Intelligent Fixtures for the Manufacturing of Low Rigidity Components

In milling of impellers and blisks (blade integrated disks), critical workpiece vibrations of thin-walled blade structures occur due to the excitation by the process forces and the dynamic compliance of the sensitive elements of the parts. Workpiece vibrations lead to inacceptable effects on the blade surfaces and thus to the production of defective parts. Also, these vibrations provoke an increased tool wear progress. Within the INTEFIX project, fixture solutions were developed which enable the detection and compensation of chatter vibrations during machining of thin-walled workpiece elements. This contribution introduces the development of an intelligent chuck for the clamping of impellers. The chuck exploits CFRP embedded piezo patch transducers for the identification of critical workpiece vibrations during milling. By means of an integrated piezo actuator, counter vibrations can be applied which disturb the regenerative chatter effect and lead to a decreased waviness of the workpiece surface. The development of the mechatronic clamping system is supported by innovative process simulation approaches.

In the aeronautic industry the manufacturing of thin-walled components is common. Occurring problems are associated to the poor dynamic performance during the machining process, causing problems like chatter, poor surface quality, low precision or distortions. Furthermore, the use of advanced materials with low machinability amplifies the cited problems. In this case study of the INTEFIX project, the fixture development was oriented to the improvement of the dynamic behaviour of the workpiece using two approaches: (1) “Active modification of mechanical impedance” deals with the use of active vibration reducers that create a counteracting inertial force with a magnetic actuator controlled in a closed loop, minimizing the vibrations during the machining of the workpiece. (2) “Controlled deformation” deals with the use of actuators integrated in the fixture to apply controlled forces in defined areas of the workpiece that modify the clamping state of the workpiece and its dynamic behaviour, leading to a reduction of the vibrations during the machining process. Additionally, the application of CFRP materials to provide a higher damping to the fixture structure is tested. This chapter covers the development and integration of the active systems in the fixture prototype, and the results obtained in the experiments.

In general rough-milling operations, unstable tool vibrations due to the interaction between process forces and tool flexibility could arise. The onset of these unstable vibrations, usually referred to as chatter, poses limitations in terms of the achievable material removal rates, hence directly impacting on the productivity. Moreover, chatter vibrations generally lead to an increase in tool wear, imposing premature tool changes and careful monitoring of the process, potentially impeding unmanned operations. Within the INTEFIX project, an active fixture prototype was developed to detect and mitigate the level of chatter vibrations in general rough-milling operations with the purpose of improving the achievable material removal rates. This contribution covers the main aspects of the global development of this prototype, from the mechanical design to the adaptive control logic used in order to drastically reduce the inputs and expertise required for its operability.

In machining of thin-walled large parts in aerospace industry, workpiece distortions occur during and after the processes due to residual stresses which are introduced or set free by the material removal process. These distortions lead to an inacceptable shape and geometric errors of the produced components and, thus, to deficient products. Considering that milling operations at large aerospace structural parts take several hours and that often expensive workpiece materials (such as titanium alloys) are used, these critical deformations cause high costs in the manufacturing companies. In some cases, post-treatments such as shot peening is applied in order to reduce the influence of residual stresses. This also means a significant increase of production costs of the parts. With the aim to overcome these challenges of part deformations, in this case study an intelligent fixture was developed which detects the tendency of workpiece distortions within sequenced processing steps and which allows an active adjustment of the clamping conditions in order to compensate for the influences of residual stresses on the final shape of the part.

Thin-walled curved workpieces are typical for structural parts of airplanes. The issue in the workpiece clamping and subsequent machining is the changeable workpiece stiffness during material removal. The fixture forces and the cutting forces deform the workpiece with dependence on the part decreasing static stiffness that causes large local surface location errors of the part. As a result, the workpiece wall thickness is out of tolerance and consequently the part’s weight is also out of tolerance. The proposed solution is based on new fixtures with integrated support and clamping function. The workpiece is clamped using a vacuum. A suitable thickness measurement sensor was integrated into the machine tool. The new fixture elements are autonomous and plug-and-produce ready, with integrated safety by monitoring the minimal workpiece clamping force. The fixture control enables fully automated operation using the specific control software. The machining process was optimized in terms of tool path strategy and cutting conditions to avoid chatter during machining and shorten the production time. The proposed manufacturing process leads to a shortening of the production time with the requested surface quality. The presented manufacturing procedure is beneficial from the productivity and cost point of view. A group of fixtures, including the necessary harness and control, offers a universal possibility for replacing a set of six specific fixtures designed as a mould with part negative shape.

