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Advanced Mechatronic System for Manufacturing and Repair of Turbine Blades
Copyright © IFAC Information Control in Manufacturing, Nancy - Metz, France, 1998
ADV ANCED MECHATRONIC SYSTEM FOR MANUFACTURING AND REPAIR OF TURBINE BLADES
U. Berger, R. Janssen, E. Brinksmeier
Institute for Material Science (IWT) , Division of Manufacturing Technologies Badgasteiner Str. 3, 28359 Bremen, Germany
Abstract: Aspects of advanced technology for individual jet engine components repair are discussed. The technological solutions lie in the domain of cutting path planning and machining of difficult-to-cut materials. The paper presents an integrated process chain for multi-axis high speed milling of airfoil materials like titanium or nickel alloys. The main development items are based on a 3D optoelectronic sensor system, simulation and cutting path planning system and a milling reference model. The results of these research developments are pursued within an ESPRIT project in the domain Intelligent Equipment and Control. Copyright © 1998IFAC
Keywords: Turbine components, Adaptive machining, Optical sensor system, Path planning, CAD/CAM.
cutting temperatures and strong adhesion between tools and chips. Increasing the cutting speed leads to high thermal load of tool and workpiece, which affects the material microstructure. This transformation has to be avoided due to the functional behaviour of this parts under load. Titanium alloys are used mainly for the stationary vanes and the rotating blades in the first compressor stages of a turbine . Their geometry varies from 25 to 500 mm total length and 20 to 200 mm total width. Since original manufacturing of turbine blades and vanes is done by contour milling under mainly automated manufacturing conditions, the repair processes are done by manual operation. The repair of turbine blades starts with the removal of the damaged area and replacement by raw block-material in a welding process. Finally the welded material has to be machined back to the original shape. This operation is highly time and cost consuming and has a lack of sufficient process monitoring in view of traceability demands. Automation of the machining process in repair stage has not progressed significantly, due to the great
1. INTRODUCTION Jet engine components are important elements of modem aircraft technology. They affect safety and economical behaviour of the engine. Especially turbine blades and vanes due to safety and functional features will lead to higher engine efficiency, reduced fuel consumption and noise emission. On the other side, are turbine blades requiring regular and certified repair and maintenance cycles. The problem related to the manufacturing and repair processes is twofold: - Due to optimised aerodynamic features modem turbine blades and vanes trend towards free form surface geometry and high fmishing accuracy in form , profile and surface. Thus the machining and assessment processes require geometric flexibility and ensurance of precise cutting paths. - Utilisation of materials with high specific strength and heat resistance, mainly condition found in difficult-to-machine materials such as titanium alloys like TiA16V4. The low heat conductivity and high chemical reactivity causes increased tool wear, high 295
improvement of surface roughness and avoidance of tensile residual stresses. The reference model implemented in the manufacturing demonstrator supports the staff involved in repair work preparation and the machine operators in selection of tools , coolant supply and machining parameters. The throughput time and machining costs can be substantially reduced. Furthermore, the part quality will be ensured at the required level.
2. DEVELOPMENT OF A PROCESS CHAIN FOR REPAIRING JET ENGINE COMONENTS
Fig. I : Jet engine components in repair status difficulties encountered when trying to control specific aspects of a component's necessary repair characteristics: - follow a precise path and adjust certain varying part parameters - program the machining path to follow the complex geometric part surface in the repair sector. Due to their exposure in the hazardous jet engine environment, airfoils in operation status can vary geometrical from other airfoils or new ones. It is therefore difficult to automate the machining processes required, because a unique repair process with its own geometrical and technological parameters is necessary for each airfoil. Fig. I shows a typical turbine blade before repair takes place. An advanced machining system based on a multiaxis general purpose high-speed milling center enables adaptive reshaping of blades with individual geometries due to the functional requirements. The RTD work performed so far to reach these goals are: Conceptualisation of a technology model combining geometrical and high speed milling features . Development of a milling reference model, based on analogous field test experiments, for selection of tool and coolant supply and setting of machining parameters. The milling reference model leads to a manufacturing database, which can be connected directly to the machine tool controller and enhances significantly the operators process assessment and understanding. Laboratory set-up of a 4-axis high speed milling center including tool and pallet changer and application oriented fixtures to avoid vibrations and chatter on the workpiece surface. The experimental field test demonstrator is equipped with force/torque measurement sensors for in-process monitoring and evaluation of machining results. Acoustic emission process surveillance is also under consideration. Experimental set-up of an optoelectronic 3D shape measurement sensor system for integration in the machining platform, based on coded light and phase shifting principles. The machining investigations and verification experiments performed so far contribute to increasing of process stability and tool life,
