A 2x4 LEGO brick from 1958 fits perfectly with one made today because LEGO maintains mold tolerances to 10 microns across billions of parts annually. The post explores how LEGO achieves this through material selection (ABS for predictable shrinkage), wire EDM mold machining, cavity tracking for traceability, and scientific molding process control. Key insights include: process stability beats tight mold tolerances alone, tolerance stack-up limits large assemblies, pigment color affects shrinkage causing issues like 'brittle brown,' and the 0.1-0.2mm interference fit is precisely tuned for child-friendly assembly force. The broader lesson for hardware engineers is to derive tolerance requirements from functional constraints rather than aspiration, and to build the entire manufacturing system around those requirements from day one.
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