To improve the geometric accuracy of CNC machined parts, dynamic machining errors due to on-line disturbances (tool deflection, tool wear, heat deformation, etc.) should be accounted for in some manner. Unless these on-line disturbances are properly handled, it is obvious that a high degree of geometric accuracy is difficult to achieve. Many attempts have been made to compensate for these on-line disturbances such as the development of engineering models; however, the models are not adequate enough for reflecting the real phenomenon and are dependent on continuous process monitoring using a variety of sensors. Closed-loop process control is a scheme for manufacturing parts and compensating for on-line disturbances and machine tool inaccuracies using error feedback. The goal has been to develop a system that automatically provides dimensional error feedback to the process machine. Closed-loop process control can be achieved before, during (in-process) or after the machining cycle. In-process control is achieved by measuring the part prior to finishing cuts while the part is fixtured to the machine tool. Although the theory behind an automated closed-loop, in-process control system would significantly reduce manufacturing costs, at the present time, machining errors typically are compensated through manual error feedback. This thesis presents a systematic approach for automatically compensating for dynamic machining errors based on a new closed-looped machining scheme. The new scheme incorporates these errors, found through in-process inspection, into a modified CAD model or "Variational Part Model". As a result, the Variational Part Model inherently contains the online disturbances associated with machining. It is important to note that this new scheme assumes the machine tool's static error (ball screw error, joint misalignment, perpendicularity error, etc.) has been addressed by some other compensating method and this scheme only addresses the dynamic machining error. To create the Variational Part Model, the machined part is measured on the machine and compared to the CAD model's theoretical data. The data is then used in conjunction with modeling functions contained in NX's Application Programming Interface (API) to interact with the CAD model and modify its feature geometry. The validity and effectiveness of the methods are presented as well as results from experimental testing. This thesis also presents the methods necessary for automatic CAM process updating to ultimately close the loop between machining and inspection.
College and Department
Ira A. Fulton College of Engineering and Technology; Mechanical Engineering
BYU ScholarsArchive Citation
Carlson, Shane A., "Development of a Variational Part Model Using In-Process Dimensional Measurement Error" (2006). Theses and Dissertations. 448.
CAD/CAM, closed-loop machining, dynamic error, in-process inspection