Screw-In Dowels and Threaded Inserts: Different Types Explained
Screw-in dowels and threaded inserts are mechanical fittings anchored into wood or wood-based materials to create a robust internal metric thread. They allow components to be repeatedly assembled and dismantled without tearing the substrate material, which often happens with a standard wood screw.
What Exactly Are Threaded Inserts?
A threaded insert acts as an adapter between two different thread types in furniture construction. While the coarse external thread cuts securely into the pre-drilled substrate (such as chipboard, MDF, or solid wood), the metallic core provides a standardised metric thread (commonly M4, M6, or M8) for machine screws.
The principle is similar to a foundation in soft soil: the tensile and shear forces of the screw are distributed across the insert's external thread flanks over a significantly larger surface area in the timber, massively increasing pull-out resistance.
Types and Variants
In furniture fitting technology, various designs are differentiated depending on construction and assembly method:
- Screw-in inserts (often known as insert nuts): Feature an internal metric thread and a coarse external wood thread. Installation is typically driven by an allen key or a continuous slot.
- Pronged tee nuts: These are not screwed in, but knocked into a through-hole from the reverse side. Four metal prongs grip into the wood, preventing rotation when tightening the bolt.
- Screw-in dowels: In furniture assembly, this term usually refers to the connecting bolt for a cam lock fitting. It features a direct wood thread on one side and a distinctive threadless head on the other, designed to engage precisely with the cam lock housing.
- Expansion dowels: Plastic threaded inserts that expand and clamp inside the drill hole only when the screw is tightened.
Dimensions, Standards, and Typical Values
The sizing of screw-in dowels and inserts depends strictly on panel thickness and the expected static load.
- Drill hole diameter: The pre-drilling dimension depends on the core diameter of the insert. For an M6 insert, pre-drilling is typically done with 8 mm to 9 mm. In hardwoods, the drill diameter should be selected slightly larger (approx. 0.5 mm) than in softer board materials.
- Spacing dimensions for cam dowels: System 32 often dictates the lengths here. Common drilling distances from the panel edge to the centre of the cam housing require specific bolt lengths, typically screw-in dowel 24mm or screw-in dowel 34mm.
- Thread and shank: A standard size for the screw-in thread into chipboard is the screw-in dowel 8mm or 5 mm, matching standard furniture manufacturing drillings.
Usage & Selection
For static, permanent joints that are never disassembled after installation, traditional wood screws and furniture screws are generally sufficient. However, whenever a piece of furniture needs to be demountable (for instance, beds, large wardrobes, or detachable table legs), threaded inserts are essential.
If one were to repeatedly drive and remove a standard wood screws 4mm directly into chipboard, the drill hole would be ruined after just a few disassemblies and the screw would spin freely. Installing inserts strictly requires accurate perpendicular pre-drilling, as a skewed thread makes subsequent insertion of metric bolts impossible.
Related Terms
- Cam lock fitting: The cylindrical counterpart to the screw-in dowel that grips the bolt head and pulls the panels together.
- Chipboard screw: For direct screw connections without an additional insert, often corrosion-protected (e.g. zinc wood screws).
- Cover cap: Used to visually conceal exposed cam lock heads or unused drill holes (e.g. self-adhesive wood cover caps).