Adhesives and fastenings for timber: HSC Industrial Technology Timber Products and Furniture Technologies

What this dot point is asking

Furniture only holds together if its joints do. NESA expects you to know the common adhesives and mechanical fastenings used in timber work, their properties and limitations, and how to select the right one for a given component and service condition. You should be able to explain why a glued mortise and tenon suits a chair rail while a knock-down fitting suits a flat-pack cabinet. This decision-making appears in the written paper and is judged in the construction quality of your Major Project.

Adhesives

A good adhesive joint can be stronger than the surrounding timber because it spreads load over the whole glued surface and leaves no visible fixing. The common types are:

• PVA (polyvinyl acetate): the everyday white or yellow woodworking glue. Easy to use, strong on clean, well-clamped joints, but it creeps under sustained load and standard grades are not waterproof. Cross-linking PVA grades resist moisture better.
• Urea formaldehyde: a strong, rigid, water-resistant adhesive supplied as a powder or two parts, used for veneering, laminating and structural joints.
• Resorcinol formaldehyde: fully waterproof and very strong, used for exterior and marine work, recognisable by its dark glue line.
• Epoxy: a two-part adhesive that bonds dissimilar materials and fills gaps, useful where joints are not perfectly mating.
• Contact adhesive: coated on both faces and pressed together for an instant bond, used to lay laminates and large veneers.

When you select an adhesive, match its strength, water resistance, gap-filling ability, open time and clamping needs to the joint and its service environment.

Mechanical fastenings

Mechanical fastenings hold components together without relying on glue, and many can be undone:

• Nails: fast and cheap, used for framing, fixing backs and bottoms, and holding glued joints while they cure. They rely on friction and pull out under load.
• Screws: much stronger holding than nails and removable, used for hinges, fittings and components that may need servicing. Pilot holes prevent splitting.
• Dowels: glued wooden pins that align and reinforce joints, common in carcase and frame construction.
• Biscuits: thin compressed beech ovals glued into slots cut by a biscuit jointer; they align panels and add strength to edge and carcase joints.
• Knock-down (KD) fittings: cam locks, screw inserts, bolts and corner blocks designed for flat-pack furniture that the user assembles and can dismantle.

Combining glue and fastenings

Many strong joints use both. The glue carries the long-term load while a nail, screw or clamp holds the parts in register until it cures. A glued and dowelled joint, or a glued biscuit joint, gains both the wide bonding area of the adhesive and the alignment and clamping of the mechanical element.

Selecting the right method

Choose by asking what the joint must do. For a structural frame joint that must resist racking, a glued mortise and tenon or dowelled joint is best. For a cabinet that ships flat and is assembled by the customer, knock-down fittings are essential. For a laminate worktop, contact adhesive bonds the surface. For an outdoor item, use a waterproof adhesive and corrosion-resistant fastenings. Always state the service conditions, then justify the strength, demountability and appearance of your chosen method.