86 Aerospace Materials Handbook surface will often occur.To avoid this condition,care should be taken to insure that as long as the cutting edge and part are touching,the tool is always feeding. The common causes of tool failure in machining superalloys are excessive flank wear,excessive groove formation at chip edges,and the inability to meet surface finish and accuracy requirements. Cutting temperatures can reach 1400-1850F(760-1010C).These temperatures are so high that oxidation and diffusion become significant contributing factors to total tool wear even for the carbide tools [5]. The literature provides recommendations for most machining operations for various superalloys as well as tool geometries.All data,however,are only general guides.A change in casting pro- cess from conventional to columnar grain directional solidification in a nickel-based superalloy will change the distribution and size of the carbides,owing to the differing heat transfer situations in each process type.Moreover,alloy chemistries may be changed to accommodate a different casting process.Machining of superalloys is so difficult that careful study should be undertaken for any alloy to develop a set of machining parameters that result in reasonable tool life as well as an economic analysis that covers speeds,feeds,tool materials,and cutting tool reconditioning costs [5]. The most common machining operations carried out on nickel-based superalloys are turning, milling,drilling,and grinding.Turning is the predominant machining operation in the manufacture of disks for gas turbines.Milling is the major operation carried out in the manufacture of jet engine mounts and blades for the compressor of jet engines [20]. To date,machining of nickel-based superalloys in the industry is mostly carried out at low cut- ting speeds(20-30 m/min)and feeds(0.15 mm/tooth)using carbides.But a rather large axial depth of cut is used for roughing operations.This is due to the toughness and reliability of carbides coupled with lower tool costs.However,carbides have poor hot-hardness;thus they are unsuitable for machining nickel-based superalloys at speeds above 30 m/min [20]. There has been extensive research in the application of high-speed machining(HSM)to various machining processes like turning,milling,and so on of nickel-based superalloys.HSM,apart from increasing productivity,also offers the advantage of better surface finish,better chip disposal,sim- plified tooling,reduction in the damaged layer,reduced burr formation,and increased machining accuracy.Ceramic inserts are capable of achieving these high cutting speeds.Silicon carbide whis- ker reinforced alumina(SiC WRA),Sialon,and CBN are promising materials for HSM of nickel- based superalloys because of their greater mechanical and thermal integrity.However,machine shops are yet to implement wide adoption of ceramic tools in the machining of nickel-based super- alloys.Many users are comfortable with machining using carbides,and the innumerable grades of ceramic tools make tool selection a very complicated task [20]. 2.3.2 TURNING Turning is a metal cutting technology in which the cutting movement is carried out by the workpiece, whereas the tool performs the auxiliary motion of feed and infeed.In machining superalloys,more heat is generated in the shear zone since the superalloys retain most of their strength at cutting temperatures,and greater tool wear occurs for a given cutting speed than with most other metals.In addition,because the cutting of superalloys requires a larger force(about twice the force for cutting medium-carbon alloy steel in turning operations),tool geometry,tool strength,and/or rigidity of the toolholder are also important concerns. Carbide tools are frequently used in turning superalloys,although ceramic,coated carbide,CBN. and HSS tools are also used.A C-2 grade is often selected for roughing.A C-3 grade is used in finishing.Standard carbide inserts with positive or negative rakes are suitable for the roughing and finishing of superalloys [21,22]. It is important to use positive rake angles for the single-point turning tools in cutting nickel alloys so that the metal is cut instead of pushed.Negative rake angles should be avoided.Positive rake angles also help to guide the chip away from the finished surface.Another important tool geometric86 Aerospace Materials Handbook surface will often occur. To avoid this condition, care should be taken to insure that as long as the cutting edge and part are touching, the tool is always feeding. The common causes of tool failure in machining superalloys are excessive flank wear, excessive groove formation at chip edges, and the inability to meet surface finish and accuracy requirements. Cutting temperatures can reach 1400–1850°F (760–1010°C). These temperatures are so high that oxidation and diffusion become significant contributing factors to total tool wear even for the carbide tools [5]. The literature provides recommendations for most machining operations for various superalloys as well as tool geometries. All data, however, are only general guides. A change in casting pro￾cess from conventional to columnar grain directional solidification in a nickel-based superalloy will change the distribution and size of the carbides, owing to the differing heat transfer situations in each process type. Moreover, alloy chemistries may be changed to accommodate a different casting process. Machining of superalloys is so difficult that careful study should be undertaken for any alloy to develop a set of machining parameters that result in reasonable tool life as well as an economic analysis that covers speeds, feeds, tool materials, and cutting tool reconditioning costs [5]. The most common machining operations carried out on nickel-based superalloys are turning, milling, drilling, and grinding. Turning is the predominant machining operation in the manufacture of disks for gas turbines. Milling is the major operation carried out in the manufacture of jet engine mounts and blades for the compressor of jet engines [20]. To date, machining of nickel-based superalloys in the industry is mostly carried out at low cut￾ting speeds (20–30 m/min) and feeds (0.15 mm/tooth) using carbides. But a rather large axial depth of cut is used for roughing operations. This is due to the toughness and reliability of carbides coupled with lower tool costs. However, carbides have poor hot-hardness; thus they are unsuitable for machining nickel-based superalloys at speeds above 30 m/min [20]. There has been extensive research in the application of high-speed machining (HSM) to various machining processes like turning, milling, and so on of nickel-based superalloys. HSM, apart from increasing productivity, also offers the advantage of better surface finish, better chip disposal, sim￾plified tooling, reduction in the damaged layer, reduced burr formation, and increased machining accuracy. Ceramic inserts are capable of achieving these high cutting speeds. Silicon carbide whis￾ker reinforced alumina (SiC WRA), Sialon, and CBN are promising materials for HSM of nickel￾based superalloys because of their greater mechanical and thermal integrity. However, machine shops are yet to implement wide adoption of ceramic tools in the machining of nickel-based super￾alloys. Many users are comfortable with machining using carbides, and the innumerable grades of ceramic tools make tool selection a very complicated task [20]. 2.3.2  Turning Turning is a metal cutting technology in which the cutting movement is carried out by the workpiece, whereas the tool performs the auxiliary motion of feed and infeed. In machining superalloys, more heat is generated in the shear zone since the superalloys retain most of their strength at cutting temperatures, and greater tool wear occurs for a given cutting speed than with most other metals. In addition, because the cutting of superalloys requires a larger force (about twice the force for cutting medium-carbon alloy steel in turning operations), tool geometry, tool strength, and/or rigidity of the toolholder are also important concerns. Carbide tools are frequently used in turning superalloys, although ceramic, coated carbide, CBN, and HSS tools are also used. A C-2 grade is often selected for roughing. A C-3 grade is used in finishing. Standard carbide inserts with positive or negative rakes are suitable for the roughing and finishing of superalloys [21,22]. It is important to use positive rake angles for the single-point turning tools in cutting nickel alloys so that the metal is cut instead of pushed. Negative rake angles should be avoided. Positive rake angles also help to guide the chip away from the finished surface. Another important tool geometric