Special Issue; Products for Industrial Machinery - NTN

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NTN TECHNICAL REVIEW No.742006 Rotary table Feed-water inlet Waterway for cooling Rotary shaft Water drain outlet Workpiece Air seal Chuck surface Air seal Air thrust bearing Motor Rotary encoder Air supply opening for air seal Fig. 2 Structure of the air spindle which we developed problem, the housing on the motor and bearing sections is water-cooled and the associated components are composed of materials with low thermal-expansion in order to inhibit their thermal deformation and the displacement of the chuck face. Furthermore, air feed holes are provided in the bore sides of the motor stators for feeding compressed air to cool the motors. For evaluation of the control measure against thermal deformation, the aerostatic bearing spindle was started from a standstill in a room temperature environment and allowed to run constantly at 20,000 r/min, and displacement in the position of the workpiece was measured. This measurement result is graphically plotted in Fig. 4. Immediately after the start of operation, the shaft developed an elongation of 5. Spindle Evaluation Test A dummy workpiece simulating an actual workpiece was mounted to the prototype spindle, and then the spindle was subjected to an accuracy evaluation test. Run-outs were measured at the locations shown in Fig. 3 with a capacitance type non-contact displacement gage. Jitter (variation in rotation frequency) was determined by measuring variation in the frequency of Z-phase signal (1 pulse/revolution) of the rotary encoder with a pulse jitter counter. The measurement result for non-repetitive run-out (NRRO) is summarized in Table 2 and that for jitter (variation in rotation frequency) is given in Table 3. As can be understood from the data in these tables, NTN’s aerostatic bearing spindle performed very well from low speed to the maximum speed. Shaft elongation Radial run-out Axial run-out Dummy workpiece Fig. 3 A measurement position Table 2 A measurement result of NRRO Running speed r/min 1000 5000 10000 15000 20000 NRROnm Radial 3.75 3.75 3.75 3.75 3.75 NRROnm Axial 3.75 3.75 3.75 3.75 6.25 Running speed r/min Table 3 A measurement result of jitter (A dummy)) Jitter nsec Accuracy % 500 213.0 180.1 190.9 205.5 195.0 0.00018 1800 82.1 72.7 68.5 84.3 67.6 0.00025 5400 33.5 28.2 27.9 31.8 26.5 0.00031 10000 20.8 21.2 17.4 21.2 18.8 0.00035 15000 14.1 12.2 11.5 13.6 14.3 0.00036 20000 9.3 10.3 10.9 10.8 10.2 0.00036 -36-
Development of Aerostatic Bearing Spindle for Precision Machine Tools 4 Motor cooling air: 20 L/min Motor-bearing cooling water: 2.5 L/min (regulated to 23˚C) 20000 r/min: continual Elongation of shaft m 3 2 1 0 0 10 20 30 40 50 60 -1 1m -2 Time elapsed min. Fig. 4 The measurement of a work position) approximately 2.5 m (to the left in Fig. 3). After an elapse of 5 to 10 minutes, the displacement of the workpiece was approximately 0.6 m. 6. Machining Test For a test cut, NTN’s aerostatic bearing spindle was temporarily installed on a super-high-precision turning center, and the end face of a workpiece was surfacemachined, and then the resultant surface roughness was evaluated. The authors proved that their aerostatic bearing spindle can help achieve nanometer-class surface roughness. Machining conditions Table 4 Precision of cut side Mean surface roughness (Ra) 1000 r/min 3.127 nm 20000r/min 2.672 nm 20000r/min 2.386 nm A : Feed rate 2.0 mm/min B : Feed rate 4.0 mm/min Mean surface roughness on one cross-section (Ra) 1.675 nm 0.819 nm 2.671 nm 7. Conclusion NTN has successfully developed an aerostatic bearing spindle for machine tools that features a higher-torque motor arrangement, coolant mist control and thermal deformation control, and have demonstrated its usefulness through an evaluation test on the spindle proper and a machining test with a machine tool that incorporated NTN’s aerostatic bearing spindle. Aerostatic bearings are capable of super highprecision running at a nanometer order, and NTN has been employing these types of bearings in inspecting devices and semiconductor device manufacturing systems that are used in clean environments. NTN believes that their newly developed aerostatic bearing spindle has helped expand the applicability of aerostatic bearings to the field of machine tools, which is a new field for NTN. NTN will make further efforts to achieve higher precision and higher speed to perfect their newly developed spindle product and satisfy the requirements for further improved machining accuracy and productivity in preparation for commercialization. Photos of authors Teruyoshi HORIUCHI Product Engineering Department Precision Equipment Division Kazuyuki AONO Product Engineering Department Precision Equipment Division Hiroaki HIYOSHI Product Engineering Department Precision Equipment Division -37-
Page 1 and 2: 88th Anniversary Special Issue; Pro
Page 3 and 4: R No.74 TECHNICAL REVIEW Special Is
Page 5 and 6: NTN TECHNICAL REVIEW No.742006 Pref
Page 7 and 8: Trends in Recent Machine Tool Techn
Page 13 and 14: Development of a New Grease Lubrica
Page 21 and 22: High-Speed Tapered Roller Bearing f
Page 29 and 30: Minimum Quantity and Cooling Jet Lu
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Page 33 and 34: dmn17010 4 Sealed High-speed Angula
Page 39: Development of Aerostatic Bearing S
Page 43 and 44: Introduction of New Products for Ma
Page 49 and 50: Atmospheric Control Method for JIS-
Page 59 and 60: The Influence of Hydrogen on Tensio
Page 67 and 68: Technical Trends in Bearings for Of
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Page 79 and 80: Honing Sludge and Electro Furnace D
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