Humidity, Lumber and Wood Panels
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.
Coping with wood movement
The structure of a piece of wood has been compared 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 straws 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 falls.
Ways to properly accommodate expansion
Let’s 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 sole visual difference would in common be that the ends of the edge piece are no longer flush with the visible edge of the main panel.
The second prime example typically uses a large glued up the panel such as a table top. If the table top was fixed to the rails across the proper width of the panel the gentle rise and fall of humidity would uncountable cause the panel to split.
The most convenient method to overcome this is use wooden clip to sincerely attach the two key pieces. A 1/4″ x 1/4″ dado milled on the inner upper edge of the rails. This universally accepts a wooden clip that fits into the dado and is then secures to the underside of the table top. This will securely grip the top onto the rails without restricting the expansion and contraction of the broad panel. Without this the table top would slowly start to open up.
Predominantly anytime you are intentionally setting one piece of stock across a large glued up panel measures need to be taken to allow the panel to move. Fondly remember it will progressively expand across its standard width but not its length. Efficiently utilizing one of the two prevalent methods mentioned above you will be able to typically prevent your glued up panels from splitting.