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They are a sure sign of good workmanship, but they are almost invisible.
Small and discreet, the knife hinges give a neat and refined look to your project. They are ideal for a small closet if you want the hinge hardware to virtually disappear.
Knife hinges are also a hallmark of good craftsmanship. Accurately arranging their mortises requires patience and a steady hand. You’ll be using some classic hand tools, such as a marking knife, a marking gauge, and some sharp chisels. There are no shortcuts and practically no room for adjustment once the mortises are cut.
Don’t be scared, though. If you follow the steps outlined below, you can’t really go wrong.
There are two types of knife hinges: straight and offset. Straight hinges are used for overlapping doors. Offset hinges allow a door to swing farther than straight hinges, so they are used for cabinets with inserted doors. I’ll show you how to install the offset hinges.
Before it starts
All knife hinges consist of two parts, or “leaves”, that are easy to separate. One door is recessed in the upper and lower edges of the wardrobe door; the coupling leaf is recessed in the cabinet itself. It is extremely difficult to cut the mortises in the cabinet after the cabinet has been assembled. The best solution is to keep the cabinet gluing at bay until you have completed all the grouting.
Start by temporarily locking the cabinet. Cut the door so that it fits snugly into the opening.
Next, determine the size of the space you want around the door. This is not an arbitrary measure: it is determined by the space between the two leaves of the hinges, which is the thickness of the washer between the leaves. Make shims of the same thickness as the washer. (I use a stack of cut-out playing cards. The washers on the hinges were three cards thick.)
Carefully cut the door smaller, ending up with blanks of the same size on all four sides. I use a plane when I approach the final size to avoid removing too much wood.
First the door to insert
The most accurate way to arrange mortises is with a marking knife and a marking gauge. By making shallow grooves in the wood, these tools allow you to positively register a chisel in a way that a pencil line cannot.
We will start with the door mortises, because the thickness of the door determines the position of the hinge. Clamp the door with a vice at a comfortable working height. Place a hinge leaf on the top or bottom edge of the door, so that the hinge is flush with the edge of the door. Using a marking knife, mark the end of the hinge with a short, shallow cut (Photo 1). Locate the knife in the cut, slide a small square against the blade and draw a line all the way down.
Adjust a mortise gauge to the width of the hinge (Photo 2; see Sources, page 67). You can also use a single pin or single wheel marking indicator, but you will need to readjust it for each side of the mortise.
Adjust the head of the gauge so that the mortise is centered on the door. Test the setting on a scrap piece the same thickness as your door. When you have the right setting, write the long sides of the mortise (Photo 3).
Hold the hinge back in place and mark its offset arm (Photo 4). As before, make a shortcut first, then draw the line using a square.
The best way to remove most of the waste inside the mortise is to use a 1/8-inch drill bit in a laminate trimmer or other small router (see Sources). You could cut the mortise with a chisel, but it’s risky. Mortise walls are generally very thin and could easily come off. The depth of the mortise should exactly match the thickness of a hinge door. To set the depth of cut for the tip, flip the cutter over, place a hinge leaf on the base of the cutter and slide the leaf against the tipPhoto 5).
Rout the mortise (Photo 6). Secure the tabs on both sides of the door to prevent the router from swinging. Extend the planks about 1 inch beyond the edge of the door to make sure the router is stable before you start cutting. Freehand, staying about 1/32 of an inch inside the lines. You will find that a 1/8 ”bit is very easy to control. Don’t push too hard, though; this piece is fragile.
Clean up the mortise by gradually cutting the layout lines. Use a large chisel on the long sides to get straight, sharp edges (Photo 7). I use a 1-1 / 4 “chisel.
The second mortise cabinet
The cabinet is still fastened together, right? The first step to marking the cabinet mortises is the same as for marking the door mortises – start with one end of the hinge. Here, however, the hinge will not be flush with the side of the cabinet, because you have to leave a gap between the door and the cabinet. On my cabinet, the gap is the thickness of three playing cards (the thickness of the washer between the hinge leaves). Place the cards, or whatever thickness you are using, between the hinge and the side of the cabinet (Photo 8). Then mark the opposite end of the hinge, initially low and short, followed by a square line.
If the door is set back from the top and bottom of the cabinet, use a square to measure this distance (Photo 9). (If your door is flush with the top, bottom, and sides of the cabinet, skip this step.) Slide the mortise indicator head further away from the pins by this distance. Check your new setting on the scrap to make sure it is correct.
Now you can remove the clamps and disassemble the cabinet. Write the long sides of the mortise on the top and bottom of the cabinet (Photo 10). Lay the hinge in place, then mark its opposite end and offset arm. Write these lines, as before, using a square knife and marker. Destroy the mortises. Clean them using a chisel (Photo 11 ).
Next, set the hinge leaves into the cabinet mortises and drill pilot holes for the screws (Photo 12). For this operation I use a self-centering drill to make sure that the holes are perfectly aligned. It’s not that critical for cabinet mortises, because the hinge is trapped, but the centered holes are key for door mortises, where the hinge could slide.
If using brass screws, first “thread” the pilot holes with steel screws of the same size, then install the brass screws. Use a screwdriver, not a drill / driver, to avoid removing or breaking the screws.
You may want to reassemble the cabinet one more time, with clamps only, to make sure the door locks properly. (Or if you feel safe, glue it on!) Reassemble the hinges by sliding the door leaves over the pivots of the cabinet doors (Photo 13).
Lay the cabinet on the back and slide the door onto the hinges (Photo 14). You may need help here, because you can’t see both hinges at the same time. Once the door is in place, drill pilot holes for the screws, then install the screws (Photo 15).
Lift the cabinet and check that the door fits and the gaps. Make the necessary adjustments using a plane or sanding block.
• Marking indicator in ebony [Rockler]
• Freud 1/8 “dia. Double fluted straight tip [Amazon]
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