Low Workpiece Research Articles

OPTIMISATION OF INTERRUPTED GRINDING PARAMETERS ACCORDING TO THE TEMPERATURE CRITERION

In the paper analytical optimization of discontinuous grinding parameters according to temperature criterion is carried out and on its basis conditions for significant reduction of cutting temperature, which consist mainly in increasing the number of contacts of working ledges of discontinuous wheel with fixed cross-section of machined workpiece, are determined. It is established by calculations that providing 20 contacts the cutting temperature may be reduced up to 3 times in comparison with the conventional grinding by a continuous wheel. This is achieved in the conditions of deep interrupted grinding with relatively low workpiece speed, as in the conditions of traditionally used multipass interrupted grinding not more than 6 contacts of working ledges of an interrupted wheel with the fixed cross-section of the workpiece are realized and the cutting temperature is reduced only in the range of 50 %. It is established by calculations that the minimum of cutting temperature at discontinuous grinding is achieved under conditions of equality of lengths of the working shoulder and the notch on the discontinuous grinding wheel and their reduction. It is also established that excess of the length of the working ledge over the length of the notch of the discontinuous wheel leads to insignificant increase of the cutting temperature. This reduces the wear of the discontinuous wheel and increases the machining capacity without actually increasing the cutting temperature. The paper shows that the obtained theoretical solutions are a necessary condition for ensuring a significant reduction of cutting temperature in discontinuous grinding. A sufficient condition should be considered as complete or partial cooling of grinding zone between contacts of machined workpiece with working jaws of discontinuous grinding wheel by intensive supply of effective technological media into the grinding zone.

Experimental study on heat-affected zones of aluminum alloys in flow drill riveting

Flow drill riveting (FDR) is a novel one-step and single-sided joining process. In FDR, a rivet penetrates through workpieces at a high rotation speed and a certain feed rate, which generates heat through friction to soften the local material. However, the frictional heat usually brings a softening effect to heat-treatable aluminum alloys, which results in heat-affected zones in joints. In this work, temperature evolution of AA6061-T6 during FDR was measured and analyzed, which exhibited a nonlinear relationship with time and was characterized by two peaks. According to the temperature evolution, the FDR process was divided into two stages: penetration of rivet (stage 1) and stop of rivet rotation (stage 2). Electron backscatter diffraction and microhardness results show that the first rise of temperature in stage 1 was attributed to friction stir between rivet and workpieces, and the friction stir at contact surface between rivet head and flash on the top surface of the upper workpiece resulted in the second rise of temperature in stage 2, which caused a larger heat affected zone on the upper workpiece. Furthermore, the effects of process parameters (i.e., rotation speed and feed rate) on temperature evolution of AA6061-T6 were investigated. A higher rotation speed results in higher temperatures, a larger heat-affected zone and a greater softening effect on the upper workpiece, while the lower workpiece is less affected, resulting in lower joint strength. A larger feed rate contributes to a smaller heat affected zone on both upper and lower workpieces, resulting in higher joint strength.

Оптимизация режимов плоского шлифования деталей малой жесткости типа колец и призматических стержней

The study introduces an algorithm and method for optimization of flat grinding modes for low-rigidity workpieces such as rings and prismatic rods. The proposed solution ensures the specified quality of the machined surface at the maximum process efficiency. The required flatness tolerance of the machined surface is provided at the second stage of optimization modes by limiting the value of the maximum elastic deformation. The variable parameters are the cutting forces, magnetic field attraction of the machine table, and the bending rigidity of the workpiece. The study introduces mathematical models to determine maximum elastic deformation of workpieces when fixing and machining, outlines the conditions for ensuring the required flatness tolerance of the machined workpiece surface, taking into account the maximum elastic deformation of the low rigidity workpiece under the influence of the magnetic field of the machine tool and the radial component of the cutting force.

Development and Investigation of an Inexpensive Low Frequency Vibration Platform for Enhancing the Performance of Electrical Discharge Machining Process.

