For residential passage and entryway doors, butt hinges are used; they require a full mortise so that the hinge surface is flush with the door and jamb. You can make the mortise either by hand, using a chisel, or with a router, with or without a template.
You first need to know what size hinge you’ll be using. Hinge dimensions are quoted in height and width, height being the dimension parallel to the hinge pin, and width the dimension across both open hinge leaves. These dimensions are driven by the door’s thickness and width.
Once you have established what size hinge you need, your first step will be to locate the hinges on the door. On a standard sized 80 inch door, the topmost edge of the upper hinge is to be 7 inches from the top of the door; the lowermost edge of the bottom hinge should be 11 inches from the bottom of the door. The third hinge should be equidistant between the top and bottom ones. This will establish the vertical placement of the hinge.
To locate the hinge leaf horizontally (along the width of door thickness) the hinge must be backset so that its casing will not interfere with the full opening of the door to 180 degrees in arc. The backset is the measurement between the front face of the doors public side and the edge of the mortise opposite from the barrel.
Use this rule of thumb to find the hinge backset required: use the backset length that locates the hinge’s barrel flush with the casing face so it allows the door to fully swing open without levering itself off the hinges. For a hinge with ¾ inch casing, ¼ inch is a good backset.
Mark the location by scribing it with sharp knife, starting with the backset line. Next, measure your verticals, and set the hinge leaf against the door to trace its outline for the other edge. Now mark the depth of the mortise on the edge of the door by setting the hinge flush with the surface and marking where it’s opposite edge falls.
You can also mark the location using a tool called a butt gauge, which is a hinge shaped steel template available in different lengths. The gauge is placed on the door face with its edge lined up to the door edge and then hit with a hammer, creating an embossed outline of the hinge.
Use a well maintained and sharpened mortise chisel, preferably a tang butt type; it should be at least as wide as the mortise width. Using hand driven strokes- no banging with mallets- begin to chip away at the wood with shallow cuts against the grain.
The bevel of the chisel should be toward the waste side of the mark lines, other wise you will tend to enlarge the removed area beyond the marks. Do not go to the full mortise depth with your cuts just yet, just define the area.
Now move the chisel toward the other end of the mortise about ¼ inch. Incline the blade at an angle of about 40 degrees and give it a fair blow toward the center of the mortise, driving it to the full depth of the mortise.
Keep moving the blade ¼ inch and repeating the full depth cuts until you do the full length of the mortise. Finish by cleaning out material along the back of the mortise, paring it out cross grain. Test for proper depth by laying the hinge leaf in the mortise, tapping it gently into place.
Once you have done all three mortises in the door, you can move on to mortising the jamb. Locating the jamb mortises is similar to locations on the door, except instead of 7 inches for the top hinge, the location is 7 1/8 inches, in order to create an eight inch gap for clearance between the top of the door and bottom of the jamb. The other jamb mortises are to be spaced in the exact dimensions from the top hinge as they are on the door, such that they will match.
Because the jamb is already installed and you will be working at awkward angles, it is much more difficult to achieve good mortises. Extra patience and a freshly sharpened chisel are a must.
Unless you have lots of practice and a steady hand, it is hard to create a perfect mortise by hand. Getting the bottom surface of the cleaned out area exactly flat so the hinge leave lays flush with no gaps is the main problem. Matching the mortise locations on the door to the locations on the jamb is also tricky. These problems can be mostly avoided by using a template and a router to make the mortise.
Pre-made mortising templates, or jigs are available from Black and Decker, Porter Cable and other manufacturers; these are adjustable to any combination door size and hinge size. They consist of a metal or plastic guide fastened by a track to a long fence. The fence clamps to the jamb or door, then the guides are slid along the track and lock in place at the hinge locations.
The actual mortising is done by the moving the router, fitted with a bushing for the template which follows the guide, along the outline of the guide. Depth is also set by the template and router bushing. This method creates a mortise with corners rounded to the radius of the router bit. Hinges with round corners are available to fit this shape mortise, or the corners can be squared off with a hand chisel.
Photo by Jennifer Slot, Creative Commons Attribution License