Browsing by Author "Karpat, Yiğit"

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Open Access
Comparative analysis of PCD drill designs during drilling of CFRP laminates
(Elsevier, 2015) Karpat, Yiğit; Bahtiyar, O.
As a result of increased use of CFRPs in the aerospace industry, the machining of CFRPs has been studied extensively. The majority of these studies consider drilling of CFRPs, since it is the most common process in the machining of structural parts used in aircraft. It has been shown that drilling process parameters and drill geometry significantly influence the quality of holes. In this study, a systematic approach has been used to compare the influence of drill geometry on process outputs such as drilling forces, torques and tool wear. Custom-made double point angle polycrystalline diamond (PCD) drills from the same manufacturer were used in the experiments. The advantage of this approach is that it eliminates the drill material and edge preparation effects on the experimental measurements, thus helps reveal the influence of drill geometry on the process outputs. The pros and cons of different drill designs are discussed and an appropriate design is identified for the drilling of thick CFRP laminate considered in this study.
Open Access
Design and fabrication of micro cutting tools using micro electro discharge machining (µ-EDM)
(2012-06) Ergür, Ali Can; Oliaei, S. N.; Karpat, Yiğit
With the advent of sophisticated electro discharge machines, design and fabrication of micro cutting tools and their employment in micro machining operations have become a popular research area. These machines have been used to fabricate micro milling, drilling, and grinding tools made from tungsten carbide and polycrystalline diamond with diameters as low as 5-10 micrometers. Successful fabrication and application of micro tools require a great deal of process knowledge. In this study, a methodology based on analytical and computational process models is proposed and some preliminary results are presented.
Open Access
Development of an iterative learning controller for polymer based micro-stereolithography prototyping systems
(IEEE, 2016) Türeyen, Erkan Buğra; Karpat, Yiğit; Çakmakcı, Melih
Additive manufacturing has become increasingly popular for a wide range of applications in recent years. Micro-stereolithography (μSLA) is a popular method for obtaining polymer-based parts. Systems using the μSLA approach usually consist of a vertical positioning system, a light source and a container where the component is built gradually as the polymer is cured at the locations where the ultraviolet light is projected. It has been noted that the motion of the positioning system and the intensity of the light source is an important factor to achieve high level dimensional precision. In this paper a three dimensional error based learning scheme is presented to improve the time varying process parameters of the system so that the dimensional accuracy of the product is improved. A mathematical model of the curing process is used for developing the error based learning algorithm. The current process parameters as a function of time and the dimensional error obtained at each layer of the production are used for increasing the quality and precision of the same part in the next iteration. Our initial simulation results show significant improvements can be obtained in a few iterations if the correct learning parameters are used based on the target parts dimensional properties.
Open Access
Energy based investigation of process parameters while drilling carbon fiber reinforced polymers
(Elsevier B.V., 2016) Karpat, Yiğit; Bahtiyar, O.
Carbon fiber reinforced polymers (CFRPs) are widely used in the aerospace industry due to their light weight, high strength, and low thermal conductivity. Drilling is a critical process that affects the quality of CFRP parts. This work studies the influence of process parameters on delamination and tool wear. Polycrystalline diamond helical drills are used in the experiments. It has been shown that drilling energy calculations can be used to set appropriate feed and speed parameters and for increasing drilling performance of CFRPs. The results also indicate the importance of thermal modeling of CFRP laminate for better understanding of the drilling process. © 2016 The Authors.
Open Access
Experimental investigations on micro milling of stavax stainless steel
(Elsevier, 2014) Oliaei, Samad Nadimi Bavil; Karpat, Yiğit
Micro mechanical milling of Stavax (modified AISI 420) stainless steel which is commonly used in the plastic injection molding industry due to its high corrosion resistance, machinability and wear resistance is studied in this paper. The goal of this study is to investigate the influence of process input parameters such as cutting speed, depth of cut, feed per tooth, radial immersion percentage, and plunging method on process outputs during circular pocketing operation. Tool wear, machining forces and surface roughness measurements are considered to identify the relationships between process inputs and outputs. It is observed that plunging method has a significant influence on tool wear which in turn affects micro milling forces and surface quality. The positive impact of micro milling with a pre-drilled hole is demonstrated. The relationship between radial immersion and feed per tooth is shown to be important in terms of tool wear and surface roughness.
Open Access
Fabrication of PCD mechanical planarization tools by using μ-wire electrical discharge machining
(Elsevier, 2016) Oliaei, Samad Nadimi Bavil; Karpat, Yiğit
Fabrication of micro components made from difficult-to-cut materials require the use of micro cutting tools which can withstand the harsh conditions during machining. Polycrystalline diamond micro tools, produced using micro wire electro discharge machining, have been used to machine silicon. In this study, fabrication of PCD planarization tools having micro-pyramid lattice structure is considered. A tungsten wire with 30 μm diameter was used, which makes it possible to obtain very precise micro-features by employing extremely low discharge energies. The performance of the tools is investigated through micro scale grinding of silicon and appropriate machining parameters which resulted in ductile regime machining of silicon are determined.
