Classification of object lines in mechanical part drawings is a critical problem for automated conversion of drawings from paper medium to CAD databases. We describe new methods for classifying object lines. These methods include section line detection, hidden line detection, centerline detection and object line extraction. A self-supervised approach which includes a spacing estimation step and a recognition step to extract section lines is described. A general purpose algorithm that not only detects dashed lines but also classifies them based on their attributes is described. These attributes are used for classification of detected dashed lines as hidden lines or centerlines. Object line extraction facilitates intelligent interpretation of geometric objects for integration with CAD/CAM systems.