In this paper, we describe a new geometric algorithm to determine the largest feasible cutter size for 2-D milling operations to be performed using a single cutter. In particular:1. We give a general definition of the problem as the task of covering a target region without interfering with an obstruction region. This definition encompasses the task of milling a general 2-D profile that includes both open and closed edges.2. We discuss three alternative definitions of what it means for a cutter to be feasible, and explain which of these definitions is most appropriate for the above problem.3. We present a geometric algorithm for finding the maximal cutter for 2-D milling operations, and we show that our algorithm is correct.