A new geometric-and-physics model of milling and an effective approach to medial axis transforms of free-form pockets for high performance machining

Mechanical part quality and productivity depend on many parameters in CNC milling processes, such as workpiece material, cutters, tool paths, feed rate, and spindle speed, etc. To pursue high performance machining, the cutting parameter optimization is in high demand in industry, though it is quite challenge. This innovative research successfully addresses some essential problems in optimizing the cutting parameters by developing a new geometric-and-physics integrated model of milling and proposing an effective approach to the medial axis transforms of free-form pockets. In this research, an original geometric model of 21/2- and 3-axis CNC milling is developed and integrated with a well-established mechanistic model. A main research contribution is that this integrated model can predict complex milling processes in higher fidelity with instantaneous material remove rates, cutting forces and spindle powers, compared to prior machining models. In the geometric model, an in-process workpiece model is introduced by using a group of discrete Z-layers and applying the B-Rep scheme to represent the workpiece shape on each layer, in order to accurately represent instantaneous cutter-and-workpiece engagement in 2Yz- and 3-axis milling. Hence, the un-deformed chip geometry can be found even for complex part milling, which is then fed to the mechanistic model to predict instantaneous cutting forces. By using this integrated model, cutting parameters can be optimized for profiling, pocketing, and surface milling to ensure steady cut and the maximum material removal rates. This model has been verified by experiments, and will be implemented into a software tool for Bombardier Aerospace. Another important research in this work is to propose aggressive roughing of free-form pockets for ultimately high cutting efficiency. For this purpose, an accurate, efficient approach to the medial axis transforms of free-form pockets and an optimal approach to multiple cutters selection and their path generation are proposed. The main contributions of this research include (1) a new mathematical model of medial axis point, (2) an innovative global optimization solver, the hybrid global optimization method, (3) an optimization model of selecting multiple cutters for the maximum material removal rate. This research can substantially promote aggressive roughing in the machining industry to increase cutting efficiency of free-form pockets. The technique has been validated using considerable number of cutting tests and can be directly implemented into commercial CAD/CAM softwar

A Practical and Optimal Approach to CNC Programming for Five-Axis Grinding of the End-Mill Flutes

For a solid carbide tapered end-mill, every flute includes a flute surface and a rake face along a helical side cutting edge, and the end-mill core is at the center and is tangent to all the flutes. The flutes significantly affect the tools cutting performance and life, and the core radius mainly affects the tools rigidity. Mainly, two methods are adopted in industry to grind the flutes; these are: the direct method and the inverse method. In the direct method, a flute is ground using a standard grinding-wheel moving in multi-axis machining to generate the rake face and the flute surface. However, the flute is the natural outcome of the grinding process without any control. On the other side, the inverse method employs the concept of inverse engineering to build a grinding-wheel that accurately grinds the end-mill flutes. This yields a free-form grinding-wheel profile that is used on a 2-axis grinding machine; however, the flute shapes are only exact on one section of the end-mill; when the grinding-wheel moves along the side cutting edge to smaller sections; the deviation of the generated flute from the designed one will be increased. Thus, neither can this method grind the rake face with the prescribed normal rake angle, nor generate the side cutting edge in good agreement with its design. Moreover, the grinding-wheel profile is very difficult and expensive to make. To address these problems, a practical and optimal approach for five-axis grinding of prescribed end-mill flutes is proposed by; first, establishing a 5-axis flute grinding theory describing the wheels locations and orientations during grinding the rake faces with constant normal rake angles; Second, introducing a simple grinding-wheel consisting of lines and circular arcs; and finally, applying an optimization algorithm to optimize the grinding-wheel shape and path. Overall, this approach significantly advances the CNC programming technique for the 5-axis flute grinding, and can substantially increase the quality of the solid carbide end-mills and lays a good foundation for the CAD/CAE/CAM of end-mills. The advantages of this approach over the other approaches are verified using computer simulation

AUTOMATED FIVE-AXIS TOOL PATH GENERATION BASED ON DYNAMIC ANALYSIS

Ph.DDOCTOR OF PHILOSOPH

Chip geometry, cutting force, and elastic deformation prediction for gear hobbing