The machining of individual jet engine components as turbine blades and vanes require different technological issues than existing ones deliver. Applied systems should combine adaption features , easy and transparent programming and shorten the throughput cycles due to cost and delivery time demands . To set up a multi-axis milling cell in view of sensor based reverse engineering and quality inspection, has been developed a process chain starting from geometry design towards quality control of machined parts. The process chain is shown in Fig. 2 and consists mainly of two loops. In the first loop a multi-axis NC programming system generates the milling path for repairing a turbine blade based on its geometry description . The geometry of the blade is generated on a CAD system and then transferred into the NC programming system via neutral data exchange formats (e.g. IGES and VDAFS). The tool paths generated by the NC programming system describes the tool location of a pre-selected tool in CLDA TA format. Then, a multi-axis postprocessor generates the necessary machine control codes for the specific selected CNC-machine. These machine control codes are checked for their correctness by a NC simulation and verification system . In the simulation only geometric aspects of machining processes are considered. These include the detection of collisions in the machining environment, the visualization of material removal of the workpiece, handling of jigs and fixtures and the numerical verification of the workpiece surface quality. If deviations between the original CAD geometry and the simulated geometry exceed a specified machining tolerance, the NC program must be corrected in the NC programming system and checked again by the simulation system . Otherwise, the simulation result is put into the technology processor in the second loop that determines necessary process parameters for the NC programme. After calibration of the multi-axis machine tool center and set-up of the fixture, tool and workpiece, a test part is machined. The machined part is fmally measured and a geometric model is generated by a reverse engineering software (Berger, et al. , 1996). If the geometry model is within the allowable tolerance zone, the real part can
296
be machined. Otherwise, machining parameters must be adaptively changed. The machining technology processor is based on a milling reference model, which contains operational parameter settings.
technology. In principle, the motion of machine axes can be displayed on a graphical screen by compiling the NC programs. The milling process (removing material away from workpiece) can be simulated by the Boolean operation of subtracting the tool swept volume from the workpiece. From a kinematic point of view, a NC-machine is mainly built up of translational and rotational segments which can be divided into two types: one is workpiece carrying and the other is tool carrying. These are represented by two kinematic chains, whose end segments are workpiece and tool respectively. Both chains start with the machine base as shown in Fig. 3 (Paul, 1981).
CAD Geometry Loop Change mllhng paths CAD geometry in IGES . VDAFS and STL formats
Since an NC-machine consists mainly of a set of segments, one can model the geometry of these segments separately. The geometric construction of the segments is done interactively by combining simple geometric primitives. The primitives currently implemented in the system are box, cylinder, sphere, cone and torus. They are geometrically dermed by a few parameters, e.g. a box can be specified by length, height, width and a corner point. A solid modelling programme based on boundary representation is used to create and manipulate these primitives. To simplify implementation of the Boolean set operations the solid modelling programme is designed to handle only planar faced objects. It is, however, easily possible to approximate analytical surfaces as multiply-connected planar faces. For user convenience, the system supports also basic geometric operations of the object manipulations in the modelling space, such as translation, rotation and scaling etc.
Technology Loop
Change technoaogy pa rameters
Quality Inspection & Reverse Engineering (3D shape measurement sensor) Manufacturing tolerance
E~ . ----------------~ Machined and approved parts 6erO l g.Jc
Fig. 2: Process chain for multi-axis milling cell
3. SIMULATION AND VERIFICATION OF MUL TI-AXIS MILLING PROCESSES Due to complex geometry of turbine blades multiaxis milling machines are needed for the repair purpose. However, the generation of error-free NC programs for these machines is still a difficult and time-consuming task despite computer-assisted tools. An NC program consists of a series of cutting tool movements which remove material from a piece of raw stock to create a prototype part, mould or stamping die. In this process one can use a high level language like APT or a CAD system to derme the geometry and cutter sequence, calculate cutter offsets and produce a cutter location data (CLDA TA) file. The CLDA TA file must then be postprocessed into a machine control data file which contains the instructions to control the specific selected machine tool. The next step is the verification of the NC programme to eliminate any errors. Generally, verification of NC programs is a timeconsuming process. Modern CAD systems provide a limited fonn of NC simulation by displaying machining paths with respect to workpiece. But it is difficult and often even impossible to imagine how the machine axes move just from scanning the cutter location data. This complicated process can be significantly simplified by using computer simulation
workpiece =
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Tool supporting chain
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Fig. 3: Kinematic configuration ofNC-machine Once the segments have been geometrically created, they can be assigned to the corresponding machine components to build the whole machine geometry. This begins with the machine base component and completes at the tool and workpiece carrying components respectively. Each segment is specified with machine axis type, moving axis and working range. In order to build the simulation environment 297
easily and quickly a library with a variety of geometric segments is also provided. A number of milling machines have been modelled by the system in (Sheng, 1992).
modelling tasks are distinguished: modelling of master parts and modelling of repaired parts. As defmed in the previous section, a master model is built for a new or a not-damaged blade. Therefore it can be modelled either according to an engineering drawing or using a scanned point set. Since this kind of modelling is off-line, available commercial tools such as CAD systems or reverse engineering software can be used. Unlike the modelling of a master part, a repaired turbine blade should be modelled on-line and with only few human interactions. The repair of turbine blades has to be performed as automatically as possible because of economical demands. A 3D sensor based on the coded light approach is used to measure boundary surfaces of turbine blades.
