Put the screws to boards that need to be flattened
Most every board requires flattening on one side before planing it to thickness. And that's easy to do if you have a jointer wide enough for the job, but many of us don't enjoy that luxury. Fortunately, with this simple jig, you can skip the jointer and successfully joint one face using a benchtop planer.
To build the jig, laminate two layers of 3⁄4 " plywood—doing that ensures a flat base with sufficient stock for the threaded dowels to bite into. (I sized the jig to fit through my planer, but you can adjust its dimensions to suit your needs. The jig and workpiece can be a heavy load, so consider carefully before making it more than 4' long.) Space the rows of threaded dowels about every 10", and include one row of 3⁄4 " holes near the center to use with a short dowel and wedge to secure the workpiece.
I made the 5⁄8 "threaded dowels and holes using a Beall Wood Threader (bealltool.com), but you can buy kits to thread dowels by hand or with a router. Or, buy birch rod already threaded (hardwaretree.com), and simply purchase taps for threading the holes. Put paraffin wax on the threaded dowels to keep them from moving during use.
To use the jig, place the board on a flat surface. Position the jig upside down on top of the board, and use a screwdriver to adjust the threaded dowels up and down to level the jig to the flat surface. Flip everything over, secure the board using a wedge, and feed the assembly into the planer, unstopped end first. For best results, feed the low end of the assembly into the planer first—doing that may require flipping the board in the jig and releveling.
—Howard Murray, Whittier, Calif.