Table top expansion - can someone explain?

[FrenchIan]

If I wanted to make a table top out of, say, oak planks glued together, and running longways, I'm told I have to allow for this "slab" to expand sideways, otherwise there's a risk it'll buckle. If I want to screw it to the carcass of the table, the screws have to be in slots.

Does this mean I have to go under the table every week, and loosen/tighten each screw? Or, are the screws tightened up just perfectly, so they can self-adjust, while still holding the top snugly?

Then, suppose I wanted breadboard ends, to hide the end grain of the "slab". I can't screw the end tightly to the "slab" across the width, because it'll expand differently. Does this mean that at different times, it may be longer or shorter that the width of the table?

Please excuse these ignorant questions, but this has been worrying me for a while.

Cheers

 

[Benchwayze]

Best way is wooden 'buttons' screwed to the underside of the top, and let into small grooves in the rails/aprons of the underframe. No glue. The buttons will move in their housings and allow the table-top to expand and contract. You might never need breadboard ends either. Shown end-grain proves the top is solid. Without allowance for movement, the top might buckle, but chances are it will split as well!

HTH

 

[Steve Maskery]

+1 for buttons.
As for BB ends, yes, you have to allow for movement and yes that means sometimes the BB ends will be shy and sometime a tad proud. The usual way of allowing for this is to have it attached by drawboring, the centre peg being in a round hole and the others being in elongated holes.
S

 

[Benchwayze]

+1 for buttons.
As for BB ends, yes, you have to allow for movement and yes that means sometimes the BB ends will be shy and sometime a tad proud. The usual way of allowing for this is to have it attached by drawboring, the centre peg being in a round hole and the others being in elongated holes.
S

Agreed on BB ends.
Arts and Crafts workers sometimes made a 'design feature' of the 'proud' ends, with say 3/8" extra length each side. That way any shrinkage or expansion wasn't really noticed.

I just saw your question re going under the table regularly.
The buttons are screwed to the tabletop tightly. NOT fixed to the underframe, other than in the small housings, so they 'float'. So no visits under the table! :mrgreen:

 

[custard]

If I wanted to make a table top out of, say, oak planks glued together, and running longways, I'm told I have to allow for this "slab" to expand sideways, otherwise there's a risk it'll buckle. If I want to screw it to the carcass of the table, the screws have to be in slots.

Does this mean I have to go under the table every week, and loosen/tighten each screw? Or, are the screws tightened up just perfectly, so they can self-adjust, while still holding the top snugly?

Then, suppose I wanted breadboard ends, to hide the end grain of the "slab". I can't screw the end tightly to the "slab" across the width, because it'll expand differently. Does this mean that at different times, it may be longer or shorter that the width of the table?

Please excuse these ignorant questions, but this has been worrying me for a while.

Cheers

If you look at antique farmhouse tables, which were originally made with air dried timber that was say 15-20% moisture content, then the breadboard ends are virtually always about 1/8" or so proud of the sides of the tables. That's because the table has now dried to 12% or less and consequently shrunk.

But as well as this one off adjustment to central heated interiors there's also a seasonal cycle of contraction and expansion.

If you take London and the non coastal south east as an example, the lowest outside relative humidity occurs April to July at about 70%, and the highest is December and January at 85%. At 70% humidity wood is stable with a moisture content of about 13% (actually 12.3% to 13.5% depending on temperature, but let's keep it simple), however at 85% humidity wood is stable at about 18% moisture content.

Indoors the situation is slightly different, and humidity levels in the UK frequently drop to 45 or 50%, which needs a moisture content of 8-9% for stability.

Bottom line is that you can never practically ensure stable humidity, therefore you have no choice but to allow for some movement in the timber.

 

[FrenchIan]

Thanks, gents.

Looks like the consensus is to use buttons and omit the breadboard ends (or make a feature of them). Now I know what to do,

Cheers

 

[Togalosh]

+1 for buttons.
As for BB ends, yes, you have to allow for movement and yes that means sometimes the BB ends will be shy and sometime a tad proud. The usual way of allowing for this is to have it attached by drawboring, the centre peg being in a round hole and the others being in elongated holes.
S

Oh b*""%$ks.. :shock:

 

[Steve Maskery]

I assure you it's not b*""%$ks at all, it's the standard way of doing it. The holes near the ends need to be elongated more than the holes nearer to the centre, too.

 

[marcros]

I think that the "Oh b*""%$ks.. " referred to a newly discovered mistake rather than questioning what you were saying.

 

[Steve Maskery]

I was creating a piece of humour.
Perhaps I failed.
S

 

[motownmartin]

I do believe that its best to store the timber in similar conditions to what the finished item will be subjected to for a few months, that way it will only move a tad, as I had to find out the hard way.

I have an Oak table with Breadboard ends that has not moved at all, I also have other Oak furniture with breadboard ends that was purchased online and was manufactured in India, this has shrunk a great deal and I guess this is due to more humid conditions in India.
I am sure that someone will put me right if I am wrong.

 

[Togalosh]

I think that the "Oh b*""%$ks.. " referred to a newly discovered mistake rather than questioning what you were saying.

yep..one I'll soon sit down to see at every mealtime if I'm not lucky... ho hum.

The table I am replacing is a poorly thrown together, pine item (as opposed to my poorly made but labouriously made oak one).. with the same end board design. This piece was screwed on along it's length & although the main section shrank to leave this board 5mm proud at either end the main section did not split...perhaps the soft timber gave way instead.

..so much to learn, so few brain cells to learn with

 

[marcros]

it may only be a matter of unscrewing and elongating some holes. post some pics up and let people advise.

 

[Togalosh]

There's nowt to see as it's all glued up.. but I've only done one end as space & clamps are not as plentiful as I'd like - so I could do the other end the correct way & see what happens. Also the top is pretty big & weighs a ton so it has to stay on my bench as I work other components... far from ideal.

It's 'rustic' English Oak, 35 thick x 960 wide end board with 5 loose tenons of 12 x 80 equally spaced & tight fitting...I had to choose loose tenons as otherwise I'd lose too much length as I've only so much wood to choose from & the end board is already over sized as it is ..I'm into old fashioned styles done out of proportion - just as well really.

I had seen a table with 6 small hand cut dowels per tenon (I assumed) along all of its boards in a semi circle.. that looked good. If it was truely old & with actual tenons or pretend, made to look old I could not tell, but I fancy copying that idea...this flies in the face of the advice given here.