We investigated the deformation of the worked materials in the punching of slanted fine holes in austenitic stainless-steel SUS304 sheets. We observed the microstructure and hardness distribution of the cross section of the worked material, and then, considered the material flow. The material flow changed the complexity of what as a result of the influence of work hardening or the transformation to strain-induced martensite. Then, the direction and magnitude of the pressure applied to the punch also changed such inn a complex manner. However, the worked material almost flowed into the hole of the die. Then, the punch was carried into the hole by this material flow.
The effect of tool wear on the fatigue strength of pierced steel sheets has been investigated through a plane bending test, in which three kinds of high-strength steel were used as specimens. The tools for this study sustained wear after 10,000 piercing shots on commercial JIS-S45C steel. The results show that the tool wear possibly reduces the fatigue strength of the pierced steel sheets. The possibility of such deterioration depends on the type of material. The holes pierced on the ferrite-pearlite or precipitation-strengthened steel do not induce the reduction in fatigue strength by the tool wear. Nevertheless, the holes on the ferrite-martensite steel induce deterioration, probably caused by residual stress. The tool wear induces tensile residual stress around the burr side of the holes; this residual stress probably accelerates the propagation of fatigue cracks. The tool wear hardly affects the roughness and hardness of the pierced surface, which are other factors related to fatigue strength.
Push-back blanking is a two-step burr-free blanking process, in which a sheet metal is half-blanked and then pushed back between platens up to the original sheet thickness. However, when conducting this process in a progressive die system, an additional step is required to eject the blanked part from the sheet scrap. In order to reduce the number of steps, the incorporation of an ejection mechanism into the push-back step is proposed in this study, with this mechanism, a burr-free product could be separated and ejected in the push-back step using the lower platen with a hole. Experiments were carried out on a mild steel sheet, and conditions for successful push-back blanking were investigated. The optimum conditions obtained were applied to the revised push-back step. It was confirmed, through the experiments and FE analysis, that the success of the process depends on the diameter of the hole in the lower platen, and that, under the optimum hole diameter, burr-free parts with the same cut edge quality as those obtained by conventional push-back blanking are obtained with a reduced number of steps.
In push-back blanking, burr-free products are produced by compressing a half-blanked sheet between platens, where success conditions are affected by the ductility of materials. In this study, the mechanism of success and the cut edge quality in the push-back blanking process using a ductile and a brittle steel sheet were discussed. Experiments were carried out, varying the punch penetration and clearance in the half-blanking step. Brittle steel showed a wider successful range and less shear droop on the cut edge than ductile steel. Corresponding simulations were conducted and the damage value based on a ductile fracture criterion was calculated around the shear zone. Negative clearance was effective in keeping the damage value low and allowed deep punch penetration without fracturing in the half-blanking step. In the following push-back step, a rapid increase in the damage was observed when the high mean stress zone prevailed over the half-blanked portion. The timing of increase in damage was earlier for brittle steel owing to the localization of deformation, resulting in a wider successful range than that of ductile steel.
In order to develop technologies to produce precision machine parts at low cost and high productivity, precision blanking technologies using a general-purpose press were studied. A PW die, which was expanded from Press Working punch (PW punch) technology, was used for blanking, and a conventional right-angled edge die (RA die) was also used for comparison. Three blanking methods, that is, conventional blanking, blanking with a counterload, and a method of adding an upsetting load, were used in the experiments. The experiments were performed using a 4.4mm- thickness mild steel plate with clearance changed in five steps from 0.42%t to 10.3%t. The main results of this study were as follows. 1) A mirrorlike smooth surface with a high rate shearing surface,such as 95%, was obtained using a ceramic-coated PW die with a minute clearance, under the conditions of conventional blanking and counterload blanking. 2) A gently sloping convex surface, which is desirable for precision machine parts, can be obtained by adding an upsetting load after blanking.
To meet the needs for weight reduction and improvement in the crash worthiness of automobiles, it is necessary to improve the press formability of high tensile strength steel sheets. The authors have developed a piercing method that improves the stretch flange ability of steel sheets, in which steel sheets are pierced under tension using a punch with a protrusion at its bottom. The method reduces straining at the pierced edge by causing early crack growth during piercing. A study of the effect of protrusion height on the pierced edge profile, work hardening, and stretch flange ability was conducted, and the following findings were obtained. 1) As protrusion height increases, stretch flange ability increases, until it exceeds a certain height after which it decreases. 2) The change in stretch flange-ability owing to the protrusion height change can be attributed to the change in the amount of strain at the surface of the pierced edges, and in the inner part of the edge. The change in strain distribution can be associated with the change in the size of the burnish zone and to rollover. 3) The change in stretch flange ability owing to the change in protrusion height can be explained by the crack growths, based on the strain distribution in each case speculated, using fracture mechanics.