4. REPAIR BASED ADAPTIVE MILLING AND ON-LINE SENSOR DA TA MODELLING In order to perform the repair procedure as automatically as possible, an adaptive milling technique was applied. The principle of the adaptive milling for the repair purpose is to generate a milling path for a turbine blade/vane (following named "blade") geometry and modify it for a defect blade according to the geometrical information provided by 3D sensors. Based on this concept the following major steps for an automatic repair process of turbine blades were defmed: - Building the master models. In this step turbine blades and vanes are classified into categories according to their geometric shapes. Following a solid model for each category is built and refered to the master model. - Generating the master milling paths. For each master model a master milling path is generated to mill out a master model from a stock, e.g. from the bounding box of the model. The master milling path is needed only once for each of master models, thus it can be generated off-line and in a previous step. Hence existing commercial solutions (CAD/CAM systems) can be used for this purpose. - Creating the actual model. An actual model is the model of a particular turbine blade to be repaired. The geometry of this particular part is captured by a 3D sensor as a set of points which are on boundary surfaces of the part. In order to generate or modify the milling path a high geometric representation is needed. - Matching the actual model with the master model. After creating the actual model the corresponding master model can be determined. By matching both models the deviation between them were calculated. The deviation will be used to modify the master milling path in the next step. Modifoing the master milling path. The modification can be done in different ways. In general sections in which changes have taken place are identified. Following the portions of paths in these sections are modified accordingly. - Adding technological parameters and postprocessing . In this step technological parameters such as feed speed, tool engagements and spindle rotations will be determined and connected to milling paths according to the blade material. The real NC programmes are fmally generated for a particular machine tool via postprocessor. The general objective of blades modelling is to generate solid models with free form surfaces as boundaries for turbine blades. For this two kinds of
. /i
.1 Workpiece
,iIF '
,p' fl'lii{"lil ,. ~. ,
d -
-
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Px
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LCO line projector
Fig. 4:
CCO camera
Principle of 3D measurement with Coded Light Approach
Fig. 5: Modelling example for a turbine vane (length appr. 150 mm) 298
The coded light approach is an absolute measurement method, that requires only a small number of images to obtain a full depth-image. This is done by projecting sequences of line-patterns (Fig. 4). The LCD-field of the projector carries switchable lines (light = "1" or dark = "0"). These lines are adressed with the Gray-code, which is a special binary code. The output of this device is a set of point clouds. To obtain a complete point set for a blade, the scanning must be performed in several views. Thus, a whole point model for the blade is obtained. Fig. 5 shows a modelling example for a turbine vane. The model is scanned in six different views and obtained contains 23152 points (Berger, 1995; Stabs and Wahl, 1992).
5. MILUNG REFERENCE MODEL MANUF ACTURING EXPERIMENTS
Fig. 6:
Experimental set-up
AND
The repair process for turbine components requires high demands on dimensional accuracy, surface roughness and material microstructure. To close the gap between simulation and verification of multi-axis milling processes, there will be established several milling reference processes according to material specifications in an experimental environment. This reference processes gain the required machining parameters, tool and workpiece behaviour and transformation of material properties under real machining conditions. Further experiments were performed in regard to increasing process stability and tool life, improvement of surface roughness and avoidance of tensile residual stresses. The reference processes will lead to an overall milling reference model, which supports both, the work preparation staff and the machine operators in choosing of machining parameters. The titanium alloy TiAI6V4 is in aerospace industrial from great importance and was therefore chosen for the first machining experiments. Titanium alloys perform high specific strength and heat resistance (strength-to-density ratio), but in any cases they are difficult to machine. The low thermal conductivity and high chemical reactivity of titanium alloys causes increased tool wear, high cutting temperatures and strong adhesion between tool and part surface (Eckstein, et al., 1991; Brinksmeier et al., 1997). Accordingly, practical milling investigations were carried out. For this a representative series of un coated and coated end mills of carbide metal and high speed steel were selected for the experiments. Experimental research focusses on cutting tool performance (e.g. breakage of cutting edges under high speed milling conditions and wear mechanisms while using different lubricants and cooling supply strategies). Fig. 6 shows the experimental set-up for the milling experiments, on the CNC-machining center.
