Negative Rake Angle Research Articles

A new value for Johnson Cook damage limit criterion in machining with large negative rake angle as basis for understanding of grinding

Grinding is one of the most complex manufacturing processes in industry and understanding its physics is still fairly limited due to the stochastic nature of the process. The physics behind the grinding process can be better understood through thermomechanical analysis of grain-material interactions, enabling the optimization of the process and grinding tool. When simulating the chip formation in machining processes, difficulties arise when the negative rake angle is large, when thermal effects on the tool and workpiece become prominent and when there is the necessity to consider damage within the constitutive equations of the workpiece material. These difficulties become further intensified given small depths of cut as in the case of grinding and the need for three-dimensional models.A coupled thermomechanical dynamic analysis is performed to simulate linear machining experiments effectuated by single diamond grains. Damage is described within a Johnson-Cook model. The damage parameters when machining Ti-6Al-4V are optimized in a three dimensional model and a new concept of applying a damage limit when machining with negative rake angle is suggested. Here, a Fortran subroutine is written to calculate the damage field variable. The simulated cutting forces and temperature in the tool as well as the workpiece are validated. Validation experiments are carried out by single grain machining experiments at a depth of cut of 30 μm and at a linear cutting velocity of 0.8m/s. Numerical challenges, such as chip separation criterion and settings of adaptive remeshing are addressed. Finally, it is shown that with an increase in the cutting edge radius, cutting forces and especially passive forces increase.

Numerical Investigation of the Influence of Tool Rake Angle on Residual Stresses in Precision Hard Turning

In precision manufacturing processes surface integrity is of the utmost importance for the performance and life-cycle of the final products. An important aspect of surface integrity is associated with residual stresses induced in the workpiece during machining. According to the relevant literature, tool rake angle plays an important role on the features of residual stresses, regarding their magnitude and distribution within the workpiece. In this paper, numerical investigations with the use of the finite elements method are presented that allow the evaluation of the influence of the tool rake angle on residual stresses for the case of hard turning of stainless steel. The investigation is performed in a wide range of positive and negative rake angles. Numerical results verify the dominant role of tool rake angle on the residual stresses. The proposed models can be used for the a priori evaluation of the characteristics of compressive stresses that are considered favorable for the produced components.

A high-dimensional design optimisation method for centrifugal impellers

The metamodel-based design optimisation (MBDO) method has been gaining increasing popularity in the field of turbomachinery design. Nevertheless, the ‘curse of dimensionality’ remains a formidable obstacle for the application of MBDO in high-dimensional design problems, which, in the context of turbomachinery design, are problems with at least 10 design variables, if expensive simulations such as computational fluid dynamics are required. To address this issue, the present study proposes to utilise a novel high-dimensional model representation (HDMR) method to support the process of MBDO and applies it to the design optimisation of a centrifugal impeller. A total of 15 geometrical design variables and three performance objectives are considered in the optimisation. In particular, the HDMR method is assisted by support vector regression (SVR) metamodels to explicitly represent the relationships between design variables and performance objectives. Apart from high metamodelling accuracy and efficiency, the SVR-HDMR metamodelling technique enables designers to refine design space as well as to identify correlations between design variables during the metamodelling process. Analysis of the final optimisation results show that a reduced hub-line curvature, a negative rake angle, a positive splitter blade lean angle and an increased splitter blade inlet angle are beneficial for enhancing efficiency of this impeller. This work provides valuable references for further high-dimensional design optimisation of turbomachine components.

