Прегледај по Аутор "Mlađenović, Cvijetin"

Сада се приказује 1 - 13 од 13
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    Comparison of different mathematical models for prediction of self-excited vibrations occurance in milling process
    (University of East Sarajevo,Faculty of Electrical Engineering, 2019) Košarac, Aleksandar; Mlađenović, Cvijetin; Zeljković, Milan; Šikuljak, Lana; Maksimović, Mirjana; Mijić, Danijel
    In modern production, despite the existence of other production methods, metal cutting still plays an important role. The performance of machine tools has a decisive role in terms of productivity and quality of production increase. Undoubtedly, productivity and quality of production are two mail requirements which are key elements to stay on top in a competitive market. One of the most influencing factor that affect the machine tools are vibrations. The most unwanted vibrations that can appear during metal cutting process are self-excited vibrations, which are one of the three kinds of mechanical vibration, free vibration, forced vibration, and self-excited vibration. When it comes to improving the performance of machine tools, the analysis of the appearance of self-excited vibrations and their isolation occupy a significant place. The aim of this paper derives from trends and limitations exists in metal production. The way to isolate the self-excited vibrations is to predict their occurrence by defining the stability lobe diagram. The paper presents two popular analytical methods for identifying stability lobe diagrams in milling, which shows the boundary between stable and unstable zone of machining operations, depending on the number of revolutions of the spindle and cutting depth. First considered method is Fourier series approach and second one id average tooth angle approach. Lather, both stability lobe diagrams were compared with results obtained experimentally.
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    CREATE SISO STATE SPACE MODEL OF MAIN SPINDLE FROM ANSYS MODEL
    (Faculty of Technical Sciences, Department for Production Engineering, 2015) Košarac, Aleksandar; Zeljković, Milan; Mlađenović, Cvijetin; Živković, Aleksandar; Kovač, Pavel
    This paper shows identification of dynamic characteristics of working unit module main spindle, based on application of APDL ANSYS and MATLAB software. Single Input Single Output (SISO) state space MATLAB model will be developed from an APDL ANSYS main spindle model. The first step in defining the state space model is to define the eigenvector elements for all modes for only the input and output degrees of freedom. The next step is analysing the each mode contribution and sort them from the largest to smallest. One of the modal reduction technique is then applied, and modes with smallest contribution are simply truncated.
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    CUTTING PARAMETERS INFLUENCE ON SURFACE ROUGHNESS IN AL 7075 MILLING
    (Faculty of Electrical Engineering, University of East Sarajevo, 2020) Košarac, Aleksandar; Šikuljak, Lana; Obradović, Čedomir; Mlađenović, Cvijetin; Zeljković, Milan
    Aluminum and its alloys are nowadays a very commonly used material in the aerospace and automotive industries. Due to its good mechanical properties, low density, high hardness, good thermal properties, and good machinability, Al7075 is one of the most commonly used aluminum alloys. This work investigates the effect of cutting parameters on surface roughness in finish milling of Al7075 aluminum alloy. Considering the negative effects of conventional cutting fluids (coolants and lubricants), oils or water emulsions on the environment and health, and the negative economic effects resulting from the foregoing, this study also analyzes the possibilities the use of different coolant-lubricant types on cutting. In this connection, the subject of research is also the influence of the cooling method on surface roughness in end milling.
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    DEFINISANJE KARTE STABILNOSTI OBRADNOG SISTEMA PRIMENOM FREKVENTNE ANALIZE SIGNALA VIBRACIJA
    (Mašinski fakultet, Univerzitet u Beogradu, 2020) Mlađenović, Cvijetin; Zeljković, Milan; Košarac, Aleksandar
    Samopobudne vibracije predstavljaju jednu od najnepovoljnijih pojava u procesu rezanja, koje kao posledicu mogu imati ubrzano habanje ili lom alata, naglo pogoršanje kvaliteta obrađene površine, povećanje bučnosti, povećanje potrošnje energije, itd. U cilju izbegavanja navedenih negativnih uticaja, pri definisanju režima rezanja se primenjuju dijagrami koji, u zavisnosti od broja obrtaja glavnog vretena i dubine rezanja, prikazuju granicu između stabilnog i nestabilnog područja rada mašine alatke sa stanovišta samopobudnih vibracija. Ovi dijagrami, nazvani - karte stabilnosti mogu biti definisani primenom matematičkih modela (analitičkih, numeričkih, ...) ili eksperimentalnim ispitivanjima (metod tangenti, zvučno mapiranje,...). U radu je prikazano definisanje karte stabilnosti obradnog sistema eksperimentalnom identifikacijom samopobudnih vibracija. Granične dubine rezanja, potrebne za definisanje karte stabilnosti, određene su metodom dekomponovanja vremenskog signala samopobudnih vibracija. Predložena metoda je primenjena za određivanje graničnih dubina rezanja pri obradi glodanjem aluminijuma Al7075 na obradnom centru EMCO ConceptMill 450. Dobijeni rezultati su verifikovani poređenjem karata stabilnosti određenih primenom navedene i više matematičkih i eksperimentalnih metoda.
