expansion coefficient?

[sunnybob]

Does anyone know how, or even if, wood expansion can be calculated by wood type and temperature rise?

For example. If I have 2 equal lengths of wood say 3ft long x 3" wide x 1" thick, one of beech and one of construction grade softwood.
If I measure them at 10 degrees c, and then at 40 degrees c, how much expansion should I expect?

 

[samhay]

Thermal expansion of wood is much smaller than expansion due to change in water content.
You can calculate the change in relative humidity upon taking air from 10 - 40 degrees, but this assumes a closed system, which is unlikely to be helpful. This will change the water content in the wood, but this takes time and movement will depending on the timber dimensions, grain orientation, species, age, etc...

So, what are you trying to predict?

 

[Cheshirechappie]

Longitudinal thermal expansion is small, but transverse can be high, and both vary with species.

One set of data I've seen suggest about 3 x 10-6 mm/mmC longitudinally, and about ten times as much transversely, but don't take those figures as absolute for all species.

(Wood has been used for longcase clock pendulum rods, because of it's low longitudinal thermal expansion. Invar is a bit better, but all the other metals need some means of compensating for temperature variation if timekeeping accuracy is to be maintained, especially for regulator clocks.)

 

[sunnybob]

outdoor recliner side table. frame of construction wood, large single ceramic tile as the top (2ft x 1ft), with a wood surround.
The problem is its outdoors. High summer is 40c in the shade, as much as 70 in direct sunlight. Winter can have torrential downpours and overnight temps can be as low as zero, but usually between 5 and 10.

i know, thats a stupid request :roll: but in mitigation, its under a pergola out of direct rain, and it can be coated with whatever waterproof finish is needed.
Its for me, no customers involved, I just want to make it. 8) 8)
Main question is if i make the hardwood frame and slide the tile in, how much gap should I leave around the tile to stop it pushing the joints apart when the wood shrinks?

 

[Astrobits]

some data here: https://www.engineeringtoolbox.com/line ... -d_95.html
Nigel

 

[sunnybob]

I'm afraid that chart is so far above my mathematical ability, I cant even understand the question :roll: (hammer)

 

[ED65]

Main question is if i make the hardwood frame and slide the tile in, how much gap should I leave around the tile to stop it pushing the joints apart when the wood shrinks?

None, tile isn't changing dimension.

Leave enough gap, like 0.5mm, around the tile just to make sure the components come together and to make sliding it in easy.

 

[sunnybob]

Thats the point. the tile doesnt change, but if I make it tight, the wood will shrink and pull the joints apart. If i make it too loose, in the summer the wood will move away from the tile completely and fall off.
possibly.

 

[Inspector]

I wouldn't use the temperature to calculate the wood movement. I would look at moisture content and use the Shrinkulator to get the numbers. No math involved. =D>

http://www.woodbin.com/calcs/shrinkulator/

Pete

 

[sunnybob]

Hmmm, this sint going to work. That one requires me to input relative humidity before and after. Thats impossible in my situation.
I think I'm going to have to wing it. 8)

 

[AndyT]

The project and the range of temperatures are pretty similar to an exterior glazed door and the conditions it gets exposed to, so if I were you I'd use a similar approach.
In your frame, make rebates of about 12mm. Make it up, then "glaze" it with the tile. Hold it in place with thin strips, filling up what was the open side of the rebate. Allow 2-3 mm of free space all around the tile. Fix it in place with glazing silicone so it stays central in the hole.

 

[J-G]

... the tile doesn't change...

Ever the pedant ! --- of course the tile changes --- everything has a coefficient of expansion.

However, of course it will be at a smaller rate than the timber. Looking at the table pointed to by Astrobits, Clay Tile is 5.9 and Oak is 54 so in that case about 9 times more for the Oak.

Having said that, I think you should not be too concerned, just make the rebate about 3mm larger which will be much more than any movement in a 2' x 1' space.

 

[Inspector]

I leave it set on moisture content of the wood and if you don't have a moisture meter you can ball park it. 4"/100mm wide White Oak going from very dry in the sun in the dry season at 6% to 15% after being rained on for a few days will move from 2 to 3 mm. It might actually be a little more or less in real life but it will move. You can always do a test yourself by taking a few inches of the chosen wood and measure it. Dump it in a bucket of water for a day or two and measure it when you pull it out. The amount it grows will show you what you need to know.

AndyT 's suggestion of setting the tile with a gap and sealant is the way to go. Breadboard ends won't open up like mitres will.

