Cutting Board Warping

[geoffo]

I have made 2 cutting boards for my daughter in law's twin sink and both boards have warped since I handed them over. ( They were ok while at my place!! ) I am looking for people's opinions on possible causes. I have taken up woodworking since retiring and have no technical knowledge about timber.

The sink is a double one that is a "drop in" type - in other words, the sink sits 19 mm below the surrounding bench surface. They wanted a board that would cover both sinks but be level with the bench. So I did my best . Details are:

Timbers used - blue gum and Tas oak 19 thick and about 48 wide laminated together to produce a striped board with contrasting colours (it looked real good till it warped!)

Main board dimensions _ approx 485 X 450

Small board - approx 270 X 420

Finish - progressively sanded back to 600 grit then 2 coats of Organoil hard burnishing oil applied to all surfaces

Both boards are warping with the centre of the top surface dipping downwards.

At 19 mm thick are the boards too thin for the sink span or is air / sink moisture causing the problem?

The boards sit on the lip of the sink with the main support being provided by the divider between the sinks.

Sorry that I did not think to take a picture.

Any help greatly appreciated.
Geoff

[Skew ChiDAMN!!]

At 19 mm thick are the boards too thin for the sink span or is air / sink moisture causing the problem?

I'd say it's the sink moisture. The air trapped underneath stays damp, while on top it's whatever humidity the kitchen is. This would cause 'em to cup regardless of thickness.

After flattening out the boards again (leave 'em upside down on a flat surface in a dry area, perhaps with a weight/platen across the top to help hurry things along) I'd try one or two things:

- Fixing rails with screws across the undersides, so that they run across the grain and help to hold it flat.

- Painting the underside with a gloss enamel paint. It's usually undesirable to finish the bottom differently to the top, but this may be one case where it's a benefit, not a mistake.

I think either - or both - would reduce the rate at which they cup again, but I doubt that anything will stop it from happening...

[mic-d]

g'day Geoff,
what you have there is a differential humidity problem. The top is experiencing a lower humidity because it is open to air circulation, while the bottom will experience high humidity due to the water in the sink, or the trap at least. One thing you can do is take them out and turn them upside down on a bench. This will allow the face to dry out and shrionk bringing the boards back flat. At that stage start recoating them with more oil to seal them better. Also, if your timber was a bit wet, the top will be drying out to equiliubrium moisture faster than the bottom for the same reasons. If that's the case, even if you flatten it as above and reoil, it will probably bow again until it reaches some kind of equilibrium moisture content.

Cheers
Michael

[wattlewemake]

Sorry about this side track but would the same happen with an end grain cutting board used in this type of application?

Just wondered thats all.

Shane

[geoffo]

To Michael and Skew - Thanks a lot for your knowledgeable replies. I will have a think about them and decide what to do.

I will most likely go Michael's way.

Michael, I had a look at the pics of your work - awesome by more standards. But I love my woodwork and I improve a little with each project.

Thanks again
Geoff

[Sigidi]

Sorry about this side track but would the same happen with an end grain cutting board used in this type of application?

Just wondered thats all.

Shane

on paper, the short answer is no, as longitudinally timber has almost no shrinkage, you should find a peice as short as 19mm to have so little movement it shouldn't make a difference.

BUT,

then we have the moisture imbalance, this will tend to 'swell' the underside of the board and not the top, this will cause it to cup.

Geoff, I'd think if the shape of boards allowed it, flip them every week, to help even the moisture exposure, or whenever they are seen to start cupping. Also, as mentioned more coats of oil.

Good luck and git ya camera out, take some pics

[geoffo]

Thanks to all for your helpful advice. The corners of the large board have different radius and can't be inverted over the sink, It is being held inverted on the bench. The grain on the boards is is in the vertical plane (Aust Woodsmith article suggests this is preferable to horizontal).

have attached some pics. The gap under the middle of the boards is about 3mm.
Thanks again
Geoff

Attachment 95608

Attachment 95609

Attachment 95610

Attachment 95611

[Sigidi]

Hey Geoffo that looks pretty special, some nice looking work, thanks for getting your camera out.

Normally the advice from woodsmith would be very appropriate, in this case, it's meant one side of end grain is exposed to moisture, this means a lot of individual timber cells can uptake moisture. In the drawing the second diagram shows boards being used that are back sawn, this exposes a lot less timber cells.

For arguments sake, say there are twice as many cells exposed in end grain as there is in face grain, and each cell uptakes 1% moisture, then the end grain cells will provide a much bigger overall distortion to the board.

One thing which may help the problem is to get the boards flat, oil them up a few more times, then get Karen to put the plugs in when she puts the boards back on top, this'll cut out a lot of the 'available' moisture the boards can uptake.

Again though, really nice striking piece of work

[Fireguard]

Just a quick question, When you're cleaning the boards, are you cleaning them by hand or are you putting then in a dishwasher.

I made a couple of boards for the wife and she washed them a few times in the dishwasher and they warped just like yours have, I managed to flatten them back out and now she washes them down by hand

[mat]

An alternate to the bottom being over humidified is the top being dried out. This could be from direct sun, airconditioning, high temps etc. If this is likely then dampening the top will flatten the boards.

[geoffo]

G'day all
Siqidi - Thanks again for the input. Your suggestions have been taken on.

Fireguard - Definitely no dishwasher. They would be too big to fit in! I think they are only being hand wiped

Mat - No air con in the house. Right from the time that I handed over the boards, Brisbane's typical sub-tropical summer high humidity has landed.

Geoff