Robust prediction of chatter stability in milling based on the analytical chatter stability

A major obstacle that limits the productivity in machining operations is the presence of machine tool chatter. Machining is a dynamic process and chatter behavior depends upon a number of different aspects including spindle speeds, material properties, tool geometry, and even the location of tool respect to the rest of machine. Many of the traditional models used to predict chatter stability lobes assume that parameters such as natural frequency, stiffness, and cutting coefficients remain constant. In reality, these parameters vary and they affect the chatter stability. The uncertainty in these parameters can be taken into consideration by employing the robust stability theory into a two degree of freedom milling model. Utilizing the Edge theorem and the Zero Exclusion condition, a robust chatter stability model, based on the analytical chatter stability milling model, is developed. This improves the reliability compared to the projected pseudo single degree of freedom model. The method is verified experimentally for milling operations while considering a changing natural frequency and cutting coefficient.     

Evaluation of near-dry machining effects on gear milling process efficiency

One of the main environmental pollution sources related to machine building industry is the huge amount of cutting fluids which are supplied during the machining processes. In order to avoid the problems induced by cutting fluids' usage, considerable progress has been recently made in the field of near-dry machining (NDM). Converting conventional processes to minimal quantity lubrication (MQL) methods imposes new tasks' classification within the tribiological system in order to guarantee the process safety and product quality. This paper gives an overview on some requirements to be considered for a successful MQL application into industrial practice. Its last part is focused on the evaluation of NDM effects on the gear milling process efficiency, with respect to hob wear, surface quality, cooling effect, and environment protection.     

Ruled Surface Machining on Five-Axis CNC Machine Tools

To achieve high precision and high productivity in machining sculptured surfaces, a new architecture for a five-axis CNC interpolator for machining ruled surfaces was developed and demonstrated on a milling machine. The objective of the five-axis interpolator is to continuously maintain the milling cutter axis in parallel with the straight lines of the ruled surface. The cutter position and orientation are calculated at each sampling period of the interpolator, and corresponding axial position commands are generated by an inverse kinematics algorithm. This real-time approach produces precise surfaces and requires substantially less machining time compared to the conventional off-line approach. Two new g-codes are also given in this paper for the new interpolator to produce part surfaces in CNC milling machines.     

Micro end mill tool temperature measurement and prediction

The objective of this work is to characterize the heat transfer in micro end mill tools during machining operations. This analysis will aid in the design of heat dissipation strategies that could potentially increase tool life and machining precision. Tool temperatures, above the unmachined workpiece surface, have been measured using an infrared camera during slot milling of aluminum (6061-T6) and steel (1018) with 300 μm-diameter two-flute tungsten carbide end mills. The measured temperatures compare favorably with temperature distributions predicted by a two-dimensional, transient, heat transfer model of the tool. The heat input is estimated by applying Loewen and Shaw’s heat partitioning analysis. Analysis of heat transfer in the tool found that 46 s into a cut conduction through the length of the tool, storage in the tool, and convection from the surface account for 41.5%, 45%, and 13.5% of the heat generated during machining. Thermal expansion and cooling strategies are discussed.     

Ball end milling mechanistic model based on a voxel-based geometric representation and a ray casting technique

Prediction of machining forces involved in complex geometry can be valuable information for machine shops. This paper presents a mechanistic cutting force simulation model for ball end milling processes, using ray casting and voxel representation methods used in 3D computer graphics field. Using this method, instantaneous uncut chip cross sectional areas can be extracted, which can be used in cutting pressure coefficient extraction and machining simulation including machining forces and geometry of the workpiece. The major advantage of the proposed scheme is that it can simulate milling processes with arbitrary cutting tool geometry on a workpiece with complex geometry, using an algorithm with constant time complexity. A series of cutting experiments were carried out to validate the model.     

Effects of magnetic field and rotary tool on EDM performance

In this work, the effects of tool rotation and various intensities of external magnetic field on electrical discharge machining (EDM) performance have been studied. Experimental trials divided into three regimes of low energy regime, middle energy regime and high energy regime. The influences of process parameters were investigated on main outputs of material removal rate (MRR) and surface roughness (SR). In order to correlate the input parameters and output values two mathematical models were developed to predict the MRR and SR according to variations of discharge energy, magnetic field intensity and tool rotational speed. Results indicated that the applying a rotational magnetic field around the machining gap improves the MRR and SR. Combination of rotational magnetic field and rotary electrode increases the machining performance, in comparison of previous conditions. This is due to better flushing debris from machining gap. This work introduces a new method for improving the machining performance, in cost and time points of view.     

