Papers by Author: Mehrdad Vahdati

Abstract: The compressed air vibration in air slide table of ultra precision machines is an obstacle for gaining Nano-metric level of accuracies in the products created by these machines. In this article, the vibration behavior of air slide table due to compressed air layer is analyzed. The air slide table fluid and dynamic analytical model has been derived. In order to have air exit velocity from clearance gap, the finite element software FLUENT was applied. Finally, in order to calculate the displacements of table due to vibration and drawing its curves via time at the presence of varieties of air pressure, the second order differential equation was solved by MATLAB. In solving differential equation the Rang-Kutta method was used.

Abstract: Nowadays, the nano-machining process is used to produce high quality finished surfaces with precise form accuracy. To understand and analyze the chip formation mechanism of nano-machining process on an atomistic scale, since the experimentation is not an easy task, numerical simulation such as molecular dynamic (MD) simulation is a very useful method. In this paper, MD simulation of the nano-metric cutting of single-crystal copper was performed with a single crystal diamond tool. The model was solved with both pair wise Morse potential function and embedded atom method (EAM) potential to simulate the inter-atomic force between the work-piece and a rigid tool. The chip formation mechanism, dislocation generation, tool forces and generated temperature were investigated. Results show that the Morse potential cannot perform an appropriate defect formation and plastic deformation in nano-metric cutting of metals. Also, tool forces in Morse potential are more than the forces in EAM potential. Furthermore, the fluctuations of resultant forces in Morse potential are greater than that of EAM. In addition, using many-body interaction potentials like EAM can lead to substantial changes in surface energies, elastic-plastic properties and atomic displacement, compared with the pair-wise potentials like Morse. Finally, the atomic displacement investigation shows that in EAM potential study, only the atoms in a local region near the cutting process are displaced, but in Morse potential a large portion of atoms has affected during cutting process. Subsequently, the chip temperature in EAM potential is more than that of Morse potential.

Abstract: Today, there is a need to understand the micro mechanism of material removal to achieve a better roughness in ultra precision machining (UPM). The conventional finite element method becomes impossible to use because the target region and grids are very tiny. In addition, FEM cannot consider the micro property of the material such as atomic defect and dislocation. As an alternative, molecular dynamics (MD) simulation is significantly implemented in the field of nano-machining and nano-tribological problems to investigate deformation mechanism like work hardening, stick-slip phenomenon, frictional resistance and surface roughness [1]. One of the machining parameters than can affect nano-cutting deformation and the machined surface quality is tool nose radius [2]. In this paper molecular dynamics simulations of the nano-metric cutting on single-crystal copper were performed with the embedded atom method (EAM). To investigate the effect of tool nose radius, a comparison was done between a sharp tool with no edge radius and tools with a variety of edge radii. Tool forces, coefficient of friction, specific energy and nature of material removal with distribution of dislocations were simulated. Results show that in the nano-machining process, the tool nose radius cannot be ignored compared with the depth of cut and the edge of tool can change micro mechanism of chip formation. It appears that a large edge radius (relative to the depth of cut) of the tool used in nano-metric cutting, provides a high hydrostatic pressure. Thus, the trust force and frictional force of the tool is raised. In addition, increasing the tool edge radius and the density of generated dislocation in work-piece is scaled up that is comparable with TEM photographs [6].

Abstract: A great deal of attention in manufacturing engineering has been focused on finishing operations of hard and brittle materials in recent years. This paper reports an experimental work on the analysis of surface roughness and material removal using design of experiment (DOE) method in magnetic abrasive finishing, (MAF) of flat surfaces. Change in surface roughness and material removal were found to increase with an increase in weight percentage of abrasive particles in magnetic abrasive brush, lubricant volume and decrease in working gap. Also, any decrease in the relative size of the abrasive particles vis-à-vis the iron particles would result into an increase of the surface roughness and decrease in material removal. It was observed that the work piece hardness had no considerable effect on the process results. The optimum parameter levels which lead into the best surface finish and highest material removal were also derived from these experimentations. Optimum levels included weight percentage of abrasive particles of 40%, Lubricant volume of 1 ml, working gap of 3 mm, relative size of abrasive particles vis-à-vis the iron particles of 0.22, and work piece hardness of 82-87 HBN. Disk type test pieces were selected from Al 7075 and their two side surfaces were under experiments. Experiments were made using a milling machine spindle as magnetic pole holder, and its table as fixture holder for work pieces.

