Browsing by Author "Oliaei, S. N. B."

Now showing 1 - 10 of 10

Open Access
Built-up edge effects on process outputs of titanium alloy micro milling
(Elsevier, 2017) Oliaei, S. N. B.; Karpat, Y.
Built-up edge (BUE) is generally known to cause surface finish problems in the micro milling process. The loose particles from the BUE may be deposited on the machined surface, causing surface roughness to increase. On the other hand, a stable BUE formation may protect the tool from rapid tool wear, which hinders the productivity of the micro milling process. Despite its common presence in practice, the influence of BUE on the process outputs of micro milling has not been studied in detail. This paper investigates the relationship between BUE formation and process outputs in micro milling of titanium alloy Ti6Al4V using an experimental approach. Micro end mills used in this study are fabricated to have a single straight edge using wire electrical discharge machining. An initial experimental effort was conducted to study the relationship between micro cutting tool geometry, surface roughness, and micro milling process forces and hence conditions to form stable BUE on the tool tip have been identified. The influence of micro milling process conditions on BUE size, and their combined effect on forces, surface roughness, and burr formation is investigated. Long-term micro milling experiment was performed to observe the protective effect of BUE on tool life. The results show that tailored micro cutting tools having stable BUE can be designed to machine titanium alloys with long tool life with acceptable surface quality. © 2017 Elsevier Inc.
Open Access
Fabrication of continuous flow microfluidics device with 3D electrode structures for high throughput DEP applications using mechanical machining
(Wiley-VCH Verlag, 2015) Zeinali, S.; Çetin B.; Oliaei, S. N. B.; Karpat, Y.
Microfluidics is the combination of micro/nano fabrication techniques with fluid flow at microscale to pursue powerful techniques in controlling and manipulating chemical and biological processes. Sorting and separation of bio-particles are highly considered in diagnostics and biological analyses. Dielectrophoresis (DEP) has offered unique advantages for microfluidic devices. In DEP devices, asymmetric pair of planar electrodes could be employed to generate non-uniform electric fields. In DEP applications, facing 3D sidewall electrodes is considered to be one of the key solutions to increase device throughput due to the generated homogeneous electric fields along the height of microchannels. Despite the advantages, fabrication of 3D vertical electrodes requires a considerable challenge. In this study, two alternative fabrication techniques have been proposed for the fabrication of a microfluidic device with 3D sidewall electrodes. In the first method, both the mold and the electrodes are fabricated using high precision machining. In the second method, the mold with tilted sidewalls is fabricated using high precision machining and the electrodes are deposited on the sidewall using sputtering together with a shadow mask fabricated by electric discharge machining. Both fabrication processes are assessed as highly repeatable and robust. Moreover, the two methods are found to be complementary with respect to the channel height. Only the manipulation of particles with negative-DEP is demonstrated in the experiments, and the throughput values up to 105 particles / min is reached in a continuous flow. The experimental results are compared with the simulation results and the limitations on the fabrication techniques are also discussed.
Open Access
Influence of tool wear on machining forces and tool deflections during micro milling
(Springer, 2016) Oliaei, S. N. B.; Karpat, Y.
Tool wear on the cutting edges of micro end mills is an important issue affecting process outputs such as tool deflections and surface roughness, especially when difficult-to-cut materials such as titanium alloys, stainless steel, etc. are machined at micro scale. An understanding of the interactions between tool wear, machining forces, tool deflections, and surface roughness is important in order to maintain component quality requirements. However, in literature, the number of studies concerning tool wear in micro end mills is limited. The goal of the paper is to better understand tool wear patterns (flank wear, edge rounding) of micro end mills and their relationship to machining parameters. In this study, first, the influence of tool wear on micro milling forces and surface roughness parameters is analyzed and favorable micro milling process parameters are identified. It is shown that, when machining with worn end mills, forces are affected by the tool wear patterns. Then, the influence of increased milling forces due to tool wear on tool deflections and tool breakage is studied using both experimental techniques and finite element analysis. The finite element model-based tool deflection and tool breakage predictions are validated through experiments. The results of this study can be used in process parameter selection in pocket micro milling operations and tool condition monitoring systems.
