Thin-Wall Machining of Light Alloys: A Review of Models and Industrial Approaches

Thin-wall parts are common in the aeronautical sector. However, their machining presents serious challenges such as vibrations and part deflections. To deal with these challenges, di erent approaches have been followed in recent years. This work presents the state of the art of thin-wall light-alloy machining, analyzing the problems related to each type of thin-wall parts, exposing the causes of both instability and deformation through analytical models, summarizing the computational techniques used, and presenting the solutions proposed by di erent authors from an industrial point of view. Finally, some further research lines are proposed

Eco-efficient process based on conventional machining as an alternative technology to chemical milling of aeronautical metal skin panels

El fresado químico es un proceso diseñado para la reducción de peso de pieles metálicas que, a pesar de los problemas medioambientales asociados, se utiliza en la industria aeronáutica desde los años 50. Entre sus ventajas figuran el cumplimiento de las estrictas tolerancias de diseño de piezas aeroespaciales y que pese a ser un proceso de mecanizado, no induce tensiones residuales. Sin embargo, el fresado químico es una tecnología contaminante y costosa que tiende a ser sustituida. Gracias a los avances realizados en el mecanizado, la tecnología de fresado convencional permite alcanzar las tolerancias requeridas siempre y cuando se consigan evitar las vibraciones y la flexión de la pieza, ambas relacionadas con los parámetros del proceso y con los sistemas de utillaje empleados. Esta tesis analiza las causas de la inestabilidad del corte y la deformación de las piezas a través de una revisión bibliográfica que cubre los modelos analíticos, las técnicas computacionales y las soluciones industriales en estudio actualmente. En ella, se aprecia cómo los modelos analíticos y las soluciones computacionales y de simulación se centran principalmente en la predicción off-line de vibraciones y de posibles flexiones de la pieza. Sin embargo, un enfoque más industrial ha llevado al diseño de sistemas de fijación, utillajes, amortiguadores basados en actuadores, sistemas de rigidez y controles adaptativos apoyados en simulaciones o en la selección estadística de parámetros. Además se han desarrollado distintas soluciones CAM basadas en la aplicación de gemelos virtuales. En la revisión bibliográfica se han encontrado pocos documentos relativos a pieles y suelos delgados por lo que se ha estudiado experimentalmente el efecto de los parámetros de corte en su mecanizado. Este conjunto de experimentos ha demostrado que, pese a usar un sistema que aseguraba la rigidez de la pieza, las pieles se comportaban de forma diferente a un sólido rígido en términos de fuerzas de mecanizado cuando se utilizaban velocidades de corte cercanas a la alta velocidad. También se ha verificado que todas las muestras mecanizadas entraban dentro de tolerancia en cuanto a la rugosidad de la pieza. Paralelamente, se ha comprobado que la correcta selección de parámetros de mecanizado puede reducir las fuerzas de corte y las tolerancias del proceso hasta un 20% y un 40%, respectivamente. Estos datos pueden tener aplicación industrial en la simplificación de los sistemas de amarre o en el incremento de la eficiencia del proceso. Este proceso también puede mejorarse incrementando la vida de la herramienta al utilizar fluidos de corte. Una correcta lubricación puede reducir la temperatura del proceso y las tensiones residuales inducidas a la pieza. Con este objetivo, se han desarrollado diferentes lubricantes, basados en el uso de líquidos iónicos (IL) y se han comparado con el comportamiento tribológico del par de contacto en seco y con una taladrina comercial. Los resultados obtenidos utilizando 1 wt% de los líquidos iónicos en un tribómetro tipo pin-on-disk demuestran que el IL no halogenado reduce significativamente el desgaste y la fricción entre el aluminio, material a mecanizar, y el carburo de tungsteno, material de la herramienta, eliminando casi toda la adhesión del aluminio sobre el pin, lo que puede incrementar considerablemente la vida de la herramienta.Chemical milling is a process designed to reduce the weight of metals skin panels. This process has been used since 1950s in the aerospace industry despite its environmental concern. Among its advantages, chemical milling does not induce residual stress and parts meet the required tolerances. However, this process is a pollutant and costly technology. Thanks to the last advances in conventional milling, machining processes can achieve similar quality results meanwhile vibration and part deflection are avoided. Both problems are usually related to the cutting parameters and the workholding. This thesis analyses the causes of the cutting instability and part deformation through a literature review that covers analytical models, computational techniques and industrial solutions. Analytics and computational solutions are mainly focused on chatter and deflection prediction and industrial approaches are focused on the design of workholdings, fixtures, damping actuators, stiffening devices, adaptive control systems based on simulations and the statistical parameters selection, and CAM solutions combined with the use of virtual twins applications. In this literature review, few research works about thin-plates and thin-floors is found so the effect of the cutting parameters is also studied experimentally. These experiments confirm that even using rigid workholdings, the behavior of the part is different to a rigid body at high speed machining. On the one hand, roughness values meet the required tolerances under every set of the tested parameters. On the other hand, a proper parameter selection reduces the cutting forces and process tolerances by up to 20% and 40%, respectively. This fact can be industrially used to simplify workholding and increase the machine efficiency. Another way to improve the process efficiency is to increase tool life by using cutting fluids. Their use can also decrease the temperature of the process and the induced stresses. For this purpose, different water-based lubricants containing three types of Ionic Liquids (IL) are compared to dry and commercial cutting fluid conditions by studying their tribological behavior. Pin on disk tests prove that just 1wt% of one of the halogen-free ILs significantly reduces wear and friction between both materials, aluminum and tungsten carbide. In fact, no wear scar is noticed on the ball when one of the ILs is used, which, therefore, could considerably increase tool life

