Chatter is an instability phenomenon in machining processes which limits productivity and results in inferior workpiece quality, noise and rapid tool wear. The increasing demand for productivity in the manufacturing community motivates the development of an active control strategy to shape the chatter stability boundary of manufacturing processes. In this work a control methodology for the high-speed milling process is developed that alters the chatter stability boundary such that the area of chatter-free operating points is increased and a higher productivity can be attained. The methodology developed in this paper is based on a robust control approach using $mu$-synthesis. Hereto, the most important process parameters (depth of cut and spindle speed) are treated as uncertainties to guarantee the robust stability (i.e., no chatter) in an a priori specified range of these process parameters. Effectiveness of the proposed methodology is demonstrated by means of illustrative examples.