<p>Precisely discerning the material separation criterion in micro-machining remains challenging yet crucial for accurately predicting cutting forces by accounting for shearing and ploughing effects. This study introduces a novel model, the instantaneous uncut chip thickness (IUCT), to enhance the accuracy of cutting force prediction in micro-milling processes. The model quantitatively integrates instantaneous shearing thickness (IST) and instantaneous ploughing thickness (IPT). The critical determinants of shearing and ploughing effects rely on the material separation point, modeled using the dead metal zone concept, which considers chip fracture caused by incomplete material accumulation. The micro-milling process is categorized into four types based on the proportion of IST and IPT within one revolution. Mechanistic cutting-force models are developed for each type and validated through experiments. The experimental results align closely with theoretical predictions, with peak force errors remaining within 10%, affirming the accuracy of the analytical force models.</p>