Thread: HELP - Table Top Cupping

Hi Rod, I notice you have it sitting flat on a bench.
I had a similar experience some time ago and found that a table top sitting on a flat surface caused uneven moisture loss?
Mine was well seasoned as well but it never stops moving.
Once I removed it from the flat surface and stood it on end overnight it went back to flat.
Don't know if this is your problem but if it is it's a simple fix and you amy not need the steel levellers.
The same thing happened to a mate not long after my experience and his was fixed the same way. Once on the fame no movement as there was balanced airflow on both sides. Hope this helps.
Cheers, Ian

You have cross braces fixed at each end. This is longitudinal grain direction. Shrinkage in this direction in Karri is usually somewhere around .1% from memory. (600 wide x .1% = .6 mm).

So the cross braces can only lengthen or shorten with moisture variation - lets say .3 mm either way from their current fixed position.

The same 200 wide Karri boards quarter sawn (radial) and edge (butt) joined, the shrinkage green to air dry is between say ~6% Radial & 10% Tangential.

Lets say the difference is only 4% - then we have a orders of magnitude difference between what the cross braces can move (.1%) compared to what the 3 x 200mm wide butt joined boards can move.

This will pull the table top into a cup every time there's any moisture change in the air - in your case 3mm.

IF the cross braces could have been fixed underneath at each end with slotted holes, so the 3 x 200 butt jointed boards could move width wise by allowing the holding down screws to slide in their slot - the braces wouldn;t pull that cup in the top.

If you try to prevent the movement with steel braces all that will happen is you'll pull the butt joins apart.

The "trick" is in designing / assembling to allow for the variations in the longitudinal radial and tangential shrinkage rates of the timber species your using and knowing their Equilibrium Moisture Content before you start, for which you need a wood moisture encounter.

Others mileage will no doubt vary.

Did it happen when this heatwave we are going through hit?
It's happened to me quite a few times when we go from nice weather to this nasty super hot type , I remember sometimes to leave the glued up tops so they dont get affected . What I do for that is I leave them sitting on a table or on the wooden floor and cover them with Boards , chip board or ply , just so only the edges are exposed.

For a fix, If I can get away with wetting a top and putting it cup down till its straight I do that. Once I had a whole extension table top , all machined and jointed up , still over size, go on me, two ends and four or five leaves all way to far to fix by wetting . I had to cut them up and start again big long hissing and cussing all the way.

Your steel idea will work if you screw it out at the ends, make the hole a little loose for some movement and make sure there is a 8mm to 10mm gap at each side . if you butted it right in tight to your added bits it would split for sure , if you give it some room to move it should work .
I'm assuming the top has cupped so that water on it would run to the middle

Nice looking top and mould by the way.

Rob

Originally Posted by Timless Timber

You have cross braces fixed at each end. This is longitudinal grain direction. Shrinkage in this direction in Karri is usually somewhere around .1% from memory. (600 wide x .1% = .6 mm).

So the cross braces can only lengthen or shorten with moisture variation - lets say .3 mm either way from their current fixed position.

The same 200 wide Karri boards quarter sawn (radial) and edge (butt) joined, the shrinkage green to air dry is between say ~6% Radial & 10% Tangential.

Lets say the difference is only 4% - then we have a orders of magnitude difference between what the cross braces can move (.1%) compared to what the 3 x 200mm wide butt joined boards can move.

This will pull the table top into a cup every time there's any moisture change in the air - in your case 3mm.

IF the cross braces could have been fixed underneath at each end with slotted holes, so the 3 x 200 butt jointed boards could move width wise by allowing the holding down screws to slide in their slot - the braces wouldn;t pull that cup in the top.

If you try to prevent the movement with steel braces all that will happen is you'll pull the butt joins apart.

The "trick" is in designing / assembling to allow for the variations in the longitudinal radial and tangential shrinkage rates of the timber species your using and knowing their Equilibrium Moisture Content before you start, for which you need a wood moisture encounter.

Others mileage will no doubt vary.

but

I'm pretty sure - that this shrinkage / expansion differential between longitudinal and radial + Tangential sawn boards was discussed a little while back, when we spoke about slabs and why they split in from the end Rod.

Fixing longitudinal grain across radial or tangential grain, without allowing movement in the fixing will cause splits or joint failures in WA Hardwoods like Jarrah and Karri.

In things like fielded panel doors you can allow for extra width wise movement of the panel, by fixing it only in the center at top and bottom allowing extra depth in your rebates on the stiles.

In Table tops, where you fix up from underneath - if you elongate the holes in the cross member support such that the screw holds the top down to its support but still allows it to expand or contract width wise to account for the difference in rates between the longitudinal grain cross member - and the sideways radial/tangential sawn top, butt joined boards (or slab), then you can avoid the tendency to cup the top, or pull but joints apart or split the slab in from the end.

A search would probably pull up the thread.

It's something we should be aware of when designing furniture from WA Hardwoods. Try to allow for the "movement differential" to occur between summer and winter - in your design - such that the movement is allowed to occur but hidden from view.

Thats my take on it for what it's worth.

I have furniture doors here, that I first made in Jarrah - 30+ years ago... where I didn't allow sufficiently for the sideways expansion of the panel in the rebate on the door styles. After 30 years the expansion of the panel has pulled the tenons from the mortice (this was back when I also used PVA glue with Jarrah coz I didn't know any better).

I've been busy 'repairing' these 4 doors recently (with epoxy) but also reducing the width of the panels a mm or so such that they don't expand enough now to push the M&T joints apart.

I admittedly got away with it for say the first 10 years, but eventually it caught up with me.

Learn from it and move on.

Further to the above the Coffee table is now complete. Its size is 1800 long x 600 wide x 600 high

Picture 1 - The Coffee table has been made from these old scaffold/formwork planks.
Picture 2 - For the shelf I did A full width (580mm) dovetailed rebate.
Picture 3 - The end panel (horizontal) and the shelf (vertical) dovetailed joint.
Picture 4 - The assembled table.
Picture 5 - To overcome the bowing of the top I used 50mm x 20mm solid steel bars.
Pictures 6 & 7 - Sanding back between coats.
Picture 8 - The finished top (it has the odd dust fleck which I can live with). The finish is Cabots CFP Floor - Satin
Picture 9 - The finished and assembled Coffee table (very heavy) in its location.
Picture 10 - The now old Coffee table (some 30 years old), I used Estapol 7008 two pack on this and it has lasted well.
Picture 11 - The new and the old side by side.

Originally Posted by Timless Timber

You have cross braces fixed at each end. This is longitudinal grain direction.

Cross braces running against the grain of the table top are THE big No-No in table building. I've seen a pine table about 800 mm wide cup so much , that a steel ruler , touching the edges of the table top, was 40 or 50 mm away from the center!

It's really beautiful timber, and a fine job, however, I wouldn't be surprised if it bends the steel braces, or pulls itself to pieces unless the braces are slotted, or attach to the top with sliding buttons.

It will want to expand most across the width with moisture, and will try its hardest to succeed.

http://www.thecoffeetable.co.uk/page32.html