Pete

 

[sunnybob]

I think Andy has it. I'll use silicone to fill the gap, rather than just wood glue everything to everything.
Thanks very much. 8)

 

[Sgian Dubh]

It sounds like you don't really need to work out likely expansion and contraction of the wood in your table because you've found a workable solution, i.e., a generous allowance around the tile or tiles, plus a flexible filler between wood and tile. And the wood you're using is relatively small in section anyway, meaning expansion and contraction is likely to be of relatively little importance, although that's not always the case because sometimes even small dimensional change in small sections of wood can matter - it depends on the structure and its use.

But, yes, if you really wanted to, or need to, you can come up with calculations for likely expansion and contraction of wood across the grain. As others have said, atmospheric relative humidity (RH) is the primary contributory factor, although temperature has a relatively small role to play.

Basically, if you know the typical seasonal range of the RH affecting the wood, whether it's used outside or inside, and you know the percentage movement factor for the species of wood you're dealing with, you can do some sums to calculate the likely maximum and minimum size of the wood across the grain over the course of a year, and use this to make necessary design and technical choices.

So, for example, if you have a tangentially sawn European oak panel 950 mm wide that's at ~7% moisture content (MC) because it's acclimatised at the driest time of year to an atmospheric RH of ~40% at an average of ~20º C, and later in the year as the RH rises to it's highest level to an average of 65% RH at a similar average 20º C+ the wood will gradually move towards about 12.5% MC. Knowing this information allows you do sums and calculate that your 950 mm wide panel is likely to expand in width by approximately 5 - 6 mm to roughly 955 - 956 mm. Then, as the seasonal average RH moves back towards about 40% RH, the panel will reduce in width towards its original 950 mm.

I'm aware of what I think is a pretty good source of information on this subject of water, water vapour, temperature, and wood, but I'd probably contravene forum rules if I pointed you towards it, so I won't, ha, ha. Slainte.

 

[sunnybob]

Thanks for the very technical advice, but I just have no way of measuring all this.
The full story here is that I already made 2 tables, with construction softwood legs and apron, 12 mm ply under the tile, and a maple edging with rebate so that the maple hung over the edge of the tile to cover the ragged cut edge. They looked really good. Untill the winter came.
Being mostly ignorant about wood characteristics, I made all of this to very tight tolerances with everything glued to everything, but it was made in May, when we had 38 - 40c every day.

Last weekend, we had massive rains and cold winds, dropping the outside temp to about 14c overnight (they are outdoors under a pergola, but I suspect humidity was 99.9%).
Both tables suffered the maple mitres pulling apart, I presume through the base and ply shrinking at a greater rate than the maple causing the tile to burst the mitre joints.

As these tables have almost no value (made them from mostly scrap wood, as a practice), theres no point in me buying meters or other equipment.
Today I have remade the edge trims in softwood to match the legs as I have no more maple. I've removed the ply and rested the tile straight to the apron, with a layer of silicone to hold it. The new edging will be glued to the apron and has a lot of room in the groove, theres no way the tile can exert pressure this time round.
Temps at the moment are mid 20's, so a nice mid range.
I'll glue it all tomorrow, apply 2 coats of shellac, and then a couple of wipe on gloss poly to seal it.

 

[Sgian Dubh]

The truth is that information about the typical average external seasonal RH variation for different parts of Cyprus is not especially hard to find. A quick search came up with this link: http://www.limassol.climatemps.com/humidity.php

Relative humidity changes constantly, i.e., by the minute, by the hour, by the day, etc, but there are seasonal averages that can be used to make estimates about likely wood expansion and contraction … if you really need to, or just want to. As mentioned before, temperature has a role to play in wood movement, but it's a relatively small role in the sort of normal liveable temperature range of, let's say, from about -5ºC to perhaps about 45 or 50ºC. And, of course, a one-off sustained downpour of perhaps a few hours (within a long period of relatively dry conditions) on exposed wood will raise its moisture content but, perhaps surprisingly, the raised moisture content will tend to be fairly localised in the outer shell of the wood, maybe a few millimetres deep. A return to drier conditions will see the wood's shell dry out fairly quickly, in perhaps a day or two, assuming drier weather returns for days or a week or two after the soaking.

However, I understand you're reasons for not being particularly interested in getting all technical about this aspect of wood science or knowledge, but if you ever change your mind, the information is out there if you need it. You just need to know where to look, and then spend a bit (maybe quite a bit, ha, ha) of time getting your head around that science. Slainte.