Thread: Shrinkage/Contraction query

Seems very excessive to me - how wide are the slabs?

20mm perhaps but 200? Maybe your adviser thought it was timber from a rubber tree

Well, firstly all wood moves as it takes on or loses moisture, that's a given. Secondly, the amount it moves will be a factor of several things including:
1. The species
2. How dry it was and whether it has to dry more or become wetter to get to equilibrium moisture content
3. How much the moisture content of the air around it fluctuates
4. How it is sealed and
5. How wide it is.

The movement is cumulative which means that the wider the slab, the greater the change in width with movement. As each fibre swells, it moves the fibre adjacent to it, which also swells etc. The boards don't just swell along their edges. So if it is very wide, then you could see a lot of movement. However 200mm is a hell of a lot and it would have to be a very wide slab to have that much movement in it.

Sorry guys.... contraction meant 20mm across.
Table being made from 6 x 200mm planks
Though 20mm still seems excessive.
Your thoughts?
A

Sounds possible as this represents 1.6%.

Why do you consider this as a problem?

You should design assuming there will be movement in all furniture.

As Silent said, the movement is caused by the timber taking in and expelling moisture from the air which varies considerably.

Well, I suppose it does still seem excessive for a well-seasoned bit of timber. It might move a fair bit until it settles down in it's new environment. I imagine the air in Sydney would be a bit drier than where it came from, so it would shrink if anything. Just keep it in mind when you're building the table to make sure you cater for movement. Don't fix the top down solidly, use sliding buttons or figure '8's or similar.

You need the wood shrinkulator.

http://www.woodbin.com/calcs/shrinkulator.htm

This page gives specs for Black Bean so you can change the shrinkage values in the calculator.

http://www2.dpi.qld.gov.au/hardwoodsqld/8230.html

You will need to pick a species that has similar characteristics to your wood. I ran the numbers for an 'Average' wood, and a 1200mm flat sawn panel drying from 14% to 10% moisture would shrink 14mm (7mm if it was quartersawn)

So you do need to allow for some movement in the table top. Out of interest, the same top made of Karri COULD move almost 20mm.

Cheers

Ian