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    Eksperimentalno - analitički metod za definisanje karte stabilnosti pri obradi struganjem
    (Elektrotehnički fakultet Univerzitet u Istočnom Sarajevu, 2014) Košarac, Aleksandar; Mlađenović, Cvijetin; Živković, Aleksandar
    Vibracije mašina alatki je pojava koja uzrokuje ne mali broj problema pri procesu obrade, kao što su habanje ili lom alata, loš kvalitet obrađene površine, povećanje potrošnje energije, povećanje bučnosti mašine, itd. Najnepovoljniji oblik vibracija predstavljaju samopobudne vibracije, koje mogu dovesti do nestabilnog rada mašine alatke, tzv. podrhtavanja (eng. Chatter). Da bi se izbjegle posljedice samopobudnih vibracija, pri radu mašine alatke potrebno je odrediti (izolovati) one režime rezanja pri kojima dolazi do pojave podrhtavanja. U tu svrhu, potrebno je formirati karte stabilnosti koje prikazuju granicu između stabilnog i nestabilnog rada mašina u zavisnosti od broja obrtaja i dubine rezanja za sve kombinacije brzine pomoćnog kretanja. Prethodno podrazumjeva čitav niz eksperimenata, što iziskuje veliko vrijeme i ne male troškove za ispitivanje. Iako je eksperimentalna metoda najpouzdanija, u posljednje vrijeme se sve više ulažu napori da se definisanje karte stabilnosti za pojedine mašine alatke izvrši analitičkim ili eksperimentalno-analitičkim metodama. U radu je prikazan eksperimentalno - analitički metod definisanja karte stabilnosti. Metod je ilustrovan na procesu obrade struganjem. Na osnovu eksperimentalno određenih modalnih parametara karakterističnih podsistema mašine alatke, analitičkim putem definisana je karta stabilnosti na kojoj je moguće jasno uočiti kritične dubine rezanja pri određenim brojevima obrtaja.
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    EXPERIMENTAL METHOD FOR DEFINING THE STABILITY LOBE DIAGRAM IN MILLING Č4732 (42CrMo4) STEEL
    (University POLITEHNICA Timisoara, Faculty of Engineering Hunedoara, 2019) Košarac, Aleksandar; Mlađenović, Cvijetin; Zeljković, Milan; Šikuljak, Lana; Imre, Kiss
    Self-exciting vibration (chatter) is an unwanted phenomenon that can occur in metal cutting. Self-exciting vibration is a phenomenon that has a negative effect on productivity, leads to accelerated wear or breakage of the tool, and in some cases can lead to breakage of elements of jigs and fixtures or elements of the machine tools. In order to predict and control the process of creating self-excited vibration, different methods have been developed. One way of predicting self-excited vibration is defining stability lobe Diagram, which shows the boundary between stable, conditionally stable and unstable cutting process, whereby combinations of cutting depth, cutting speed, i.e. spindle speed and feed rate are observed. Methods for defining the stability lobe diagram can be analytical, and experimental. This paper shows the experimental method of defining the stability lobe diagram, on machining steel Č4732 (42CrMo4). Metal cutting is carried out on a vertical machining center. The methodology for defining the stability lobe diagram implies that a series of experiments were done, were workpiece surface to be milled is made with a slope of 3°. In this way, when moving the tool, the cutting depth gradually increases until the moment of self-excited vibration occurs. The occurrence of the vibration is registered by measuring the acceleration, where the accelerometer was being mounted on the main spindle carrier, as close as possible to the tool. After the self-excited vibration occurred, which is manifested by a sudden jump of acceleration amplitude, and by the change in the sound, the cutting process is stopped and then the axial depth of the cut at which the vibration occurs determined by the tangent method in Matlab. The obtained stability lobe diagram has two dimensions, which means that all measurements were performed for one, a constant value of feed rate per tooth.
