One problem in the blanking of a motor core has been that products tend to become oval shaped. In this research, a method of expressing dimensional changes was investigated using the distributions of stress and strain in a deformed material by finite element method (FEM), to manufacture products of high circularity by circular blanking. The effects of the clearance between the punch and the die, and the material anisotropy property, namely, r, on the dimensional change of the blank were examined. In addition, the validity of the method was confirmed by comparing the calculated and experimental values. The result showed that the configuration factors of the process of the dimensional change included the timing of fracture, the position of fracture, and the extent of plastic elongation, and that the process was affected markedly by clearance and r. The extent of plastic elongation decreased when clearance increased; it increased with decreasing r, because the extent of plastic deformation increased, therefore taking time to the start of fracture. The analysis results showed good correlation to the experimental ones. It is suggested that the method can be used to predict dimensional changes of products.
A hot metal gas forming process for an aluminium alloy tube using sealing pressure and resistance heating was developed. In this process, the sealed tube is bulged by resistance heating without controlling internal pressure. A hot metal gas forming process for an aluminium alloy tube without and with axial feeding was carried out. The deformation behaviour of the tube inside the die was observed through a heatproof glass plate attached to the side wall of the die. The sealing pressure, current density, timing of axial feeding, feeding velocity and amount of axial feeding were optimized. For excessive sealing pressure, fracture occurred, and the tube partially melted at a small sealing pressure and a large current density. Since the temperature of the tube rapidly drops after coming into contact with the die, the axial feeding before the contact is effective in preventing fracture. A ceramic die with a small thermal conductivity is useful for improving the temperature decrease and die filling.
Laser peen forming is an incremental sheet metal forming using shock waves induced by a short pulse laser. The authors have adopted laser peen forming with a femtosecond laser for thin-sheet-metal bending. To investigate properties of such forming, several thin sheets of pure aluminum, phosphor bronze and austenitic stainless steel were bent. Results showed that there is a relationship between bending angle and the product of spot diameter and fluence (density of pulse energy). Forming a spot shape was effective in changing bending efficiency. These results are useful for controlling bending deformation. The influences of material properties and sheet thickness were investigated. It was found that thinner and softer sheet metal was easy to bend. Moreover, the forming properties of phosphor bronze and stainless steel were easily affected by the condition of the laser emitter.
Recently, the adjustment of working conditions in a manufacturing process is indispensable for realizing manufacturing accuracy in the V bending of a sheet metal. This adjustment is called "Trial Processing", and is a factor for lowing productivity. Therefore, "Trial Processing Free", a method that can obtain accuracy within tolerance but without the adjustment, is required. In the V bending of a sheet metal, there are two types of bending process, i.e., (i) air bending and (ii) bottom bending, using two types of punch control, i.e., (a) stroke control and (b) force control. In this research, a series of analysis and experiments were carried out to clarify that (ii) bottom bending with (b) force control is most appropriate for achieving Trial Processing Free. It was also established that bottom bending at an appropriately controlled force can absorb the effect of uncontrollable factors such as the dispersion of thickness and material properties of sheet metals and bend sheets within a certain tolerance, because the bending angle is stable even in a very wide range of bending forces. This bending method with force control was called the "FCSS (Force Control to Sweet Spot) method" and the advantage of this new process was verified experimentally.
The effect of bottoming load on the reduction of springback was investigated by performing U-bending experiments on a 590MPa level high-strength steel sheet for three different forming gaps (i.e., the gaps between the punch and die, which were 0, 5 and 10% less than the sheet thickness) at the punch corner. From the experiment, it was found that springback decreases with increasing bottoming load to some extent but a certain amount of springback remains even under a higher load. In the 3D Finite Element (FE) simulation of the U-bending/bottoming, it was clarified that bending stresses at a punch R-corner are markedly reduced by bottoming; however, those in the vicinity of the end of the corner cannot be eliminated. This is why it is so difficult to completely eliminate the springback by bottoming in U-bending. Thus, it is recommended in real press forming operations to apply a certain amount of bottoming load to reduce springback, but it should be very large. One of the important conclusions for an accurate FE simulation of springback is that the proper choice of a material model is essential, and furthermore, consideration of the elastic deformation of tools will give better results. The best combination, found in the present study, is the use of the Yoshida-Uemori kinematic hardening law for the material model and 3D deformable solid model for tools.
Highly corrosion-resistant stainless-steel sheets have been increasingly used in various applications. In press forming, however, workpieces of stainless-steel sheets more easily adhere to die tools than to other metals. In this study, we propose to apply sintered polycrystalline diamond (PCD) to die tools used for press forming. Die tools made of sintered PCD can be fabricated by electric discharge machining. We carried out a deep drawing experiment on a thin SUS304 stainless-steel sheet using die tools made of sintered PCD (a die and a blank holder). It was found that the drawing limit was improved using the PCD tools compared with the conventional tool made of alloy tool steel (SKD11), because the coefficient of friction between the PCD tools and the workpiece was reduced. Moreover, we carried out 5,000 continuous deep drawing experiments without lubricant. For the conventional tool, the workpiece started adhering to the die immediately after the start of drawing, and large flaws were observed on the side wall of the drawn products. For the PCD tools, the workpiece never adhered to the tools during drawing, and high-precision- drawn products were stably obtained for up to 5,000 drawing experiments.