Another Way to Flatten Warped Boards with Your Planer
One of the most common ways to flatten a board in a planer is to glue sacrificial rails to the board and then rip off the rails once the planing work is done (see Jerry Lyons’s method in FWW #208, p. 16). My approach eliminates those rails in favor of reusable 1/2-in. by 1/2-in. skid strips dry-fitted into grooves routed into one surface of the board.
Joint the edges of the board and clamp it between two guide rails (I used aluminum channel, but you could use wood beams or steel angle iron) so that the two high opposite corners are near the top of the rails. Attach the router to a piece of stiff plywood wide enough to span the guide rails, make a 1/2-in. clearance hole for the bit, and screw a couple of guide cleats to the edges. Adjust the bit so that it makes a shallow, flat-bottom groove for the whole length of the board. Now flip the jig and rout a second groove along the opposite side.
You now have grooves that are exactly coplanar at their bottoms. Remove the warped board from the jig and insert the skid strips into the grooves. Secure them with some double-faced tape or a few dabs of hot glue. Run the warped board through the planer with the strips down. When the upper surface is flat, remove the strips, and plane the other side. Save the strips for next time.
1. Clamp board between guide rails
2. Rout grooves for skid strips
3. Run board through planer