Open Access
Fabrication of polymer micro needles for transdermal drug delivery system using DLP based projection stereo-lithography
(Elsevier, 2016) Ali, Z.; Türeyen, E. Buğra; Karpat, Yiğit; Çakmakcı, Melih
Fabrication of micro needles, which reduce pain during insertion and lessen tissue injury, has recently attracted great interest. Different manufacturing systems have been utilized for the advancement of micro needles such as two-photon photo polymerization, bulk lithography, droplet-borne air blowing and injection molding [1]. This paper proposes a micro fabrication process for polymer micro needles, using DLP based projection-based stereo lithography that is capable of fabricating micro-needles using biocompatible polymers. The fabrication in the experimental setup is performed with continuous movement of the platform in the vertical direction hence good surface quality is obtained. The influence of polymerization time, light intensity of DLP projector and chemical composition of the resins on the production speed and the geometrical accuracy of the micro needles have been studied. The length and the tip diameter of the micro needle are shown to be controlled through these factors. The length and tip diameter of the fabricated micro needles were observed using SEM and optical microscope and measured to be around 520 μm and 40 μm, respectively. The results indicate that polymer micro needles with appropriate geometry can be fabricated using this technique. © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.
Open Access
Influence of diamond tool chamfer angle on surface integrity in ultra-precision turning of singe crystal silicon
(Springer, 2019) Karpat, Yiğit
Ultra precision diamond machining enables the economical production of freeform optics on infrared materials such as silicon. To produce optics with acceptable surface integrity, it is important to have a good understanding of process-work material interaction between diamond tool and brittle and hard single crystal IR materials. Chamfered cutting edges are known to have high strength, which makes them suitable for machining difficult-to-cut materials. This study investigates the influence of chamfer angle on the surface integrity of silicon. Diamond tool chamfer angles of − 20°, − 30°, and − 45° are considered under practical diamond turning conditions of single crystal silicon. State-of-the-art techniques were used to investigate the surface integrity of the machined silicon surfaces. The results show that chamfer angle of 30° yields more favorable results compared to 20° and 45° under the conditions tested. The results indicate the complex interplay between tool geometry and process parameters in reaching an acceptable level of surface integrity. A machinability map indicating ductile and brittle machining conditions for 30° chamfered diamond tool has been presented which includes directly transferable knowledge to the precision machining industry.
Embargo
Investigating elastic recovery of monocrystalline silicon during plunging experiments with chamfered diamond tools
(Elsevier Inc., 2023-08-03) Karpat, Yiğit
This study investigates the relationship between elastic recovery and friction during ductile mode machining of monocrystalline silicon. Plunging experiments were performed on an ultra-precision CNC machining center as the silicon work material was rotated. This approach allowed for investigating the combined effect of varying crystal directions and increasing depth on elastic recovery, pressures, and friction. A diamond cutting tool with a chamfered edge that maintains a constant negative rake along the cutting edge was used in plunging tests. Considering the importance of flank face contact area on friction and pressure distributions at the cutting zone, a non-linear expression is proposed to model the elastic recovery ratio as a function of groove depth. It is then employed in an analytical machining model to calculate the variation of pressure and coefficient of friction. The influences of pressure variation and changes in pressure direction as the machining mode transitions from ductile to brittle are investigated based on resolved stresses on the identified active slip plane. The methodology proposed in this study can be extended to study other monocrystalline materials, and the findings can be utilized in the process planning of advanced optics manufacturing.
Open Access
Investigating flank face friction during precision micro cutting of commercially pure titanium via plunging tests with diamond grooving tools
(Elsevier, 2022-01) Karpat, Yiğit
This study investigates flank face friction while micro machining commercially pure titanium (cp-Ti grade 2) work material considering size effects. It is important to understand friction phenomena at the tool flank and work material surface since they affect the surface integrity of the machined parts. A single crystal diamond grooving tool is used in machining experiments to reduce the influence of cutting edge radius. In addition, plunging type of cutting experiments were performed to investigate the influence of flank face contact on the machined surface. A friction model which is based on work and tool material properties is proposed to model the contribution of adhesion and deformation of the flank face coefficient of friction. The results show that for the cp-Ti and diamond tool pair, adhesion seems to be the dominant model of friction and also contributes to the size effect. The deformation friction becomes more dominant during the chip formation stage. When cutting edge effect is eliminated, the influences of flank and rake face friction on the size effect are shown.
Open Access
Mechanistic force modeling for milling of unidirectional carbon fiber reinforced polymer laminates
(2012-06) Karpat, Yiğit; Bahtiyar, O.; Değer, B.