Open Access
Investigating the influence of built-up edge on forces and surface roughness in micro scale orthogonal machining of titanium alloy Ti6Al4V
(Elsevier, 2016) Oliaei, S. N. B.; Karpat, Y.
The edge geometry of cutting tools directly influences the chip formation mechanism in micro-mechanical machining, where the edge radius and uncut chip thickness are in the same order of magnitude. An uncut chip thickness that is smaller than the cutting edge radius results in a large negative rake angle during machining, and built-up edge formation then affects the mechanics of the process. In this study, micro-scale orthogonal cutting tests on titanium alloy Ti6Al4V were conducted to investigate the influence of built-up edge formation on the machining forces and surface roughness. Cutting edges in these tests are engineered using wire EDM technique to have an edge radius of around 2 μm and clearance angles of 7° and 14°. It is observed that machining process inputs (uncut chip thickness, cutting speed, and clearance angle) affect the size of the built-up edge, which in turn affect the process outputs. It is observed that built-up edge formation protects the cutting edge from flank and crater wear under micro machining conditions and the influence of built-up edge on the surface roughness varies depending on the cutting speed and uncut chip thickness. Our findings also indicate a close relationship between the minimum uncut chip thickness and the mean roughness depth (Rz) of the machined surface. The minimum uncut chip thickness is found to be around 10% of the edge radius in the presence of built-up edge.
Open Access
Investigating the influence of friction conditions on finite element simulation of microscale machining with the presence of built-up edge
(Springer, 2017) Oliaei, S. N. B.; Karpat, Y.
In micromachining, the uncut chip thickness is less than the cutting tool edge radius, which results in a large negative effective rake angle. Depending on the material properties, this large negative rake angle promotes built-up edge (BUE) formation. A stable BUE acts like a cutting edge and affects the mechanics of the process. The size of the BUE increases with increasing uncut chip thickness and cutting speed. It also creates a positive rake angle, but it decreases the clearance angle of the tool. A method of including BUE formation in finite element simulations is to use sticking friction conditions at the tip of the tool. However, this approach is shown to be insufficient to simulate BUE formation in microscale machining. Therefore, the cutting edge is modified with the experimental BUE size in the finite element simulations based on experimental measurements. The influence of friction models between BUE and the work material has been investigated, and the study identifies friction coefficients that yield good agreements with experimental results. The finite element model is shown to be capable of simulating process forces and chip shapes for uncut chip thickness values larger than minimum uncut chip thickness.
Open Access
Micro tool design and fabrication: a review
(Elsevier, 2018) Oliaei, S. N. B.; Karpat, Yiğit; Davim, J. P.; Perveen, A.
Mechanical micromachining is considered as a cost-effective and efficient fabrication technique to produce three dimensional features and free-form surfaces from various engineering materials. Micro cutting tools are an essential part of mechanical micromachining and they are exposed to harsh conditions which reduces tool life and adversely affect the economics of the process. The challenge is therefore to maintain the tool rigidity and cutting edge sharpness for extended period of time. Thus, the design, fabrication and durability of micro cutting tools are of significant importance for successful micromachining operations. This review paper aims to provide a comprehensive understanding about the capabilities, characteristics, and limitations of different fabrication techniques used in the manufacturing of micro cutting tools. State-of-the-art micro cutting tool design and coating technology has been presented for various micromachining applications. Possible future research direction and development in the field of micro tool design and fabrication has also been discussed.
Open Access
Mikro frezeleme işleminin takım eksen sapması göz önüne alınarak mekanistik modellemesi
(Gazi Üniversitesi Mühendislik-Mimarlik, 2018) Karpat, Yiğit; Kanlı, Muammer; Oliaei, S. N. B.