Active fixturing: literature review and future research directions

Fixtures are used to fixate, position and support workpieces and represent a crucial tool in manufacturing. Their performance determines the result of the whole manufacturing process of a product. There is a vast amount of research done on automatic fixture layout synthesis and optimisation and fixture design verification. Most of this work considers fixture mechanics to be static and the fixture elements to be passive. However, a new generation of fixtures has emerged that has actuated fixture elements for active control of the part–fixture system during manufacturing operations to increase the end product quality. This paper analyses the latest studies in the field of active fixture design and its relationship with flexible and reconfigurable fixturing systems. First, a brief introduction is given on the importance of research of fixturing systems. Secondly, the basics of workholding and fixture design are visited, after which the state-of-the-art in active fixturing and related concepts is presented. Fourthly, part–fixture dynamics and design strategies which take these into account are discussed. Fifthly, the control strategies used in active fixturing systems are examined. Finally, some final conclusions and prospective future research directions are presented

An experimental validation and optimisation tool path strategy for thin walled structure

This work was carried out with the aim to optimise the tool path by simulating the removal of material in a finite element environment which is controlled by a genetic algorithm (GA). To simulate the physical removal of material during machining, a finite element model was designed to represent a thin walled workpiece. The target was to develop models which mimic the actual cutting process using the finite element method (FEM), to validate the developed tool path strategy algorithm with the actual machining process and to programme the developed algorithm into the software. The workpiece was to be modelled using the CAD (ABAQUS CAE) software to create a basic geometry co-ordinate system which could then be used to create the finite element method and necessary requirement by ABAQUS, such as the boundary condition, the material type, and the element type

Recent research on flexible fixtures for manufacturing processes

Fixtures, are used to fixate, position and support workpieces, and form a crucial tool in manufacturing. Their performance influences the manufacturing (and assembly) process of a product. Furthermore, fixturing can form a significant portion of the needed investment and total process planning time for the manufacturing system. Many fixturing concepts, as contribution to increase the flexibility of the manufacturing system, are reported in the literature. The flexible fixturing designs can be classified into the following seven categories: modular fixtures, flexible pallet systems, sensor-based fixture design, phase-change based concepts, chuck-based concepts, pin-type array fixtures and automatically reconfigurable fixtures. It is observed that the more intelligent and automated fixturing systems are designed with the demands for automation in certain industries in mind. Furthermore, different fixturing solutions suit the engineering demands for different manufacturing areas, this means that for the foreseeable future all technologies will remain current. From the self-reconfigurable fixturing techniques a new fixturing capability is emerging: in process reconfigurability for the optimal placement of clamps and supports during the whole process time. These several concepts together with some recent patents are studied here. The paper concludes with some prospective research directions in the field of flexible fixturing

