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