Ductile cutting of silicon microstructures with surface inclination measurement and compensation by using a force sensor integrated single point diamond tool

This paper presents a measurement and compensation method of surface inclination for ductile cutting of silicon microstructures by using a diamond tool with a force sensor based on a four-axis ultra-precision lathe. The X- and Y-directional inclinations of a single crystal silicon workpiece with respect to the X- and Y-motion axes of the lathe slides were measured respectively by employing the diamond tool as a touch-trigger probe, in which the tool-workpiece contact is sensitively detected by monitoring the force sensor output. Based on the measurement results, fabrication of silicon microstructures can be thus carried out directly along the tilted silicon workpiece by compensating the cutting motion axis to be parallel to the silicon surface without time-consuming pre-adjustment of the surface inclination or turning of a flat surface. A diamond tool with a negative rake angle was used in the experiment for superior ductile cutting performance. The measurement precision by using the diamond tool as a touch-trigger probe was investigated. Experiments of surface inclination measurement and ultra-precision ductile cutting of a micro-pillar array and a micro-pyramid array with inclination compensation were carried out respectively to demonstrate the feasibility of the proposed method.

Cryogenic machining of porous tungsten for enhanced surface integrity

Cryogenic machining of porous tungsten is being developed as an alternative sustainable process to current industry practice of machining plastic infiltrated workpieces. Eliminating the use of plastic infiltrant is contingent on the ability to control as-machined surface properties of porous tungsten. To this end, the influence of various machining parameters and cooling conditions on machining mechanisms, particularly as a function of cutting temperature, is explored. By employing modified polycrystalline diamond cutting tools, high speed cryogenic machining of porous tungsten by ductile shear was achieved. Cutting speeds of up to 400 m/min are possible with this novel approach, and very low surface roughness of Ra≈0.4μm is realized, at the expense of surface porosity. At low cutting speeds, rake angle has a key impact on surface morphology, with more negative rake angles increasing the amount of brittle fracture occurring on the machined surface and thereby increasing surface porosity. An excellent mix of open surface porosity and low surface roughness may be achieved at low cutting speeds of 20m/min when a sharp edged cutting tool with a rake angle of −5° is used. Using a separate set of machining parameters for rough and finish machining may offer an optimum approach to realize economical cryogenic machining of porous tungsten.

Force Controlled Grinding of Ceramic Materials

Brittle materials like ceramics or glass can be machined by cutting with negative rake angles and by abrasive machining processes. Especially grinding allows for low surface roughness and high shape accuracy. Conventional path-controlled grinding processes may damage functional surfaces if brittle fracture occurs and may thus lead to lateral, radial and axial cracks. High grinding forces can be a reason for brittle fracture when grinding ceramic materials. A solution for this effect may be the application of force controlled grinding processes. In this paper adapted control algorithms were implemented for force controlled grinding and verified in grinding experiments. As an example, cylindrical grooves were ground with an injection moulded spherical grinding tool in alumina and zirconia ceramics. After grinding surface roughness, shape accuracy and process forces are analysed and discussed.

Material Plastic Side Flow and White Layer in Hardened Steel GCr15 Precision Turning

Turning instead of grinding of the hardened steel is an environmentally cleaner production technology and green fabrication technique. Using the negative rake angle basic shape blades of Mountain of Sweden Sandvik Coromant PCBN7015 cut hardened bearing steel GCr15 and observe fine surface under microscopic condition.. Microscopic observation showed that there was side stream between hard turning two knife marks forming carinata. Side stream level aggravate on the fine surface with the enlargement of Tool Nose radius. The White layer of hard turning fine surface is divided into three areas. White-bright zone is made up of Cryptocrystalline martensite, Dark gray zone is made up of tempered martensite, Grayish zone is made up of the original martensite. This research will enrich theory of hard turning.

Effect of Machining Configurations on the Electrochemical Response of Mild Steel in 3.5% NaCl Solution

The present work is based on the study of the electrochemical response of mild steel as a function of machining configurations. The variable parameters were rake angle and turning speed, while feed rate and depth of cut remained fixed. Dynamic polarization tests and electrochemical impedance spectroscopy in 3.5% NaCl solution were done to analyze the electrochemical behavior of mild steels with the variation of rake angle and turning speed. The electrochemical response showed that the steel machined at higher speed and positive rake angle had higher resistance to charge transfer. Similarly, steel machined at lower speed and negative rake angle showed lower resistance to charge transfer. The results obtained in this study suggest that machining on mild steel should be carried out at positive rake angle and at higher speed to have smoother surface finish, strain-relieved surface grains, and subsequently better corrosion resistance, which was measured from corrosion current as determined by the Tafel extrapolation from the polarization plots.

