I’ve seen complicated jigs for flattening a wide board in a thickness planer, but I have a simpler method that uses rails glued to the boards. I use basswood for the rails because it’s inexpensive and soft. First, cut the board into the shortest length possible and joint or handplane the edges straight. Position the board cupped side up on a flat work surface. Insert wedges at each end near the corners until the board is lifted off the table with between 1/16 in. and 1/8 in. of clearance and doesn’t rock.

Now glue the 3/4-in.-thick rails to the board’s edge so their bottoms rest flat on the work table. After the glue has cured, remove all clamps and double-check that the side rails are still sitting flat on the work table. Remove any high spots with a handplane.

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Now run it through the planer. Once the concave section is flat, flip the board and plane the other side. Then rip off the side rails.

Drawings by Jim Richey

Drawings by Jim Richey

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