Final Shape Accuracy Research Articles

Precise machining of disk shapes from thick metal substrates by femtosecond laser ablation

Femtosecond-pulsed lasers have proven to be versatile for machining of microscale features in a variety of materials with minimal residual stress and heat-affected zone. However, machining of components from 200-μm-thick metal substrates by femtosecond laser ablation requires repetitive scanning of the laser beam focus along multiple adjacent paths numerous times to completely remove the part. In this study, optimization of multi-pass ablation parameters for efficient cutting and optimum final shape accuracy of disks of Stellite, aluminum, and stainless steel was investigated. The ablation depth of Stellite was measured with different pulse energies and groove widths, and 200 μm of diameter and thickness of Stellite disk were machined. The tapered cross-section of the disk was observed and increased pulse energy as the laser beam approached the bottom of the sample helped to minimize the taper angle of the sides of the disk. Moreover, the comparison of ablation depth in Stellite, aluminum, and stainless steel were conducted with various pulse energies and groove widths.

자동차 초고강도 강판 패널의 스프링백 저감에 관한 연구

The very big springback of advanced high strength steel(AHSS) sheets invokes undesired shape defects, which can be generally eliminated by die correction or process parameter control. The springback reduction by controlling the forming process parameters is easy for the application, but limited for the bulky achievement. In this study, the effective die correction method, which obtains the modification of tool shape from the relationship between die design variable and springback, is introduced and is applied to the TWB tool of automotive side rail to show the validity and usefulness. Among the die correction trials repeatedly performed, the first trial is carried out by correcting the tool shape to the opposite direction to the springbacks of several tool sections. Next trials are done by extrapolating the springbacks of among the original tool uncorrected and the tools corrected negative amounts of the springback and by finding tool shapes without springbacks. After the angle of side wall and radius of curvature of horizontal bottom floor are chosen as design variables in the tool design of side rail, the tool shape is corrected 3 times. The accuracy of final shape within the assembly limit of 1mm and the springback reduction of 75.8% compared to the uncorrected tool are achieved.

The mechanics of multi-point sandwich forming

Multi-point sandwich forming (MPSF) is a sheet metal forming process of recent innovation, in which a reconfigurable bottom die and a rubber top die are used. Also, a reusable die sheet is placed on the pins of the reconfigurable die to form a continuous surface. Because a simple press may be used and the cost of altering the tooling to make a new part shape is small, MPSF is a suitable method for forming small-batch quantities of a particular part. However, the basic deformation mechanics of MPSF have not yet been investigated and thus the best configuration of tool components to make a given part shape has to be determined by trial and error, which detracts from the low-cost aspect of the process. Therefore, in this paper, the results of investigation of the effect of workpiece and tool component dimensions and workpiece properties on the characteristics of the formed part shapes are presented. To enable the effect of tool and process parameters to be revealed more readily, plane strain deformation is used and a cylindrically shaped die surface was chosen for the bottom die. Three kinds of metal sheet and two kinds of rubber were used in the experiments. Experimental results show that for the die sheet, forming force increases and dimple size reduces with increase in the thickness of the sheet. Dimples occur on the surface of 1 mm LF21 aluminum sheet workpieces, however no dimples are formed on 2-mm-thick Q235B and ST12 steel sheet workpieces. The stiffness of the interpolator has an effect on the final shape accuracy. Also, the forming force of the workpiece reduces and shape accuracy increases, when lubrication is used in the surfaces of a workpiece.

A Numerical Thermoplastic Analysis of Line Heating Processes for Saddle-type Shells with the Application of an Artificial Neural Network

Line heating (LH) is a process for forming compound-curved shells of a ship's hull, and is carried out by skilled workers. The accuracy of final shape and the production time are based solely on the experience and intuition of the workers. Many attempts have been tried to analyze the LH mechanism theoretically and experimentally, in order to achieve productivity. The nature of the LH process involves a three-dimensional transient thermal conduction phenomenon, followed by temperature-induced permanent plastic deformation. Due to the complexity of the physical problem, a theoretical analysis is not presently available. Previous studies have been limited to simplified models or two-dimensional analyses, which are inadequate for applications in current shipyard practices. In addition, a final manufactured shape is dependent on many factors involved in the process, such as torch speed and position, type of heat, cooling method (air or water), as well as plate dimensions. The effect of each factor on a final deformed shape con not be obtained using simplified modeling. With a practical application in mind, a numerical approach was employed, to simulate the LH process. Based on the mechanics of LH, temperature and stress fields are uncoupled, and each field is solved using a general finite element program. Heat flux for the heating torch and convection condition for the cooling hose are modeled for temperature analysis. The plate to be fabricated is descretized using three-dimensional solid elements. In order to verify the validity of the present model, a temperature distribution is obtained for a flat plate problem and compared with published data. The results are in good agreement with the published data. A numerical simulation is then carried out for forming saddle-type shells according to current shipyard practices. Currently, the line heating is applied to a cylindrically curved shell to produce a doubly curved shape, that is saddle-type shell. Thus, singly curved shells are modeled to simulate the forming process of saddle-type shells and temperature distribution, permanent plastic deformation, and residual stress are calculated for the obtained final shells. Parametric studies are given and discussed relative to the effects of forming parameters, such as torch speed, cooling method, and plate dimensions. It should be noted that each piece of shell of a ship's hull is not identical. In addition, each ship is different. This means that every single piece of flat plate is fabricated using different combination of forming parameters. Thus, it is necessary to generate new parameters from the calculated results for the automation of the LH process. An artificial neural network (ANN) algorithm is applied to generate new parameters, and is verified with several actual examples.