In-situ assessment of machinability and chip formation through the dynamic contact electrical resistance measurement method (DyCERM)

• A novel monitoring method measures tool-workpiece contact during milling. • DyCERM tracks chip formation, tool wear, chatter, and surface quality. • A 15% error was observed in shear band spacing predictions from chip analysis. • Tool wear increases tooth contact time by up to 62% during milling. • DyCERM offers low-cost, real-time monitoring for industrial machining needs. This work introduces a novel methodology for high-precision online monitoring of the tool-workpiece interaction in milling operations. The proposed Dynamic Contact Electrical Resistance Measurement (DyCERM) method for machining monitoring, combines user-friendly software with cost-effective hardware to measure the voltage drop across the dynamic tool-workpiece contact interface. Theoretically, DyCERM can be used to monitor milling operations that are applied to conductive materials. The system features high-acquisition rates (2 MHz), enabling the precise monitoring of fundamental features, including tool wear, chatter, surface waviness, workpiece material, cutting speed, and depth of cut. Notably, this study introduces an innovative capability to analyse the chip formation process, allowing accurate measurement of the distance between shear bands within the generated chip, an unprecedented feature in machining process monitoring technologies. The results revealed strong correlation between chip formation patterns, theoretical predictions and signal data, achieving a maximum error of 15 % in shear band spacing analysis. The system reliably detects tool wear and chatter, indicated by increased tool contact time and confirmed by higher surface roughness under unstable conditions. When applied to additively manufactured parts, DyCERM identifies unmachined areas due to surface waviness, signalling the need for additional milling. Sensitivity to cutting speed variations was demonstrated, especially at high spindle speeds up to 20,000 rev/min, where increased speeds reduced tooth contact time deviations in ductile materials, aligning with machinability indicators. Overall, DyCERM offers a versatile, non-invasive, and cost-effective solution that advances traditional monitoring techniques by delivering accessible, high-precision online diagnostics for demanding industrial machining applications.