Surface roughness prediction in milling based on tool displacements

In this paper, an experimental device using non-contact displacement sensors for the investigation of milling tool behaviour is presented. It enables the recording of high frequency tool vibrations during milling operations. The aim of this study is related to the surface topography prediction using tool displacements and based on tool center point methodology. From the recorded signals and the machining parameters, the tool deformation is modeled. Then, from the calculated deflection, the surface topography in 3D can be predicted. In recent studies, displacements in XY plane have been measured to predict the surface topography in flank milling. In this article, the angular deflection of the tool is also considered. This leads to the prediction of surfaces obtained in flank milling as well as in end milling operations. Validation tests were carried out: the predicted profiles were compared to the measured profile. The results show that the prediction corresponds well in shape and amplitude with the measurement.     

Macro-level environmental comparison of near-dry machining and flood machining

This paper focuses on investigating several aspects of the machining process from an ecological perspective, the result being a macro-level analysis. The analysis presented here considers not only the environmental impact of the material removal process itself, but also the impact of the associated processes such as the material preparation, and the scrap processing. A macro-level assessment of the comparative life cycle environmental performance of the near-dry machining (NDM) using TiN-coated carbide tools and the flood machining (FM) is performed by a case study referring to the gear milling. The assessment, using the SimaPro 7.1.5 software and the ecoinvent1.5 database, includes combined Life Cycle Assessment (LCA) of the workpiece material, the scrap processing, the use of lubrication, and the energy consumption.     

An Approach to Theoretical Modeling and Simulation of Face Milling Forces

A new approach to theoretical modeling and simulation of face milling forces is presented. The present approach is based on a predictive machining theory in which machining characteristic factors in continuous cutting with a single-point cutting tool can be predicted from the workpiece material properties, tool geometry, and cutting conditions. The action of a milling cutter is considered as the simultaneous work of a number of single-point cutting tools, and the milling forces are predicted from input data of workpiece material properties, cutter parameters and tooth geometry, cutting condition, cutter and workpiece vibration structure parameters, and types of milling. A predictive force model for face milling is developed using this approach. In the model, the workpiece material properties are considered as functions of strain, strain rate, and temperature. The ratio of cutter tooth engagement over milling is taken into account for the determination of temperature in the cutting region. Cutter runout is included in the modeling for the chip load. The relative displacement between the cutter and workpiece due to the cutter and workpiece vibration is also included in the modeling to consider the effect on the undeformed chip thickness. A milling force simulation system has been developed using the model, and face milling experimental tests have been conducted to verify the simulation system. It is shown that the simulation results agree well with experimental results.     

Improving the environmental performance of machine-tools: influence of technology and throughput on the electrical energy consumption of a press-brake

Machine-tools have been identified as one of the main energy-using products to be analyzed in an Ecodesign perspective, targeting the reduction of their environmental impact. Following this, machine-tool manufacturers are committed to anticipate eminent regulations and are looking for guidelines to improve their products on an effective and low-cost manner. This paper describes part of the energy consumption study followed in the frame of the Ecodesign of a commercial press-brake. All-hydraulic and all-electric commercially available systems, of different capacities and working in real production scenarios, have been included. From this study, a preliminary version of an LCI dataset for the bending process is proposed, structured in technology, machine capacity and usage mode categories, and using the bending cycle as the reference unit. The energy consumption per category was estimated based on a specific process energy model built as a function of the referred parameters. The contribution of the respective machine-tool structure to the environmental impact of the machining process is to be included, targeting the completion of such machining unit process dataset. The full LCA of an all-hydraulic system revealed the significant contribution of the machine-tool structure to the global life-cycle environmental impact of the machine (about 40%), while electricity during use phase contributes with about 46% to the total impact. This contradicts the general results published for other metal and non-metal forming processes, and is understood to be related with the discrete loading character of such forming processes here discussed.     

