Chapter 2-Process Development and Approach for 3D Profile Grinding/Polishing 29 Table 5 Comparison of milling,grinding and belt polishing. Potential Surface Removal Cycle Tool life Tool Machine process finish rate time (no.of unit cost cost(US$) (g/min) (min.) vanes) (USS) 5-axis Micron 0.8 >56 0.22 6① 350.000 milling 5-axis Sub-micron 0.15 300 <32) 150 >500.000 grinding Belt Micron or 15 2.5 7.5 <300.000 polishing Sub-micron with 6-axis robot Note: (1)The price is for the ball end mill insert. (2)The wheel wears faster that the material removal.It is not practical to grind more than three parts for one wheel with tool wear compensation. (3)The price is for the conventional abrasive wheel having a size of 200 mm in diameter. Due to the inherent difficult-to-machine nature of superalloy,any process would incur severe tool wear,with the end mill cutter being the worst.Fast tool wear means frequent tool changing and tooling cost.Again, polishing belts are much cheaper than grinding wheels,resulting in lower tool cost.Although an ultra-hard cutter would improve milling efficiency considerably and bring the cutter life close to a polishing belt,its high cost, in the range of a few thousand US dollars,hardly justifies such an automation system.Taking into account all these considerations,it can be concluded that the belt polishing process is most suitable for automating airfoil blending despite its large size and poor position controllability. 3. Force Control in Material Removal In contrast to CNC milling where material removal depends on position control,abrasive machining processes like belt polishing have the contact force as the prime factor determining the material removal and final profile finish quality.Force control is particularly important for avoiding overcutting and undercutting.To achieve the desired finish profile,it is crucial to control the contact force and compliance between tool and vane. as the profile of the workpiece is rather irregular (after brazing)and distorted.An operator carries out the job skilfully with his/her visionChapter 2 - Process Development and Approach for 3D Profile Grinding/Polishing 29 Table 5 Comparison of milling, grinding and belt polishing. Potential process Surface finish Removal rate (g/min) Cycle time (min.) Tool life (no. of vanes) Tool unit cost (US$) Machine cost (US$) 5-axis milling Micron 0.8 >56 0.22 6 (1) 350,000 5-axis grinding Sub-micron 0.15 300 <3 (2) 150 (3) >500,000 Belt polishing with 6-axis robot Micron or Sub-micron 15 2.5 3 7.5 <300,000 Note: (1) The price is for the ball end mill insert. (2) The wheel wears faster that the material removal. It is not practical to grind more than three parts for one wheel with tool wear compensation. (3) The price is for the conventional abrasive wheel having a size of 200 mm in diameter. Due to the inherent difficult-to-machine nature of superalloy, any process would incur severe tool wear, with the end mill cutter being the worst. Fast tool wear means frequent tool changing and tooling cost. Again, polishing belts are much cheaper than grinding wheels, resulting in lower tool cost. Although an ultra-hard cutter would improve milling efficiency considerably and bring the cutter life close to a polishing belt, its high cost, in the range of a few thousand US dollars, hardly justifies such an automation system. Taking into account all these considerations, it can be concluded that the belt polishing process is most suitable for automating airfoil blending despite its large size and poor position controllability. 3. Force Control in Material Removal In contrast to CNC milling where material removal depends on position control, abrasive machining processes like belt polishing have the contact force as the prime factor determining the material removal and final profile finish quality. Force control is particularly important for avoiding overcutting and undercutting. To achieve the desired finish profile, it is crucial to control the contact force and compliance between tool and vane, as the profile of the workpiece is rather irregular (after brazing) and distorted. An operator carries out the job skilfully with his/her vision