Fig. 7: Relation between tool life travel and cutting speed The effect of the cutting speed on the tool life travel of un coated HSS tools is shown in Figure 7. It can be established that an increasing cutting speed reduces the tool life of HSS tools rapidly. An increase of the cutting speed to 90 m/min shortens the tool life travel down to 0,50 m. A further increase to 100 m/min is almost impossible. In this case, the tool failed after a milling path of 0,20 m. It can be established that milling processes under high speed conditions with cutting speeds over 100 m per minute are impossible with the HSS tools. For additional investigations, the experiments were extended with carbide tools . Compared with HSS tools the carbide tools have shown clearly better performance under high speed cutting conditions. Increasing cutting speeds also lead to reduced tool life travel, but compared with HSS tools higher cutting speed can be achieved. Generally, the correct use of coolants during machining operations greatly extents tool life. The investigations have shown that the use of overflow lubrication with a 6% emulsion and also the use of minimum quantity lubrication strategies reduces the tool wear visible. The reduction of the heat generated by the process reduces also the tendency of welded chips on the tool surface. Especially in the case of milling the use of minimum quantity lubrication is suitable to reduce the tool wear (Brinksmeier, et al., 1997). In this case a special device atomizes the 299
Program with the title: Advanced Mechatronics Technology For Turbine Blades Repair (AMA TEUR). The other project partners are ZENON S.A. (GR), Hellenic Aerospace Industry (GR), APS Gesellschaft fur Automatisierung, ProzeBsteuerung, Schweilltechnik m.b.H . (DE), Bremen Institute of Industrial Technology and Applied Work Science at the University of Bremen (DE), Staubli France S.A. (FR).
coolant and supplies it with very low flow rates ..For the use of HSS tools, it can be established that the tendency of the titanium to pressure weld to the tool flank can almost completely avoided by the use of oil mist lubrication.
REFERENCES
Fig. 8:
Berger, U. (1995). Development of a Sensor-Based Planning and Programming System for Industrial Robot Manufacturing of One-of-a-Kind and Small Batch Production, PhD Dissertation , University of Bremen. Berger, U., Sheng, X., WaIter, A. (1996). Simulation and Verification of 5-Axis Milling Processes for Repairing Turbine Blades, SURFAIR Xl, ll eme lourmies Inlernalionales dElude sur les Trailements de Surfaces dans l 'Industrie Aeronautique et Spatiale, Cannes. Brinksmeier, E., Berger, U., Janssen, R. (1997). High-Speed Milling of TiAI6V4 for Aircraft st Applications, 1 French and German Conference on High Speed Machining, 17. - 18. June 1997, University of Metz. Eckstein, M. et a/. (1991). Schaftfrasen von Titanlegierungen mit hohen Schnittgeschwindigkeiten. VDl-Z 133 12, pp. 28-34. Paul, R.P. (1981). Robot Manipulators : Mathematics, Programming and Control, MIT Press, Cambridge, MA . Sheng, X. (1992). Solid Modelling Based Geometric and Graphical Simulation of Multi-axis Milling Processes, PhD Dissertation, University of Bremen. Stahs, T. G. , Wahl, F. M. (1992). Fast and Versatile Range Data Acquisition in a Robot Work Cell. Proceedings of the IEEEIRSJ International Conference on Intelligent Robots and Systems. pp. 1169-1174. Raleigh, USA.
Welded chips on the tool flank
6. SUMMARY The basic issue of the described developments is the integrated process chain for multi-axis milling of turbine blades and vanes. Due to the requirements in machining of the individuality of the jet engine components as turbine blades and vanes specific attention is paid to high-speed milling aspects. The geometrical loop concerning cutting path planning and adaptation is based on a mathematical model for a generic machine tool system. Features like collision avoidance, jig and fixture modelling and universal postprocessing are included. For quality inspection and reverse engineering purposes a 3D shape measurement system based on Coded Light Approach and phase shifting techniques is integrated in the information loop. The presented results of the investigations carried out show an overview of the possibilities for influencing the milling process of titanium alloys. Due to the results performed so far it can be established that higher cutting speeds on the machining of TiAI6V 4 can be realized according to aerospace specification. The appropriate selection of tool materials, cutting parameters and coolant lubrication constitutes the basis for a safe machining process. Further investigations will be performed to achieve a wide range of machining data for suitable milling conditions to ensure increasing process stability. The pursued parameter database supports the machine operators involved in repair work preparation and selection of cutting data.
