Screw-In Threaded Inserts vs. Tee Nuts: Threaded Inserts in Furniture Construction

A screw-in threaded insert features a coarse external thread and is screwed into a pre-drilled hole, whilst a tee nut has prongs that are secured into the timber from the rear of the workpiece using a hammer. Both fittings are designed to create resilient metric internal threads in engineered wood and timber.

What are threaded inserts in furniture construction?

Conventional wood screws and furniture screws cut their thread directly into the material. If such a joint is loosened and refastened repeatedly, the timber wears away and the screw loses its grip. To prevent this, a threaded insert is used. It provides a robust metal internal thread (typically zinc-plated steel or brass) into which metric machine screws can be driven and removed as often as required.

The screw-in threaded insert (insert nut): technology and installation

The screw-in threaded insert (often referred to in joinery as an insert nut) is a cylindrical fitting featuring a metric internal thread and a coarse, self-tapping external thread. It works similarly to an expansion plug in masonry, yet forms a positive mechanical interlock in timber.

Installation is usually carried out in a blind hole. The drilled hole must match the core diameter of the external thread precisely. The insert is then screwed straight into the wood using an internal hexagon (hex socket) or slot with an Allen key or screwdriver. The external thread bites directly into the chipboard, MDF, or solid timber, providing an exceptionally secure hold.

The tee nut: technology and installation

Tee nuts do not have an external thread. They consist of a cylindrical barrel with an internal thread and a flat flange equipped with three or four sharp prongs.

A through-hole is mandatory for installation. The hole diameter corresponds to the outer diameter of the barrel. The nut is inserted into the hole from the rear of the workpiece and driven in with a hammer until the prongs are fully flush with the timber. The crucial technical factor: the load applied by the screw must act under tension. As the screw is tightened, it pulls the flange of the nut even tighter against the timber.

Dimensions, standards, and typical values

Threaded inserts are classified according to their internal metric thread. Standard dimensions in furniture making include:

• M4 / M5: For lightweight connections, handles, or small hardware fittings.
• M6: The standard for medium loads.
• M8 / M10: For heavy static loads, typically for securing table legs, furniture feet, or castors.

The lengths of these inserts usually vary between 8 mm and 25 mm depending on board thickness. Important: standard wood screws, such as 4mm wood screws, must never be used with these nuts; only metric threaded machine screws are suitable.

Application and selection: which fitting to choose?

The choice between the two options depends on accessibility, material thickness, and load direction.

• Screw-in inserts are chosen when the reverse side of the component remains visible or the material is very thick (e.g. legs on a wooden desk). They are ideal for concealed joints because they fit neatly into blind holes.
• Tee nuts are used where high tensile forces occur and the rear side of the timber is concealed (e.g. upholstery frames, climbing walls, or structural frames). If the through-hole is visible on the reverse side of visible furniture, it is often concealed using self-adhesive screw cover caps for wood.

For example, when assembling a dark wood chest of drawers and fitting height-adjustable plinth feet, a screw-in insert in the base panel is the professional choice, as it does not break through the visible upper surface of the bottom board.

Related terms

• Machine screw: A screw with a metric thread that strictly requires a matching mating thread (nut or insert).
• Blind hole: A drilled hole that does not penetrate completely through the workpiece.
• Through-hole: A hole drilled all the way through the material.
• Furniture foot thread: Usually an M8 or M10 threaded stud screwed directly into a threaded insert.