Knife hinge

Overview

A knife hinge has two thin blades that pivot on a pin where they cross, like a pair of scissors. One blade is let into the door edge, the other into the carcass top or bottom. Because the pivot is at the edge, the hinge is almost invisible when the door is closed, leaving a thin line. It is used on small, high-quality inset doors where the hardware should not show.

Types and sizes

• Surface-mount and mortise versions exist; the mortise type is described here.
• Offset versions swing the door clear of a frame stile that projects.
• Size is blade length, about 1-1/4 to 2 in (30 to 50 mm), and thickness about 1.5 to 3 mm (1/16 to 1/8 in).
• Brass blades match antique pieces; steel is stronger and cheaper.
• Always bought as a pair for a door.

Where it is used

Not named in the style guides; it appears on lightweight inset doors on small cabinets and display cases in a range of formal styles. In the library there is no catalogue item for it.

Choosing and sizing

Choose it for a door under about 8 kg (18 lb) and about 600 mm (24 in) high. It will not carry a heavy door, because the weight hangs from two thin blades fixed into end grain at top and bottom. For a heavier door use Butt hinge or European concealed hinge.

Installation

1. Cut the door slightly narrower than the opening; the hinge needs a gap of about 1.5 mm (1/16 in) at the pivot side.
2. Mark and cut the blade mortises into the door's top and bottom edges: depth equals blade thickness, width equals blade width.
3. Mark the matching mortises in the carcass top and bottom, with the pivot point directly over the door's hinge-edge line.
4. Drill pilot holes (2 to 2.5 mm, 5/64 to 3/32 in) before the screws; edge and end-grain holes hold poorly.
5. Set the door on the lower blade first, lower the top blade over the pin, check the swing, then tighten.

Wood movement and species

The blades are fixed to the door's end grain at top and bottom, which has the weakest holding for screws, so the door's width movement is acceptable but the fixings are not. Use a dense wood or drive the screws into a hardwood plug or insert glued into the end grain. Brass in oily woods like Teak is fine; steel in the oaks stains.

Common mistakes

• Door binds: pivot not aligned with the door edge, or door too tight at the swing side.
• Screws pull out of end grain: use shorter, thinner screws set in side grain wherever possible.
• Heavy door sags: switch hinge type.

Related

• Butt hinge: the visible, stronger alternative.
• Pivot hinge: another way to hang a door without a side hinge.
• Wood screw: pilot-hole sizes.