A new metric is presented to automatically establish the stability limit for time domain milling simulation signals. It is based on periodically sampled data. Because stable cuts exhibit forced vibration, the sampled points repeat over time. Periodically sampled points for unstable cuts, on the other hand, do not repeat with each tooth passage. The metric leverages this difference to define a numerical value of nominally zero for a stable cut and a value greater than zero for an unstable cut. The metric is described and is applied to numerical and experimental results.