Joints and construction: carcass, frame, dowel, mortise-and-tenon and sheet-material work
What the exam tests
Choosing a joint that suits the material, load and appearance, and setting it out so it assembles square and strong.
Core rules
- Carcass joints: butt (weak, needs fixings), rebate, housing/dado, dovetail, dowel, biscuit and cam/confirmat. Dovetails resist pull-out; housings resist racking.
- Frame joints: mortise and tenon, haunched tenon for a housed or narrowed stile, stub tenon for glazing bars, bridle and dowel. Tenon thickness is about one-third of the rail thickness.
- Dowel joints need accurate hole centres, adequate depth and glue spread; dowels align and reinforce but do not replace a shoulder.
- Sheet materials: biscuits, dominoes, confirmats and cam fittings suit manufactured boards; edges of MDF and particleboard are weak, so use a rebate, loose tenon or special fittings.
- Allow movement: solid wood panels in frames must float; sheet goods need expansion gaps at hidden edges.
- Dry-fit, check square and mark faces before gluing.
Worked example
A table apron into a leg: rail 80 mm × 25 mm. Tenon thickness ≈ 8 mm, width about 50 mm, with a haunch 10 mm deep if the mortise would otherwise break out. Cut the mortise first, then fit the tenon to it. The reasoning: the tenon must be thick enough to resist bending but leave enough cheek material on the leg; the haunch protects the mortise edge.
Traps
Making a tenon too thick and weakening the stile; gluing end grain without a sized or reinforced joint; trapping a solid panel so it splits; assuming biscuits give structural strength; cutting joints without a face-side mark.