Coping with wood movement
All wood workers need to be aware of wood movement when
building furniture projects. Failure to consider this
important factor can cause wood panels to crack or split.
My high school biology teacher compared the structure
of a piece of wood with a bundle of straws. The straws
represent the cells and capillaries of the wood. When
a tree is growing water and food are transferred up through
the cells and are carried throughout the tree.
Once a tree is fell the free water in the wood is lost
rather quickly but the bound water trapped in the straw
like cell is slower to dissipate. As the level of bound
water is reduced these straw like cells get narrower
in diameter but maintain their length. This is the fundamental
reason that wood expands and contracts across its width
but has almost no expansion length wise.
When working with wood it is very important to allow
the wood to expand and contract with changes in humidity.
As humidity rises the straw like cells expand in diameter
with the opposite effect occurring as the relative humidity
Ways to accommodate expansion
Lets use a cutting board for the first example. A solid
wood cutting board with a bread board edge secured with
a tongue and groove joint. One edge has the bread board
edge secured on both ends with a dowel through the tongue
The other bread board edge is secured with the same tongue
and groove joint but this end only has a dowel in the
center of the piece.
The end secured with two dowels will develop a crack
in the main panel. Since the length of the edge piece
doesn't change having it fixed across the width of the
main panel from expanding.
Alternatively, the opposite end of the cutting board
is only fixed in the center. The main panel is free to
expand and contract towards the ends. The only visual
difference would be that the ends of the edge piece are
no longer flush with the edge of the main panel.
The second example uses a large glued up panel such
as a table top. If the table top was fixed to the rails
across the width of the panel the rise and fall of humidity
would undoudtable cause the panel to split.
The best method
to overcome this is use wooden clip to attach the two
pieces. A 1/4" x 1/4" dado
milled on the inside upper edge of the rails. This accepts
a wooden clip that fits into the dado and is then secures
to the underside of the table top. This will security
hold the top hold the top onto the rails without restricting
the expansion and contraction of the large panel. Without
this the table top would slowly start to open up.
In general anytime you are fixing one piece of stock
across a large glued up panel measures need to be taken
to allow the panel to move. Remember that it will expand
across its width but not its length. Using one of the
two methods mentioned above you will be able to prevent
your glued up panels from splitting.