Joints, Assembly and Construction Methods
This domain tests whether you can choose a joint that suits the load, the material and the production method, and then assemble it without introducing error. Examiners like proportion rules, glue-up sequence and movement allowances.
Joint families
- Butt joints need reinforcement: dowel, biscuit, domino or pocket screw.
- Halved joints for frames and crossing members; housing (dado) for shelves; rebate for backs and drawer bottoms.
- Mortise and tenon: through, haunched (door rails), stub (stopped), wedged. Strongest joint for frames.
- Dovetail: through, lapped, secret mitre. Slope 1:6 in hardwood, 1:8 in softwood; pins and tails cut from a single reference face.
- Mitre with spline or key for corners; loose tenon and domino for fast production work; cam-and-dowel or confirmat for knock-down carcases.
Proportions and fit
- Tenon thickness is about one third of the rail thickness: a 40 mm rail takes a 12–13 mm tenon. Shoulders must be square and the joint should slide together with light hand pressure.
- Dowel diameter is roughly one third to one half of the stock thickness; dowels spaced no more than about three diameters apart.
- Allow 0.1–0.2 mm clearance so glue is not scraped off; a joint that must be hammered in is too tight.
Construction methods
- Frame and panel: the panel floats in a groove, with 2 mm clearance all round. Solid panels must never be glued across the grain.
- Carcase work: 32 mm system for shelf pins (5 mm holes), back panel housed in a rebate, face frame or frameless construction.
- Table tops fixed with buttons or figure-eight fasteners that allow movement; breadboard ends pinned only at the centre.
- Drawers: dovetailed or dowelled boxes, bottom in a groove, runners or undermount slides set parallel.
Clamping and glue-up
Dry-fit first, then glue in the correct order (carcase before shelves, frame before panel), apply even pressure with cauls, and check diagonals before the glue sets.
Worked example
A frame opening is 600 × 400 mm; each side has a 10 mm groove and the panel needs 2 mm clearance per side. Panel width = 600 + (2 × 10) − (2 × 2) = 616 mm; height = 400 + 20 − 4 = 416 mm. Panel thickness should be about 1 mm less than the groove width so it can move. If the panel were cut flush and glued, it would split the frame as it shrank in a dry winter.
Traps that cost marks
Gluing end grain without sizing; over-clamping and starving the joint; gluing a solid panel all round; cutting the tails before establishing a reference face; assuming a mitre is strong without a spline.
State the joint, the dimension rule and the movement allowance - three marks in one sentence.