Threaded Inserts & Screw-in Dowels: Definition & Guide to Threaded Bushings

A threaded insert (often referred to as a Rampa insert or threaded bushing) is a cylindrical connector featuring a coarse external thread for timber and a metric internal thread for machine screws. It is used to create a heavy-duty, reusable screw connection in soft or porous base materials such as chipboard or softwood.

What is a threaded insert exactly?

When furniture components must be dismantled frequently or need to withstand high tensile forces, conventional wood screws and furniture screws reach their limits. The thread cuts directly into the wood; repeatedly screwing and unscrewing shears the material, causing the screw to lose its grip.

A threaded insert resolves this problem by dividing the load transmission into two parts. Much like a deep foundation in soft ground, the coarse and wide external thread of the insert distributes the tensile forces over a significantly larger surface area within the wood. Internally, the component provides a standardised metric internal thread (such as M6 or M8), into which a standard machine screw can be inserted and removed as often as required without wearing down the base material. The term Rampa insert has become widely used in carpentry as a generic term for these fittings, though it originally stems from a specific brand name.

Types and variants

Threaded inserts differ primarily by their material, drive type, and collar:

• Material: For high loads in furniture construction, inserts made of galvanised steel or die-cast zinc are used. The latter are corrosion-resistant and provide a clean thread guide.
• Drive: Installation is usually carried out via a hex socket (Allen key) or a continuous slot on the top face. The hex socket provides superior force transmission and prevents the tool from slipping during insertion.
• Collar: Inserts with a flush collar sit level with the timber surface and prevent the insert from being driven too deep. Collarless variants can be countersunk completely beneath the material surface.

A specific subcategory consists of screw-in dowels. These often feature a wood thread on one end and a dedicated head for cam connectors on the other. They are used as standard in industrial furniture manufacturing to join carcass elements at right angles.

Typical dimensions and standards

When selecting fittings, two dimensions are crucial: the metric internal thread and the overall length of the component.

• Internal thread: Common sizes are M4, M5, M6, M8, and M10.
• Length: The screw-in depth typically varies between 10 mm - 30 mm.
• Screw-in dowels for cam locks: These feature standardised drilling dimensions. A screw-in dowel 24mm or a screw-in dowel 28mm refers to the dimension from the wood surface to the centre of the cam lock housing (clamping dimension).

Pre-drilling is strictly necessary. The drill diameter depends on the core diameter of the external thread and the hardness of the timber. For chipboard or softwood, a hole matching the exact core diameter is generally drilled, whereas for hardwood it should be 0.5 mm larger.

Application and installation

Threaded inserts are fitted wherever high pull-out resistance is required. Typical applications include attaching table legs, mounting height-adjustable furniture feet, or suspending heavy cabinet elements.

Installation steps:

1. Drill the hole precisely perpendicular.
2. Chamfer (countersink) the edge of the drilled hole slightly so the timber does not split during insertion.
3. Drive in the threaded insert using the appropriate hex key.

If the screw heads of the metric screws need to be concealed on visible joints afterwards, self-adhesive cover caps for wood are used. If dismantling is not planned and components are to be fixed permanently, fitters often opt directly for classic, visually matching screws such as black wood screws with a suitable drive, for instance wood screws PZ2.

Related terms

• Pronged tee nut: An alternative to the screw-in bushing. It is driven from the reverse side into a through-hole and grips into the timber with prongs.
• Cam connector: A two-part joining fitting (such as Minifix) that is frequently paired with a screw-in dowel.
• Machine screw: A screw with a metric thread and blunt end that is driven into the threaded insert.
• System 32: An international standard for hole line patterns in furniture construction with a pitch of 32 mm, to which many connecting fittings are standardised.