Abstract: At 9:25 a.m. on February 14, 2020, an employee was working for a firm that manufactured high-precision, complex components and assemblies for the aerospace industry. He worked in machinery maintenance. He was troubleshooting the tool changer on a 188C Niigata SPN50 50 CNC horizontal mill machine work center. This was a numerical controlled horizontal mill machine center. As the employee was inside the machine with the door closed, the tool changer cycled. The tool-changer arm struck the employee in the chest. The employee suffered a fractured sternum, two fractured ribs, and small fractures to two bones on his spine (vertebrae). He was hospitalized for two or more days. There was a sensor that was supposed to have stopped the tool-changer arm, but the employee had moved it. The relocation of the sensor allowed the tool-changer cycle to complete and the tool-changer arm to strike the employee in the chest. There was one eyewitness to the incident.