CHAPTER 2 PROCESS DEVELOPMENT AND APPROACH FOR 3D PROFILE GRINDING/POLISHING XiaoQi Chen*,Zhiming Gong*,Han Huang*,Shuzhi Ge**,Libo Zhou*** *Singapore Institute of Manufacturing Technology, 71 Nanyang Drive,Singapore 638075 **Department of Electrical Computer Engineering,The National University of Singapore,10 Kent Ridge Crescent,Singapore 119260 **Department of System Engineering,Ibaraki University,Japan 1. Introduction Industrial robots are gaining widespread applications in manufacturing process automation.Their applications can be classified into two broad categories,namely non-constrained and constrained manipulation.The former does not involve force interaction or control between the end- effector and the environment that the robot acts on.Examples of non- constrained processes are inspection,laser cutting,welding,plasma spraying and many assembly tasks.Typically,the position control with or without external sensors is sufficient to accomplish these tasks.On the other hand,constrained robotic tasks such as machining,deburring, chamfering,grinding,and polishing involve force interaction between the tool and the workpiece to be processed.In addition to position control,the contact force and process parameters must be controlled to achieve the desired output. For the past decade the theory of force control for constrained robotic applications has been extensively researched [1].By and large,there are three approaches to force control,and these are impedance [2],hybrid position/force control [3]and constrained motion control [4].Instead of
CHAPTER 2 PROCESS DEVELOPMENT AND APPROACH FOR 3D PROFILE GRINDING/POLISHING XiaoQi Chen*, Zhiming Gong*, Han Huang*, Shuzhi Ge**, Libo Zhou*** *Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075 **Department of Electrical & Computer Engineering, The National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 *** Department of System Engineering, Ibaraki University, Japan 1. Introduction Industrial robots are gaining widespread applications in manufacturing process automation. Their applications can be classified into two broad categories, namely non-constrained and constrained manipulation. The former does not involve force interaction or control between the endeffector and the environment that the robot acts on. Examples of nonconstrained processes are inspection, laser cutting, welding, plasma spraying and many assembly tasks. Typically, the position control with or without external sensors is sufficient to accomplish these tasks. On the other hand, constrained robotic tasks such as machining, deburring, chamfering, grinding, and polishing involve force interaction between the tool and the workpiece to be processed. In addition to position control, the contact force and process parameters must be controlled to achieve the desired output. For the past decade the theory of force control for constrained robotic applications has been extensively researched [1]. By and large, there are three approaches to force control, and these are impedance [2], hybrid position/force control [3] and constrained motion control [4]. Instead of
20 X O Chen,Z M Gong,H Huang,S Z Ge,and L B Zhou tracking the motion and force trajectory,the impedance control regulates the dynamic behaviour between the motion of the manipulator and the force exerted on the environment.In [5],the impedance control of robot manipulators using adaptive neural network is proposed.Without an explicit force error loop,the desired dynamic behaviour is specified to obtain a proper force response. Hybrid position/force control combines force and torque information with positional data to simultaneously satisfy position and force trajectory constraints that are specified in a task-related coordinate system.In [6,an adaptive controller for force control with an unknown system and environmental parameters is examined.Force control,impedance control, and impedance control combined with a desired force control are treated using model-reference adaptive control (MRAC).A recent work [7]has examined the stability of the most basic hybrid control,which requires no robot dynamic model.Many other researchers have studied various learning methods for hybrid force/position control [8-12].Attempts have been made to apply constrained robotic control for deburring and chamfering [13-18].A research work on polishing sculptured surface using a 6-axis robot is reported [19].Generally speaking,constrained robotic applications are largely confined to laboratory explorations,particularly for an unknown environment.Only a handful of commercial systems such as Yamaha finishing robots are available for 3D profile polishing. Furthermore they are limited to processing new parts in simple operations, which rely on teaching and play-back or CAD-driven off-line programming. One of the hurdles in automating constrained robotic tasks is that it is difficult,indeed very often impossible,to derive an analytical model to describe the process to be controlled.Successful execution of the empirical process relies heavily on human knowledge.The problem escalates when part and process variables must be considered,such as part distortions (typical in aerospace component overhaul),severe tool wear (most pronounced in processing superalloys such as Inconel),and process optimisation to meet stringent standards required by the industry.All these factors must be vigorously studied before proposing a feasible approach to such an intriguing problem. This chapter discusses the perspective and approach of 3D profile grinding and polishing in general,and blending of overhaul jet engine components specifically.The JT9D first stage turbine vane is selected as a case study.Section 2 discusses the surface finishing processes including
