My Dining Table (Photo Thread)

[Stu Pocknee]

You're looking at relative humidity there,

Oops, sorry, I assumed you were talking RH as from what I have read it's RH that is the important factor with regard to wood moisture content, not absolute?

However, given performance since your build, it might be you will be OK regarding seasonal. I would be more doubtful if you were to move to a significantly drier climate, though.

Interesting. Thanks for sharing Alastair. Sorry about your turned stuff And your BIL's experience

Wow. I like this forum already! Plenty of good advice and interested and supportive people. Where do I send the cheque?

[Andy Mac]

Nice work Stu! Should be a thing of beauty when its finished.

Cheers

[silentC]

My basic understanding is that relative humidity refers to the potential moisture content of air at a given temperature - so colder air can hold less moisture than hotter air but relative humidity ranges from 0 to 100% for both, even though the actual water content is different at the same RH readings.

Apparently this is important when you are talking about local variations (seasonal change) and in particular when you are air-drying timber because the RH indicates how much water the air can 'absorb' from the timber and thus affects the drying time.

On the other hand, absolute humidity refers to the actual amount of water content by volume - and that would reflect how much water is actually available in the air. So higher absolute humidity means that the environment in which the wooden item is located contains more water than an area with lower absolute humidity. Since the air can hold more water at higher temperatures than it can at lower temps, it follows that higher temperatures lead to higher absolute humidity than would be possible at colder temps.

So in terms of moving an object from one place to another, it seems reasonable to expect that higher absolute humidity means the object will need to absorb water to reach equilibrium and conversely lower absolute humidity means the object will have to lose water.

Anyway, that's my layman's understanding of it. I'm sure there is a lot more to it than this basic and possibly flawed interpretation. No doubt someone else will have all the science.

[Rocker]

I would be more doubtful if you were to move to a significantly drier climate, though.

regards

It seems to me that the table would be at risk of damage, whether it were moved to a drier climate, in which case the interior of the top might shrink and crack longitudinally, or whether it were moved to a more humid one, in which case it may expand, and so rupture the mitre joints. It seems that it must stay where it is

Rocker

[Alastair]

It seems to me that the table would be at risk of damage, whether it were moved to a drier climate, in which case the interior of the top might shrink and crack longitudinally, or whether it were moved to a more humid one, in which case it may expand, and so rupture the mitre joints. It seems that it must stay where it is

Rocker

The more complete answer

[Stu Pocknee]

For anyone still following this thread here's a table I came across on the weekend that nicely illustrates the problem with a mitered border on a table.

Click thru to see full size.


I took this at a friends place on the weekend. The cracks from shrinking are big enough to stick your finger in.


As a an amateur I figure I can get away with doing dumb things in my woodshop. The only person who suffers is me.

This was a reasonably expensive, store bought, table however.


It's usually only glass tables that you can see the floor through...


This is a pretty good example of the differential shrinkage rates along the grain, and across the grain.


The end result is that the internal planks have broken their glue joints as they have shrunk. Was it an issue with the wood being too green during manufacture? Or a change in relative humidities from where it was made to where it ended up? Either way, it's a poor design...

BTW, my own table remains as solid as ever...

[Expat]

Love the table and I'm hoping it doesn't move too much. I've got a 100+ year old dining table that was my great-grandparents wedding present to their daughter - my paternal grandmother - sitting in the garage waiting to be restored - once I get myself some more space out there! It's opened up along the top so I'm reading all about fixes for things like that and if I come across anything uplifting I'll pass it along. I have a feeling I'll be redoing a lot of the hand work on my table but I'm hoping you won't have to.

[silentC]

Yes that's a very good example of what can go wrong. You can see how the mitres have opened up. Notice how they are closed at the outer edges though - that's because the board has shrunk across it's width, which pulls the angle away from 45 degrees.

The frame has been joined with a mitred bridle joint by the look and has been pinned as well. I'd say, without knowing the history of it, that the boards probably weren't that well dried for it to have shrunk that much. Or it was made somewhere tropical and moved somewhere much drier.

I've seen plenty of furniture like that though and some people like the rustic look of it.

[Stu Pocknee]

Notice how they are closed at the outer edges though - that's because the board has shrunk across it's width, which pulls the angle away from 45 degrees.

I was trying to get my head around why this had happened. Your explanantion makes perfect sense.

@Expat: good luck with the restoration. I hope we can see some pics in this forum as you go along?

[Wongo]

There are plenty of members who want to use mitre joints for their table top. These pictures will be very useful in showing them the problem.

[silentC]

It's important to note that there are two issues here though and only one of them is of concern in designing a top.

The first issue is that the central panel has been glued to the frame, and the frame effectively locks the width, so when the central panel has shrunk, it had no option but to split along the joins. This is the main issue with some of these designs - failing to allow the central panel to float.

There's no reason you couldn't build a top with a mitred frame, so long as the central panel floats. This opens up the possibility for gaps around the edges though.

The other issue is the way the mitres have opened up. That shouldn't necessarily happen and I think it is an extreme example - possibly due to improperly dried timber. Or maybe it has been left out in the weather. If the timber is dimensionally stable, then there's nothing wrong with a mitred frame, although the wider the stiles and rails, the more chance there is of seeing it happen.

I have a coffee table that I built over 20 years ago. It has a mitred frame with 4" rails/stiles and a floating glass panel in the centre. The mitres haven't moved at all. It was built from kiln-dried meranti though, which is very stable.

[Scally]

I like the look of your table. Nice work.

Your pics of the friends table are an excellent demonstration of wood movement.
The timber has a tropical look and it may not have been well seasoned before it came to your relatively drier climate.

It is amazing how much pressure timber can exert. I saw a deck made with 6" wide blackbutt boards. They had heavy tech screw through the boards near the edges to stop the boards cupping. The shrinking and swelling of the boards snapped several tech screws.
They salvaged the deck by drilling oversized holes for the tech screws to allow the boards to move.

Your oversided holes in the blocks used to fix the top to the base should work fine.

[blockhed]

your table looks better than a bought one Stu.
glad it's still lookin' good.
regards
the block