Petra Wiederkehr

Tool wear-dependent process analysis by means of a statistical online monitoring system

Simulating milling processes can provide numerous optimization possibilities regarding process stability and surface quality. In tool and die manufacturing often long-running processes are necessary. In contrast to very time-consuming Finite-Element-based approaches, geometric physically-based simulation systems allow predictions for such processes because of their relatively short runtime. The machining of hardened material and varying engagement conditions between the tool and the workpiece provoke a gradually increasing influence of tool wear on the cutting edges. To consider these alterations while simulating milling processes, different approaches can be used. Because of the complex characteristics of tool wear, methods, which result in an increased simulation runtime, have to be used for the geometric modeling of tool wear. In this paper, a novel approach for monitoring a milling process is presented, which utilizes an online-selection of pre-calculated simulation data to predict the process stability for different states of tool wear. To achieve this, measured data are compared to simulated data, which result from offline simulation conductions for each defined state of tool wear. As tool wear changes when the process is progressing, different simulation data for different states of tool wear have to be selected to ensure a valid stability prediction.

Improvement strategies for the formfilling in incremental gear forming processes

Incremental Sheet-Bulk Metal Forming offers an innovative and flexible approach for the manufacturing of gears. An insufficient formfilling of the generated gearing, especially of the first tooth formed, is observed. Aiming for a formfilling improvement of the first tooth element, three influencing factors were investigated. First, the prevailing friction is analyzed and a possibility for its adjustment is offered by a tailored adaption of the tool surface topographies. These were manufactured by micromilling, EDM and polishing processes and partially covered by CrAlN PVD-coatings. Based on ring-compression tests, which were performed to determine the resulting friction conditions, the analyzed topographies were transferred onto real tool surfaces and used in the incremental gear forming process. Second, the influence on the formfilling of the blank cutting process and the resulting sheet edge properties were investigated. The third aspect to enhance the formfilling of the gear elements was the modification of the process strategy of the incremental forming process. Due to different conditions for the initial and the following indentations, a preforming operation was investigated in order to realize a similar material flow for all indentations. With the combination of the best parameters regarding the tool surface, the blank cutting process and the forming strategy, an improvement of the formfilling of the first formed gear element by up to 33% and for the following gears by up to 13% was achieved.

An integrated macroscopic model for simulating SLM and milling processes

Due to their flexibility to also build up highly complex geometries, Additive Manufacturing (AM) processes are increasingly applied. Although near net-shape components can be manufactured using, for example, the Selective Laser Melting (SLM) process, the required surface quality can often not be achieved. In order to manufacture contact areas or functional surfaces, subsequent machining processes can be used to achieve the required accuracy in shape and dimension as well as the desired surface quality. In order to reduce the experimental effort during process design and optimization, simulation systems that are able to efficiently model both processes are required. In this paper, an empirical geometry-based model for SLM and milling processes will be presented. Due to the usage of an empirical model, based on the analysis of a set of reference structures, the simulation of macroscopic geometries can be achieved and used in subsequent milling simulations. Furthermore, an experimental validation of the combination of the two simulation models will be presented.

Case Study 1.1: Identification and Active Damping of Critical Workpiece Vibrations in Milling of Thin Walled Workpieces

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.

Case Study 2.1: Detection and Compensation of Workpiece Distortions During Machining of Slender and Thin-Walled Aerospace Parts

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.

Simulation of surface structuring considering the acceleration behaviour by means of spindle control

Due to the increasing demands on surface quality of machined surfaces, deviations of the feed velocity, which can occur in complex 5-axis milling processes and are caused by the insufficient acceleration and deceleration behaviour of the machining centre, have to be avoided. This is crucial in the case of surface structuring by means of high-feed milling. The high-feed rate can be used to generate quasi-deterministic surface structures on forming tools. Applying surface structures for forming processes, the friction can be tailored to improve the form filling of small cavities. However, in order to generate homogeneous surface structures, it is important to ensure a constant feed per tooth during the milling process. In this work, a novel approach for the predicting surface structures using a geometric physically-based simulation system is shown. Furthermore, an empirical model was developed which represents the acceleration and deceleration behaviour of the used machining centre for predicting the deviations of the feed rate. Therefore, it is possible to take the alternating feed rate into account when simulating the milling process. In addition, a control approach, for adapting the spindle speed online, is used to keep the tooth feed constant.