Jointing, carcass and frame construction
Jointing, carcass and frame construction
This domain tests joint selection and construction of boxes and frames that are strong, square and fit for purpose.
Core rules
- Joint strength depends on glue area, fit and mechanical interlock. End grain glues poorly; long grain glues well.
- Common joints: butt (weak, needs fasteners), halving (frames), housing (shelves), mortise and tenon (frames and legs), dowel (cheap alignment), biscuit (edge joints), dovetail (drawers and boxes).
- Carcass: sides, top, bottom, back and shelves. Use housings, dados or biscuits for shelves; check diagonals to square; fit a back to resist racking.
- Frame: rails and stiles, mortise and tenon or dowel; haunch tenons resist twisting; shoulders give a clean fit.
- Dry-fit before glue, mark mating parts, and clamp evenly without crushing.
Worked example
A dining table needs a rail joined to a 70 mm square leg. Choose a mortise-and-tenon joint: the tenon gives long-grain glue surfaces and mechanical resistance to racking. Mark the mortise from the rail's face side, keep it centred, and cut the tenon with a shoulder. Dry-fit, then glue and clamp. Add a draw-bore peg or corner block if the design allows. Reasoning: a butt joint would rely on screws and end grain; the tenon spreads load into the leg.
Traps
Gluing end grain without sizing; making joints too tight and starving the glue; over-clamping; forgetting to check diagonals; cutting the back after assembly; ignoring grain direction when choosing rails.