How to Flatten Cupped Boards
A cupped board is warped across its width so that the board is slightly U-shaped along its entire length. When you turn the same board over it is said to be "crowned." Cupping and crowning occur when a board is dried incorrectly so that one side of the board is wet and the other side is dry. The board always expands toward the wet side. Sometimes boards less than 1/4-inch thick can be fixed by clamping. Very tight joinery may also flatten cupped boards used in cabinetry. Most of the time the only way to flatten the board is by making it thinner.
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Instructions
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Set the depth of cut on your jointer to 1/32 inch. A jointer is a machine used to make board surfaces and edges flat. Scribble pencil lines across the cupped side of the board you want to flatten.
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Turn on the jointer according to the instructions that came with your jointer. Set the warped board, cupped side down on the jointer feeder table.
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Push the cupped board through the jointer with a push block. A push block is a block of wood you use to push wood through sharp, fast moving, steel blades instead of using your hands.
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Thin the cupped side of the board by 1/32 inch at a time until all the pencil marks disappear.
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Scribble pencil lines across the crowned side of the board and turn the board crowned side down. Thin the crowned side of the board by 1/32 inch at a time until all the pencil marks disappear. Depending on the severity of the cupping, your board may be thinner by 1/8 inch or more when you are done.
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Tips & Warnings
Always use a push block when using a table saw, jointer or bench planer.
References
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