The machining industry is constantly challenged through increasing demands for productivity and stringent part quality requirements such as dimensional accuracy and surface quality. Physics-based models are becoming more commonly employed in the manufacturing industry for traditional machining processes like turning, milling, and drilling. By utilizing such models, machining process planners can optimize productivity while preserving or improving part quality, through virtual manufacturing of the components ahead of time via realistic simulations. In this context, cutting force prediction models are essential for machining process simulations. For traditional machining operations, where the cutter and workpiece geometries and kinematics are simple, cutting forces can be calculated via analytical equations. However, in complex processes like 5-axis milling, turn-milling and gear machining, the cutter-workpiece engagement is very complex and is best calculated using geometric CAD modelers. This engagement information allows for cutting forces along the cutting edge of the tool to be computed and summed up. Modeling the cutting forces also provides insight into the torque/power requirement, elastic deformation, vibrations, and machining stability (chatter) during the process, which are the primary factors that contribute to dimensional inaccuracies, surface location errors, and poor surface finish. By integrating these models, a comprehensive physics-based approach to machining processes can be developed, allowing for accurate simulation, prediction, and optimization of part quality. The main objective of this thesis is to establish the very first steps of such an integrated simulation environment for the gear hobbing process, by investigating the efficient prediction of cutting forces and elastic deformations. Hobbing is a high-speed and accurate gear cutting process used extensively to produce external gears – which are essential components in power transmission, automotive, aerospace, and automation (e.g., robotics) applications. The hobging process involves feeding a rotating cutting tool (known as a ‘hob’) into a workpiece (referred to as blank gear) that is rotating while the two are meshed together, as would be in worm-gear mechanism. This results in the continuous removal of chips during the process. Unlike conventional machining operations, hobbing has complex tool and workpiece geometries, and complicated kinematics with multi-axis motions. In this thesis, a mathematical model of the hobbing kinematics is developed and validated through collected CNC signals obtained using the Siemens 840D controller of Liebherr LC500 hobbing machine. The cutter-workpiece engagement is calculated using an efficient discrete geometric modeler in tri-dexel format. Using Delaunay triangulation and alpha shape reconstruction, the 2D cross-section of the uncut chip is created from its internal data. This cross-section is then utilized to approximate the local chip geometry along the discretized cutting edge of the tool. Each node along the cutting edge represents a generalized oblique cutting force model with specific rake and inclination angles, and principal directions (i.e., tangential, feed, and radial). At each time step, the incremental forces for the engaged cutting edge nodes are computed and ultimately integrated to obtain the total cutting forces. Using a rotary dynamometer, the proposed cutting force model has been validated through cutting trials on a Liebherr LC500 CNC hobbing machine. The tests involved cutting of several spur and helical external gears with varying process parameters in single and two-pass processes. The model reasonably captures the overall behavior of the measured forces, min/max force envelopes and cutting strokes with the RMS error being 7-21% for roughing passes and 24-36% for finishing passes throughout the tests, which is reasonable for machining process planning. In the finishing cut, due to the forces being smaller, the signal-to-noise ratio and apparent prediction accuracy are worse. The elastic deformation is modeled based on the static stiffness of the tooling and workpiece assemblies. The stiffness is approximated from experimentally-measured mechanical frequency response functions (FRFs). The expected elastic deformations are computed by dividing the cutting forces by the static stiffness values. The calculated deflections are then used to superpose the tool’s nominal position in the time-domain simulation of the gear machining operation, thereby gears to be ‘virtually-machined’ with errors originating both from the kinematics of the hobbing feeding process, as well as the mechanical elastic deformations. The virtually-produced gears are then measured according to the ANSI/AGMA standard for gear inspection, using the integrated gear cutting simulation and metrology software developed at the University of Waterloo, and the prediction results are compared with the quality inspection measurements taken from physically machined gears, using a GLEASON 300GMS Lead & Involute Checker. The lead deviation predictions showed good correlation, while profile deviations require further research. Overall, this thesis has achieved a detailed physics-based model for hobbing, which focuses on the kinematics, chip geometry, cutting forces, and elastic deformation. Future research will explore error sources in the cutting force model prediction, enhancing the elastic deformation model, and developing models for vibrations and chatter

Time Localization of Abrupt Changes in Cutting Process using Hilbert Huang Transform

Cutting process is extremely dynamical process influenced by different phenomena such as chip formation, dynamical responses and condition of machining system elements. Different phenomena in cutting zone have signatures in different frequency bands in signal acquired during process monitoring. The time localization of signal’s frequency content is very important. An emerging technique for simultaneous analysis of the signal in time and frequency domain that can be used for time localization of frequency is Hilbert Huang Transform (HHT). It is based on empirical mode decomposition (EMD) of the signal into intrinsic mode functions (IMFs) as simple oscillatory modes. IMFs obtained using EMD can be processed using Hilbert Transform and instantaneous frequency of the signal can be computed. This paper gives a methodology for time localization of cutting process stop during intermittent turning. Cutting process stop leads to abrupt changes in acquired signal correlated to certain frequency band. The frequency band related to abrupt changes is localized in time using HHT. The potentials and limitations of HHT application in machining process monitoring are shown

SPATIAL TRANSFORMATION PATTERN DUE TO COMMERCIAL ACTIVITY IN KAMPONG HOUSE

ABSTRACT Kampung houses are houses in kampung area of the city. Kampung House oftenly transformed into others use as urban dynamics. One of the transfomation is related to the commercial activities addition by the house owner. It make house with full private space become into mixused house with more public spaces or completely changed into full public commercial building. This study investigate the spatial transformation pattern of the kampung houses due to their commercial activities addition. Site observations, interviews and questionnaires were performed to study the spatial transformation. This study found that in kampung houses, the spatial transformation pattern was depend on type of commercial activities and owner perceptions, and there are several steps of the spatial transformation related the commercial activity addition. Keywords: spatial transformation pattern; commercial activity; owner perception, kampung house; adaptabilit