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
24 X O Chen,Z M Gong,H Huang,S Z Ge,and L B Zhou employed to remove excessive material on surfaces of new jet engine parts or overhaul turbine airfoils.Within the blending process,we further define rough grinding as the process step to remove the excessive material with the profile generation as the primary aim.The aim of fine polishing is to achieve the desired surface roughness.In this sense,the term blending is often interchangeable with grinding and polishing. In the aircraft overhaul industry,the blending process is intended to remove excessive braze material for the brazed area to be flush with the original surface within a tight tolerance.Current manual blending (also called belt polishing)uses belt machine to remove the braze,within tolerated undercuts and overcuts,as illustrated in Figure 3 (a).After belt polishing,the part is polished with a flap wheel,as shown in Figure 3(b), to achieve the final surface finish.Table 3 lists quality requirements of the blending operation.They are achieved with the operator's skills and knowledge: Manipulate the part correctly in relation to the tool head. Exert correct force and compliance between the part and the tool through wrists,and control the force interaction based on process knowledge. Adapt to part-to-part variations through visual observation and force feedback sandy belt convex concave (a) (b) Figure 3 Manual polishing of brazed airfoils with (a)belt polishing tool,(b)flap wheel One can imagine that a possible automation solution is to develop a machine which can mimic the operator's capabilities.In an abrasive machining process such as belt polishing,the amount of material removed
24 X Q Chen, Z M Gong, H Huang, S Z Ge, and L B Zhou employed to remove excessive material on surfaces of new jet engine parts or overhaul turbine airfoils. Within the blending process, we further define rough grinding as the process step to remove the excessive material with the profile generation as the primary aim. The aim of fine polishing is to achieve the desired surface roughness. In this sense, the term blending is often interchangeable with grinding and polishing. In the aircraft overhaul industry, the blending process is intended to remove excessive braze material for the brazed area to be flush with the original surface within a tight tolerance. Current manual blending (also called belt polishing) uses belt machine to remove the braze, within tolerated undercuts and overcuts, as illustrated in Figure 3 (a). After belt polishing, the part is polished with a flap wheel, as shown in Figure 3 (b), to achieve the final surface finish. Table 3 lists quality requirements of the blending operation. They are achieved with the operator’s skills and knowledge: • Manipulate the part correctly in relation to the tool head. • Exert correct force and compliance between the part and the tool through wrists, and control the force interaction based on process knowledge. • Adapt to part-to-part variations through visual observation and force feedback. contact wheel sandy belt convex concave buttress (a) (b) Figure 3 Manual polishing of brazed airfoils with (a) belt polishing tool, (b) flap wheel. One can imagine that a possible automation solution is to develop a machine which can mimic the operator’s capabilities. In an abrasive machining process such as belt polishing, the amount of material removed
Chapter 2-Process Development and Approach for 3D Profile Grinding/Polishing 25 not only depends on tool position,but also the contact force between the tool and workpiece.Such an automation system requires position control as well as force control so that the desirable amount of material can be removed to avoid excessive overcut or undercut.The complexity of such automation further escalates in consideration of process dynamics and part- to-part variations typified by near-net-shape new parts and overhaul parts. The first hard choice is what material removal process is most suitable for the intended automation system.Hence,the machining processes for superalloy materials must be carefully evaluated. Table 3 Quality requirements of airfoil blending. Items Specifications Overcutting ≤100 microns Undercutting ≤100 microns Trailing edge Absolutely no overcutting Leading edge 0 to 200 microns gap from the template. Smooth curvature. Wall thickness Greater than minimum wall thickness at specified check points Surface roughness ≤1.6 microns Ra Transition from brazed No visible transition lines to non-brazed area Blending path No visible path overlapping marks Part integrity No burning marks 2.2 CNC Milling A four or five-axis CNC system with a hard cutting tool would be able to satisfy the position control required by 3D profile finishing.The material removal can depend solely on position control.Other researchers have proposed the CNC milling process for cutting superalloy materials. However,the key issue to be addressed is tool wear and tool life in processing difficult-to-machine materials such as Inconel. To evaluate the feasibility of hard tool machining,experiments were conducted with conventional cutting tools on the sample material.The objective of the experiments was to monitor the tool conditions by