7. ACKNOWLEDGEMENT The presented paper is based on a RTD project funded by the European Commission in the ESPRIT
300
ADV ANCED MECHATRONIC SYSTEM FOR MANUFACTURING AND REPAIR OF TURBINE BLADES
U. Berger, R. Janssen, E. Brinksmeier
Institute for Material Science (IWT) , Division of Manufacturing Technologies Badgasteiner Str. 3, 28359 Bremen, Germany
Abstract: Aspects of advanced technology for individual jet engine components repair are discussed. The technological solutions lie in the domain of cutting path planning and machining of difficult-to-cut materials. The paper presents an integrated process chain for multi-axis high speed milling of airfoil materials like titanium or nickel alloys. The main development items are based on a 3D optoelectronic sensor system, simulation and cutting path planning system and a milling reference model. The results of these research developments are pursued within an ESPRIT project in the domain Intelligent Equipment and Control. Copyright © 1998IFAC
Keywords: Turbine components, Adaptive machining, Optical sensor system, Path planning, CAD/CAM.
cutting temperatures and strong adhesion between tools and chips. Increasing the cutting speed leads to high thermal load of tool and workpiece, which affects the material microstructure. This transformation has to be avoided due to the functional behaviour of this parts under load. Titanium alloys are used mainly for the stationary vanes and the rotating blades in the first compressor stages of a turbine . Their geometry varies from 25 to 500 mm total length and 20 to 200 mm total width. Since original manufacturing of turbine blades and vanes is done by contour milling under mainly automated manufacturing conditions, the repair processes are done by manual operation. The repair of turbine blades starts with the removal of the damaged area and replacement by raw block-material in a welding process. Finally the welded material has to be machined back to the original shape. This operation is highly time and cost consuming and has a lack of sufficient process monitoring in view of traceability demands. Automation of the machining process in repair stage has not progressed significantly, due to the great
1. INTRODUCTION Jet engine components are important elements of modem aircraft technology. They affect safety and economical behaviour of the engine. Especially turbine blades and vanes due to safety and functional features will lead to higher engine efficiency, reduced fuel consumption and noise emission. On the other side, are turbine blades requiring regular and certified repair and maintenance cycles. The problem related to the manufacturing and repair processes is twofold: - Due to optimised aerodynamic features modem turbine blades and vanes trend towards free form surface geometry and high fmishing accuracy in form , profile and surface. Thus the machining and assessment processes require geometric flexibility and ensurance of precise cutting paths. - Utilisation of materials with high specific strength and heat resistance, mainly condition found in difficult-to-machine materials such as titanium alloys like TiA16V4. The low heat conductivity and high chemical reactivity causes increased tool wear, high 295
improvement of surface roughness and avoidance of tensile residual stresses. The reference model implemented in the manufacturing demonstrator supports the staff involved in repair work preparation and the machine operators in selection of tools , coolant supply and machining parameters. The throughput time and machining costs can be substantially reduced. Furthermore, the part quality will be ensured at the required level.
2. DEVELOPMENT OF A PROCESS CHAIN FOR REPAIRING JET ENGINE COMONENTS
Fig. I : Jet engine components in repair status difficulties encountered when trying to control specific aspects of a component's necessary repair characteristics: - follow a precise path and adjust certain varying part parameters - program the machining path to follow the complex geometric part surface in the repair sector. Due to their exposure in the hazardous jet engine environment, airfoils in operation status can vary geometrical from other airfoils or new ones. It is therefore difficult to automate the machining processes required, because a unique repair process with its own geometrical and technological parameters is necessary for each airfoil. Fig. I shows a typical turbine blade before repair takes place. An advanced machining system based on a multiaxis general purpose high-speed milling center enables adaptive reshaping of blades with individual geometries due to the functional requirements. The RTD work performed so far to reach these goals are: Conceptualisation of a technology model combining geometrical and high speed milling features . Development of a milling reference model, based on analogous field test experiments, for selection of tool and coolant supply and setting of machining parameters. The milling reference model leads to a manufacturing database, which can be connected directly to the machine tool controller and enhances significantly the operators process assessment and understanding. Laboratory set-up of a 4-axis high speed milling center including tool and pallet changer and application oriented fixtures to avoid vibrations and chatter on the workpiece surface. The experimental field test demonstrator is equipped with force/torque measurement sensors for in-process monitoring and evaluation of machining results. Acoustic emission process surveillance is also under consideration. Experimental set-up of an optoelectronic 3D shape measurement sensor system for integration in the machining platform, based on coded light and phase shifting principles. The machining investigations and verification experiments performed so far contribute to increasing of process stability and tool life,
