The intensive competition has forced the manufacturing industry to improve product quality and production efficiency, and meanwhile reduce production cost. Detecting potential manufacturability problems in early design stages would prevent unnecessary cost involved in re-work and/or re-design activities at later manufacturing stages. This paper presents an algorithm for determining re-design areas to feedback the designer when the design model involves machining holes and pockets using a three-axis computer numerical control machine. The algorithm takes into account the various sizes of tools and tool holders, and therefore allows a designer to test the manufacturability of their available cutters on their design models. A color-based obstacle-detecting method, based on layer-slicing of re-design areas, helps a designer to re-evaluate a design model for increasing manufacturability. Practical examples are provided as the output of the process.