The structural components employed for the aeronautical industry show highly restrictive requirements concerning their weight and strength capabilities. Following these requirements, they usually have a slender nature, showing a highly unsymmetrical ribbed geometry. This point coupled with the residual stresses present on the stocks from which these parts are machined, cause that the final machined parts show distortion problems that can make them unacceptable and, thus, generate the rejection of the manufactured component. Within the INTEFIX project, a solution has been developed comprising the use of fixtures coupled with calculation engines for tackling the manufacturing of this kind of parts. Taking into account the actual residual stress state for the stock, a software tool has been developed that is capable to automatically clamp the part in the optimal way and machine the part assuring the minimum distortion for the machined component. When developing this tool, the requirement for its usage by low skilled operators at workshop level has been taken into account, guaranteeing its usability in actual manufacturing environments.

The manufacturing chain of metallic components normally involves various processes like welding, machining or thermal treating. This combination leads to internal residual stresses that commonly result in deformations and distortions of the workpiece affecting the manufacturing precision and the quality of the component. This problem is especially relevant in large, slender and complex workpieces like those manufactured in the aeronautic industry. In this case study, the fixture development was oriented to the control and minimization of the deformation of a large component of an aircraft engine during the setup and clamping process to improve the precision during the machining process. The initial state of the component is an already deformed geometry as a consequence of previous welding processes; so, the objective is the development of an intelligent fixture able of clamping the deformed component without producing additional geometrical distortions. This chapter covers the development of a solution for the clamping of deformed components and the results obtained in the experiments.

The Case Study is focused on the improvement of the productivity and accuracy of large workpieces production thought the shortening of the setup and clamping time. Current situation in the workpiece adjustment is based on extremely time demanding manual setting, when the operator has to find optimal zero point of the workpiece manually—scribing operation and to fix the workpiece. The main goal of the Case Study is the development of a low time consuming clamping system with low requirements for the operator. This objective was achieved by the development and testing of a comprehensive solution that automates the workpiece adjustment process by active fixture units with centralized control and uses a machine tool touch probe for workpiece current state automatic inspection. This approach is more effective than manual setting and also reduces the risk of errors. In addition, the developed solution allows the workpiece automatic clamping in an adjusted position and with a pre-deformation if necessary.

For large parts machining, workpiece set-up is a time-consuming and usually an error prone process. Large parts of unitary or very small batches are located in the machine using previously designed references in its surfaces according to machining specifications (marking out process). Both, the marking out process and the alignment in the machine are normally manual processes. The part has to be aligned according to machine axes. This process is done normally for each workpiece using touch probes or dial indicators. Zero point systems are conventionally not used in large parts machining. A 3D metrology system based on photogrammetry technology has advantages with respect to the marking out process: quick response, high precision in the captures, and the easiness to use. A photogrammetric application was developed here with the goal of producing parts with minimum material removal. An on-board solution was developed to control positioning of the part in the machine according to photogrammetric application requirements. Also, a kinematic alignment table was developed with the ability to rotate in three degrees of freedom simplifying the alignment process of the part in the machine. Finally, the developed system was validated in an industrial environment.

The fixtures are used to locate, clamp and support the workpiece during the manufacturing process. Their performance affects the results attending to the quality, cost and performance. Thus requirements like accuracy, reliability, short set-up time and cost-effectiveness are demanded for the fixtures. Within the INTEFIX project, the design of an active fixture is presented for the accurate positioning of a planet carrier with very strict requirements of tolerances and for an intelligent adjustment during the machining process when required. This device will allow the manufacturer to reduce the manual inspections, to automatize the adjustment tasks and to improve the machining process setup time. This will increase consequently the productivity and achieve the required accuracy and the required geometrical quality of the part. The development of the active fixturing focused mainly on the conception of a high precision actuator capable of moving the large part with the required tolerance. The active fixture was implemented into the machine and the obtained results proved that the proposed active fixture is able to centre the workpiece within the tolerance of 10 μm assuring the quality requirements.