The machining of individual jet engine components as turbine blades and vanes require different technological issues than existing ones deliver. Applied systems should combine adaption features , easy and transparent programming and shorten the throughput cycles due to cost and delivery time demands . To set up a multi-axis milling cell in view of sensor based reverse engineering and quality inspection, has been developed a process chain starting from geometry design towards quality control of machined parts. The process chain is shown in Fig. 2 and consists mainly of two loops. In the first loop a multi-axis NC programming system generates the milling path for repairing a turbine blade based on its geometry description . The geometry of the blade is generated on a CAD system and then transferred into the NC programming system via neutral data exchange formats (e.g. IGES and VDAFS). The tool paths generated by the NC programming system describes the tool location of a pre-selected tool in CLDA TA format. Then, a multi-axis postprocessor generates the necessary machine control codes for the specific selected CNC-machine. These machine control codes are checked for their correctness by a NC simulation and verification system . In the simulation only geometric aspects of machining processes are considered. These include the detection of collisions in the machining environment, the visualization of material removal of the workpiece, handling of jigs and fixtures and the numerical verification of the workpiece surface quality. If deviations between the original CAD geometry and the simulated geometry exceed a specified machining tolerance, the NC program must be corrected in the NC programming system and checked again by the simulation system . Otherwise, the simulation result is put into the technology processor in the second loop that determines necessary process parameters for the NC programme. After calibration of the multi-axis machine tool center and set-up of the fixture, tool and workpiece, a test part is machined. The machined part is fmally measured and a geometric model is generated by a reverse engineering software (Berger, et al. , 1996). If the geometry model is within the allowable tolerance zone, the real part can
296
be machined. Otherwise, machining parameters must be adaptively changed. The machining technology processor is based on a milling reference model, which contains operational parameter settings.
technology. In principle, the motion of machine axes can be displayed on a graphical screen by compiling the NC programs. The milling process (removing material away from workpiece) can be simulated by the Boolean operation of subtracting the tool swept volume from the workpiece. From a kinematic point of view, a NC-machine is mainly built up of translational and rotational segments which can be divided into two types: one is workpiece carrying and the other is tool carrying. These are represented by two kinematic chains, whose end segments are workpiece and tool respectively. Both chains start with the machine base as shown in Fig. 3 (Paul, 1981).
CAD Geometry Loop Change mllhng paths CAD geometry in IGES . VDAFS and STL formats
Since an NC-machine consists mainly of a set of segments, one can model the geometry of these segments separately. The geometric construction of the segments is done interactively by combining simple geometric primitives. The primitives currently implemented in the system are box, cylinder, sphere, cone and torus. They are geometrically dermed by a few parameters, e.g. a box can be specified by length, height, width and a corner point. A solid modelling programme based on boundary representation is used to create and manipulate these primitives. To simplify implementation of the Boolean set operations the solid modelling programme is designed to handle only planar faced objects. It is, however, easily possible to approximate analytical surfaces as multiply-connected planar faces. For user convenience, the system supports also basic geometric operations of the object manipulations in the modelling space, such as translation, rotation and scaling etc.
Technology Loop
Change technoaogy pa rameters
Quality Inspection & Reverse Engineering (3D shape measurement sensor) Manufacturing tolerance
E~ . ----------------~ Machined and approved parts 6erO l g.Jc
Fig. 2: Process chain for multi-axis milling cell
3. SIMULATION AND VERIFICATION OF MUL TI-AXIS MILLING PROCESSES Due to complex geometry of turbine blades multiaxis milling machines are needed for the repair purpose. However, the generation of error-free NC programs for these machines is still a difficult and time-consuming task despite computer-assisted tools. An NC program consists of a series of cutting tool movements which remove material from a piece of raw stock to create a prototype part, mould or stamping die. In this process one can use a high level language like APT or a CAD system to derme the geometry and cutter sequence, calculate cutter offsets and produce a cutter location data (CLDA TA) file. The CLDA TA file must then be postprocessed into a machine control data file which contains the instructions to control the specific selected machine tool. The next step is the verification of the NC programme to eliminate any errors. Generally, verification of NC programs is a timeconsuming process. Modern CAD systems provide a limited fonn of NC simulation by displaying machining paths with respect to workpiece. But it is difficult and often even impossible to imagine how the machine axes move just from scanning the cutter location data. This complicated process can be significantly simplified by using computer simulation
workpiece =
'"1
:-~
tool -;
0
I- r ~
Workpiece supporting chain
e- ~ .