Difficulty in debris removal and the transport of fresh dielectric into discharge gap hinders the process performance of electrical discharge machining (EDM) process. Therefore, in this work, an economical low frequency vibration platform was developed to improve the performance of EDM through vibration assistance. The developed vibratory platform functions on an eccentric weight principle and generates a low frequency vibration in the range of 0–100 Hz. The performance of EDM was evaluated in terms of the average surface roughness (Ra), material removal rate (MRR), and tool wear rate (TWR) whilst varying the input machining parameters viz. the pulse-on-time (Ton), peak current (Ip), vibration frequency (VF), and tool rotational speed (TRS). The peak current was found to be the most significant parameter and contributed by 78.16%, 65.86%, and 59.52% to the Ra, MRR, and TWR, respectively. The low frequency work piece vibration contributed to an enhanced surface finish owing to an improved flushing at the discharge gap and debris removal. However, VF range below 100 Hz was not found to be suitable for the satisfactory improvement of the MRR and reduction of the TWR in an electrical discharge drilling operation at selected machining conditions.

Investigations on grain size characteristics in microstructure during grinding of maraging steel 3J33

Due to the nonlinear and tight coupling of the process parameters in grinding, the surface integrity of the parts is influenced by multiple factors. At present, most studies have macroscopically analyzed the effect of grinding force and heat caused by grinding parameters on surface morphology and mechanical property, while the investigation of dynamic evolution in the material microstructure during grinding and its influencing mechanism on the surface integrity is not yet fully understood. In this study, a novel microscopic grain size model comprehensively considering grinding temperature, heating rate and strain is proposed to investigate the phase transformation, surface hardness and roughness of maraging steel 3J33. The key factors affecting the change of the grain size are deeply studied and the model coefficients are calibrated by heat treatment and grinding experiments at first. Then the relationship between grain size and phase transformation is revealed based on the proposed model and the sensitivity of grain size to different process parameters is quantitatively analyzed. The surface integrity of the workpiece, such as hardness and roughness, is further investigated by comparing the variation of the grain size and phase transformation before and after the grinding process. The theoretical and experimental results show that large cutting depth and low workpiece speed are more beneficial to reduce the grain size. The phase transformation is strongly correlated with grain size and higher wheel speed is supposed to restrain the phase transformation of the material. Based on this analysis, the guideline of process optimization is provided for the grinding of maraging steel to obtain better surface integrity.

An Active Control Method for Chatter Suppression in Thin Plate Turning

This paper presents an active control method, consisting of an adaptive sliding-mode controller (ASMC) and a displacement field reconstruction (DFR) method, for chatter suppression in turning of thin-walled workpieces (such as compressor disks and casings in aircraft engines) where low workpiece stiffness renders machining with potential regenerative chatter. Due to the presence of multi-modal dynamics, variant modal parameters, and measurement difficulties, active chatter control of thin plate turning has been challenging. Unlike existing controls based on a lumped-parameter single degree-of-freedom cutting model, a distributed-parameter dynamic model of a rotating thin plate with multiple vibration modes is used to analyze the machining stability with the designed controller. Moreover, model parameters of the plate are not needed to construct the controller. The DFR is employed to capture the plate dynamic behavior for feedback to the ASMC during turning, overcoming the long existing difficulties to measure plate vibration at the cutting point. A fast tool servo is utilized in the control implementation. Theoretical analyses, numerical simulations, and experimental evaluation on a lathe demonstrate that chatter in thin plate turning can be effectively attenuated with the proposed active control method.

Prediction of stable depth of cuts in turning and milling operations: a new probabilistic approach

Chatter vibration in turning and milling operations is one of the most critical problems that causes low workpiece quality and manufacturing efficiency. Therefore, the determination of stable cutting depths is crucial for these operations. Several vibrational characteristics [natural frequency (ωn), stiffness coefficient (k), and damping coefficient (s)] affect stable cutting depths. The vibration characteristics of these operations show randomness for every setup condition. For this reason, the randomness of the vibration characteristics should be modeled. In this study, a probabilistic approach and regression model are combined for turning operation. Also, a probabilistic approach and analytical model are integrated for milling operation. The purpose of these models is to establish confidence intervals for stability diagrams. As a result, the operators can work in a secure region during the operations.