20 X Q Chen, Z M Gong, H Huang, S Z Ge, and L B Zhou tracking the motion and force trajectory, the impedance control regulates the dynamic behaviour between the motion of the manipulator and the force exerted on the environment. In [5], the impedance control of robot manipulators using adaptive neural network is proposed. Without an explicit force error loop, the desired dynamic behaviour is specified to obtain a proper force response. Hybrid position/force control combines force and torque information with positional data to simultaneously satisfy position and force trajectory constraints that are specified in a task-related coordinate system. In [6], an adaptive controller for force control with an unknown system and environmental parameters is examined. Force control, impedance control, and impedance control combined with a desired force control are treated using model-reference adaptive control (MRAC). A recent work [7] has examined the stability of the most basic hybrid control, which requires no robot dynamic model. Many other researchers have studied various learning methods for hybrid force/position control [8-12]. Attempts have been made to apply constrained robotic control for deburring and chamfering [13-18]. A research work on polishing sculptured surface using a 6-axis robot is reported [19]. Generally speaking, constrained robotic applications are largely confined to laboratory explorations, particularly for an unknown environment. Only a handful of commercial systems such as Yamaha finishing robots are available for 3D profile polishing. Furthermore they are limited to processing new parts in simple operations, which rely on teaching and play-back or CAD-driven off-line programming. One of the hurdles in automating constrained robotic tasks is that it is difficult, indeed very often impossible, to derive an analytical model to describe the process to be controlled. Successful execution of the empirical process relies heavily on human knowledge. The problem escalates when part and process variables must be considered, such as part distortions (typical in aerospace component overhaul), severe tool wear (most pronounced in processing superalloys such as Inconel), and process optimisation to meet stringent standards required by the industry. All these factors must be vigorously studied before proposing a feasible approach to such an intriguing problem. This chapter discusses the perspective and approach of 3D profile grinding and polishing in general, and blending of overhaul jet engine components specifically. The JT9D first stage turbine vane is selected as a case study. Section 2 discusses the surface finishing processes including
Chapter 2-Process Development and Approach for 3D Profile Grinding/Polishing 21 manual blending,CNC milling and wheel grinding.Consideration of poor machinability of the material leads to the determination of a suitable process to automate the profile blending operation.Section 3 establishes the model of the contact force between the tool and the workpiece,which is crucial to desired material removal and surface finish.Section 4 generalises model-based robotic machining systems,highlighting their advantages and deficiencies.Section 5 details part variations and process dynamics that are predominant in robotic blending.Section 6 proposes a knowledge-based adaptive robotic system by integrating various intelligent software modules,such as Knowledge-Based Process Control (KBPC)and Data- Driven Supervisory Control(DDSC).Section 7 presents results of tool path optimisation and tool wear compensation,which can be incorporated into the knowledge-based process control to address process dynamics.Finally, the chapter is concluded with some remarks on the proposed approach and concept. 2. Profile Grinding and Polishing of Superalloys 2.1 Superalloy Components and Manual Blending Superalloys are widely used,in the aerospace industry for example,to meet the following demanding engineering requirements: High strength-to-weight ratio High fatigue resistance High corrosion resistance Superior high-temperature strength Jet engine turbine vanes and blades are often made of Inconel materials However,these materials have poor machinability,long recognised by manufacturers.Figure 1 shows the schematics of a high-pressure turbine (HPT)vane. The vane consists of an airfoil having concave and convex surfaces,an inner buttress and an outer buttress.After operating in a high-temperature and high-pressure environment,vanes incur severe distortions as large as 2 mm in reference to the buttress.On the airfoil surface there are hundreds of cooling holes.After a number of operational cycles,defects such as fully or partially blocked cooling holes,micro cracks and corrosions begin to occur. Because of the high cost of the components,it is common practice to repair
Chapter 2 - Process Development and Approach for 3D Profile Grinding/Polishing 21 manual blending, CNC milling and wheel grinding. Consideration of poor machinability of the material leads to the determination of a suitable process to automate the profile blending operation. Section 3 establishes the model of the contact force between the tool and the workpiece, which is crucial to desired material removal and surface finish. Section 4 generalises model-based robotic machining systems, highlighting their advantages and deficiencies. Section 5 details part variations and process dynamics that are predominant in robotic blending. Section 6 proposes a knowledge-based adaptive robotic system by integrating various intelligent software modules, such as Knowledge-Based Process Control (KBPC) and DataDriven Supervisory Control (DDSC). Section 7 presents results of tool path optimisation and tool wear compensation, which can be incorporated into the knowledge-based process control to address process dynamics. Finally, the chapter is concluded with some remarks on the proposed approach and concept. 