A bit of a problem with my benchtop

[Virgil]

A bit? Read on.

Evening workers of wood.

I am fairly new into the world of wood (did sign up to this forum just this evening). I have been intent on building a bench for about three years now. Did happen to acquire some recycled 4x2 blackbutt, got it sawn and dressed by a good friend, glued it together to make a great slab about 6'x2'. Then I read the instructions. The timber seems to be plain sawn and I in my unknowing haste glued it together (and quite nicely too for a beginner I might add) edge to edge. This the books say would mean I might end up with a benchtop that will bow and cup like blazes as the weather changes. The weather has been hot these last couple of days and the books are right. When viewed from the end my beautiful blackbutt top is now a beautful blackbutt banana.

Now, this top was the first step in the construction of my bench. I would like to use it as the top of the carcass I have yet to construct. If I use the top as is and just make sure it screwed down real tight to the carcass would it eventually pull the carcass apart?

If I were to clamp the top flat, cut a tenon on each end, rout a mortice in a pair of end caps and whack them on each end would it stay flat or would I end up with a split end cap?

Should I rip the top up into 50mm bords rotate them through 90deg and reglue?

Should I donate the top to my neighbour's open fire this winter and start again?

Any advice would be greatly appreciated.

Cheerio,
Virg.

[The Bleeder]

First of all Virgil, welcome to the forum. I wouldn't be donating it to the fire just yet. Post a pic of the top and lets have a look. How much of a banana is it?

Trying to screw it down to flatten it out (depending on the banana) may cause more problems later on.

[AlexS]

Ripping along the joints and flipping every 2nd one end for end may be the solution, but let's see some pics first. Include at least one showing the end grain.
Also, if you do fix it, don't leave it sitting around flat without allowing circulation all around it.

[old pete]

Hi Virgil,

There's no reason per se that back sawn blackbutt would behave in that fashion if its properly dry and has minimal drying gradient from core to case. Almost always one side of a benchtop being made is exposed to more drying than the other. Try reversing the upside of you benchtop. If the shape returns close to flat then your timber isn't dry but you will probably get away with it if the top is secured to the chassis in such a fashion that it can move cross grain. All standard joinery practice.

The other potential cause of your problem is that the edges weren't quite square, or they weren't straight and you have built in heaps of stress with cramping forces designed to force them together with zero gap. THIS NEVER WORKS:no:.

You can't force out bow in thick,high density stock such as this if the reversing trick doesn't work: it will just transfer the problem to somewhere else.

You may have to rip and start again. If you do that then do the glue up in two sections. That is a far more forgiving approach if things arent quite right in the square and straight department.

Good Luck.

Old Pete:roll:

[echnidna]

You can probably straighten it by wetting the hollow face and weighting down on a flat surface.
If you cut it up to redo you could cut the 4 x 2's into 2 x 2's so you can select the quartersawn alignments to glue it up again.

However I dislike QUARTERSAWN timber for a benchtop as quartersawn is easier to slit with a chisel etc than backsawn timber.

Its mainly a matter of fixing the top to a solid base so the timber can expand and contract with humidity changes.

[Calm]

Have you done any "treament" to one side - remember whatever is done to one side must be done to the other ie oil, vanish etc.

Waiting on the photos - if you need help to add photo to post just ask.

Cheers

[Virgil]

Many thanks folks for your responses to my benchtop prob. I'll extract digit from date tomorrow, get some photos and post them up. We can then go from there.

Cheerio,
Virg.

[Wongdai]

This exact problem has happened to my bench top also.

I sanded it perfectly flat last weekend with a floor sander. I noticed last night after the week in my hot shed, that it has cupped probably 1.5mm from the centre to the edges.

Is this significant in terms of bench tops. I understand that they are meant to be perfectly flat, but what is perfect at the end of the day on a round earth.

Would it help straighten it out if I painted the back side?

[echnidna]

straighten it before you paint it

[Wongdai]

straighten it before you paint it

There wouldn't be much point. I would only paint it, if the paint straightened it out.

[blackhole]

sealing both sides would be to prevent it from twisting again.

[Virgil]

I've not applied any finish to it as yet. I was going to do that once I had fixed it to the bench. I was going to use a liberal application of Organoil's Danish Oil.

Haven't got those photos yet have I?

Cheers,
Virg.

[Ben from Vic.]

You have fastened the boards to allow for expansion and contraction?

[Wongdai]

Virgil, I have managed to fix my benchtop up so that it is not cupped any longer, and thought I would share what I did in case it is useful for you.

It was very simple.

I wet the side of the top that was concave quite thoroughly, and then left it out in the sun convex side facing towards the sun. About a day and a half later the top was almost perfectly flat - in fact it had just started to bow in the opposite direction.

Hope this helps.

[Johncs]

Hi Virgil,


You may have to rip and start again. If you do that then do the glue up in two sections. That is a far more forgiving approach if things arent quite right in the square and straight department.



Some advice I read recently and which seems good to this beginner is, when edge-laminating, glue and join & clamp two pieces.

When the glue has set (half an hour or whatever), remove clamps and clean off the still-soft glue.

Glue and join and clamp the third.

Repeat until done.

Remember to clamp in t dimensions, one to press the edges together and the other to ensure they stay aligned.

Doing this, you can evaluate the progress. I think my wife's glue-up would have gone better has we known to do this.

[old pete]

Hi Virgil,

The advice from Johncs is essentially sound re gluing up a piece at a time being a lot easier for beginners to manage. You should be able to add two pieces at each operation depending on width without difficulty. Please do not follow johncs advice re clamping time unless you are using a radio frequency curing resin glue. Depending on glue and conditions clamping needs to be at least overnight. I organise myself to do clamp ups coming up to knock off time so the job can just sit on the bench or floor and does not interfere with on going work. Remove excess glue the next morning with a sharp cabinet scraper. Keep the excess glue to a sensible minimum; there is no value at all in having the job swimming in glue durin glue up. A squeeze out bead of 2mm square should be your max. excess and none should drip off from the lower face. Happy glueing; its always satisfying to prepare a flat panel with near invisible joints. Old Pete

[Johncs]

Hi Virgil,

Please do not follow johncs advice re clamping time unless you are using a radio frequency curing resin glue. Depending on glue and conditions clamping needs to be at least overnight.

Read the directions for your glue. I have to hand these:
1. PVA. Clamp 30 minutes softwood, 60 for hardwood.
2. Titebond and Titebond III. Clamp time 30 minutes.

See http://titebond.com/FaqTB.asp
What is the clamping and drying time of Titebond Wood Glues?