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A New Method for Multi-objective Optimal Design of Milling Parameters by Considering Chatter Vibrations

K.N.Toosi University of Technology-A. Sohani
Sharif University of Technology-E. Jafarzadeh, S. Khodaygan
Published 2019-05-13 by SAE International in United States
The desired milling process with high material removal rate (MRR) and low surface roughness of the product can be achieved only if machining chatter is absent. Incorporating chatter into the optimal selection of the machining parameters leads to a complex problem. Therefore, the approach of selecting conservative intervals for the machining parameters is usually employed instead. In this paper, a practical approach is proposed to specify the optimal machining parameters (depth of cut and spindle speed) in order to maximize MRR and minimize forced vibrations by considering machining chatter. Firstly, the worst-case scenario-based optimization problem in terms of the surface quality is solved to find the critical time at which maximal amplitude vibrations occur. Then, the time dependency of the problem is eliminated. Secondly, the multi-objective optimization is conducted to achieve the Pareto Optimal Front (POF). The Stability Lobe Diagram (SLD) is obtained independently through well-established analytical methods. Optimal machining parameters on the obtained POF are mapped into the SLD to represent optimal results for the cases at which machining chatter is absent. Finally, these…
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A Method for System Identification in the Presence of Unknown Harmonic Excitations Based on Operational Modal Analysis

Sharif University of Technology-S. Khodaygan
Published 2019-01-23 by SAE International in United States
Operational modal analysis techniques classically have been developed based on the assumption that the input to the system is a stationary white noise. While, in many practical cases, the systems are excited by combination of white noise and colored noises (harmonic excitations). Consequently, in conditions where non-white noises are present, the existing OMA methods cannot completely distinguish between the system poles and the induced poles due to colored noises. In order to overcome this weakness of OMA methods, some researches have been conducted in the field. In this paper, a new method is proposed for identifying the modal parameters of the system under the unknown colored noises, based on the Power Spectral Density Transmissibility (PSDT) function. In this work, the proposed methodology is established upon applying the auxiliary force, which can re-excite the system under operational conditions. In order to identify the modal parameters through the PSDT function, an appropriate parametric identification method such as the Poly-reference Least Squares Complex Frequency-domain method (PLSCF), or Poly-Max method, is utilized. Thus, modal parameters of the system poles…
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Multi-Objective Optimal Tolerance Allocation of the Mechanical Systems under the Thermal Gradients

Sharif University of Technology-S. Khodaygan, Javad Hemati-Nik
Published 2018-04-03 by SAE International in United States
Tolerance allocation is a key tool to reach a product with the minimum cost and the maximum performance. Since the thermal effects can cause the dimensional and geometrical variations in the components of mechanical assemblies, the tolerance allocation may be inefficient in the optimal tolerance design at the nominal conditions without including the thermal impacts. In this paper, a new optimal tolerance design of mechanical assemblies with the thermal effects is proposed. According to the proposed method, the tolerance allocation procedure is modeled as a multi-objective optimization problem. The functional objective, the manufacturing cost, and the quality loss function are considered as the corresponding objectives multi-objective optimal tolerance design problem. Using the computational results from the finite element simulations and based on the Artificial Neural Network (ANN) method, the design function as functional objective can be modeled. The optimal tolerance design problem in a multi-objective optimization form can be solved using Non-dominated Sorting Genetic Algorithm-II (NSGA-II). The Pareto optimal front (POF) of the proposed multi-objective optimization problem can be obtained. Finally, to illustrate the efficiency…
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A New Approach for the Reliability-Based Robust Design Optimization of Mechanical Systems under the Uncertain Conditions

Sharif University of Technology-S. Khodaygan, M. H. Sharafi
Published 2018-04-03 by SAE International in United States
A mechanical system inherently affected by the conditions, factors, and parameters of uncertainties. Without including the uncertainty effects in the design procedure, the designs may not be robust and reliable. Robust design optimization (RDO) method is a procedure to find the insensitive design with respect to the variations. On the other hand, reliability is measured by the probability of satisfying a specific design criterion. Therefore, a reliable design is a design that satisfies the specified criteria even with some uncertainties in variables and parameters. Reliability-based design optimization (RBDO) is an optimization procedure that incorporates reliability requirements to find the proper design. Since RDO and RBDO are usually the expensive computational approaches, the Reliability-Based Robust Design Optimization (RBRDO) may be difficult to apply. In this paper, a new model for the reliability based robust design optimization is introduced. First, two new factors that are called “chance - penalty function” and “reliability multiplier” are introduced. Based on these new factors, the optimality, the robustness, and the reliability functions as three objective functions are modeled. Then, a combined…
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Modal Parameter Identification of Rotary Systems Based on Power Spectral Density Transmissibility Functions

Sharif University of Technology-S. Khodaygan
Published 2018-04-03 by SAE International in United States
Operational modal analysis based on power spectral density transmissibility functions (PSDT) is a powerful tool to identify the modal parameters with low sensitivity to excitations. The rotor systems may have the asymmetric damping or stiffness matrices which can lead to increase the difficulties of the identification procedure. In this paper, a new method is proposed to identify the modal parameters of the asymmetric rotary systems by the operational modal analysis based on the power spectral density transmissibility functions. For pole extraction from the PSDT function, a proper parametric identification method such as the Poly-reference Least Squares Complex Frequency-domain method (PLSCF) or poly-Max method can be used. Then, the rotary system poles can be identified from a Stabilization Diagram (SD) with overestimating the system model order. The proposed algorithm is validated by a computer simulation.
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Statistical Tolerance Analysis of Flexible Assemblies with Contact Effects