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    Identifikacija samoizavanih vibracija pri obradi struganjem primenom savremene dijagnostičke opreme
    (Institut IMK "14. oktobar", Kruševac, 2013) Košarac, Aleksandar; Zeljković, Milan; Mlađenović, Cvijetin; Živković, Aleksandar; Gašić, Milomir
    Pojava samoizazvanih vibracija pri obradi rezanjem je odavno uočena, ali i danas predstavlja vrlo aktuelno polje istraživanja kako sa stanovišta matematičkog modeliranja i predikcije, tako i sa stanovišta eksperimentalnog ispitivanja i upravljanja obradnim procesom. Razvojem NU mašina alatki i sve većom primenom visokobrzinske obrade, pomenuti problemi su postali još izraženiji, pa su detekcija i prevencija nastanka samoizazvanih vibracija postale suštinski važne za eksploataciju ovih obradnih sistema. U radu je prikazan način utvrđivanja trenutka nastanka samoizazvanih vibracija pri obradi struganjem, kao jednog od indikatora dinamičke nestabilnosti obradnog sistema, primenom savremene dijagnostičke opreme National Instruments i LabView i Matlab softvera. Takođe, na osnovu eksperimentalno dobijenih rezultata, primenom metode tangenti, određene su granične vrednosti dubine rezanja pri kojima dolazi do nastanka samoizazvanih vibracija, kao osnove za definisanje karte stabilnosti obradnog sistema, i identifikaciju najosetljivijeg elementa sa stanovišta dinamičkog ponašanja mehaničke strukture mašine alatke.
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    Machining Simulation and Verification of Tool Path for CNC Machine Tools with Serial and Hybrid Kinematics
    (Faculty of mechanical and civil engineering Kraljevo, University of Kragujevac, 2017) Zivanović, Saša; Tabaković, Slobodan; Zeljković, Milan; Mlađenović, Cvijetin; Košarac, Aleksandar; Gašić, Milomir
    Development of modern machine tools basically is directed on improvement of kinematic structures and exploitation characteristics. As a result of this in last two decades industry more and more uses machine tools based on parallel and hybrid kinematics with significant increasing of speed of main and movement and feederate. In order to provide more efficient exploitations of these machines, reduce of the preparation time of production, increase of the safety of users and machines is necessary application of software for simulation and verification programs. This paper presents the concept of modern technological preparing of manufacturing in the case of the definition of virtual machine tools based on conventional - serial and hybrid kinematics, simulation of machining and verification of programs for machining of characteristic workpiece.
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    Next-Gen Manufacturing: Machine Learning for Surface Roughness Prediction in Ti-6Al-4V Biocompatible Alloy Machining
    (MDPI, 2023-11-15) Košarac, Aleksandar; Tabaković, Slobodan; Mlađenović, Cvijetin; Zeljković, Milan; Orašanin, Goran; Liang, Steven Y.
    Mechanical engineering plays an important role in the design and manufacture of medical devices, implants, prostheses, and other medical equipment, where the machining of bio-compatible materials have a special place. There are a lot of different conventional and non-conventional types of machining of biocompatible materials. One of the most frequently used methods is milling. The first part of this research explores the machining parameters optimization minimizing surface roughness in milling titanium alloy Ti-6Al-4V. A full factorial design involving four factors (cutting speed, feed rate, depth of cut, and the cooling/lubricating method), each having three levels, implies the 81 experimental runs. Using the Taguchi method, the number of experimental runs was reduced from 81 to 27 through an orthogonal design. According to the analysis of variance (ANOVA), the most significant parameter for surface roughness is feed rate. The second part explores the possibilities of using different ML techniques to create a predictive model for average surface roughness using the previously created small datasets. The paper presents a comparative analysis of several commonly used techniques for handling small datasets and regression problems. The best results indicate that the widely used machine learning algorithm Random Forest excels in handling regression problems and small datasets.