Machining assessment of nano-crystalline hydroxyapatite bio-ceramic

Hydroxyapatite (HAP) is a widely used bio-ceramic in the fields of orthopedics and dentistry. This study investigates the machinability of nano-crystalline HAP (nHAP) bio-ceramic in end milling operations, using uncoated carbide tool under dry cutting conditions. Efforts are focused on the effects of various machining conditions on surface integrity. A first order surface roughness model for the end milling of nHAP was developed using response surface methodology (RSM), relating surface roughness to the cutting parameters: cutting speed, feed, and depth of cut. Model analysis showed that all three cutting parameters have significant effect on surface roughness. However, the current model has limited statistical predictive power and a higher order model is desired. Furthermore, tool wear and chip morphology was studied. Machined surface analysis showed that the surface integrity was good, and material removal was caused by brittle fracture without plastic flow.     

Parametric glass milling with simultaneous control

Parametric glass milling is presented to machine periodical circular channels on the glass plates for manufacturing micro testing devices. An end mill traverses in the linear motion during the workpiece rotation, which are synchronized by simultaneous control. The glass milling is controlled by 4 parameters in a mathematical model without NC program. Based on the principle of the parametric machining and the effect of the cutter axis inclination on the cutting process, a milling machine was developed to perform the parametric glass milling with an inclined ball end mill. The cutter axis inclination and the actual feed rate are associated with the critical feed rate, the maximum feed rate at which a crack-free surface is finished. As a machining example, a periodical circular channel was machined with a transparent surface by the simultaneous control.     

The effect of spindle speed,feed-rate and machining time to the surface roughness and burr formation of Aluminum Alloy 1100 in micro-milling operation

This paper describes the characteristics and the cutting parameters performance of spindle speeds (n, rpm) and feed-rates (f, mm/s) during three interval ranges of machining times (t, minutes) with respect to the surface roughness and burr formation, by using a miniaturized micro-milling machine. Flat end-mill tools that have two-flutes, made of solid carbide with Mega-T coated, with 0.2 mm in diameter were used to cut Aluminum Alloy AA1100. The causal relationship among spindle speeds, feed-rates, and machining times toward the surface roughness was analyzed using a statistical method ANOVA. It is found that the feed-rate (f) and machining time (t) contribute significantly to the surface roughness. Lower feed-rate would produce better surface roughness. However, when machining time is transformed into total cut length, it is known that a higher feed-rate, that consequently giving more productive machining since produce more cut length, would not degrade surface quality and tool life significantly. Burr occurrence on machined work pieces was analyzed using SEM. The average sizes of top burr for each cutting parameter selection were analyzed to find the relation between the cutting parameters and burr formation. In this research, bottom burr was found. It is formed in a longer machining time compare the formation of top burr, entrance burr and exit burr. Burr formation is significantly affected by the tool condition, which is degrading during the machining process. This knowledge of appropriate cutting parameter selection and actual tool condition would be an important consideration when planning a micro-milling process to produce a product with minimum burr.     

Application of Taguchi method to selection of optimal lubrication and cutting conditions in face milling of AlMg3

This paper outlines the Taguchi optimization methodology, which is applied to optimize the cutting parameters in face milling when machining AlMg₃ (EN AW 5754) with HSS (high speed steel) tool under semi-finishing conditions in order to get the best surface roughness and the minimum power consumption. Beside the conventional flood lubrication, the investigations include the minimal quantity lubrication and the dry milling. These environment-friendly cutting techniques are considered two practical ways to the cleaner manufacturing in the context of the sustainable production. The parameters evaluated are the cutting speed, the depth of cut, the feed rate and the cooling lubrication techniques (cutting fluid flow). The appropriate orthogonal array, signal to noise (S/N) ratio and Pareto analysis of variance (ANOVA) are employed to analyze the effect of the mentioned parameters on the good surface finish (surface roughness). This paper illustrates the application of the techniques for single performance characteristics optimization, which employs the weighting factors to each of the S/N ration of the responses to obtain a multi-response S/N ratio for each trial of the orthogonal array and, finally, a single optimal process parameters setting. Using Taguchi method for the design of experiments (DOE), it is investigated the significant influence and the parameters interaction effect with minimum number of trials as compared with a full factorial design.     

