Machining processes, especially milling, place several requirements on monitoring systems. Two prevalent ones are minimal implications towards the machine tool system and direct proximity to the machining zone to acquire high signal quality. Thereby, sensitive detection of even minor process alterations is enabled. This paper presents a suitable monitoring approach based on a sensory tool holder. An innovative wireless energy harvesting technology using spindle rotation is implemented. Obtained signal data for exemplary milling processes as well as corresponding signal analysis strategies are presented.