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    Određivanje frekvencijskih karakteristika sklopa glavnog vretena mašine alatke primjenom Ojler – Bernulijeve teorije greda i pozicionog sprezanja
    (Elektrotehnički fakultet Univerzitet u Istočnom Sarajevu, 2016) Košarac, Aleksandar; Zeljković, Milan; Mlađenović, Cvijetin; Živković, Aleksandar; Milojković, Slobodan
    U radu je prikazana analitička metoda određivanja prenosne funkcije sklopa glavnog vretena mašine alatke, kao osnova za određivanje karte stabilnosti. Glavno vreteno se modeluje iz segmenata, uključujući i mjesta oslonaca (ležajeve), koristeći pri tome Ojler – Bernulijevu teorija greda. Prenosna funkcija sklopa glavnog vretena se određuje metodom poziciong sprezanja prenosnih funkcija segmenata za određene granične uslove, odnosno krutosti i prigušenja mjesta spoja segmenata, zatim oslonaca (ležajeva) i sl. Verifikacija dobijenih matematičkih modela izvršen je numerički, primjenom metoda konačnih elementata, kao i eksperimentalnim ispitivanjem, tj. eksperimentalnom modalnom alalizom.
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    Prediction of self-excited vibrations occurance during aluminium alloy AL 7075 milling
    (Faculty of Electrical Engineering, University of East Sarajevo, 2019) Košarac, Aleksandar; Šikuljak, Lana; Šalipurević, Miloš; Mlađenović, Cvijetin; Zeljković, Milan
    Despite the existence of other production methods, metal cutting still plays an important role in modern production. The performance of machine tools has a decisive role in terms of productivity and quality of production increase. When we are talking about improving the performances of machine tools, the analysis of the appearance of self-excited vibrations and their isolation take a significant place. The aim of this paper comes from trends and limitations that are present in metal production. The paper presents analytical and experimental methods for identifying stability lobe diagrams in the aluminum alloy Al 7075 milling. The influence of certain regime on the occurrence of selfexcited vibrations, i.e. the definition of vibrational stable processing regimes, is considered.
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    STATE SPACE MODELING FROM FEM MODEL USING BALANCED REDUCTION
    (Technical faculty "Mihajlo Pupin", Zrenjanin, 2016) Košarac, Aleksandar; Zeljković, Milan; Mlađenović, Cvijetin; Živković, Aleksandar; Prodanović, Saša
    This paper shows discretized model of main spindle working unit module obtained byANSYS software. Based on the conducted modal analysis for first ten bending modes within frequency range from 0 to 10 kHz,main shapes and natural frequency (eigenfrequency) have been determined,and modal matrix was formed. Obtained results were afterwards used to determinemodel dynamic behavior in the state space using MATLAB. State space model wasfurther used as reference onefor modal reductionwhereby balanced reduction method was applied. Frequency response of full model (all oscillatory modes) and reduced model for direct FRF and cross FRF are presented through Bode diagram. Reliability of the model is verified by comparing the impulse response offull modeland reduced modelin time domain.
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    Thermal Behavior Modeling Based on BP Neural Network in Keras Framework for Motorized Machine Tool Spindles
    (MDPI, 2022) Košarac, Aleksandar; Cep, Robert; Trochta, Miroslav; Knežev, Miloš; Zivković, Aleksandar; Mlađenović, Cvijetin; Antić, Aco
    This paper presents the development and evaluation of neural network models using a small input–output dataset to predict the thermal behavior of a high-speed motorized spindles. Different neural multi-output regression models were developed and evaluated using Keras, one of the most popular deep learning frameworks at the moment. ANN was developed and evaluated considering the following: the influence of the topology (number of hidden layers and neurons within), the learning parameter, and validation techniques. The neural network was simulated using a dataset that was completely unknown to the network. The ANN model was used for analyzing the effect of working conditions on the thermal behavior of the motorized grinder spindle. The prediction accuracy of the ANN model for the spindle thermal behavior ranged from 95% to 98%. The results show that the ANN model with small datasets can accurately predict the temperature of the spindle under different working conditions. In addition, the analysis showed a very strong effect of type coolant on spindle unit temperature, particularly for intensive cooling with water