IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 15, NO. 1, FEBRUARY 1999 57 Feature-Based Reverse Engineering of Mechanical Parts William B. Thompson, Senior Member, IEEE, Jonathan C. Owen, H. James de St. Germain, Stevan R. Stark, Jr., and Thomas C. Henderson, Senior Member, IEEE Abstract— Reverse engineering of mechanical parts requires used to create CAD models of a part based on sensed data extraction of information about an instance of a particular part acquired using three-dimensional (3-D) position digitization sufficient to replicate the part using appropriate manufacturing techniques [1]. Part-to-CAD reverse engineering produces techniques. This is important in a wide variety of situations, since functional CAD models are often unavailable or unusable models which allow up to date NC fabrication and facilitate for parts which must be duplicated or modified. Computer design modification. Successful instances include everything vision techniques applied to three-dimensional (3-D) data ac- from sporting goods to aircraft parts. quired using noncontact, 3-D position digitizers have the potential Reverse engineering of solid objects traces its roots back to for significantly aiding the process. Serious challenges must be overcome, however, if sufficient accuracy is to be obtained and the pantograph, which uses a mechanical linkage to duplicate if models produced from sensed data are to be truly useful for arbitrary geometric shapes at any predetermined scale. Copy manufacturing operations. This paper describes a prototype of lathes and mills are more contemporary and automated ver- a reverse engineering system which uses manufacturing features sions of the pantograph. In a copy lathe, a mechanical stylus is as geometric primitives. This approach has two advantages over moved along a template specifying a 1-D profile. The position current practice. The resulting models can be directly imported into feature-based CAD systems without loss of the semantics and of the cutter is adjusted based on this template, producing a topological information inherent in feature-based representations. revolute object with the same profile. A copy mill typically In addition, the feature-based approach facilitates methods capa- moves a stylus over a surface, using the height of the surface ble of producing highly accurate models, even when the original to set the -axis in a three-axis mill, thus making a copy of 3-D sensor data has substantial errors. the original object. Several vendors have produced copy mills Index Terms— Feature-based CAD, reverse engineering, sur- which use noncontact sensors. These systems have the added face fitting. advantage of storing the sensed profile, so that an object can be duplicated many times without repeated scanning. I. INTRODUCTION Copy lathes and mills duplicate a physical part without producing any intermediate model of the geometry of the AD MODELS are often unavailable or unusable for part, other than stylus position or 3-D points acquired with a Cparts which must be duplicated or modified. This is a particular problem for long life cycle systems for which spare noncontact sensor. While some can produce NC code capable part inventories have been exhausted and original suppliers of driving other lathes and mills, none can produce a CAD are unable or unwilling to provide custom manufacturing runs model of an existing part. Such models are desirable for a of spare parts at affordable prices and in a timely manner. number of reasons. Modifications to the part cannot easily For many parts, either CAD systems were not used in the be done at the level of NC code. Even if the part is to be original design or the documentation on the original design is duplicated as is, refixturing and hidden concavities often lead otherwise inadequate or unavailable. For a variety of reasons, to situations in which multiple scans of an object’s shape must CAD models, even when they exist, may not be sufficient to be combined into a single, consistent representation. Some support modification or manufacturing using modern methods. shape properties such as deep holes will not be accurately Finally, shop floor changes to the original design may mean measured by either mechanical styli or noncontact sensors. that the original CAD model no longer accurately reflects the The most straightforward approach to generating a reverse geometry of the part. Reverse engineering techniques can be engineered geometric model of a mechanical part involves a designer or engineer making measurements using traditional Manuscript received February 18, 1997; revised October 3, 1998. This devices such as calipers and gauges and entering the results work was supported by the Advanced Research Projects Agency under Army into a standard CAD system. When high precision is required, Research Office Grant DAAH04-93-G-0420. This paper was recommended contact coordinate measuring machines (CMM’s) are often for publication by Associate Editor A. Kusiak and Editor V. Lumelsky upon evaluation of the reviewers’ comments. used. Positional accuracy on the