As with many other woodworking tasks, routing hinge mortises requires precision and consistency for flawless results. So build this made-to-size jig specific to the hinges you're using, and you'll get repeatable, accurate results with little effort. Here's how to make it using just a few scraps of 1⁄2 " medium-density fiberboard (MDF) or Baltic birch plywood.
First, select the right bit
This jig works with a pattern bit which has the bearing mounted above the cutters. As the bearing rides against the edges of the jig opening, the cutters rout the mortise to the same size as the opening. Choose a bit with a cutter length of 3⁄8 " or less (sometimes called a mortising or dado-cleanout bit) for best results. Longer bits require thicker jig material to work in the same way.
Now, build a jig
Capture the hinge leaf with the jig's center strips and side pieces, apply glue to their edges, and clamp them together to make the jig's face.
Begin by measuring the short edge of a hinge leaf, excluding the barrel. Double that measurement to determine the width of the jig's center strips [Drawing, below]. Rip a 12"-long piece of MDF to that width, as well as a 2 1⁄2 x20" piece of MDF. Crosscut both in half. With those four pieces cut, assemble the jig's face, photo below.
Locate the hinges, mark the jig
Next, clamp the jig's face to the door, photo below. Use a pencil to mark the jig along the door's ends; then, cut along those lines to bring the jig to final length. Mark the center of the hinge's barrel on the jig face, photo below. Then, cut a piece of plywood to 41⁄2 " wide and the exact length of the jig face to make the support arm. Glue and screw this piece perpendicular to the underside of the jig, edge abutting the marked line, covering the hinge-barrel-side of the bit opening.
Put the jig to use
First, use a hinge to set the router bit's cutting depth, photo below. Then, align the support arm's bottom edge with the bottom edge of the door, clamp it in place, and rout the mortise (as shown above. Repeat at the top edge of the door.
To rout mating mortises in the cabinet, clamp the jig's support arm to the face frame's stile, see photo below. Finally, with the mortises roughed out, square up their corners with a chisel. Then, install your hinges.
Quick Tip! Use brass screws that come with the hinge only after drilling pilot holes and applying wax to the screws to prevent them from binding and breaking.