Hardened steel is used for dies and molds, and is quenched and tempered to improve its mechanical properties and wear resistance. For dimensional accuracy, hardened steel is machined by a metal removal process. High-speed cutting is an effective method of improving productivity. As, the cutting temperature rises very high in high-speed cutting, the tool materials require both good wear-resistance and heat-resistance, and the cutting parts must be cooled for efficacy and efficiency. In this study, hardened steel was turned with a high-pressure coolant supplied, the chip configurations, the tool wear and the surface roughness were experimentally investigated. The hardened steel used was an ASTM D2 coldworked die steel (60HRC). The results are as follows: (1) In turning with a highpressure coolant supplied, the effectiveness of chip breaking performance was improved. In this case, the chip length was shorter with the increase of the coolant pressure, and the chip length was longer with the increase of the cutting speed. (2) In the case of a cutting speed of 10.00 m/s, large wear on the flank face was observed in the dry cutting. It was possible to suppress the tool wear on the flank face with a high-pressure coolant supplied. (3) In the high-pressure coolant cutting method of hardened steel with a cBN tool at a cutting speed of 10.00 m/s, a cBN grain size of 5.0 µm, 45 cBN grain/55 binding phase and main element of the binder phase of TiCN-Al was an effective tool material. The surface roughness by cutting with this cBN tool was almost constant up to a cutting distance of 1080 m.

The tool life in cutting high-nickel alloy is shorter than that in turning of carbon steel. In order to identify an effective tool material for the cutting of high-nickel alloy, the chip configurations, the tool wear and the surface roughness were experimentally investigated. The high-nickel alloy was turned with high-pressure coolant supplied. The results are as follows: (1) In turning with high-pressure coolant supplied, the effectiveness of chip breaking performance was improved. In this case, the chip length was shorter with the increase of the coolant pressure. (2) In the case of finish cutting high-nickel alloy, in the dry cutting large notch wear on the depth of the cut line was observed. In wet cutting with the cBN content of 60 % to 65 % under high cutting speed and high-pressure coolant supplied, it was possible to suppress the large notch wear. (3) In the high-pressure coolant cutting method of high-nickel alloy with a sintered cubic boron nitride compact tool, the cBN content of 60 % and the main element of the binder phase of Al 2 O 3 -Al was an effective tool material. (4) In the high-pressure coolant cutting method of high-nickel alloy with the cBN content of 60 % and the main element of the binder phase of Al 2 O 3 -Al, if the cutting coolant pressure is high and the cutting speed is low, large tool damage occurs to the notch boundary of the flank, and the tool life is short.In conclusion, the tool life of the cBN content of 60 % and the main element of the binder phase of Al 2 O 3 -Al can be improved by increasing coolant pressure in turning high-nickel alloy with a sintered cubic boron nitride compact tool under high cutting speed. KEY WORDScutting, high-pressure coolant supplied, high-nickel alloy, machinability, sintered cubic boron nitride compact

Because of its extremely high strength to weight ratio, 7075 aluminum alloy is used for highly stressed structural parts including aircraft fittings, gears and shafts and various other commercial aircraft, aerospace and transportation equipment. In the case of cutting aluminum alloys, the chip breakability is the most important feature to ensure reliable operation in automated machining. In this study, in turning of 7075 aluminum alloy with a high-pressure coolant supply, the chip configurations, and the mass and thickness of chips were experimentally investigated. The following results were obtained:(1) In the case of a cutting speed of 5.0 m/s, a feed rate from 0.05 mm/rev to 0.50 mm/rev and a depth of cut from 0.1 mm to 3.0 mm, chips were not broken at a feed rate of 0.15 mm/rev or less with the conventional coolant supply. With a high-pressure coolant supply, the combination area of feed rate and depth of cut in which chips were broken was wider than that with the conventional coolant supply. With the high-pressure coolant supply at a coolant pressure of 7 MPa, there was a combination area of feed rate and depth of cut in which chips were not broken. However, chips were broken in all areas at a coolant pressure of 20 MPa.(2) In the case of both the high-pressure coolant supply, which has a coolant pressure of 7, 14 or 20 MPa, and the conventional coolant supply, the thickness of chip increased with the increase of the depth of cut. And, the thickness of chip did not change depending on the cutting method, namely the high-pressure coolant supply cutting method and the conventional coolant supply cutting method.