Chapter 2 - Process Development and Approach for 3D Profile Grinding/Polishing 25 not only depends on tool position, but also the contact force between the tool and workpiece. Such an automation system requires position control as well as force control so that the desirable amount of material can be removed to avoid excessive overcut or undercut. The complexity of such automation further escalates in consideration of process dynamics and partto-part variations typified by near-net-shape new parts and overhaul parts. The first hard choice is what material removal process is most suitable for the intended automation system. Hence, the machining processes for superalloy materials must be carefully evaluated. Table 3 Quality requirements of airfoil blending. Items Specifications Overcutting ≤ 100 microns Undercutting ≤ 100 microns Trailing edge Absolutely no overcutting Leading edge 0 to 200 microns gap from the template. Smooth curvature. Wall thickness Greater than minimum wall thickness at specified check points Surface roughness ≤ 1.6 microns Ra Transition from brazed to non-brazed area No visible transition lines Blending path No visible path overlapping marks Part integrity No burning marks 2.2 CNC Milling A four or five-axis CNC system with a hard cutting tool would be able to satisfy the position control required by 3D profile finishing. The material removal can depend solely on position control. Other researchers have proposed the CNC milling process for cutting superalloy materials. However, the key issue to be addressed is tool wear and tool life in processing difficult-to-machine materials such as Inconel. To evaluate the feasibility of hard tool machining, experiments were conducted with conventional cutting tools on the sample material. The objective of the experiments was to monitor the tool conditions by
26 X O Chen,Z M Gong,H Huang,S Z Ge,and L B Zhou measuring the cutting force,and establish the cycle time required.The following cutting conditions were applied: .Machine tool:Hitachi Seiki (VG 45)5 axis Machining Centre. WC ball end cutter (insert)-UX 30(maximum rotation diameter 10 mm). Rotation speed of cutter:1200 rpm. Depth of cut:0.06 mm. Transverse feed:150 mm/min. Length of cutting pass:58 mm(removal rate:1 gram/min.). The cutting force was recorded to check the tool life.Figure 4 shows the cutting force along Z-axis as a function of the cutting pass.It is apparent that the cutting force has a large increase after 30 passes,indicating significant tool wear. 250 200 N 150 100 50 0+ 0 20 40 60 Cutting pass Figure 4 Milling force as a function of cutting pass. Given that 80%of the vane surface is covered by the braze material and the braze layer is about 1.5 mm,the weight of braze material to be removed is about 70 grams.Thus the number of cutting passes required for one piece of vane is 157.This indicates that we have to change inserts 5 times for milling one vane.The effective cutting time for one vane is about 70 min. (i.e.70 grams 1 gram per min.).The fast tool wear would incur considerable tooling cost.The slow removal rate compares unfavourably with the manual belt polishing which takes about 10 minutes to polish away the braze material.It was therefore concluded that the CNC milling method
26 X Q Chen, Z M Gong, H Huang, S Z Ge, and L B Zhou measuring the cutting force, and establish the cycle time required. The following cutting conditions were applied: • Machine tool: Hitachi Seiki (VG 45) 5 axis Machining Centre. • WC ball end cutter (insert) – UX 30 (maximum rotation diameter 10 mm). • Rotation speed of cutter: 1200 rpm. • Depth of cut: 0.06 mm. • Transverse feed: 150 mm/min. • Length of cutting pass: 58 mm (removal rate: 1 gram/min.). The cutting force was recorded to check the tool life. Figure 4 shows the cutting force along Z-axis as a function of the cutting pass. It is apparent that the cutting force has a large increase after 30 passes, indicating significant tool wear. 0 50 100 150 200 250 0 20 40 60 Cutting pass Milling Force-Z (N) Figure 4 Milling force as a function of cutting pass. Given that 80% of the vane surface is covered by the braze material and the braze layer is about 1.5 mm, the weight of braze material to be removed is about 70 grams. Thus the number of cutting passes required for one piece of vane is 157. This indicates that we have to change inserts 5 times for milling one vane. The effective cutting time for one vane is about 70 min. (i.e. 70 grams / 1 gram per min.). The fast tool wear would incur considerable tooling cost. The slow removal rate compares unfavourably with the manual belt polishing which takes about 10 minutes to polish away the braze material. It was therefore concluded that the CNC milling method
Chapter 2-Process Development and Approach for 3D Profile Grinding/Polishing 27 is not suitable for the blending of turbine vane because of fast tool wear and long cycle time. 2.3 Wheel Grinding The second process investigated was the wheel grinding process that offers an alternative solution to belt polishing.Clearly,controlling a grinding wheel with a 5-axis CNC is much easier that controlling a large polishing belt.Again,grinding wheel wear and material removal rate must be examined before introducing such a process for an automation solution. Air cylinder(2)） Grinding wheel with motor Figure 5 Experimental set-up for grinding test Figure 5 shows the experimental set-up of the grinding test.The grinding wheel is mounted on a pneumatic rig,which has an upward pressure and a downward pressure.The contact force between the grinding wheel and workpiece is determined by the differential air pressure of the two cylinders as follows: F=(Pup-Pdoun)A-W (1) where F is the contact force,Pup the upward air pressure,Pdowm the downward air pressure,A the surface of the piston,and W the weight of the grinding motor and the grinding wheel.The experiments were conducted under the following conditions: 。Pp:2.0 Kgf/cm2
