Wood expansion? - Woodworking Talk - Woodworkers Forum
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post #1 of 8 Old 08-03-2020, 03:34 PM Thread Starter
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Wood expansion?

Short version: How much will wood expand when moving from a very hot, dry garage in central California, to the inside of a house on the cool Pacific coast?

Longer version: I'm building a bedframe, complete with a headboard. The main panel of the headboard is 34 inches wide across the grain (beech wood), and it's framed with cherry. I know that the frame is going to restrict the wood movement, so I'm building it so the top 8 inches or so will be fixed in place, and then sliding tenons and pegs with elongated grooves will hold it in place horizontally while allowing it to move vertically.

So...how deep of a groove in the bottom part of the frame do I need to leave? All the calculators online assume that you can measure the humidity with precision, and I assume most of the "rules of thumb" aren't working with the extremes of going from a hot, dry garage in the central valley, with a humidity of 22% today, to a house on the cool, wet Pacific coastline. It's just going to be finished with tung oil, so no help there. Is the frame just going to explode? Or will I be alright allowing a half inch of movement? A full inch? Do I need to leave the bottom part of the frame off entirely? I don't need an exact answer here, just general reassurance or warning.

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post #2 of 8 Old 08-03-2020, 04:13 PM
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there are internet sites that will give you the expansion coefficients for each species.
some give just the coefficient - which is x.y inches per inch of swell/shrink
and some like http://www.woodbin.com/calcs/shrinkulator/ factor in the actual finished dimension(s)


all fine and good, but the missing bit is what are the usual/average conditions "here" and "there"
https://www.wunderground.com/ has all the statistical data.

you can look up your 'average' where you're building and the 'averages' where the headboard will live.


beware the assumptions. if wunderground says your average humidity is X%, but your shop area is air conditioned.... the actual humidity experienced by the work piece in your shop will be less.
the other issue is the rough sawn cut - flat, quarter sawn, plus a couple more. how the board is sawn out of the tree trunk affects the grain orientation and hence the shrink/swell coefficient to be observed...
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post #3 of 8 Old 08-03-2020, 05:16 PM Thread Starter
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Originally Posted by TomCT2 View Post
there are internet sites that will give you the expansion coefficients for each species.
some give just the coefficient - which is x.y inches per inch of swell/shrink
and some like http://www.woodbin.com/calcs/shrinkulator/ factor in the actual finished dimension(s)


all fine and good, but the missing bit is what are the usual/average conditions "here" and "there"
https://www.wunderground.com/ has all the statistical data.

you can look up your 'average' where you're building and the 'averages' where the headboard will live.


beware the assumptions. if wunderground says your average humidity is X%, but your shop area is air conditioned.... the actual humidity experienced by the work piece in your shop will be less.
the other issue is the rough sawn cut - flat, quarter sawn, plus a couple more. how the board is sawn out of the tree trunk affects the grain orientation and hence the shrink/swell coefficient to be observed...
Excellent! This site is perfect. My shop is NOT air-conditioned: it's literally a garage with an open door, so I imagine the humidity is more or less exactly what the weather site says it is. So assuming a humidity of around 20% here, and 60% there, it says the overall change will likely be about 1 inch (or a little over). So...I should probably give the panel 1.25 inches of free space in the groove, to avoid issues? I think I can manage that. Thanks a ton!

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post #4 of 8 Old 08-03-2020, 05:43 PM
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Quote:
Originally Posted by mackman View Post
So assuming a humidity of around 20% here, and 60% there...
Haha! If you add those two together you still won't be where we are on a daily basis! LOL! This morning the temp was 76, humidity 94%, weather app said it 'feels like 85. I don't recall a day in the last few months when the humidity was below 88% to start each day.

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post #5 of 8 Old 08-03-2020, 05:52 PM
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I treat expansion contraction the same. I use the cabinet door cap. Never failed me...
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post #6 of 8 Old 08-03-2020, 06:23 PM
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For years I've used 1/8 inch per foot, cross grain.

Let me explain.
You start with KD wood. And it sits in your shop for a few weeks. It has reached equilibrium. You build your project and move the piece to the Pacific North West or to Phoenix, Arizona. Here in the north side of Huntington Beach the humidity rarely gets above 30%. (We are almost on the border of Westminster.)

I made a couple of solid cherry nightstands and moved them to the Portland, Oregon area. No problems.

You mentioned a panel 34 inches wide or 3 feet for rule of thumb. The wood is going to move 3/8 inch maximum, equilibrium to new equilibrium. Now before we jump in and leave the extra 3/8 all around, lets be smart and reduce the effect of wood movement to 3/16. All it takes is a bit of glue and a 23 gauge pin nail gun to reduce the effects of the wood movement.

Cut the channel for the panel an extra inch deep. As you install the panel put about 1 inch of glue, top and bottom of the panel and then shoot a very short pin nail through the frame and into the panel to keep the panel from moving. Do this in the center of the panel only. What you have done is to divide the effect of wood movement in half. The panel will still move 3/8 but only 3/16 on each side of center. I use 5/8 inch pin nails for this and remember only 1 each top and bottom.
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Last edited by NoThankyou; 08-03-2020 at 06:25 PM.
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post #7 of 8 Old 08-04-2020, 12:30 AM
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I would consider using an additional rail and make the panel smaller
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post #8 of 8 Old 08-04-2020, 09:09 PM
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When doing large cope and stick panels, they make a product called "spaceballs". They are silicone or rubber balls that come in different diameters to fit into the cope. They allow the panel to expand and contract while keeping it centered. I have used them before with success. I have also used any type of compressible cord I may have around that will fit in the cope. Again, keeps the panel centered while allowing it to expand and contract. I usually will not do panels over 24" in solid wood. If paint grade with mdf panels the width does not really matter to me.
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