Carbon or glass fibre reinforced polymer composites are widely used, notably for lightweight structures, due to their interesting material properties. However, the machining of composites represents a challenge. Their excessive abrasiveness leads to tool wear, which can put workpiece material integrity at risk. In the case of lightweight structure, the monitoring of trimming operation is difficult due to vibrations of the flexible workpiece and scrap offcut. This paper presents a monitoring approach based on vibration and power signals that is compatible with the machining of lightweight composite structure. Tap tests are performed to analyze the experimental set-up and the machining tests. Experiments validate the approach with detections of tool breakage and chatter occurrence.