VOL. 15 January ISSUE YEAR 2014
in Vol. 15 - January Issue - Year 2014
Robot-Guided Finishing - Current Research Topics
Figure 1: Multipurpose robot-cell at PTZ Berlin
Figure 2: Comparison of robot-guided drag finishing and vibratory finishing of a workpiece preprocessed by adaptive belt grinding
Industrial robots today have a wide range of applicability. Typical tasks for robots in the field of production technology include welding, assembly and handling. A constant increase in flexibility and accuracy has enabled robots to perform more delicate manufacturing operations as well. Especially, the increase in accuracy has put a focus on researching and developing robot-guided cutting and finishing applications. A great potential for robot-guided cutting and finishing operations is seen in machining of parts with small batch sizes and changing dimensions and geometries. Robot-guided grinding and finishing operations tend to be especially challenging due to ever-increasing demands concerning surface roughness and close tolerances. Besides applications like deburring, robot-guided finishing processes can also be used for automation of manual processes, such as the repair of turbine blades from aircraft. Repair processes for aircraft turbines have a great economic relevance, due to the use of more and more high-cost, complex integrated parts such as blade-integrated disks (Blisks). For these parts, even elaborate automated repair processes are economically sensible. A typical repair process chain for turbine blades includes cleaning, inspection, removal of the defect area, built-up welding, recontouring, finishing and quality assurance. The recontouring of the repaired area is usually carried out by milling or belt-grinding. After this, the surface of the repaired area has to be finished so that no trace of the previous machining is still visible and the requirements concerning surface roughness are once again met. Due to the complex geometry of turbine blades and the differing sizes and positions of the repair areas, finishing of these surfaces is today still a manual process. Through automation of the process steps of recontouring and finishing, reproducibility of the repair processes can be increased and costs reduced.
Multipurpose Robot-Cell For Finishing Processes
At the Production Technology Center (PTZ) Berlin the applicability of robot-guided finishing processes such as belt-grinding and robot-guided drag finishing is examined and applications are being developed by researchers from the Institute for machine Tools and Manufacturing of the Technical University Berlin. For this, a multi-purpose robot cell was set up consisting of a Comau NJ 370 6-axis industrial robot, an adaptive belt-grinding station and belt-grinding head, three vibratory finishing bowls and a 3D-measuring device for inspection. Figure 1 gives an overview of the setup at the PTZ. The inspection of the parts before, during and after machining plays an important role in an automated repair process for turbine blades. At the PTZ Berlin, this is done with a 3D-measuring device GOM ATOS SO. This system has an accuracy of 20 µm and is capable of fully digitalizing a part for a set-actual-comparison with CAD-data. Based on the data provided by the 3D-measurement, finishing processes can be defined.
Adaptive Robot Guided Belt-Grinding
For robot-guided grinding in general, some difficult challenges have to be overcome. Usual industrial robots do not provide the positioning accuracy and stiffness needed for grinding operations. For this reason, all robot-guided grinding processes at PTZ Berlin, in this case belt grinding, are force-controlled. Two setups for force-controlled processes are available. The first system is a pneumatic belt grinding station for a one-axis force-controlled process in which the robot holds the workpiece. This setup is very similar to commercially available and industrially widely applied systems. Great possibilities lie in the force-controlled application of a grinding head attached to the robot. The system, developed at the PTZ Berlin and installed in the multipurpose robot cell, is capable of controlling forces and torque in all three axes and has a fast enough reaction time for grinding operations.
With these systems, an adaptive machining strategy for recontouring of turbine blades was developed at the PTZ Berlin. Based on the 3D-measurements, trajectories are planned automatically for workpieces to be repaired. The technological parameters for the process such as feed rate and cutting speed are retrieved from an extensive database correlating processing results such as material removal rates with process parameters. The database is the result of the research in belt-grinding processes at the PTZ Berlin. The adaptive belt- grinding process is iterative, meaning that it includes several steps from rough to fine grinding with subsequent 3D-measurement and recalculation of process parameters after every step.
Robot-Guided Drag Finishing
Powerful turbines require great surface qualities on their blades. This has to be ensured after repair as well. No traces of the repair processes should be visible and usually no aligned machining traces are allowed on the blades. For this reason, mass finishing with loose abrasives is very suitable for the last step in turbine blade repair. At the PTZ Berlin, mass finishing is applied as vibratory finishing, robot-guided drag finishing, and a combination of the two. Robot-guided drag finishing has a kinematic flexibility that allows a controlled machining of complex parts combined with the possibility of achieving high relative velocities between workpieces and the abrasive media leading to improved material removal rates. The potential of robot-guided drag finishing is exemplarily shown in the results in Figure 2: here vibratory finishing with stationary workpieces is compared to robot-guided drag finishing. The test workpieces are made of the Ni-base alloy Inconel 718 and were preprocessed by robot-guided belt finishing. For the vibratory finishing, the work pieces were affixed in the bowl at a depth of at = 100 mm and slowly rotated to ensure an even material removal. For drag finishing, a sine-wave dipping movement with an amplitude of as = 100 mm at an average depth of at = 100 mm was programmed; the workpiece speed was set to vW = 30 m/min. In this experiment, the surface roughness Ra was measured on three positions of the workpiece. The media consisted of non-abrasive ceramic particles in combination with a polishing slurry. Figure 2 shows the surface roughness development over time for both processes.
The results from the experiment show a 40 % decrease in surface roughness Ra after a processing time of tp = 35 min for drag finishing in comparison to a rather slow improvement during vibratory finishing. Furthermore, due to the kinematics of the drag-finishing process, the surface roughness was very similar for all measured positions on the workpiece.
Conclusion And Outlook
The multipurpose robot-cell at PTZ Berlin provides the possibility to research and develop different finishing processes such as adaptive belt grinding and mass finishing. Investigations into automation of repair processes as described in this article pose a great starting point for further research in this area. Exemplarily, the possibilities for a combination of adaptive belt grinding and robot-guided drag finishing were shown. A comparison of robot-guided drag finishing with the state of the art process vibratory finishing shows the potential of drag finishing with individually designed kinematics.
Looking beyond the described repair processes, a wide range of applications in finishing is possible for multipurpose robot cells. Parts with very different sizes and shapes can be machined with one setup. Especially, the deburring of complex parts seems very promising, as this is oftentimes still a manual process. At PTZ Berlin, basic research in the area of finishing is currently focused on understanding fundamental material removal processes and developing numerical and empirical models. In the area of finishing, research is focused not only on mass finishing and belt-grinding but also on machining with abrasive brushes, abrasive flow machining, lapping and honing. Based on the knowledge acquired through research, the overall goal at PTZ Berlin is to be able to design individual, flexible and precise processes and machining systems for a broad range of applications, parts and materials.
Dipl.-Ing. Arne Dethlefs
Head of Finishing Technologies
Institute for Machine Tools and Factory Management
Technical University Berlin