Routing Hinge Mortises
For when you end up hanging a new door in an existing frame and have to match the door’s hinge locations to the frame’s.
In most cases, carpenters buy doors that are prehung in their jambs with the hinges already in place. But sometimes, you end up hanging a new door in an existing frame and have to match the door’s hinge locations to the frame’s, or you find yourself hanging a door in a new frame on site. The old carpenters I learned from would have just chiseled out hinge mortises—and that can be fast if you have only one door to do, and if you have a sharp chisel. But if you have more than one door to hang, making a router jig doesn’t take long, and the results are as close to perfect as a carpenter needs to get.
Jigs are single purpose—they work with one router and one size bit to cut a specific size mortise. If I find myself on a job without a jig, I’ll sometimes freehand a hinge mortise or three with the router, cleaning up the edges with a chisel (it’s easier to cut a mortise to a consistent depth with a router than with a chisel). But that’s a last resort, because making a jig is pretty easy. In this case, the edge of the round router base is guided by three fences on the jig. Other routers with bases that aren’t a consistent distance from the bit can still work with a jig, though it’s tougher. For example, my Bosch laminate trimmer has a square base with one side closer to the bit than the other three, so I need to remember which sides of the base should contact the jig’s fences. You can also make jigs that work with guide collars or bearing-guided bits, but the approach shown here is simple and works with an inexpensive straight bit that you probably already have laying around. The one here is a 1⁄2-in. straight bit chosen to create a 1⁄4-in. radius at the corners to match the hinges.
The jig base used here is 1⁄2-in. plywood, but you could also use 3⁄8-in. or 3⁄4-in. stock. The base dimensions don’t need to be exact, but the length should be about twice the diameter of the router base plus the height of the hinge, and the width approximately that of the router base plus the width of the hinge mortise. The fences are made from scrap 1x stock.