The purpose of this work is to investigate a new method for detecting chatter in milling. In this method, the spindle drive current signal of a vertical milling machine is used for monitoring process instability. Both simulations and experimental work are conducted. Results show that current signals can transmit chatter frequencies reliably. The sensitivity of the current signal to slight process instability is assessed. A statistical indicator, the R-value is used to evaluate this sensitivity. Statistical analysis of experimental data shows that the R-value is insensitive to variations in speed, feed and geometry of cut. Also, experiments show that slight variations in the process instability results in a significant increase in the R-value.