Woodworking Joints and Carpentry Techniques
The job of a joint
A joint must locate the parts, carry load, provide glue area and allow for seasonal movement. End grain glues poorly, so butt joints are always reinforced - dowel, biscuit, plate or a proper joint.
Box and frame joints
Half-lap - two members reduced to half thickness; strong in compression, used for crossing frames. Mortise and tenon - through, blind (stopped), haunched, fox-wedged. Standard proportion: tenon thickness = one-third of the rail thickness, mortise the same width, tenon length up to five times its thickness. A haunch is used where the mortise sits close to the end of a stile (door frames): it stops the stile twisting and fills the groove. Bridle joints for three-way frames. Dovetail - through, lapped, secret-mitre. Tails and pins; angle roughly 1:8 in hardwood, 1:6 in softwood. Tails go on the member that resists the pulling load, e.g. drawer sides. Housing/dado for shelves, rebate for backs and glass, tongue and groove for solid tops and floors where movement must be accommodated, mitre for frames (reinforce with spline or key), scarf to lengthen, dowel, biscuit and domino for modern work.
Worked example
A 30 mm thick table apron meets a 45 mm leg. Tenon thickness = 30/3 = 10 mm; cut a haunch because the mortise is near the top of the leg; dry-fit, check diagonals, then glue and cramp.
Traps
A tenon too thick weakens the stile; dovetail angle cut the wrong way; a stopped mortise left with rounded corners; a mitre left unreinforced; forgetting to check the frame for square before the glue sets.