مجله مهندسی ساخت و تولید ایران
سال هفتم شماره 11 (بهمن 1399)

Stewart robot falls into the category of parallel robots, consisting of two upper and lower platforms. The top platform, the moving platform and the bottom platform, are fixed, with the two arms connected by six spheres with spherical joints. In this study, we present a novel method for controlling robots and a comprehensive model for vibration detection. After developing the governing equations of the robot arms, we performed a modal analysis to control the robot position. The results of rigid and flexible arms in different parts of the robot arm using the aforementioned controller were studied. Comparing the results, it can be said that in the robot model with elastic joints, due to the increase in degrees of freedom, the level of vibrations increased, thus, controlling the robots with elastic joints is more demanding than the that in the corresponding robot with rigid joints.. Also, by applying perturbations to the system, it was found that the performance of the controller was affected by disturbances to the system and the location error was increased. Finally, it can be stated that in the control system, determining the optimal coefficients in the relationships plays a major role in the performance of the controller. Keywords

Nowadays, in metal forming industries, the production of parts with proper thickness distribution and geometric accuracy is very important and significant. In this research, all parts are formed by a new process called two-point incremental forming process using a flexible die. Flexible die, is a die that consists of a number of separate overhead pins that can be adjusted by the height of each pin according to the desired geometry. First, the effect of the thickness (0.5, 1.5, 3 and 6mm) and hardness (60 and 90Shore A) of rubber layers on the two parameters of thickness distribution and geometric accuracy of the formed samples has been studied. The effect of the arrangement of the pins in a flexible die with full and partial layout on the two mentioned parameters has been studied. The results show that the geometric accuracy of the sample formed on the rubber layer with a thickness of 0.5mm has been improved by 78.3% compared to the thickness of 6mm, but the degree of thickness of this workpiece is more than other parts. Geometric accuracy of the workpiece is also improved by 61.77% when using a rubber layer with a hardness of 90Shore A compared to a hardness of 60Shore A, but the thickness distribution of the workpiece is the same. The pins with full arrangement have a better geometric accuracy and thickness distribution than the partial arrangement in a flexible die. Using a multi-step strategy in a flexible die has the disadvantage of wrinkling.

Smoothness of the machined surface is an important feature in machining. Fractal geometry is a creative and practical branch of modern mathematics. One of the most important features of fractal fractal geometry is the fractal dimension. The purpose of this study is to investigate the effect of current intensity on the surface hardness and roughness of steel workpiece in the process of electric discharge using fractal analysis. For this purpose, parts of alloy steel AISI316 L were machined by Spark. Then, the samples were imaged microscopically to evaluate the quality of the machined surfaces by scanning electron microscopy (SEM). The results of experiments and fractal analysis showed that with 10 times increase in current intensity, average surface roughness increased about twice and surface hardness decreases 0.8 times.Also, increasing current intensities increased the standard and amplitude of surface signal while, the frequency of changes is decreased. This indicates that the level and height of the surface are increased and points of higher altitude and depth are created at the surface which means that the surface has become rougher. As a result, increasing the surface roughness and reducing the surface hardness of the workpiece has increased the fractal dimension of machining surfaces.

Composite lattice structures are widely used in the aerospace, automobile and marine industries due to their benefits such as high stiffness and strength with light weight. In this study buckling strength after impact of grid stiffened composite panels was investigated experimentally. For this purpose, grid stiffened composite panels with iso-grid reinforced lattice and three types of skin thickness of 6, 12 and 18 layers were designed, fabricated and tested. E-glass fibers and epoxy resin was used for both ribs and skin. At first, the specimens were subjected to a low velocity impact and then the buckling test was carried out. From results it is concluded that with increasing the thickness of skin, failure mechanisms such as fiber breakage, delamination, matrix cracking consumes more energy. But, by considering the weight, the buckling strength per weight of the specimen was maximum for the specimen with 12 layers for skin. More explanation about failre mechanisms are included in the manuscript.

Advanced ceramics have special applications in the manufacture of special and strategic parts due to their characteristics such as high strength, stability at high temperatures, high corrosion resistance and unique magnetic and electrical properties. Boron carbide is a dark crystalline material that is the hardest known material after diamond and cubic boron nitride. Due to the high characteristics and hardness of boron carbide and the special ability of the electrical discharge machining process in machining hard materials and creating complex shapes, in this study, the effect of peak current parameters and pulse -on time on surface roughness and material removal rate of samples prepared from boron carbide has been investigated experimentally and statistically. The results of the experiments showed that the peak current had the greatest effect on the surface roughness and material removal rate of the samples and also the comparison of microscopic images of the surface of the machined samples showed that the peak current has increased the cracks and the size of the cavities. Thus, with the increase of peak current from 16 ampere to 24 ampere, the average size of the cavities has increased by about 30% and the continuation of the process of increasing the current intensity to 32 ampere has increased the average size of the cavities by about 8 %. Also, during the factorial optimization, the optimal values for the sampling material removal rate and surface roughness of the samples were 2.45 mm3 / min and 4.30 μm, respectively.

In this work, the laser bending of tailor machined blanks has been investigated with experimental tests and finite element simulations. It should be noted because of presence of different thicknesses in the tailor machined blanks, their laser bending process is more complicated than the monolithic sheets. In addition, multi-curvature bending phenomenon that is the non-uniformity of deformations in the transverse direction can appear more considerably in the laser bending process of tailor machined blanks than the monolithic sheets. In this paper, a new irradiating scheme has been proposed based on laser scanning speed variations for laser bending of tailor machined blanks. It is proved that with the proposed irradiating method, the differences in the bending angles of thin and thick sections of tailor machined blanks and also multi-curvature bending phenomenon has been extremely decreased. The results of bending angle and multi-curvature bending phenomenon with considering the different mechanisms in thin and thick sections of tailor machined blank demonstrate that the proposed irradiating scheme is an appropriate method for laser bending of tailor machined blanks.