Sink EDM. 

Sink EDM uses specifically shaped electrodes which are “sunk” into the workpiece in an automated and controlled manner by thousands of electrical sparks per second, each of which melts away a tiny amount of workpiece material. Sinker EDM is inherently a 3D processes capable of producing blind cavities.

The process is an excellent problem-solving tool for features and configurations that are difficult or impossible to produce using conventional machining processes. Medical parts often demand such configurations, especially in optical and orthopedic surgical instruments and assemblies