order of 3 m locally and W. B. Thompson, H. J. de St. Germain, and T. C. Henderson are with the 14 m corner to corner is possible, but sensing of a large Department of Computer Science, University of Utah, Salt Lake City, UT 84112 USA. number of points is extremely slow and expensive damage J. C. Owen was with the University of Utah, Salt Lake City, UT 84112 can be done if the probe is not maneuvered toward the object USA. He is now with Red Storm Entertainment, Morrisville, NC 27560 USA. along an appropriate path. More recently, noncontact CMM’s S. R. Stark, Jr., was with the University of Utah, Salt Lake City, UT 84112 USA. He is now with Orbtek, Inc., Salt Lake City, UT 84115 USA. produced by companies such as Cyberware, Digibotics, and Publisher Item Identifier S 1042-296X(99)01227-6. Laser Designs have significantly increased the speed with 1042–296X/99$10.00 1999 IEEE Authorized licensed use limited to: The University of Utah. Downloaded on December 2, 2008 at 15:48 from IEEE Xplore. Restrictions apply. 58 IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 15, NO. 1, FEBRUARY 1999 which data can be collected. These devices project a spot or commonly found on such parts. For example, a large portion line of light and use triangulation to determine range. While of milled parts consist of planar surfaces containing -D less accurate than contact CMM’s, the best are capable of features such as holes, pockets, bosses, and the like. positional accuracy exceeding 50 m. Nonoptimal surface In this paper, we describe an alternate approach for effi- properties can degrade this, while deep concavities, discontin- ciently creating a CAD model of a part with a significant num- uous surface orientation, surface geometries forcing oblique ber of such specialized manufacturing features. The system viewing angles, or outright occlusion will cause data to be is interactive, since some aspects of the reverse engineering missing entirely. (See [2] for methods which position sensors process cannot be done based on the part alone and other in ways that minimize these effects.) For comparison, com- aspects of the process can benefit significantly from a small monly available NC milling machines can achieve precisions amount of human intervention. In a sense, we provide a of 2–10 m for hole and bore spacings and can produce set of electronic calipers to be used as a smart measuring cutting accuracies on the order of 50–250 m depending on tool, specialized to the job of creating CAD/CAM models. the feature being cut and the tool being used, though special The system is effective because it analyzes 3-D sensor data measures can be used to obtain higher precision. using knowledge of manufacturing processes and modeling Many of the commercially available systems for the reverse techniques. engineering of mechanical parts using automatically acquired Our main innovation is to use manufacturing features as 3-D position data use rather unsophisticated geometric models. the geometric primitives fit to scanned data, rather than using Often, a digitizer is moved along parallel scanning paths and triangulated meshes or parametric surface patches. This leads NC code is generated to move a cutter along the same 3-D to four important advantages. path. In effect, no model other than the raw scan data is 1) Appropriateness for Manufactured Parts: Many com- used, though preprocessing to remove noisy data points, align plex parts can be described naturally and compactly in scan lines from multiple scans, etc., is usually necessary. terms of manufacturing features. A feature-based reverse More recently, techniques have been developed for fitting engineering system can more easily generate models of parametric surface patches to 3-D position data [3]. The such parts than can a system intended for more general geometric primitives that are used range from simple planes free-form geometries. and cylinders [4] to piecewise smooth surface parametric sur- 2) Ease of Importation into Feature-Based CAD Systems: face patches [5]–[7]. Sometimes, triangulated meshes are used Several commercially available CAD systems allow as an intermediate representation [8]–[13]. Several software parametric modification of manufacturing features in surfacing packages, including Imageware Surfacer, Parametric their models. This functionality is lost, however, if Pro/SCAN-TOOLS, and Cyberware Cyserf, have recently be- imported models consist only of surface patches, with- come available. These packages fit spline patches to raw data out the additional semantics and topology inherent in points and format the result for importation of the surfaces feature-based representations. into commercial CAD systems. 3) Reduced Need for Complete, Robust, Geometric Com- Models acquired in any of these ways represent the as- putations: Substantial effort is involved in converting measured geometry of the sample part. Substantial manual a collection of surface patches obtained by fitting to intervention is still required in order to convert these models scanned data into a form usable by a solid modeler.