When a single micron of error on a plastic component can mean the difference between a successful surgical outcome and a catastrophic failure, the machining process must be flawless. Drawing from a decade of experience machining critical components for Class III medical devices, this article reveals the hidden challenges of plastic instability, the data-driven strategies to overcome them, and a real-world case study where we slashed rejection rates from 18% to 0.3% on a complex implant delivery system.