Threaded Inserts & Insert Nuts — Construction & Wood Threads Explained

September 29, 2025

An insert nut (also known as a threaded insert or screw-in bush) is a cylindrical fastener driven into wood or timber-based materials. It features a coarse external thread for a secure hold within the base material and a metric internal thread to allow machine screws, adjustable feet, or other hardware fittings to be mounted repeatedly and securely under load.

What is a threaded insert exactly?

When components in furniture construction need to be disassembled frequently or require high tensile strength, standard wood screws and furniture screws reach their physical limits. If a wood screw is screwed in and out of the same hole multiple times, the thread grinds down the wood fibres. The drill hole strips out and the screw loses its grip.

Here, the threaded insert acts like a steel anchor within the material. It is screwed once into a pre-drilled hole. The deep, coarse external thread cuts firmly into chipboard, MDF, or solid wood. On the inside, the component provides a standardised metric thread. This allows furniture parts to be bolted together tightly and detached as often as required without damaging the base material.

Types and variants

In professional furniture construction and joinery installation, several types of these furniture connectors are utilised:

Hex-drive insert nut: The most common design. It features an internal hexagon (Allen key drive) at the top, allowing it to be screwed flush into the timber.
Slotted insert nut: An older but still widely used design. It is installed using a wide flat-head screwdriver.
Pronged tee nut: This variant has no external thread, but instead features drive-in prongs. It is driven into the drill hole from the rear of the workpiece. When tensile load is applied to the bolt, the prongs draw deeper into the wood.
Materials: Most inserts are made from zinc-plated steel or brass. A threaded insert made of plastic is rarely used for load-bearing structures, but is found on lightweight fascias or specialised dowel systems.

Dimensions, standards, and typical values

The technical specification of a threaded insert is defined by its internal thread, length, and the required pilot hole diameter.

Internal thread: Typical metric sizes in furniture making are M4, M6, M8, and M10.
Lengths: These components are typically available in lengths of 10 mm - 25 mm (e.g. 12 mm, 15 mm, 20 mm). The length must be matched to the material thickness of the wooden board to prevent bursting through the surface.
Pilot hole diameter: This value is critical for holding strength. It depends on the core diameter of the external thread and the hardness of the timber. For a threaded insert with an M6 internal thread, a pilot hole of 8 mm is often drilled in soft chipboard, whilst 9 mm is used in hard solid wood (such as oak).

Application & installation

Insert nuts are utilised wherever metric bolts are required in timber constructions. A classic example is the installation of heavy furniture legs. These often feature a threaded stud (e.g. M8) that is screwed directly into the threaded insert in the base of the cabinet carcass. The inserts are also used when securing heavy handles where a standard furniture handle with simple through-bolts is insufficient.

Installation steps:

Pre-drill the pilot hole perfectly vertically.
Lightly countersink the hole edges to prevent the wood from splitting as the external thread is driven in.
Drive in the insert nut slowly and straight using the appropriate tool (usually a hex key) until it sits flush with the timber surface.
Should the hole be in a visible area and not later concealed by an attached fitting, self-adhesive wood cover caps can be used for visual concealment.

Related terms in furniture construction

Cam and dowel connectors: Another system for carcass joints consisting of a connecting bolt (often screwed into a small threaded insert) and a cam lock housing.
Machine screw: The metric counterpart driven into the insert nut.
Wood screw: A screw with a self-tapping thread for direct timber fastening without an insert. It is commonly driven with tool profiles such as PZ (e.g. wood screws PZ2) or TX.