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.

Click to enlarge

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

Fine Woodworking Recommended Products

Double Sided Tape

A thin woven core makes XFasten tape strong yet not too thick (0.008 in.), and helps it hold powerfully without being difficult to peel off.

Buy at Amazon

Starrett 12-in. combination square

This accurate, robust, well-machined, easy-to-read combo square is as great as ever.

Buy at Amazon

Bessey EKH Trigger Clamps

Bessey EKH series quick-release trigger-style clamps are simple to use and maintain consistent pressure. Trigger clamps that perform the way you've always wished they would!

Buy at Amazon