Enhancement of computer aided manufacturing systems using analytical curves

• Main Author: Lotfi, Behrooz
• Other Authors: Khoo Li Pheng
• Format: Theses and Dissertations
• Language: English
• Published: 2012
• Subjects: DRNTU::Engineering::Mechanical engineering::Kinematics and dynamics of machinery
• Online Access: https://hdl.handle.net/10356/47846
• Institution: Nanyang Technological University

In curvilinear contour machining, it is important to reduce the contour error during milling. Contouring error is mainly attributed to unexpected changes in the material removal rate and the cutting force. Application of variable feed rates to achieve a constant material removal rate has been previously studied. However, these investigations are not sufficient to achieve a precise and error-free machining. It is necessary to consider possible tool deflections due to excessive machining forces. Moreover, the effect of changing direction on cutting forces is neglected in conventional methods. It is important to predict the cutting forces profile along the moving axes before machining. It helps determine if the forces required to produce the desired curvilinear motion are within the capabilities of the machine. A high-performance motion positioner is another major requirement for generation of curvilinear motion. Parallel mechanisms offer good candidates as positioners due to their high stiffness and low inertia which enables transmission of large forces. However, they may lead to motion error due to inherent limitation of driving motors, such as backlash motions. Frequent direction reversals and rapid changes in rotation speed of the driving motors result in undesired errors in the generated path. In this work, a novel dynamical model for generation of curvilinear paths is presented. The proposed model investigates the relationship between the forces of a machine tool and the feed rate to achieve constant cutting forces as well as a constant material removal rate. A new approach for predicting cutting forces for curvilinear machining is also established. This approach could predict the cutting forces along the moving driving axes of a machine tool for both constant and variable feed rates during machining curved geometries. Moreover, a three-degree-of-freedom parallel mechanism with unlimited rotational capability is fabricated. The fabricated mechanism is able to reach any point in any orientation within the singularity-free circular workspace. The system was used to verify a motion generation algorithm for producing smooth and backlash-free motions for complex geometrical paths including those with very sharp edges. The proposed algorithm does not require the driving motors to perform stop-reverse and high accelerated motions. This results in improved dynamic performance and reduced motion errors. Besides that, another algorithm for motion generation based on rotating coordinate system for exploiting the best performance of a manipulator was formulated. It was shown that the proposed method could improve the dynamic performance of the manipulator and reduce the undesired errors.