Fabrication of Hyper-Hemisphere of Cobalt-Chromium Alloys Using Curve Generator Machining

High-precision machining is required for manufacturing hyper-hemispherical artificial joints made of difficult-to-cut metals such as cobalt-chromium (Co-Cr) alloys to provide wear resistance in the human body. The hyper-hemisphere of Co-Cr alloys is finished by curve generator machining, in which the rotation axes of the cutter and workpiece intersect each other at the center of the sphere to be machined. This paper presents a kinematic model to simulate the shape and surface topography on hyper-hemispheres with the cutter loci in curve generator machining. The kinematic model was validated with a cutting test, in which the surface profiles were measured around the pole and equator of the sphere. Simulations were performed to study the cutting process and surface finish. The appropriate cutting parameters were determined to improve the surface finish based on a kinematic simulation. A smooth surface was obtained when small inclinations of the workpiece, large nose radii of the cutter, low workpiece rotation speeds, and non-integer ratios of the tool spindle speed to the workpiece rotation speed were employed. The effects of the setting errors, such as the mounting error of the cutter and alignment error of the spindle and workpiece axes, were estimated via the kinematic simulation. It was found that the surface topography and radius of the sphere depended on the setting errors. The radius and center of the spherical shape were different from those of an ideal sphere by an error in the X-axis in the global coordinate system. The oval shape was caused by an error in the Y-axis. An error in the Z-axis affected the radius of the machined sphere.

Effects of process parameters and drill point geometry in deep drilling of SAE 4144M under MQL

Deep drilling is a difficult machining operation, it often presents problems related to chip evacuation and efficiently cooling and lubrication of the cutting zone. Minimum quantity of lubrication (MQL) is an ecological alternative to wet machining. It provides better lubrication, however, it has low cooling effect and the chips are not removed away from the cutting zone by coolant. In this paper a combination of cutting speed, feed-rate and tool geometry is determined to produce smaller chips and lower workpiece temperature for deep drilling of SAE 4144 M steel under MQL lubrication condition. Full factorial design of experiment was used to plan the set of the experiments. Tool wear behavior and cross sections of the drilled holes were compared to a pressurized oil condition. Split point geometry, lower cutting speed and higher feed-rate resulted in the best chip shape and relatively low workpiece temperature in deep drilling of SAE 4144 M with MQL. The worn drill presented microchipping, adhered workpiece material on the rake face and exposed substrate resulted from abrasion of TiNAl coating. MQL resulted in longer tool life and more surface defects compared with the pressurized oil condition.

Modification of surface morphology during cryogenic turning of metastable austenitic steel AISI 347 at different parameter combinations with constant CO2 consumption per cut

For numerous cryogenically machined materials a hardness increase in the workpiece surface layer has already been reported due to strain hardening mechanisms and compressive residual stress. When machining metastable austenitic steels, these hardening mechanisms are overlaid by a deformation induced phase transformation from austenite to martensite. This phase transformation is favored by low thermal and high mechanical loads in the workpiece surface layer; thus the martensite content can be modified by varying the thermo-mechanical load.In this study, the cryogenic turning process is designed to maintain low workpiece temperatures during machining and to maximize the passive forces and consequently the martensite content. The passive forces are adjusted by using indexable inserts with strongly varying chamfer angles and therefore, also varying effective rake angles. The feed rate and thus the cutting cross-section are varied as well to further adjust the passive forces. As higher feed rates lead to shorter cutting times, the cooling time is also shortened, which increases the thermal load and worsens the surface morphology. Therefore, for each variation, the coolant supply was controlled to maintain a constant quantity of CO2 during each cut. Hence, low workpiece temperatures can be achieved even with high feed rates. By controlling the thermo-mechanical load, significantly more martensite can be generated while maintaining a constant CO2 consumption per cut.

