This paper presents an enhanced marching cubes algorithm to construct an iso-boundary for in-process geometric modeling for material removal processes. The author first analyzes the tool motion and the geometric properties in material removal processes. The result shows that the in-process geometry is the complement of the tool swept volume from the raw material. The in-process geometry can be determined by continuously updating itself from the swept volume of the tool. This study uses a three-dimensional G-buffer to update the intersection information between the tool swept volume and the in-process geometry. Rather than traditionally searching for all intersection points ranging in a cube, the developed algorithm uses certain specific intersection points that are selected based on the removal geometry properties to construct the iso-boundary. It avoids the unfavorable ambiguities and holes on constructed boundaries. In addition, the developed algorithm is able to handle multiple intersection points in a cubical edge. This study also discusses material removal volume and tool collision issues. The computer implementation shows that the developed method is superior to the traditional ones in material removal applications.