Joint choice for a school chair side frame
A school workshop must produce 12 side frames for classroom chairs. Each frame has two 45 mm × 45 mm legs and two rails 70 mm × 25 mm, with the rail faces flush to the outside of the legs. The available equipment is a hollow-chisel mortiser, a router table, a dowelling jig and a bench drill; there is no domino machine. The chairs will be used by teenagers and must resist racking when tipped back on two legs. The timetable allows 35 minutes of machining and dry-fitting per frame, and the school wants no metal fasteners visible. A trial dowel joint with four 8 mm × 40 mm dowels in one rail pulled out after a drop test. The head of department asks for a joint that can be cut accurately by learners and glued with PVA.
1. Select a joint for the rail-to-leg connection and justify it against the dowel joint and a biscuit joint, taking account of the available equipment and the loading.
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Use a mortise-and-tenon joint, not dowels. The rail is 70 mm wide and 25 mm thick, so a tenon about 50 mm wide and 8 mm thick can be cut with the hollow-chisel mortiser and a router table. The tenon has mechanical interlock and a large glue area on the cheeks and shoulders; four 8 mm dowels have much less resistance to the racking moment because they act as short pins in end grain and can pull out. Biscuits are worse: they are short, compressed beech wafers with little continuous grain and are not designed for structural chair loads. The mortiser is available, and the 35-minute allowance is realistic if a plywood setting jig is made. Keep the joint dry-fit before glue so learners do not force a tight tenon and split the leg.
2. Specify the tenon and mortise dimensions, including a haunch, and explain why the haunch matters. Check that the joint fits within the 45 mm leg and the 25 mm rail.
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Tenon thickness 8 mm, about one third of the 25 mm rail. Tenon width 50 mm, leaving 10 mm shoulders each side of the 70 mm rail. Mortise depth 25 mm in the 45 mm leg, leaving 20 mm of sound wood behind. Because the rail face is flush to the outside of the leg, offset the tenon toward the outside and keep at least 10 mm of mortise wall on the inner side; mark the face side and face edge on both parts. Add a 10 mm long × 8 mm thick haunch at the top of the tenon. The haunch fills the end of the mortise, stops the rail twisting under racking, and increases the glued shoulder area. It also prevents the tenon from rotating when the chair is tipped back. Do not make the mortise wider than the tenon to ease assembly; that reduces glue strength.
3. Describe a dry-fit and glue-up check for one frame, and identify one common learner error that would cause early failure.
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Dry-fit each frame on a flat assembly table and measure both diagonals; they should be within 2 mm. Check that the shoulders close evenly with no gap over 0.2 mm and that the rail is not proud of the leg. During glue-up, apply PVA to the tenon cheeks and the mortise walls, not just the end grain, assemble within the open time, and clamp across the joint, not along the rail. A common learner error is to cut the mortise too close to the leg face, leaving a thin wall that splits when the clamp is tightened; another is to rely on the haunch as a stop and leave the shoulder open. After curing, rack-test one frame before making all 12.