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-A punching process including thickening of a hole edge of an ultra-high strength steel sheet with a taper punch and step die was developed to improve fatigue strength of the punched sheet. In this process, the sheet was punched by the bottom of the punch, and subsequently the hole edge was thickened by the taper of the punch and the corner step of the die. The taper angle of the punch and the step height of the die were optimised to increase the amount of thickening. The quality of the sheared edge for the thickened punched sheet was improved by ironing with the taper of the punch, and compressive residual stress was generated around the hole edge. Seizure on the surface of the punch was prevented by VC-coating for repeated punching and realistic punching speed. It was found that the present punching process including thickening is effective in improving the fatigue strength of the punched ultra-high strength steel sheet and in preventing occurrence of delayed fracture.

The edge of a punched hole was thickened by burring processes to improve the fatigue strength of punched ultra-high strength steel sheets. In this process, the sheet was punched by the bottom of the punch, and subsequently the hole edge was thickened by the taper of the punch and the corner step of the die. The fatigue strength of the hole edge thickened by burring was higher than that of the punched hole edge. The punching and the burring were combined into the punching process including thickening. The step height of the die was optimised to increase the height of the hole edge. The quality of the sheared edge for the thickened punched sheet was improved by ironing with the taper of the punch, and compressive residual stress was generated around the hole edge. It was found that the punching process including thickening is effective in improving the fatigue strength of the hole edge in ultra-high strength steel sheets and in preventing the occurrence of delayed fracture.

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Gear precision measurement was carried out for gear with tooth number z=17, gear module mn=0.5, tooth width b=10mm, spur gear design and made using JIS C 3604 brass material. Cutting force measurement was carried out for gear with z=30, mn=1.75, spur gear, and JIS C 3604 brass material. Main results are as follows: (1) Gear precision and cutting force was almost the same in both dry and wet cutting. (2) Process capability index Cp of tooth profile errors were almost the same in both dry and wet hobbing gear cutting, and 5.9 to 8.1. Cp of tooth lead errors was 5.8 to 10.7. It was concluded that dry hobbing gear cutting of brass material has a sufficient production capacity. (3) Almost the same gear precision in both dry and wet hobbing gear cutting was considered because cutting forces were also same.