber to start with.
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scsmith42

There is not much to gain by preplanning and a lot to risk. 

Presuming that you cut your lumber to be a 1" dry rough sawn board, after a year it is fully air dried and probably around 12 to 15% moisture content. You only need a few days in a kiln to bring it down below 10%. 
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