Matthew T. Bement

In‐process gap detection in friction stir welding

Purpose – This paper aims to investigate methods of implementing in‐process fault avoidance in robotic friction stir welding (FSW).Design/methodology/approach – Investigations into the possibilities for automatically detecting gap‐faults in a friction stir lap weld were conducted. Force signals were collected from a number of lap welds containing differing degrees of gap faults. Statistical analysis was carried out to determine whether these signals could be used to develop an automatic fault detector/classifier.Findings – The results demonstrate that the frequency spectra of collected force signals can be mapped to a lower dimension through discovered discriminant functions where the faulty welds and control welds are linearly separable. This implies that a robust and precise classifier is very plausible, given force signals.Research limitations/implications – Future research should focus on a complete controller using the information reported in this paper. This should allow for a robotic friction stir ...

Vibration Suppression in Cutting Tools Using a Collocated Piezoelectric Sensor/Actuator With an Adaptive Control Algorithm

The machining process is very important in many engineering applications. In high precision machining, surface finish is strongly correlated with vibrations and the dynamic interactions between the part and the cutting tool. Parameters affecting these vibrations and dynamic interactions, such as spindle speed, cut depth, feed rate, and the parts material properties can vary in real-time, resulting in unexpected or undesirable effects on the surface finish of the machining product. The focus of this research is the development of an improved machining process through the use of active vibration damping. The tool holder employs a high bandwidth piezoelectric actuator with an adaptive positive position feedback control algorithm for vibration and chatter suppression. In addition, instead of using external sensors, the proposed approach investigates the use of a collocated piezoelectric sensor for measuring the dynamic responses from machining processes. The performance of this method is evaluated by comparing the surface finishes obtained with active vibration control versus baseline uncontrolled cuts. Considerable improvement in surface finish (up to 50%) was observed for applications in modern day machining.

Compatible, total energy conserving and symmetry preserving arbitrary Lagrangian–Eulerian hydrodynamics in 2D rz – Cylindrical coordinates

We present a new discretization for 2D arbitrary Lagrangian–Eulerian hydrodynamics in rz geometry (cylindrical coordinates) that is compatible, total energy conserving and symmetry preserving. In the first part of the paper, we describe the discretization of the basic Lagrangian hydrodynamics equations in axisymmetric 2D rz geometry on general polygonal meshes. It exactly preserves planar, cylindrical and spherical symmetry of the flow on meshes aligned with the flow. In particular, spherical symmetry is preserved on polar equiangular meshes. The discretization conserves total energy exactly up to machine round-off on any mesh. It has a consistent definition of kinetic energy in the zone that is exact for a velocity field with constant magnitude. The method for discretization of the Lagrangian equations is based on ideas presented in [2,3,7], where the authors use a special procedure to distribute zonal mass to corners of the zone (subzonal masses). The momentum equation is discretized in its “Cartesian” form with a special definition of “planar” masses (area-weighted). The principal contributions of this part of the paper are as follows: a definition of “planar” subzonal mass for nodes on the z axis (r=0) that does not require a special procedure for movement of these nodes; proof of conservation of the total energy; formulated for general polygonal meshes. We present numerical examples that demonstrate the robustness of the new method for Lagrangian equations on a variety of grids and test problems including polygonal meshes. In particular, we demonstrate the importance of conservation of total energy for correctly modeling shock waves. In the second part of the paper we describe the remapping stage of the arbitrary Lagrangian–Eulerian algorithm. The general idea is based on the following papers [25–28], where it was described for Cartesian coordinates. We describe a distribution-based algorithm for the definition of remapped subzonal densities and a local constrained-optimization-based approach for each zone to find the subzonal mass fluxes. In this paper we give a systematic and complete description of the algorithm for the axisymmetric case and provide justification for our approach.1 The ALE algorithm conserves total energy on arbitrary meshes and preserves symmetry when remapping from one equiangular polar mesh to another. The principal contributions of this part of the paper are the extension of this algorithm to general polygonal meshes and 2D rz geometry with requirement of symmetry preservation on special meshes. We present numerical examples that demonstrate the robustness of the new ALE method on a variety of grids and test problems including polygonal meshes and some realistic experiments. We confirm the importance of conservation of total energy for correctly modeling shock waves.

Workpiece dynamics during stable cutting in a turning operation

In this investigation, workpiece and cutting tool dynamics in a turning operation are investigated. It is shown that dynamic information from the workpiece propagates to the cutting tool more consistently in the tangential cutting direction than in the radial cutting direction since there is a preload that seeks to keep the tool in contact with the workpiece. Finally, trajectories in the state space comprised of workpiece accelerations parallel to the radial and tangential cutting directions were investigated and it is graphically shown that stable cutting results in more organised dynamics and unstable cutting results in less organised dynamics, which agrees with previous work.

Excitation Design for Damage Detection Using Iterative Adjoint-Based Optimization

This paper presents a method for designing excitations for the purpose of enhancing the detectability of damage. The field of structural health monitoring (SHM) seeks to assess the integrity of structures for the primary purpose of moving from time-based maintenance to a more cost effective condition-based maintenance strategy. Consequently, most approaches to SHM are nondestructive in nature. One common nondestructive approach is known as vibration-based SHM. In this approach, a structure is instrumented with an array of sensors at various locations. The structure is then excited and its dynamic response recorded. This response is then interrogated to extract features that are correlated with damage. A survey of the SHM literature [1], [2], reveals that a great deal of attention has been paid to the data interrogation portion of the SHM process, with almost no attention paid to the excitation design. This focus is quite understandable in many applications where only ambient excitation is available, such as most civil engineering applications. However there are many applications where the excitation is selectable (e.g., most wave propogation approaches to SHM), and, indeed, where proper excitation selection is essential. As a simple example, consider a beam or column with a crack that is nominally closed due to a preload. If the provided excitation is not sufficient to open and close the crack, the detectability of the crack in the measured output will be severely limited.© 2008 ASME

Inferring Hardness From High-Speed Video of the Machining Process

This paper presents the results of a study to assess the feasibility of inferring workpiece material hardness from high-speed video data of chip formation obtained during a turning operation. The motivation for assessing hardness in situ comes from the fabrication of shaped charges, where spatial variation in hardness is known to affect the performance of the shaped charge. While other in-process data could be used for this purpose, video data are analyzed here because of the stand-off, non-contact advantages afforded. This is especially relevant for highly qualified machining processes for small-lot, high value parts where any interference with the process (e.g., introduction of cables near the machine tool) is undesirable. A multistep image processing procedure is presented which is used to extract several features from the video data. These features are then used to develop a classifier which can be used to predict work-piece hardness. Multiple classifier designs (Knn and Ratchet) are considered.Copyright

A Geometrically Comprehensive Approach to Modeling Dynamic Cutting Forces in Turning: Application to Regenerative Chatter

Present chatter models in turning lack physical insight because they do not model the process in a geometrically rigorous manner. Many of the models are linear and produce unrealistic, unbounded vibration amplitude growth after the onset of chatter. Those that are nonlinear are typically reverse engineered in order to predict bounded vibration. The current approach models the forces in machining due to chip formation, plowing, and interference between the flank of the cutting tool and the machined workpiece surface in a geometrically comprehensive fashion. Additionally the effects of strain, strain rate and temperature on the chip formation process are captured. In doing so, accurate predictions can be made for both the occurrence of chatter and its vibration amplitude growth over time. The proposed model is validated with machining experiments on a compliant workpiece to explore the effect of tool nose radius on chatter.Copyright