Installing Straight Knife Hinges
There are two main types of knife hinges; straight and offset. Straight knife hinges are usually used when the top and bottom project beyond the two gables, and when the door will overlay the gables. Offset knife hinges are used when the front edge of the top, bottom and gables are all inline, and the door will be inset. The cabinet in this issue calls for straight knife hinges.
I would recommend mocking-up the joint, whether this is the first time or 20th time you’ve installed knife hinges. Once routed, the mortise is very hard to conceal if you’ve made a mistake. Here you can see the dry-assembled cabinet in the background and the mock-up in the foreground.
Mark the Gable
Use a fine pencil to mark the gable location before taking the cabinet apart. If you can’t get a fine enough pencil to accurately mark the location, it might be a good idea to mark the location of the hinge while the gable is still in place.
Locate the Hinge Mortise
With the cabinet apart, mark a gap line 1/16" in front of the gable end. Then add another line the width of the door away from the gap line. Centered between those two lines is the hinge mortise, which can be marked now. The next step in layout is to mark the location of the rounded end. The center of the pivot point should be centered over the outer edge of the gable. With that marked you can place the hinge over the layout lines to mark the straight end of the hinge.
Set the Depth
If the surface in which you’re routing the mortise has been rough-sanded, set the router bit’s depth of cut to exactly that of the thickness of the hinge. If you’re using solid wood, and it hasn’t been rough-sanded, I would aim to set the bit about 1/64" deeper, as you will sand the wood down afterwards.
Once the mortise is routed you can square up one end with a chisel. You might want to sneak up on a perfect fit, as it’s hard to glue wood back on to fill a gap at the end of the hinge.
In theory, the thickness of the spacer on each knife hinge should be equal to the thickness of the gap between the doors and the cabinet. Brown erred on the wide side, as a gap that’s slightly too small will cause the door to bind, while a slightly oversized gap will not be noticeable under the overhanging top.
With the pivot pin hole overhanging the edge of the door by half you should mark where the straight end of the hinge is located.
Rout the Mortise
Once the support block is clamped to the side of the door, and the edge guide is set to the correct distance, Brown routs the hinge mortise in the doors.
Use a chisel to square up the end of the routed mortise.
Hang the Door
Once the cabinet hinges have been installed Brown places the door hinge leaves on the cabinet hinge pivots and slides the door into place over the hinges.
A Slight Shift
If the doors are out of alignment one way to adjust them is to shift the hinges slightly. In this photo Brown is shifting the hinge towards the outside of the door, and will install the second screw to secure the hinge. With it in place Brown adds the other screw. The old hole sometimes has to be filled, and redrilled, before installing the final screw.
Hand Tools vs. Power Tools
I have never even considered making knife hinge mortises with hand tools, but that doesn’t mean it’s impossible. As long as you have a straight router bit the same diameter as the width of the hinge leaf, the final fit should be perfect with one pass. Even using a bit that is narrower than the hinge leaf and making two passes works out okay. Depth of cut is also constant. If you use hand tools, there is a greater chance of breaking out an edge, as the mortises are typically quite close to the edge of a workpiece. This is especially true when creating the mortises on the doors. The bottom line is that unless you’re a wizard at sharpening and using hand tools, I’d recommend using a small router for this operation.
Because improperly routed knife hinge mortises are very hard to hide, I always do a quick mock-up of the joint. Often you want doors on one cabinet to act differently than doors on another cabinet, so this is the time to play around with slightly different locations to ensure you have the final pivot action you want. In addition to making a mock-up, it’s crucial you plan ahead and cut the hinge mortises before gluing up the cabinet.
Mark the cabinet mortises
With the cabinet assembled, but not glued, mark the locations of the front of the gables then unassemble the cabinet. Next, mark a 1/16″ gap in front of the gable. This is the gap between the back of the door and the front of the gable. Moving further towards the front of the top and bottom, mark the width of the door. The next step is to mark the location of the knife hinge with it centered on the door. Position one half of the knife between these lines, and move it left or right until the center of the pivot point is centered on an imaginary line that extends forward from the outer edge of the gable. Mark the outline of the hinge now.