Abstract: Air Spindles have been used in ultra precision machines for several years due to their advantages such as high speed rotation, low friction, and low vibration, [1]. Air spindles are widely used in these machines for producing precise work pieces. Although, spindles function on a very complicated theoretical basis, [2, 3], their structure is very simple and consists of mainly a rotor and a stator. The rotor/stator could be made of different shapes. A cylindrical shape is the one commonly in use. The spindle designed in this work has a spherical configuration. It has been designed so that it could be moved without application of electric motor and only by a wind turbine system, [4]. The spindle studied in this research uses compressed air for rotor suspension, and has an air turbine for rotating its shaft. A thin air film acts as bearing layer between rotor and stator. In design procedure, operation parameters such as air inlet pressure for turbine, air inlet pressure for bearing, diameter of turbine nuzzles, diameter of bearing nuzzles, clearance between rotor and stator and etc. have been considered, [5]. A prototype spindle has been manufactured using design criteria. The influence of above mentioned parameters have been recognized through experiments.

Abstract: Technological requirements urges high technology industries using materials with advanced properties. Although it is compulsory using these materials due to their advantages, some of them should be tolerated because of their weak machining and mechanical properties. Attempts for qualitative machining of complex curves on many work pieces may result in the advent of new finishing techniques. One of the most newly introduced techniques is Magnetic Abrasive Finishing, MAF. In this paper a study on the effects of tool and work piece gap, and lubricant volume, on the roughness and material removal rate have been studied. The results show that setting the working gap to 3 mm and the lubricant volume to 0.7 ml, yields a change in surface roughness (Ra), and the material removal (MR) will gain its best result.

Abstract: Ultra precision machine tools are used in nano machining technology. Two main assemblies creating rotational and linear motion, called air spindle and linear air table are used in these machines. The linear air table has been simulated experimentally like a linear air bearing. This bearing moves in the main direction, X, while it has straightness error motions in Y and Z directions. The error values vary due to different parameters. This investigation deals with the influential parameters and their proportion in the error value. As far as the two mating surfaces are separated by a thin layer of pressurized air, the air pressure, location and amount of external load, are some of the parameters which have been studied. Results show that the more air pressure, the more stability and stiffer table. There will be less error motions as well. The error amount is not the same in X and Y directions. Finally, in order to have linear stiffness, the optimum air pressure, external load, and load location have been decided.

Abstract: Air spindles in ultra precision machines produce rotational movements for the cutting tool or work piece. The common combination of a simple cylindrical rotor and stator is the design for most spindles. If the length of such a spindle is longer than usual, it will deviate from its stable situation and start vibrating during operation, especially in high rotational speeds. In order to overcome the vibration problem, one of possible solutions is the application of a spherical rotor and stator. The manufacturing and assembly limitations do not allow obeying the spherical shape exactly. Thus, the design has been committed according to a quasi-sphere. This form of the rotor will be more stable. The subsequent result of stability improvement will be less air pressure and power consumption. There are some specific characterizations of the spindle which must be calculated for the spherical case. For this purpose a computer model of the object was made. Then, the model was put under finite element study to find the best air pressure and air flow velocity condition.

Abstract: Ultra precision machines are used for very precise machining as well as small parts. Due to their application, the accuracy of products has been upgraded in recent years. Thus, dimensional accuracies could be compared with surface texture dimensions like roughness and etc. In order to attain dimensions with surface texture accuracy, usually micro/nano meter, it is necessary to adopt ordinary machining technologies with micro/nano techniques. This measuring by adoption leads to nano-machining. Nano-machining researches deal with all three basic components of, machine tools, work piece, and cutting tool, which have fundamental importance for development of this technique. Despite of wide range of possible researches, only part of design points of ultra precision machines have been considered in this report. Air operated systems, like air slide table, and air spindle are examples of ultra precision machine tool components. These two components have been analyzed for some of their characteristics. Experiments have been planed to extract the relationship between stiffness, employing some of the effective parameters such as air pressure and air gap. The results describe the performance condition of air table and air spindle under different loadings.