Flat-panel doors are practical and attractive. And making the panels from plywood is economical and labor-saving. No need to plane down stock, glue up narrow boards, then rip and crosscut individual door panels, and finally back-cut their edges. You can cut the panels for a dozen doors from a sheet of plywood in minutes. In addition, because plywood is stable, you can glue it into the frame to reinforce the door.
But the panel grooves cut by conventional cope-and-stick cutters have a definite shortcoming.
They’re 1/4"-wide. That’s too wide for conventional 1/4" plywood, which is on the order of 7/32" thick, and too narrow for 1/2" plywood, which is thinner than its nominal thickness, too.
Several years ago, the tooling engineers at Amana came up with cope-and-stick bits incorporating adjustable slotters for the panel groove. The two-piece slotter can be shimmed to expand it from 3/16" to 9/32" for thin ply. For thicker plywood, you add a third element to the slotter. Using the shims, you can expand its basic 7/16" cut to 17/32" for 1/2" plywood.