Chapter 2 - Process Development and Approach for 3D Profile Grinding/Polishing 27 is not suitable for the blending of turbine vane because of fast tool wear and long cycle time. 2.3 Wheel Grinding The second process investigated was the wheel grinding process that offers an alternative solution to belt polishing. Clearly, controlling a grinding wheel with a 5-axis CNC is much easier that controlling a large polishing belt. Again, grinding wheel wear and material removal rate must be examined before introducing such a process for an automation solution. Workpiece Force control device Air cylinder (x2) Grinding wheel with motor Figure 5 Experimental set-up for grinding test. Figure 5 shows the experimental set-up of the grinding test. The grinding wheel is mounted on a pneumatic rig, which has an upward pressure and a downward pressure. The contact force between the grinding wheel and workpiece is determined by the differential air pressure of the two cylinders as follows: F = (Pup – Pdown)A - W (1) where F is the contact force, Pup the upward air pressure, Pdown the downward air pressure, A the surface of the piston, and W the weight of the grinding motor and the grinding wheel. The experiments were conducted under the following conditions: • Pup : 2.0 Kgf/cm2
28 X O Chen,Z M Gong,H Huang,S Z Ge,and L B Zhou ·Pdown:0.4 Kgf/cm2. Wheel rotation speed:7546 rpm. Three types of grinding wheels were tested: .Norton C150D5BTM No:06380.Size:101.6x19.1x6.4 mm. Cratex 302 C(rubberised abrasives),Size:76.2x3.2x6.4 mm. Cratex 304 M (rubberised abrasives),Size:76.2.4x6.4 mm. The test results are shown in Table 4.The three grinding wheels offer similar material removal rate between 0.13 to 0.15 grams per minute.Even if Cratex 304 M is most resistant to wear,its wear rate (0.25 g/min)is still higher than the material removal rate (0.15 g/min).For one workpiece,the total amount of material to be removed is about 45 grams.Assuming the removal rate of 0.15 g/min,the total polishing time required is 300 min. Table 4 Material removal and tool wear for grinding wheels. Workpiece Grinding wheel Conditions Weight Weight Removal WeightWeight Wear before after rate before after rate (g) (g) (g/min) (g) (g) (g/min) Norton C150D5BTM 358.7 358.5 0.133 147.6 147.0 0.40 Duration:1.5 min. Cratex 302 C 358.3 358.0 0.15 42.2 40.6 0.80 Duration:2.0 min Cratex 304 M 358.0 357.7 0.15 66.2 65.7 0.25 Duration:2.0 min The evaluation results for milling,grinding and belt polishing are summarised in Table 5.Clearly,all possesses can meet the surface finish requirements of 1.6 micron Ra.However,they differ greatly in terms of material removal rate,hence the cycle time.For milling and grinding processes,the removal rates are 0.15 g/min and 0.8 g/min respectively.The resultant long cycle times are unacceptable for production use when there are large amount of materials to be removed.On the other hand,belt polishing offers a superior removal rate of 15 g/min,and each belt can process three pieces.However its position control is less accurate due to its area contact and belt vibration
28 X Q Chen, Z M Gong, H Huang, S Z Ge, and L B Zhou • Pdown : 0.4 Kgf/cm2 . • Wheel rotation speed: 7546 rpm. Three types of grinding wheels were tested: • Norton C150D5BTM No: 06380, Size: 101.6×19.1×6.4 mm. • Cratex 302 C (rubberised abrasives), Size: 76.2×3.2×6.4 mm. • Cratex 304 M (rubberised abrasives), Size: 76.2.4×6.4 mm. The test results are shown in Table 4. The three grinding wheels offer similar material removal rate between 0.13 to 0.15 grams per minute. Even if Cratex 304 M is most resistant to wear, its wear rate (0.25 g/min) is still higher than the material removal rate (0.15 g/min). For one workpiece, the total amount of material to be removed is about 45 grams. Assuming the removal rate of 0.15 g/min, the total polishing time required is 300 min. Table 4 Material removal and tool wear for grinding wheels. Conditions Workpiece Grinding wheel Weight before (g) Weight after (g) Removal rate (g/min) Weight before (g) Weight after (g) Wear rate (g/min) Norton C150D5BTM Duration: 1.5 min. 358.7 358.5 0.133 147.6 147. 0 0.40 Cratex 302 C Duration: 2.0 min. 358.3 358.0 0.15 42.2 40.6 0.80 Cratex 304 M Duration: 2.0 min. 358.0 357.7 0.15 66.2 65.7 0.25 The evaluation results for milling, grinding and belt polishing are summarised in Table 5. Clearly, all possesses can meet the surface finish requirements of 1.6 micron Ra. However, they differ greatly in terms of material removal rate, hence the cycle time. For milling and grinding processes, the removal rates are 0.15 g/min and 0.8 g/min respectively. The resultant long cycle times are unacceptable for production use when there are large amount of materials to be removed. On the other hand, belt polishing offers a superior removal rate of 15 g/min, and each belt can process three pieces. However its position control is less accurate due to its area contact and belt vibration