Skills · skill
Mortise and Tenon Joint
Making · Intermediate
- Difficulty
- Intermediate
- Safety
- Chisels are sharp and slip: clamp the work, keep the free hand behind the edge. Mallet blows off the chisel end.
- Done When
- It goes together by hand with light friction, the shoulders close tight all round with no gap, and it stays square under clamping.
- Tools
- Marking gauge, square, marking knife, mortise chisel, mallet, tenon saw, shoulder plane, PVA glue, clamps.
- Category
- Making
- Summary
- The oldest strong wood joint: a tongue on one piece fitting a slot in the other, glued and sometimes pegged — chairs, doors, timber frames.
Steps
Mark the mortise from the tenon stock: a third of its thickness, gauged from the same face on both pieces.
Chop the mortise: chisel bevel-toward-the-waste, levering out chips, working from the middle to the ends.
Saw the tenon cheeks just outside the line, then the shoulders.
Pare the tenon to a snug hand-push fit — never a hammer fit, that splits the mortise.
Dry-fit, check square, glue the cheeks, clamp, check square again.
Peg through if it will take load in tension.
The Science
Wood is strong along the grain and weak across it, and it moves with humidity across the grain but hardly at all along it. The joint works because the tenon's long-grain faces glue to the mortise's long-grain walls; end grain glues badly. Sizing the tenon a third of the stock thickness balances the strength of the tenon against the walls left either side of the mortise.
How It Fails
A loose joint from sawing on the wrong side of the line — glue does not fill gaps. A split mortise from a fat tenon. Shoulders that do not close because the tenon bottoms out first.
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