Thread: benchtop with wide boards -cupping prevention advice?

Originally Posted by elver

Hi, a local recycler supplied 170 wide by 40 thick tallowwood boards for my new kitchen island. Its to be 2800 x1000 finished dimensions. The woods been dressed and has a little bow in the length -say 4 mm over the 3m length of the boards. This bow means I can't flip the boards to alternate the growth rings.
I'm really worried if I don't flip alterate boards it will cup badly in the future ( the boards are live sawn so the grain looks like harbour bridges across each board).

If I did flip the boards the ends will need to be forced down by about 10mm to retain the thickness I need, and I'm sure the stress will do very bad things in time.

I don't understand where the 10 mm comes from - If the boards are bowed by 4 mm over the length and you flip every alternate one that means each board only has to be forced by 2 mm on average into a flat position. this perfectly doable over 2.8M of length with a decent set of clamps and support pieces,

They are all bowed the same way, so when you flip alternate boards the bow is 4mm down on one and 4mm up on another, so that adds up to about 8mm or so.

My worry isn't about whether I can force them down- its more whether doing so is a bad idea?

Originally Posted by elver

They are all bowed the same way, so when you flip alternate boards the bow is 4mm down on one and 4mm up on another, so that adds up to about 8mm or so.

My worry isn't about whether I can force them down- its more whether doing so is a bad idea?

That's not how it works. The boards are forced to a centre line in between the two boards so the ends and the middle get pushed only 2mm towards the mid line.
Like this



What you will need is a flat strong surface to do the clamping and glue up on. I've done it on the frame of the piece being made - that is if it has a frame.

Makes sense bob. Its a big kitchen benchtop but i can clamp it to my benchtop. I spent the last fee hours jointing some old windy 4x 2s to use as cauls and beams for clamping to ( with some to go under the bench so i can pull it down to a flat level plane).

looking at BobL's sketch, there's really no need to force the boards into horizontal alignment
glued up like a "banana lounge" as Bob has sketched, you'd only have to take a few mm off the top surface to get a truly flat bench

Ok, got it glued. No biscuits just plenty of west system epoxy and thickener. With a bunch of fresh panel clamps (with the nifty veritas dog thingies) and a bundle of other clamps it came together ok. A fair bit of force needed to get it flat, but it looks ok in the end.

My company makes timber bench tops every week. Keep it simple.
If you put too much stress/pressure into a bench top when you glue it up, it will come back and bite you. A couple of mm's is nothing to worry about so do not stress. We never use epoxy's as they are not 'flexible' and are more subject to cracking, we use AV 515 as it dries in 20mins and sands up very well. Remember timber wants to move! if you try to stop it I will crack.
We only ever straighten the timber on the gluing edge only and then clamp. We then put it through the thicknesser/wide belt sander turning it over on every pass until it is dead flat.

1 year update...

Well, the joints in the bench have opened up at one end - the end away from the waterfall. The epoxy has just lifted straight off the joints. I used West System with 207 hardener. There are now gaps of about 1mm.

I needed to make a bench from the same timber, so to avoid problems I ran a few trials.

1) I glued the same timber with epoxy,
2) I then I tried acetone wipe down then epoxy,
3) then acetone wiped, with titebond III glue.
4) and lastly titebond III glue on plain bare timber.

Interestingly the titebond III beaded up on the wood a little when applied, both on the acetone wiped and non wiped samples.

All surfaces were freshly machine jointed. All samples were clamped to dry to 24 hrs after gluing.

The samples were then cracked by hammer blows. Every one failed on the glue line - almost no breaking of the timber fibers. I decide that the acetone wiped tietebond worked the best, so used that in my bench with lots of biscuits. We'll see how it turns out in time.

I think this wood might be spotted gum, even though it was sold as tallowwood. It behaves far worse than any spotted gum I've ever used before though..

I need to make a dining table out of this stuff. Any thoughts on how I can edge joint it better? I was thinking of using a hefty spline to assist add more surface area to the glue joint.

A fine sawn joint would be preferable to a planed joint in these types of timber.

Yes, I guess it would be. Hard to get a 3m board jointed straight off the saw though.

I suppose I could try roughing up the faces with a wire brush and see if that helps in a test.

Polyurethane is best for waxy timbers like spotted gum.

Don't use epoxy, it's not designed for that kind of work; it's a surface glue that doesn't penetrate the timber, so when you tighten the clamps you squeeze it all out and there's nothing left to actually stick. The more you tighten, the worse it gets.

Why not do a tongue and groove?

The boards are 40mm thick and there was plenty of glue left on the surfaces in the test pieces. Normally I laminate with west system epoxy and I've never had it not stick to any timber. This stuff is really strange. My laminations are normally cedar and jarrah and they're always good, so I think Ive got clamping pressure dialled in right.

I'd thought about trailing polyurethane. I'll do a few more tests.

John.G - I was going to use a spline. Is there an easy way to cut a tongue and groove?

Moulder or spindle moulder.. Most cabinet shops will have the latter.