Carbon fiber reinforced polymer (CFRP) usage in the aerospace industry has been steadily increasing due to its superior material properties such as high strength, low weight, high resistance to corrosion, and a low thermal expansion coefficient. In addition, CFRP parts are produced near-net-shape, a process that eliminates rough machining operations. However, machining operations such as drilling, side milling, and slotting are still necessary to give the CFRP parts their final shape. A majority of the studies on machining of CFRP laminates are on drilling. The number of studies on milling of CFRPs is quite limited. In this study, a mechanistic cutting force model for milling CFRPs is proposed based on experimentally collected cutting force data during slot milling of unidirectional CFRP laminates using two different polycrystalline diamond cutters. Cutting force coefficients in radial and tangential directions are calculated as a function of fiber cutting angle. The relationship is represented with simple sine functions. The mechanistic model is shown to be capable of predicting cutting forces during milling of multidirectional CFRP laminates. The experimental milling force measurements and predicted milling forces agree well with each other. Surface milling experiments were also conducted to investigate the relationship between milling forces and surface quality. Some suggestions on surface milling of CFRP laminates are given based on these observations.
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
Microstructure effects on process outputs in micro scale milling of heat treated Ti6Al4V titanium alloys
(Elsevier, 2018) Ahmadi, M.; Karpat, Yiğit; Acar, O.; Kalay, Y. E.
This study investigates the influence of materials’ microstructural characteristics, including grain size and phase fractions, in micro end milling of heat treated Ti6Al4V titanium alloys. Micro milling process conditions such as feed, depth of cut, and the cutting edge radius of the micro end mill are in the same order of magnitude as the grain size of the material, which gives rise to the anisotropic behavior of the multiphase materials and their deformation characteristics considering their grain size, grain boundaries, and phase fractions. A good understanding of such relationships is believed to be instrumental in developing predictive models of machining based on computational techniques. The influence of micro milling process on the crystallographic texture and microstructure of Ti6Al4V alloys is the subject of this study. For this purpose, heat treatment was performed on the Ti6Al4V samples to obtain two different microstructures: fine equiaxed and enlarged equiaxed microstructures. Micro milling experiments were performed on each sample and process outputs such as cutting forces, areal surface texture, built-up edge (BUE) formation, and alterations in the microstructure were investigated. Electron backscatter diffraction (EBSD) analysis was used to investigate the microstructure of the machined surfaces. It was observed that smaller grain size (both α and β) and lower fraction of β phase in the material yielded higher cutting forces. BUE formation and its size were affected by the microstructure of the samples. The results of this study may be useful in developing microstructure-based, predictive modeling of micro milling process.
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
Milling force modelling of multidirectional carbon fiber reinforced polymer laminates
(Elsevier, 2012) Karpat, Yiğit; Bahtiyar, O.; Deger, B.
Carbon fiber reinforced polymer (CFRP) usage in the aerospace industry has been steadily increasing due to its superior material properties such as high strength, low weight, high resistance to corrosion, and a low thermal expansion coefficient. In addition, CFRP parts are produced near-net-shape, a process that eliminates rough machining operations. However, machining operations such as drilling, side milling, and slotting are still necessary to give the CFRP parts their final shape. A majority of the studies on machining of CFRP laminates are on drilling. The number of studies on milling of CFRPs is quite limited. In this study, a mechanistic cutting force model for milling CFRPs is proposed based on experimentally collected cutting force data during slot milling of unidirectional CFRP laminates using a polycrystalline diamond cutter. Cutting force coefficients in radial and tangential directions are calculated as a function of fiber cutting angle. The mechanistic model is shown to be capable of predicting cutting forces during milling of multidirectional CFRP laminates and capable of investigating stability of machining. © 2012 The Authors.
Open Access
A model-based investigation of tool-chip friction during precision micro cutting of commercially pure titanium alloy
(Inderscience Publishers, 2022) Aksin, Alp; Karpat, Yiğit
Understanding interaction between the cutting tool edge radius and the work material is essential to identify the conditions leading to superior surface finish during the micromachining process. The interaction between friction angle and effective rake angle has been investigated based on a slip-line field-based machining model from the literature. Machining forces and cut chip thickness values were obtained from orthogonal cutting tests and employed in the process model. The proposed model also allows for calculating material properties such as shear flow stress and fracture toughness. The proposed model can successfully simulate machining forces during shearing-dominated machining conditions. The results showed the importance of flank and rake face friction in micro-scale machining.
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
The morphological changes upon cryomilling of cellulose and concurrent generation of mechanoradicals
(Elsevier, 2019) Aksın, Alp; Karpat, Yiğit
Microstructural features such as grain size, grain morphology, and phase fractions have significant influence on process outputs of machining processes. During micro end milling where feed per tooth and depth of cut values are set comparable to grain size, process material interactions become more significant. The goal of this study is to investigate the influence of microstructural effects on process outputs during micro milling of cp-Ti work material. Micro milling experiments are performed to observe the process outputs as a function of grain size and grain morphology and a mechanistic approach has been used to explain their effects during micro milling.
Open Access
Multi-scale milling force modeling through a finite element simulation based metamodel
(2012-06) Oliaei, S. N.; Karpat, Yiğit
In this study, a milling force model based on finite element simulation of orthogonal cutting process has been proposed. Design of computer experiments approach is used to obtain a metamodel based on finite element simulation results of machining titanium alloy Ti6AL4V. The metamodel is integrated into an analytical milling force model including tool runout effect to calculate milling forces at micro and macro scales. The proposed model yields results that are in good agreement with measured milling forces.
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.