Estimation Method of Machining Error on Low Rigidity Workpiece for Tool Posture Planning

During cutting of low rigidity workpieces, avoiding elastic deformation due to machining forces is important. In multi-axis cutting of jet engine blades, tool posture is typically determined by trial and error based on the computer-aided machining operator’s experience. In this study, we developed a system of quantitative evaluation of machining error for blade surface finishing process by estimating the cutting force at each cutting point of the blade with different relative tool postures, and analyzing the deformation at each point with a finite element analysis. With the system, we become be able to perform simulations to evaluate tool posture with less shape error, maintaining the machining efficiency.

Relationship between cutting process parameters and white layer thickness in orthogonal cutting

ABSTRACTIn this paper, a 2D orthogonal cutting of hardened AISI52100 steel is developed to investigate the influence of cutting process parameters (CPP) on white layer (WL) thickness. A finite element model (FEM) based on the phase transformation mechanism and takes into account the mechanical effects is adopted to predict the WL depth. Several numerical simulations are performed in various conditions of tool geometry, tool flank wear, material properties, and cutting conditions. The predicted results of cutting forces, chip morphologies, and WL depth are physically consistent with the previous experimental investigations. The numerical results prove that formation of thick WLs is unavoidable in all cases, except for low workpiece hardness, very thin WLs are noticed. The results also confirm that the WL thickness depends on all used CPP. In particular, the cutting speed, feed, tool flank wear, and workpiece hardness are the most influencing parameters.

Influence Of Procedure-Related Cutting Edge Micro Geometry Modification On The Production Quality When Milling Fibre Reinforced Plastics

The importance of carbon or glass fibre reinforced plastics in aviation and automotive industry has increased considerably over the last twenty years. The latest generation of commercial aircrafts for example exhibits a composite weight proportion of up to 50 %. To generate the final contour and to realize appropriate interfaces for following assembly processes, modern tools and manufacturing technologies are required to ensure high workpiece qualities. The combination of strength enhancing fibres, a ductile matrix and various laminate structures highly strains the cutting edge during the cutting process. For this field of application various cutting edge and tool geometries has been developed by several tool manufacturers to maximize wear resistant and shape accuracy. The wear mechanism and course of different cutting materials and its influence on workpiece quality during trimming of carbon and glass fibre reinforced plastics were analysed. It could be shown that an increasing tool wear does not cause necessarily low workpiece qualities. Additionally, the influence of different process parameters and strategies on the cutting edge micro geometry forming and the workpiece quality were analysed.

Estimation of Dynamic Grinding Wheel Wear in Plunge Grinding

Using conventional grinding wheels, self-excited vibrations are one of the most limiting factors in terms of productivity and process stability in cylindrical plunge grinding. Depending on the dynamic behavior of the workpiece and machine, vibrations of the workpiece copy on the grinding wheel's surface, caused by uneven wear. This results in increasing waviness of the grinding wheel and by that, increasing workpiece vibration. Electromagnetic actuators are capable of influencing the dynamic process forces and therefore, the wear. The authors pursue the objective, to achieve an active control of the tool wear for low workpiece vibration and high workpiece quality. Therefore, a tool-wear-model which enables the estimation of the grinding wheel's surface is proposed. The parameterization of the model is realized carrying out a set of reference processes with subsequent identification. Aside from the dynamic tool wear, the workpiece oscillation is simulated by the model. A Kalman Filter is utilized to adjust the model onto the current process using the measured workpiece oscillation. Thus, it is possible to achieve an online estimation of the wave amplitude and phase angle on the grinding wheel's surface as well as their progression.

Experimental and numerical study of grind-hardening-induced residual stresses on AISI 1045 Steel

A finite element model has been developed for the prediction of the thermo-mechanical response during grind-hardening of AISI 1045 steel. Grind-hardening process utilizes the heat generated in the grinding area for the surface heat treatment of the workpiece. The workpiece surface is heated above the austenitization temperature by using large values of depth of cut and low workpiece feed speeds. The workpiece undergoes martensitic phase transformation increasing its hardness in the surface layer. This paper establishes a finite element model for predicting the grind-hardening temperature, resulting crystal structure of the workpiece material and residual stresses distribution. In the present paper, modeling and prediction of the residual stresses profile as a function of the process parameters is presented. The model’s results are validated for two cases; a dry grind-hardening and a coolant-assisted grind-hardening of AISI 1045 steel.

