High-tolerance prototype machining is a game of microns, where a single thermal expansion can scrap a $5,000 part. This article reveals a counterintuitive strategy—controlling the machine’s coolant temperature, not just the part—that reduced our prototype rejection rate by 40%. Backed by a detailed case study from a medical device project, you’ll learn how to preempt material instability and achieve tolerances of ±2 microns consistently.