Process capability study of laser assisted micro milling of a hard-to-machine material

Laser assisted micro milling (LAMM) is capable of generating three-dimensional micro scale features in hard-to-machine materials. This paper compares the process capability of LAMM with conventional micro milling of a hardened tool steel. In particular, the potential advantages of LAMM over micro milling with respect to cutting forces, tool wear, material removal rate, burr formation and surface roughness are investigated when micro milling hardened A2 tool steel (62 HRC). The results show that LAMM has significant advantages over micro milling, especially in terms of cutting forces, material removal rate and tool wear. The average reduction in the resultant cutting force is found to be up to 69% with laser assist. In addition, tool wear is found to be substantially less with laser assist even when the material removal rates are increased by a factor of six over the tool manufacturer recommended cutting conditions.     

Transitioning to sustainable production – Part I: application on machining technologies

This two part paper presents general issues, methods and a case study for achieving production sustainability on a machining technology level. In order to tackle these issues, the paper promotes sustainable production via the alternative machining technologies, namely cryogenic and high pressure jet assisted machining that have a high potential to cut costs and improve competitiveness by reducing resource consumption and thus creating less waste. The general issues of sustainable technologies pointed out with a comparative case study life cycle assessment performed for alternative machining processes are covered in part I of the work, concluding that future of sustainable production is going to entail the use of alternative machining technologies to reduce consumption rates, environmental burdens, and health risks simultaneously, while increasing performances and profitability. As an upgrade to this part, overall cost evaluation is covered by a case study in part II of this work.     

Transitioning to sustainable production – part II: evaluation of sustainable machining technologies

This paper presents a case-study that highlights the importance of sustainable machining technologies in achieving sustainable development objectives. A technology evaluation was undertaken to understand the likely impacts of the use of technology on sustainability performance measures. The evaluation is more than an experimental method for supporting the design of technology and an instrument for supporting decision-making. It is also a tool for supporting technology policy and for encouraging its adoption and application in industry. More specifically, a sustainability evaluation of cryogenic and high pressure jet-assisted machining in comparison to conventional machining is examined. Sustainability performance measures refer to environmental impact, energy consumption, safety, personal health, waste management, and cost. The case-study refers to the machining of high-temperature Ni-alloy (Inconel 718). It is shown that tooling costs represent the major contribution to the overall production cost, which contradicts previous analyses, and that sustainable machining alternatives offer a cost-effective route to improving economic, environmental, and social performance in comparison to conventional machining.     

Analysis of machined surface quality in a single-pass of ball-end milling on Inconel 718

The ball-end milling process is widely used for generating three-dimensional sculptured surfaces with definite curvature. In such cases, variation of surface properties along the machined surface curvatures is not well understood. Therefore, this paper reports the effect of machining parameters on the quality of surface obtained in a single-pass of a ball-end milling cutter with varying chip cross-sectional area. This situation is analogous to generation of free form cavities, pockets, and round fillets on mould surfaces. The machined surfaces show formation of distinct bands as a function of instantaneous machining parameters along the periphery of cutting tool edge, chip compression and instantaneous shear angle. A distinct variation is also observed in the measured values of surface roughness and micro-hardness in these regions. The maximum surface roughness is observed near the tool tip region on the machined surface. The minimum surface roughness is obtained in the stable cutting zone and it increases towards the periphery of the cutter. Similar segmentation was observed on the deformed chips, which could be correlated with the width of bands on the machined surfaces. The sub-surface quality analysis in terms of micro-hardness helped define machining affected zone (MAZ). The parametric effects on the machining induced shear and residual stresses have also been evaluated.     

Sustainable machining: selection of optimum turning conditions based on minimum energy considerations

The aim of the work reported in this paper was to develop a new model and methodology for optimising the energy footprint for a machined product. The total energy of machining a component by the turning process was modelled and optimised to derive an economic tool-life that satisfies the minimum energy footprint requirement. The work clearly identifies critical parameters in minimising energy use and hence reducing the energy cost and environmental footprint. Additionally, the paper explores and discusses the conflict and synergy between economical and environmental considerations as well as the effect of system boundaries in determining optimum machining conditions.     

Multiaxis Machining: PKMs and Traditional Machining Centers

Since the first CNC-type hexapod machine tool prototypes were presented at the 1994 International Machine Tool Show (IMTS) in Chicago, much debate has ensued on whether or not these machine tools will ever reach the place where they challenge traditional machining centers. This paper presents a review of research topics in the field of parallel kinematic machining for manufacturing, as well as a parallel view of the state of the art of traditional multiaxis machining. After discussing similarities as well as differences in issues faced by parallel kinematic machines and traditional machining centers, a survey of existing prototypes is provided.