I'm referring to tacit knowledge - the nuances of a subject that are difficult or impossible to codify, express or transmit. As a machinist, I rely a great dea

I'm referring to tacit knowledge - the nuances of a subject that are difficult or impossible to codify, express or transmit. As a machinist, I rely a great deal on intuition. I can hear a bad cut across the room. I couldn't tell you exactly what it is that I'm listening for, but I can tell just by sound that a cutting insert is worn out or that the feeds and speeds aren't quite right. When I look at a drawing to plan out a sequence of cutting operations, I'm balancing dozens of parameters and thousands of subtle factors to achieve a suitable compromise between material removal rate, process stability, tool wear, energy consumption, precision and complexity. I can teach you all of the facts that I'm weighing up, but I can't fully explain all of the heuristics and instincts that guide that process of optimisation. Essentially all production machining is done with automated CNC machines, but most apprentices still start out learning on manually-operated machines. There are things you learn by standing at a machine and turning the handwheels that nobody knows how to teach.