Investigations on Cryogenic Turning to Achieve Surface Hardening of Metastable Austenitic Steel AISI 347

Metastable austenitic steels offer the opportunity of a surface hardening during machining due to a deformation induced martensite formation, substituting downstream hardening-processes. To maintain the necessary low process and workpiece temperatures for a phase transformation from austenite to martensite, cryogenic cooling using CO2-snow was examined in this study. The influence of workpiece diameter, coolant flow rate as well as pre-cooling and pre-surface hardening on the obtainable phase content of martensite in the surface layer was investigated.

Nanomilling mechanism on Cu surfaces investigated using atomistic simulation

The nanoscale milling and scratching processes of copper workpieces are studied using molecular dynamics simulations based on the tight-binding and Morse potentials. The effects of the rotation velocity of the tool and the workpiece temperature are evaluated in terms of atomic trajectories, slip vectors, flow field of chips, cutting forces and groove characteristics. The simulation shows that a slip system in the ⟨110⟩ direction on the workpiece surface occurs for milling with a tool rotation velocity of ω = 0°/fs. However, no apparent slip system appears for ω = 0.005°/fs or higher; instead, the number of amorphous areas increases. At ω = 0°/fs (nanoscratching), most of the removed atoms pile up in front of the tool and some gradually backfill when the tool rotates due to the effects of rotational friction and adhesion between the tool and the removed atoms. The largest number of removed atoms that piled up in front of the tool were obtained for milling with ω = 0°/fs; the number of removed atoms that piled up in front of the tool decreased with the increasing ω value. The component forces corresponding to the feed direction of the tool are the largest for the nanodrilling and nanomilling processes. High-precision grooves can be obtained at a low workpiece temperature (e.g. room temperature) with ω = 0°/fs.

Systematic Simulation Method for the Calculation of Maximum Deflections in Peripheral Milling of Thin-Walled Workpieces with Flexible Iterative Algorithms

Due to the deflection of tool and workpiece induced by cutting force, there is a high complexity associated with the prediction of surface form errors in the peripheral milling process of thin-walled workpieces. And the prediction of surface form errors induced by cutting deflection is the precondition for process optimization and error compensation. This paper proposes a systematic simulation procedure suitable for surface form errors prediction in peripheral milling of low rigid thin-walled workpiece. Some key algorithms with the judgment of contacts between the cutter and the workpiece, the flexible iterative algorithm as well as the tool/workpieces deflection prediction using FE model are developed and presented in detail. Comparisons of the form errors and cutting forces obtained numerically and experimentally confirm the validity of the proposed algorithms and simulation procedure.

On Surface Grind Hardening Induced Residual Stresses

Grind hardening process utilizes the heat generated in the grinding area for the surface heat treatment of the workpiece. The workpiece surface is heated above the austenitizing temperature by using large values of depth of cut and low workpiece feed speeds. The workpiece undergoes martensitic phase transformation increasing its hardness in the surface layer. Usually compressive residual stresses are induced in the surface layer. In the present paper, modeling and prediction of the residual stresses profile as a function of the process parameters is presented. The model's results are validated for two cases; a dry grind hardening and a coolant assisted grind hardening of AISI 1045 steel.

External grind-hardening forces modelling and experimentation

Grind hardening process utilizes the heat generated in the grinding area for the surface heat treatment of the workpiece. The workpiece surface is heated above the austenitizing temperature by using large values of depth of cut and low workpiece feed speeds. However, such process parameter combinations result in high process forces that inhibit the broad application of grind hardening to smaller grinding machines. In the present paper, modelling and predicting of the process forces as a function of the process parameters are presented. The theoretical predictions present good agreement with experimental results. The results of the study can be used for the prediction of the grind hardening process forces and, therefore, optimize the process parameters so as to be used with every size grinding machine.