When using large slabs of wood in your shop, you’ll often encounter pieces that are too large to flatten on the jointer or run through the thickness planer. You could certainly tackle this with hand tools, but that can be very time consuming. A simpler solution is to use the router to flatten one side of the board. With a simple sled and rail set up, and a straight cutting bit, you can transform your router into a board flattening machine.
Construct the two jigs
Lay the board onto open pipe clamps. You will need one clamp at each end of the board. Cut two wooden rails slightly longer than the length of the board. On one face of each rail, screw a second piece of wood that is 1 inch thick, 1 1/2 inches wide, and the same length as the rails. These spacers will allow the router bit to clear the board on each side.
Set these rails on each side of the board. Use a level and some shims to level the rails across the whole length. Once the rails are level, tighten the clamps. Check for level one more time, to make sure nothing shifted while clamping.
Construct the router sled using two rails of 3/4-inch plywood that are 2 inches wide and at least 12 inches longer than the width of the board you are planing. Drill a 1 1/4-inch hole in the middle of a 2-inch wide piece of 3/4-inch plywood. This length of the piece is the same as the diameter of your router base. Screw this piece between the two plywood rails.
Plane the wood
Mount either a 3/4-inch or 1-inch straight cut bit into your router. Attach the router base to the middle plywood piece in the sled using two screws. Make sure the bit is centered over the 1 1/4-inch hole.
Turn on the router and lower the bit until it barely touches the wood. You want to remove the wood in small increments to prevent burning the bit and tearing out the wood. Move the router and sled back and forth across the width of the board, while simultaneously pushing the jig forward down the length of the board.
Lower the bit in 1/16-inch increments and continue planing the board until you have taken wood from the entire surface. Remove the board from the clamps and level the other side through your surface planer.
Read and follow all manufacturer's instructions that come with your tools. Always wear eye and ear protection.