Babol Noshirvani Univ. of Technology-Amir Ghasemali, Hamed Afrasiab
Sharif Univ. of Technology-S. Khodaygan
Published 2016-04-05 by SAE International in United States
One of the most important characteristics of industrial products, especially mechanical set-ups, is considering the tolerances of production and assembly of these set-ups, which directly influences the products’ operations. In sheet metal structures, due to the high flexibility of the sheets, the errors appeared while assembly will be as highly influential as the errors due to the production tolerance of the sheets. As a result, having a comprehensive model which could analyze the assembly process of these structures and also clarifies the relation between the tolerance of the parts and the ultimate changes of the set-up will be of considerable importance. During the assembly process, the contact effect between the components is inevitable. If such effect is not considered, the contact surfaces will permeate. The purpose of this paper is to present a method to analyze the tolerance of flexible sheet structures, considering the contact effect between surfaces. It is performed by the Method of Modified Influence Coefficients (MMIC) for calculating the outcome error appeared in the assembly set-up. To do so, the method has…
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An Efficient Method for Workpiece Locating Error Prediction in Machining Process

Sharif Univ. of Technology-S. Khodaygan
Published 2016-04-05 by SAE International in United States
Fixtures play a key role in locating workpieces to manufacture high quality products within many processes of the product lifecycle. Inaccuracies in workpiece location lead to errors in position and orientation of machined features on the workpiece, and strongly affect the assemblability and the final quality of the product. The accurate positioning of workpiece on a fixture is influenced by rigid body displacements and rotations of the workpiece. In this paper, a systematic approach is introduced to investigate the located workpiece position errors. A new mathematical formulation of fixture locators modeling is proposed to establish the relationship between the workpiece position error and its sources. Based on the proposed method, the final locating errors of the workpiece can be accurately estimated by relating them to the specific dimensional and geometric errors or tolerances of the workpiece and the related locators. The proposed method is developed for error analysis based on worst case and statistical approaches. The application of the presented method is illustrated through presenting an example and the computational results are compared with the…
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Error Reduction in Spatial Robots Based on the Statistical Uncertainty Analysis

SAE International Journal of Materials and Manufacturing

K.N. Toosi University of Technology-M. Hafezipour
Sharif University of Technology-S. Khodaygan
  • Journal Article
  • 2015-01-0435
Published 2015-04-14 by SAE International in United States
Kinematic accuracy of the robot end-effector is decreased by many uncertainties. In order to design and manufacture robots with high accuracy, it is essential to know the effects of these uncertainties on the motion of robots. Uncertainty analysis is a useful method which can estimate deviations from desired path in robots caused by uncertainties. This paper presents an applied formulation based on Direct Linearization Method (DLM), for 3D statistical uncertainty analysis of open- loop mechanisms and robots. The maximum normal and parallel components of the position error on the end-effector path are introduced. In this paper, uncertainty effects of both linear and angular variations in performance of spatial open-loop mechanisms and robots are considered. Based on the relations for the percent contributions of manufacturing variables, for the position error reduction, the tolerances that have the most significant effects on the commutated uncertainty zone of the end-effector position can be modified. The proposed method is illustrated using a spatial manipulator with three-revolute joints and verified with a Monte Carlo simulation method. Finally, normal and parallel distances…
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Robust Tolerance Design of Mechanical Assemblies Using a Multi-Objective Optimization Formulation

Sharif University of Technology-S. Khodaygan, Mohammad R. Movahhedy
Published 2014-04-01 by SAE International in United States
The design process always has some known or unknown uncertainties in the design variables and parameters. The aim of robust design is minimization of performance sensitivity to uncertainties. Tolerance allocation process can significantly affect quality and robustness of the product. In this paper, a methodology to minimize a product's sensitivity to uncertainties by allocating manufacturing tolerances is presented. The robust tolerance design problem is formulated as a multi-objective optimization based on the combined function-uncertainty-cost model. Genetic algorithm is utilized to solve the multi-objective optimization and a case study is presented to illustrate the methodology.
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A Method for Locator Errors Compensation in the Fixture - Workpiece System

SAE International Journal of Materials and Manufacturing

K. N. Toosi University of Technology-S. Khodaygan
  • Journal Article
  • 2013-01-1382
Published 2013-04-08 by SAE International in United States
Inaccuracies in workpiece location lead to errors in position and orientation of machined features on the workpiece, and strongly affect the assemblability and the final quality of the product. The accurate positioning of workpiece on a fixture is influenced by rigid body displacements and rotations of the workpiece due to locator errors. In this paper, a new mathematical modeling of a fixture - workpiece system is proposed to establish the relationship between the workpiece location error and its sources. For the purpose of eliminating the locator errors of the fixtured workpiece, the resultant errors due to several sources are modeled in the locator error. Then the displacement and rotation errors of the located workpiece can be compensated by adjusting the length of locators. The proposed method is proper for error analysis based on both worst case and statistical approaches. The application of the presented method is illustrated through presenting an example and the computational results are compared with the simulation results of the case study in a CAD system.
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