2. Profile Grinding and Polishing of Superalloys 2.1 Superalloy Components and Manual Blending Superalloys are widely used, in the aerospace industry for example, to meet the following demanding engineering requirements: • High strength-to-weight ratio • High fatigue resistance • High corrosion resistance • Superior high-temperature strength Jet engine turbine vanes and blades are often made of Inconel materials. However, these materials have poor machinability, long recognised by manufacturers. Figure 1 shows the schematics of a high-pressure turbine (HPT) vane. The vane consists of an airfoil having concave and convex surfaces, an inner buttress and an outer buttress. After operating in a high-temperature and high-pressure environment, vanes incur severe distortions as large as 2 mm in reference to the buttress. On the airfoil surface there are hundreds of cooling holes. After a number of operational cycles, defects such as fully or partially blocked cooling holes, micro cracks and corrosions begin to occur. Because of the high cost of the components, it is common practice to repair
22 X Q Chen,Z M Gong.H Huang,SZ Ge,and L B Zhou these parts instead of scrapping them.The repairing process starts with cleaning and covering the defective areas with the braze material.The purpose of brazing is to fill up the defects,but unavoidably,the brazed areas will be higher than the original surface.Figure 2 shows a cross section of the airfoil brazed with a repair material. inner buttress outer buttress featherseal slots airfoils baffle (concave side) inner platform outer platform concave side of mateface mounting lugs Figure 1 Schematics of a HPT Vane. Leading edge Cavities Cooling holes Trailing edge Braze material Parent material Figure 2 Turbine airfoils repaired with braze material. Table 1 summarises the overhaul conditions of typical jet engine high- pressure turbine vanes.The wall thickness of the airfoil ranges from 0.8 mm in the trailing edge to 2 mm in the leading edge.The braze material, similar to the parent material in composition,is laid down on the airfoils manually,and its thickness is about I mm.The compositions of the braze
22 X Q Chen, Z M Gong, H Huang, S Z Ge, and L B Zhou these parts instead of scrapping them. The repairing process starts with cleaning and covering the defective areas with the braze material. The purpose of brazing is to fill up the defects, but unavoidably, the brazed areas will be higher than the original surface. Figure 2 shows a cross section of the airfoil brazed with a repair material. Figure 1 Schematics of a HPT Vane. Braze material Parent material Leading edge Trailing edge Cavities Cooling holes Figure 2 Turbine airfoils repaired with braze material. Table 1 summarises the overhaul conditions of typical jet engine highpressure turbine vanes. The wall thickness of the airfoil ranges from 0.8 mm in the trailing edge to 2 mm in the leading edge. The braze material, similar to the parent material in composition, is laid down on the airfoils manually, and its thickness is about 1 mm. The compositions of the braze
Chapter 2-Process Development and Approach for 3D Profile Grinding/Polishing 23 material are shown in Table 2.The three major chemical elements of both parent and braze materials are cobalt(Co),chromium(Cr)and nickel (Ni). Table 1 Overhaul conditions of jet engine turbine vanes. Items Conditions Airfoil material Inconel Airfoil curvature 3D,concave and convex Braze material Similar to parent material Hardness 20 to 30 HRC Machinability Poor Part dimension 150×140×80mm(max.) Part weight 0.68 kg (max) Wall thickness 0.8(trailing edge)to 2 mm (leading edge) Part distortion Up to 2.0 mm Braze thickness 0.5-1.5mm Braze pattern Defined sets Braze coverage About 80%of airfoil surface Brazing operation Manually done with dispenser Table 2 Compositions and properties of braze material. Chemical Atomic Atomic Density Weight elements Number Weight (g/cm3) Percentage Cobalt 27 58.93 8.90 45.6 Chromium 24 51.996 7.19 23.75 Nickel 28 58.693 8.902 25 Tungsten 74 183.84 19.30 3.5 Tantalum 73 180.95 16.654 1.75 Titanium 22 47.90 4.54 0.1 Carbon 6 12.01 3.513 0.3 Baron 5 10.811 2.34 1.48 Blending can be defined as the material removal process to achieve the desired finish profile and surface finish roughness.The process is often
Chapter 2 - Process Development and Approach for 3D Profile Grinding/Polishing 23 material are shown in Table 2. The three major chemical elements of both parent and braze materials are cobalt (Co), chromium (Cr) and nickel (Ni). Table 1 Overhaul conditions of jet engine turbine vanes. Items Conditions Airfoil material Inconel Airfoil curvature 3D, concave and convex Braze material Similar to parent material Hardness 20 to 30 HRC Machinability Poor Part dimension 150 × 140 × 80 mm (max.) Part weight 0.68 kg (max) Wall thickness 0.8 (trailing edge) to 2 mm (leading edge) Part distortion Up to 2.0 mm Braze thickness 0.5 – 1.5 mm Braze pattern Defined sets Braze coverage About 80% of airfoil surface Brazing operation Manually done with dispenser Table 2 Compositions and properties of braze material. Chemical elements Atomic Number Atomic Weight Density (g/cm3 ) Weight Percentage Cobalt 27 58.93 8.90 45.6 Chromium 24 51.996 7.19 23.75 Nickel 28 58.693 8.902 25 Tungsten 74 183.84 19.30 3.5 Tantalum 73 180.95 16.654 1.75 Titanium 22 47.90 4.54 0.1 Carbon 6 12.01 3.513 0.3 Baron 5 10.811 2.34 1.48 Blending can be defined as the material removal process to achieve the desired finish profile and surface finish roughness. The process is often