İşlenmesi zor malzemelerden yapılan mikro parçaların hassas üretiminde mikro frezeleme yaygın olarak kullanılan bir yöntemdir. Malzeme işleme hızının yüksek olması ve işleme sonunda iyi yüzey kalitesi üretilmesi bu yöntemin en belirgin avantajlarıdır. Mikro frezeleme işleminin sonuçlarının tahmin edilebilmesi için ilk adım hassas bir mekanistik kuvvet modelinin geliştirilmesidir. Mikro frezelemede oluşan ortalama kesme kuvvetlerinin ilerlemeye göre değişimi makro ölçek frezelemede olduğu gibi doğrusal bir karakteristik göstermez. Bu çalışmada, kübik polinom karakteristiğine sahip ortalama kuvvet modeline dayalı bir mekanistik model geliştirilmiştir. Ek olarak mikro takımın eksenel sapması geliştirilen mekanistik modele dahil edilmiştir. Geliştirilen model ile titanyum Ti6Al4V alaşımının işlemesi sırasında tahmin edilen kuvvetlerin yapılan ölçümler ile uyumlu olduğu görülmüştür. Farklı işleme şartlarında takım eksenel sapmasının değeri geliştirilen model vasıtası ile araştırılmıştır.
Open Access
Modelling and analysis of tool deflections in tailored micro end mills
(Inderscience Enterprises, 2019) Oliaei, S. N. B.; Karpat, Yiğit
The deflection of micro end mills has a detrimental effect on surface quality of the machined micro components and adversely affects the achievable dimensional and geometrical tolerances. In this paper, the analysis and modelling of tool deflections of tailored micro end mills have been considered. The tool deflections are obtained using analytical models as well as finite element simulations and verified using a dedicated measurement setup, which uses a capacitive sensor with a nanometre resolution for static tool deflection measurements. The optimisation of the micro end mill geometry has been performed to determine optimum neck taper angle and transition radius of the single edge micro end mill to have minimum tool deflections. With the developed model, tool failure predictions for a given process parameter set can be performed and it can be used for better micro milling process planning.
Open Access
Polycrystalline diamond end mill cutting edge design to improve ductile-mode machining of silicon
(Elsevier, 2018) Oliaei, S. N. B.; Karpat, Yiğit
Silicon is a commonly used material in optoelectronics and micro fluidics devices. Micro mechanical milling of silicon with polycrystalline diamond (PCD) tools has the potential to produce three-dimensional surfaces with good surface finish and an increased material removal rate. PCD micro end mill geometry is known to influence process outputs yet its effect has not been studied in detail. In this study, a PCD end mill with a hexagonal geometry has been considered, and its micro cutting geometry has been modified to have a parallelogram shape featuring a large negative rake angle on the bottom of the tool. The proposed micro geometry also reduces the contact area between the tool and the work material. The proposed geometry was fabricated using wire electric discharge machining (WEDM). Ductile-to-brittle transition conditions and areal surface roughness have been investigated as a function of tool geometry and feed during micro milling of silicon. A significant improvement in material removal rate and surface roughness has been obtained compared to a commercially available PCD end mill having hexagonal geometry with flat bottom. The results show that PCD micro end mill geometry significantly affects the process outputs.
Open Access
Sterilization of PMMA microfluidic chips by various techniques and investigation of material characteristics
(Elsevier, 2016) Yavuz, C.; Oliaei, S. N. B.; Cetin, B.; Yesil-Celiktas, O.
The sterilization of microfluidic chips is a vital step of the fabrication process prior to the customer use in biomedical applications. The aim of this study was to analyze the influence of different sterilization techniques and to compare the characteristics of the material before and after sterilization of polymethylmethacrylate (PMMA) microchips. For this, supercritical carbon dioxide (SC-CO2) along with standard sterilization methods such as ultraviolet (UV), heat (autoclaving), ethylene oxide (EtO) and hydrogen peroxide (H2O2) were applied. The treated microchips were analyzed by Scanning Electron Microscopy, Differential Scanning Calorimetry, Fourier Transform Infrared Spectroscopy and Laser Scanning Microscopy in order to ascertain any changes in the chemical structure and surface morphology. The optimum sterilization parameters for SC-CO2 were elicited as 120 bar, 40°C and 60 min which provided complete sterility and did not alter the main properties of the polymer along with EtO and H2O2 sterilizations unlike heat and UV treatments. However, surface roughness and microchannel profiles were negatively affected. Although complete sterility was achieved, each protocol has its own strengths and weaknesses. © 2015 Elsevier B.V. All rights reserved.