Tool supporting chain
&,018 1
Fig. 3: Kinematic configuration ofNC-machine Once the segments have been geometrically created, they can be assigned to the corresponding machine components to build the whole machine geometry. This begins with the machine base component and completes at the tool and workpiece carrying components respectively. Each segment is specified with machine axis type, moving axis and working range. In order to build the simulation environment 297
easily and quickly a library with a variety of geometric segments is also provided. A number of milling machines have been modelled by the system in (Sheng, 1992).
modelling tasks are distinguished: modelling of master parts and modelling of repaired parts. As defmed in the previous section, a master model is built for a new or a not-damaged blade. Therefore it can be modelled either according to an engineering drawing or using a scanned point set. Since this kind of modelling is off-line, available commercial tools such as CAD systems or reverse engineering software can be used. Unlike the modelling of a master part, a repaired turbine blade should be modelled on-line and with only few human interactions. The repair of turbine blades has to be performed as automatically as possible because of economical demands. A 3D sensor based on the coded light approach is used to measure boundary surfaces of turbine blades.
4. REPAIR BASED ADAPTIVE MILLING AND ON-LINE SENSOR DA TA MODELLING In order to perform the repair procedure as automatically as possible, an adaptive milling technique was applied. The principle of the adaptive milling for the repair purpose is to generate a milling path for a turbine blade/vane (following named "blade") geometry and modify it for a defect blade according to the geometrical information provided by 3D sensors. Based on this concept the following major steps for an automatic repair process of turbine blades were defmed: - Building the master models. In this step turbine blades and vanes are classified into categories according to their geometric shapes. Following a solid model for each category is built and refered to the master model. - Generating the master milling paths. For each master model a master milling path is generated to mill out a master model from a stock, e.g. from the bounding box of the model. The master milling path is needed only once for each of master models, thus it can be generated off-line and in a previous step. Hence existing commercial solutions (CAD/CAM systems) can be used for this purpose. - Creating the actual model. An actual model is the model of a particular turbine blade to be repaired. The geometry of this particular part is captured by a 3D sensor as a set of points which are on boundary surfaces of the part. In order to generate or modify the milling path a high geometric representation is needed. - Matching the actual model with the master model. After creating the actual model the corresponding master model can be determined. By matching both models the deviation between them were calculated. The deviation will be used to modify the master milling path in the next step. Modifoing the master milling path. The modification can be done in different ways. In general sections in which changes have taken place are identified. Following the portions of paths in these sections are modified accordingly. - Adding technological parameters and postprocessing . In this step technological parameters such as feed speed, tool engagements and spindle rotations will be determined and connected to milling paths according to the blade material. The real NC programmes are fmally generated for a particular machine tool via postprocessor. The general objective of blades modelling is to generate solid models with free form surfaces as boundaries for turbine blades. For this two kinds of
. /i
.1 Workpiece
,iIF '
,p' fl'lii{"lil ,. ~. ,
d -
-
"
,>
Px
Fp
LCO line projector
Fig. 4:
CCO camera
Principle of 3D measurement with Coded Light Approach
Fig. 5: Modelling example for a turbine vane (length appr. 150 mm) 298
The coded light approach is an absolute measurement method, that requires only a small number of images to obtain a full depth-image. This is done by projecting sequences of line-patterns (Fig. 4). The LCD-field of the projector carries switchable lines (light = "1" or dark = "0"). These lines are adressed with the Gray-code, which is a special binary code. The output of this device is a set of point clouds. To obtain a complete point set for a blade, the scanning must be performed in several views. Thus, a whole point model for the blade is obtained. Fig. 5 shows a modelling example for a turbine vane. The model is scanned in six different views and obtained contains 23152 points (Berger, 1995; Stabs and Wahl, 1992).
5. MILUNG REFERENCE MODEL MANUF ACTURING EXPERIMENTS
Fig. 6:
Experimental set-up
AND
The repair process for turbine components requires high demands on dimensional accuracy, surface roughness and material microstructure. To close the gap between simulation and verification of multi-axis milling processes, there will be established several milling reference processes according to material specifications in an experimental environment. This reference processes gain the required machining parameters, tool and workpiece behaviour and transformation of material properties under real machining conditions. Further experiments were performed in regard to increasing process stability and tool life, improvement of surface roughness and avoidance of tensile residual stresses. The reference processes will lead to an overall milling reference model, which supports both, the work preparation staff and the machine operators in choosing of machining parameters. The titanium alloy TiAI6V4 is in aerospace industrial from great importance and was therefore chosen for the first machining experiments. Titanium alloys perform high specific strength and heat resistance (strength-to-density ratio), but in any cases they are difficult to machine. The low thermal conductivity and high chemical reactivity of titanium alloys causes increased tool wear, high cutting temperatures and strong adhesion between tool and part surface (Eckstein, et al., 1991; Brinksmeier et al., 1997). Accordingly, practical milling investigations were carried out. For this a representative series of un coated and coated end mills of carbide metal and high speed steel were selected for the experiments. Experimental research focusses on cutting tool performance (e.g. breakage of cutting edges under high speed milling conditions and wear mechanisms while using different lubricants and cooling supply strategies). Fig. 6 shows the experimental set-up for the milling experiments, on the CNC-machining center.
