A new analytical chip formation model is proposed for micro-end-milling operations. The model calculates an instantaneous uncut chip thickness by considering the combination of exact trochoidal trajectory of the tool tip and tool run-out, while the simplified circular trajectory and the neglected run-out create negligible change in conventional-scale chip formation models. Newton-Raphson iterative method is employed during the calculation to obtain quadratic convergence. The proposed approach allows the calculation of instantaneous uncut chip thickness to be done accurately and rapidly, and the prediction accuracy of this model is also verified by comparing the simulation results to experimental cutting forces.