We present Robotic Modeling Assistant (RoMA), an interactive fabrication system providing a fast, precise, hands-on and in-situ modeling experience with an augmented reality CAD editor and a robotic arm 3D printer. As a designer creates a new model using RoMA AR CAD editor, features are constructed concurrently by a 3D printing robotic arm sharing the same design volume. The partially printed physical model then serves as a tangible reference for the designer as she adds new elements to her design. With RoMA, users can integrate real-world constraints into a design rapidly, allowing them to create well-proportioned tangible artifacts. Users can even directly design on and around an existing object, and extending the artifact by in-situ fabrication.