Modelling and design methodology for fully-active fixtures

Fixtures are devices designed to repeatedly and accurately locate the processed workpiece in a desired position and orientation, and securely hold it in the location throughout the manufacturing process. Fixtures are also charged with the task of supporting the workpiece to minimise deflection under the loads arising from the manufacturing process. As a result, fixtures have a large impact on the outcome of a manufacturing process, especially when the workpiece presents low rigidity. Traditionally, in manufacturing environments, where thin-walled components are produced, the utilised fixtures are dedicated solutions, designed for a specific workpiece geometry. However, in the recent decades, when the manufacturing philosophy has shifted towards mass customisation, there is a constant technological pull towards manufacturing equipment that exhibits high production rates and increased flexibility/reconfigurability, without any compromise in the quality of the end result. Therefore, fixtures have been the focal point of a plethora of research work, targeting mainly towards either more reconfigurable, or more intelligent/adaptive solutions. However, there have been no attempts so far to merge these two concepts to generate a new fixturing approach. Such an approach, referred to in this work as fully-active fixrturing, would have the added ability to reposition its elements and adapt the forces it exerts on-line, maximising the local support to the workpiece, and thus reducing vibration amplitude and elastic deformation. This results in a tighter adherence to the nominal dimensions of the machined profile and an improved surface-finish quality. This research work sets out to study the impact of such fixturing solutions, through developing suitable models which reflect the fixture-workpiece system behaviour, and a design methodology that can support and plan the operation of fully-active fixtures. The developed model is based on a finite elements representation of the workpiece, capturing the dynamic response of a thin-walled workpiece that is being subjected to distributed moving harmonic loads. At the same time, the workpiece is in contact with an active element that operates in closed-loop control. An electromechanical actuator is charged with the role of the active elements, and it is modelled via first-principle based equations. Two control strategies are examined experimentally to identify the best performing approach. The direct force/torque control strategy with a Proportional-Integral action compensator is found to lead to a system that responds faster. This control architecture is included in the model of the active elements of the fixture. The behaviour of the contact between the fixture and the workpiece is approximated via a combination of a spring and a damper. The overall model is assembled using the impedance coupling technique and has been verified by comparing its response with the time-domain response of an experimental set-up. The developed model serves as the backbone of the fully-active fixture design methodology. The latter is capable of establishing important fixturing parameters, such as the pattern of motion of the movable fixture element, the points on the surface of the workpiece that formulate the motion path of the fixture element, the time instant at which the element needs to change position, and the clamping forces the fixture needs to apply and maintain. The methodology is applied on a thin plate test case. Such a plate has been also used in a series of machining experiments, for which the fixturing parameters used are those that resulted from the test case. A very good quantitative agreement between both experiments and theory was observed, revealing the capabilities of the methodology itself and of the fully-active fixturing approach in general

Modelling and design methodology for fully-active fixtures

Fixtures are devices designed to repeatedly and accurately locate the processed workpiece in a desired position and orientation, and securely hold it in the location throughout the manufacturing process. Fixtures are also charged with the task of supporting the workpiece to minimise deflection under the loads arising from the manufacturing process. As a result, fixtures have a large impact on the outcome of a manufacturing process, especially when the workpiece presents low rigidity. Traditionally, in manufacturing environments, where thin-walled components are produced, the utilised fixtures are dedicated solutions, designed for a specific workpiece geometry. However, in the recent decades, when the manufacturing philosophy has shifted towards mass customisation, there is a constant technological pull towards manufacturing equipment that exhibits high production rates and increased flexibility/reconfigurability, without any compromise in the quality of the end result. Therefore, fixtures have been the focal point of a plethora of research work, targeting mainly towards either more reconfigurable, or more intelligent/adaptive solutions. However, there have been no attempts so far to merge these two concepts to generate a new fixturing approach. Such an approach, referred to in this work as fully-active fixrturing, would have the added ability to reposition its elements and adapt the forces it exerts on-line, maximising the local support to the workpiece, and thus reducing vibration amplitude and elastic deformation. This results in a tighter adherence to the nominal dimensions of the machined profile and an improved surface-finish quality. This research work sets out to study the impact of such fixturing solutions, through developing suitable models which reflect the fixture-workpiece system behaviour, and a design methodology that can support and plan the operation of fully-active fixtures. The developed model is based on a finite elements representation of the workpiece, capturing the dynamic response of a thin-walled workpiece that is being subjected to distributed moving harmonic loads. At the same time, the workpiece is in contact with an active element that operates in closed-loop control. An electromechanical actuator is charged with the role of the active elements, and it is modelled via first-principle based equations. Two control strategies are examined experimentally to identify the best performing approach. The direct force/torque control strategy with a Proportional-Integral action compensator is found to lead to a system that responds faster. This control architecture is included in the model of the active elements of the fixture. The behaviour of the contact between the fixture and the workpiece is approximated via a combination of a spring and a damper. The overall model is assembled using the impedance coupling technique and has been verified by comparing its response with the time-domain response of an experimental set-up. The developed model serves as the backbone of the fully-active fixture design methodology. The latter is capable of establishing important fixturing parameters, such as the pattern of motion of the movable fixture element, the points on the surface of the workpiece that formulate the motion path of the fixture element, the time instant at which the element needs to change position, and the clamping forces the fixture needs to apply and maintain. The methodology is applied on a thin plate test case. Such a plate has been also used in a series of machining experiments, for which the fixturing parameters used are those that resulted from the test case. A very good quantitative agreement between both experiments and theory was observed, revealing the capabilities of the methodology itself and of the fully-active fixturing approach in general