Fig. 7: Relation between tool life travel and cutting speed The effect of the cutting speed on the tool life travel of un coated HSS tools is shown in Figure 7. It can be established that an increasing cutting speed reduces the tool life of HSS tools rapidly. An increase of the cutting speed to 90 m/min shortens the tool life travel down to 0,50 m. A further increase to 100 m/min is almost impossible. In this case, the tool failed after a milling path of 0,20 m. It can be established that milling processes under high speed conditions with cutting speeds over 100 m per minute are impossible with the HSS tools. For additional investigations, the experiments were extended with carbide tools . Compared with HSS tools the carbide tools have shown clearly better performance under high speed cutting conditions. Increasing cutting speeds also lead to reduced tool life travel, but compared with HSS tools higher cutting speed can be achieved. Generally, the correct use of coolants during machining operations greatly extents tool life. The investigations have shown that the use of overflow lubrication with a 6% emulsion and also the use of minimum quantity lubrication strategies reduces the tool wear visible. The reduction of the heat generated by the process reduces also the tendency of welded chips on the tool surface. Especially in the case of milling the use of minimum quantity lubrication is suitable to reduce the tool wear (Brinksmeier, et al., 1997). In this case a special device atomizes the 299
Program with the title: Advanced Mechatronics Technology For Turbine Blades Repair (AMA TEUR). The other project partners are ZENON S.A. (GR), Hellenic Aerospace Industry (GR), APS Gesellschaft fur Automatisierung, ProzeBsteuerung, Schweilltechnik m.b.H . (DE), Bremen Institute of Industrial Technology and Applied Work Science at the University of Bremen (DE), Staubli France S.A. (FR).
coolant and supplies it with very low flow rates ..For the use of HSS tools, it can be established that the tendency of the titanium to pressure weld to the tool flank can almost completely avoided by the use of oil mist lubrication.
REFERENCES
Fig. 8:
Berger, U. (1995). Development of a Sensor-Based Planning and Programming System for Industrial Robot Manufacturing of One-of-a-Kind and Small Batch Production, PhD Dissertation , University of Bremen. Berger, U., Sheng, X., WaIter, A. (1996). Simulation and Verification of 5-Axis Milling Processes for Repairing Turbine Blades, SURFAIR Xl, ll eme lourmies Inlernalionales dElude sur les Trailements de Surfaces dans l 'Industrie Aeronautique et Spatiale, Cannes. Brinksmeier, E., Berger, U., Janssen, R. (1997). High-Speed Milling of TiAI6V4 for Aircraft st Applications, 1 French and German Conference on High Speed Machining, 17. - 18. June 1997, University of Metz. Eckstein, M. et a/. (1991). Schaftfrasen von Titanlegierungen mit hohen Schnittgeschwindigkeiten. VDl-Z 133 12, pp. 28-34. Paul, R.P. (1981). Robot Manipulators : Mathematics, Programming and Control, MIT Press, Cambridge, MA . Sheng, X. (1992). Solid Modelling Based Geometric and Graphical Simulation of Multi-axis Milling Processes, PhD Dissertation, University of Bremen. Stahs, T. G. , Wahl, F. M. (1992). Fast and Versatile Range Data Acquisition in a Robot Work Cell. Proceedings of the IEEEIRSJ International Conference on Intelligent Robots and Systems. pp. 1169-1174. Raleigh, USA.
Welded chips on the tool flank
6. SUMMARY The basic issue of the described developments is the integrated process chain for multi-axis milling of turbine blades and vanes. Due to the requirements in machining of the individuality of the jet engine components as turbine blades and vanes specific attention is paid to high-speed milling aspects. The geometrical loop concerning cutting path planning and adaptation is based on a mathematical model for a generic machine tool system. Features like collision avoidance, jig and fixture modelling and universal postprocessing are included. For quality inspection and reverse engineering purposes a 3D shape measurement system based on Coded Light Approach and phase shifting techniques is integrated in the information loop. The presented results of the investigations carried out show an overview of the possibilities for influencing the milling process of titanium alloys. Due to the results performed so far it can be established that higher cutting speeds on the machining of TiAI6V 4 can be realized according to aerospace specification. The appropriate selection of tool materials, cutting parameters and coolant lubrication constitutes the basis for a safe machining process. Further investigations will be performed to achieve a wide range of machining data for suitable milling conditions to ensure increasing process stability. The pursued parameter database supports the machine operators involved in repair work preparation and selection of cutting data.
7. ACKNOWLEDGEMENT The presented paper is based on a RTD project funded by the European Commission in the ESPRIT
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