The Principle
In the 1930s workshops of Canton, we didn't guess whether a joint would hold. We calculated it. A mortise-and-tenon fails not from lack of glue, but from shear stress exceeding the wood's parallel-to-grain strength.
Formula: τ = F / A
Where:
• τ = Shear stress (MPa)
• F = Applied force (Newtons)
• A = Tenon shear area (mm²) = width × length
Oak baseline: Red oak parallel-to-grain shear strength ≈ 9.0 MPa (Forest Products Lab, 1938).
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Why This Matters
I've seen young volunteers rush a cut because "it looks close enough." Close enough gets people killed. This tool forces you to measure twice.
In my day, we carved these joints for church floors, bridge trusses, and the frames of houses that would shelter generations. Today, the stakes are the same — only the materials change.
Safety Margin Rule: Any joint calculating > 7.2 MPa (80% of oak's limit) requires reinforcement. No exceptions.