Kyriakos Efstathiou

Determination of the chip geometry, cutting force and roughness in free form surfaces finishing milling, with ball end tools

Abstract CAD/CAM systems offer various possibilities for finishing milling of parts such as dies and moulds, turbine blades and other high quality components, but most of them do not take into account the surface topomorphy expected, which is significantly affected, among others, by the milling kinematics and the contact conditions between the tool and the workpiece. In order to predict the workpiece roughness in multiaxis finishing milling with ball end tools, the computer supported milling simulation algorithm ‘ ballmill ’ was developed. By means of this algorithm, considering the individual movements of the cutting tool and of the workpiece due to the milling kinematics, the undeformed chip geometry, the cutting force components, the tool deflections and the final surface topomorphy expected are determined. Numerous investigations concerning the parameters mentioned above, with various workpiece materials have been carried out in order to determine the correlation of the experimental results with the corresponding calculated ones with the aid of the ballmill algorithm. Moreover the algorithm validity was extensively evaluated in milling of free form surfaces of large hydroturbine blades. The convergence between the experimental and the related calculated surface topomorphies by means of the ballmill computer program was found out to be satisfactory. Thus, the prediction of appropriate cutting conditions and milling kinematics to fulfill surface topomorphy requirements was enabled.

Improvement of PVD coated inserts cutting performance, through appropriate mechanical treatments of substrate and coating surface

Abstract The wear behavior of coated cemented carbides (HM) inserts with various substrate and coating surface mechanical treatments were investigated experimentally in milling and analytically through a finite-elements method (FEM) simulation of the cutting process. The treatments carried out were polishing and micro- and glass-blasting at various pressures. The initiation and progress of the tool failure was depicted through scanning electron microscopy (SEM) and energy-dispersive X-ray microspectral investigations of the cutting edges used. Furthermore, FEM simulation of the contact between the tool and the workpiece enabled the development of a quantitative description of the influence of the substrate and coating surface roughness on the mechanical stresses occurring, and thus on fatigue-induced coating failure and wear. The coating stress–strain curves and their fatigue endurance were determined through FEM-supported nanoindentation measurements and impact test result evaluations, respectively. In all FEM calculations conducted, multilinear constitutive laws were applied for modeling the coating and the substrate. The experimental and computational results obtained quantitatively exhibit the influence of the substrate, as well as of the coating surface treatments, on the cutting performance of the HM insert.

Effect of the Cutting Edge Radius and its Manufacturing Procedure, on the Milling Performance of PVD Coated Cemented Carbide Inserts

Abstract The fatigue and wear behaviour of PVD coatings on cemented carbide inserts with various cutting edge radii are investigated experimentally and analytically in milling. The inserts with cutting edge radii from 8 up to 35 μm were manufactured by honing and micro-blasting. The tool wear progress was depicted through Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) microspectral analysis. The Finite Elements Method (FEM) simulation of the contact between the tool and the workpiece highlights the effect of the cutting edge radius on the first coating fracture and the further wear development. The wear behaviour of the cutting edge radii manufactured by honing, in comparison to the corresponding ones by means of micro-blasting, is significantly enhanced, whereas the cutting edge radius increasing can lead to a higher tool life.

The influence of the coating thickness on its strength properties and on the milling performance of PVD coated inserts

Abstract The evolution of the physical vapour deposition (PVD) process has contributed to the wide application of thin hard coatings on cutting tools. The film thickness can significantly affect the tool cutting performance. In the present paper, PVD (Ti46Al54)N coatings with thickness from 2 to 10 μm were deposited on cemented carbide inserts. The coating material properties and especially their stress–strain relationship for the various coating thicknesses were determined by means of a FEM-based evaluation procedure on nanohardness measurement results. An increasing of the coating thickness deteriorates the coating mechanical strength, however it can lead to higher effective cutting edge radii, thus inducing lower stresses on the cutting edge, as the related FEM simulation results of the cutting edge region during the material removal show. Moreover, the substrate is better protected against abrasive wear and thermal loads occurring during the cutting process. The tool wear investigations conducted in milling are depicted by the numerically extracted dependencies, explaining the increased cutting performance of thicker coatings which, on the other hand, cause higher PVD costs.