Get ready to rout
With a straight bit of the exact width of the hinge chucked in your router, install and position an edge guide to the correct dimension from the inner cutting edge of the bit. It’s easiest to use a plunge bit, but if all you have is a fixed-base router, a steady hand will help the process go fairly well. Set the depth of the bit to cut the exact depth as the thickness of the hinge.
Now the scary part – rout the mortise. Though you can use stop blocks, the easiest and quickest approach is to ensure you have good lighting and stop when the bit is flush with the pencil mark. You can stop 1/32″ earlier on the straight end (towards the center of the cabinet) if you want some insurance, but I would strongly encourage you to run the spinning router bit to the exact point on the curved end, as a perfect arc will be left for the hinge to fit against. Repeat this process for all the mortises. Square up the one end of the mortise and test the fit of the hinge.
Now the doors
At this point I usually apply a finish to the cabinet and assemble it so the door opening is square, and set in stone. Once the glue has dried, I focus on the mortises in the doors. I like to keep the width of my doors as large as possible, so I can make any trim cuts after the doors have been installed. The height of the doors has to be trimmed to fit the opening with proper gaps on top and bottom. Aim to have the size of the gaps equal to the thickness of the spacers on the hinges you’re using. In practice the door can be a bit narrow, but not too wide, or it will not fit in the opening. The width of the door will be trimmed to final dimension after the doors have been installed and tested.
These mortises should be centered on the side edge of the door, so set your router’s edge guide to the correct measurement now. The depth of the bit will remain the same.
With the doors clamped in your workbench, lay out the mortise location. Again, the center of the pivot point should be centered over the outer edge of the door.
If you were to run your router along the narrow edge of your door, you would likely tip side to side as you cut the mortise. To prevent this from happening, clamp a piece of straight stock to the back of the door, perfectly flush with the door’s edge. It will help support the router during the cut.
With good lighting, and the edge guide referencing off the front of the door, cut the mortise. Repeat the process for all the other mortises. Square the round ends up with a chisel.
Pre-drill one hole in each hinge then attach each hinge with a screw. You’ll probably want to use brass screws when the piece of furniture is complete, but for now stick to anything but brass, as they are very soft and their heads will snap off. It’s very frustrating trying to remove a broken screw.
There are two ways to install the door; both are very similar, but which one you use will depend on how tall the doors are. For a tall door, screw the two cabinet hinge leaves in the top and bottom of the cabinet, and screw the upper door hinge leaf to the upper edge of the door. Place the final lower door leaf on its mate, which is installed to the cabinet. With the door as vertical as possible, place the pivot hole in the upper hinge over the pivot pin in the cabinet leaf, then slightly rotate the door onto the extended lower hinge leaf. Add a screw to the lower door leaf and it’s time to test the fit. The one adjustment you must make if the door is not tall is as follows. Don’t install the upper leaf on the top of the door. Just position the lower and upper door leaves on their mates. Hopefully the upper one will not fall off. Now slide both the bottom and top door leaves into the mortises at the same time, then screw them in place.
Close the door to realize a perfect gap all around the piece. If, by some chance, this isn’t the case, you can trim the doors to fit perfectly. If the doors are out far enough that you don’t have enough material to remove, you should shift the hinges in or out slightly. Loosen the single screw in one or more of the hinges and figure out which door needs to be shifted in which direction. At this point you may have to remove a bit of wood with a chisel so the hinge can sit further under a door, or you may reveal a slight gap at the end of a hinge. Both of these situations are preferable to having ugly, uneven gaps around the doors. Since only one screw was installed on each hinge, you can adjust the hinge slightly and then drill another pilot hole and install the other screw. When the gaps are even you can install any remaining screws. A trim of the mating edges of the doors can now be done to give you an even gap all around.
Rob is a studio furniture maker and the editor at Canadian Woodworking & Home Improvement.