Optimisation of the cutting edge roundness and its manufacturing procedures of cemented carbide inserts, to improve their milling performance after a PVD coating deposition

The fatigue and wear behaviour of PVD coatings on cemented carbide inserts with various cutting edge radii are investigated experimentally and analytically in milling. The inserts with cutting edge radii from 8 up to 35 μm were manufactured by honing and micro-blasting. The tool wear progress was depicted through Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) microspectral analysis. The Finite Elements Method (FEM) simulation of the contact between the tool and the workpiece highlights the effect of the cutting edge radius on the first coating fracture and the further wear development. The wear behaviour of the cutting edge radii manufactured by honing, in comparison to the corresponding ones by means of micro-blasting, is significantly enhanced, whereas the cutting edge radius increasing can lead to a higher tool life.

Experimental and FEM Analysis of the Fatigue Behaviour of PVD Coatings on HSS Substrate in Milling

Abstract The new generation of single and multilayer thin hard coatings ensures the improved tool life and the enhanced superficial quality of the workpiece material. However, in interrupted cutting processes such as in milling, the tool coating life time may be limited by fatigue phenomena. Nowadays, the fatigue performance of coatings is quantitatively predictable. In the present paper, the fatigue prospects of PVD coatings on High Speed Steel (HSS) inserts are examined experimentally and analytically in milling. The coating failure is depicted by means of Scan Electron Microscopy (SEM) investigations and Energy Dispersive X-ray (EDX) microspectral analyses. The cutting process is simulated by means of a Finite Elements Method (FEM) parametric model and the stress results illustrate a fatigue prediction that fits to the experimental ones. Data required for the FEM simulation, such as material properties, cutting forces, chip compression ratio etc. are determined experimentally.

NC-Code Preparation with Optimum Cutting Conditions in 3-Axis Milling

For the optimization of the cutting speed and feed rate in 3-axis NC-milling a computer supported procedure is presented. Input to that procedure is the NC- code off a part. The tool motions, derived from the NC-code, are grouped in subprocesses. Considering the manufacturing cost and accuracy, using developed models, which enable the calculation of the cutting force components and the tool wear, the optimum feedrate and cutting speed values along the tool path are calculated. These optimum cutting conditions are automatically implemented into the NC-code. which is properly modified. An application example of the developed procedure is presented.

Optimization of the cutting edge radius of PVD coated inserts in milling considering film fatigue failure mechanisms

The fatigue and wear behavior of PVD coatings on cemented carbide substrates in milling are investigated experimentally and analytically through Finite Elements Method (FEM) simulation of the cutting process. Cutting inserts with different cutting edge radii and at various feedrates were examined. The initiation and progress of the tool failure is depicted through Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray microspectral investigations of the used cutting edges. The FEM simulation of the contact between the tool and the workpiece enables a quantitative description of the influence of mechanical stress components on the coating fatigue failure. Hereby critical coating fatigue stresses determined by means of the impact test were considered. The experimental and computational results exhibit quantitatively the effect of tool radius and feed rate on the coating fatigue failure as well as on the overall cutting performance for various substrates and film structures. In order to utilize the superior characteristics of coatings towards improving the cutting performance, it is highly recommended to optimize the cutting insert wedge radius, as well as the cutting conditions. Herewith a premature coating failure and a consequent rapid wear development can be prevented.

Supernitrides: A novel generation of PVD hardcoatings to meet the requirements of high demanding cutting applications

Abstract Based on a unique sputtering technology using highly ionized plasmas a novel grade of high performance hard coatings, the Supernitrides with macro free morphology were developed. Within the scope of the investigations described, a characteristic (Ti, AI)N based Supernitride variant, with an AIN content doss to the conductivity limit of 65-67mol-% AIN was deposited on cemented carbide inserts. the films mechanical properties were extracted by means of nanoindentations and impact tests and compared to the corresponding ones of an effective state of the art (Ti46AI54)N coating. Milling investigations, conducted with both previous mentioned coatings, demonstrated the enhanced cutting performance of the applied Supernitride coating, especially at elevated cutting temperatures.

The Antikythera Mechanism: The Construction of the Metonic Pointer and the Back Plate Spirals

The main dials of the back face of the Antikythera Mechanism have partially survived together with the pointer of the upper dial and a few remains of the mechanism that supported and rotated it. The reconstruction of this mechanism, described in this article, fits perfectly its description in the Mechanism’s inscriptions. Our results also show that both spirals were Half Circles spirals, drawn from two different centres. The unwanted eccentricity that would be produced from the pointer’s being placed at one of the centres is proven to have been ingeniously avoided with the appropriate drawing of the cell divisions.