Experimental investigation of specific cutting energy and surface quality based on negative effective rake angle in micro turning

Since the uncut chip thickness is comparable to the cutting edge radius, in fact, micro cutting is a negative rake angle cutting process. The minimum chip thickness has great effect on the cutting process. This paper presents an experimental investigation of specific cutting energy and surface quality based on the negative effective rake angle in micro turning. A new model is developed to calculate the negative effective rake angle in micro cutting. The effective rake angle is found to be more negative with the decreasing ratio of uncut chip thickness to cutting edge radius. The minimum chip thickness is calculated to be about 0.2–0.3 times of cutting edge radius based on the critical negative rake angle. The turn point of the nonlinear increase of specific cutting energy is observed to be at the critical negative rake angle. The minimum surface roughness also is found to be achieved near the critical negative rake angle.

Energy Cost Optimization in High Speed Hard Turning Using Simulated Annealing Algorithm

Selecting the cutting conditions to optimize the economics of machining process as assessed by energy machining cost is essential. The aim of this research is to determine the optimum cutting parameters that minimize the energy cost needed for removing one cubic centimetre of material in High Speed Hard Turning (HSHT) process. To achieve that, a set of experimental machining data to cut hardened steel AISI 4340 was obtained with different ranges of cutting speed, feed rate, depth of cut and negative rake angle using mixed ceramic as a cutting tool. Regression models have been developed by using Box-Behnken design as a design of experiment. Then, the Simulated Annealing Algorithm (SAA) has been used to optimize the cutting parameters. The data collected was statistically modelled. The results show that the range of minimum energy cost to remove one cubic centimetre of material for the three techniques can be achieved in the range of 300 to 308 as a cutting speed, -12 for cutting rake angle, 0.125 as a feed rate and 0.15 as a depth of cut.

In situ analysis of flow dynamics and deformation fields in cutting and sliding of metals

The flow dynamics, deformation fields and chip-particle formation in cutting and sliding of metals are analysed, in situ , using high-speed imaging and particle image velocimetry. The model system is a brass workpiece loaded against a wedge indenter at low speeds. At large negative rake angles, the flow is steady with a prow of material forming ahead of the indenter. There is no material removal and a uniformly strained layer develops on the workpiece surface—the pure sliding regime. When the rake angle is less negative, the flow becomes unsteady, triggered by formation of a crack on the prow free surface and material removal ensuing—the cutting regime. The strain on the prow surface at crack initiation is found to be constant. Chip morphologies, such as discrete particle, segmented chip and continuous chip with mesoscale roughness, are shown to arise from a universal mechanism involving propagation of the prow crack, but to different distances towards the indenter tip. The simple shear deformation in continuous chip formation shows small-angle oscillations also linked to the prow crack. Implications for material removal processes and ductile failure are discussed.

Evaluation of subsurface damage caused by ultra-precision turning in fabrication of CaF2 optical micro resonator

The optical micro-resonator, which stores light at a certain spot, is essential in next-generation optical signal processing. Single-crystal calcium fluoride (CaF2) is the most suitable material for this element. Ultra-precision turning is a feasible fabrication process for CaF2 optical micro-resonators. In this study, the influence of subsurface damage on the resonator's Q factor is investigated. TEM observation shows that the subsurface layer of up to several tens of nanometers thickness changed from single-crystal to polycrystalline morphology due to ultra-precision turning. A diamond tool with 0° rake angle results in lower damage than one with negative rake angle, which enhances the resonator's performance.

0706 uctile Cutting of a Microstructure Array on Silicon Surface by Using a Diamond Tool with a Negative Rake Angle

0706 uctile Cutting of a Microstructure Array on Silicon Surface by Using a Diamond Tool with a Negative Rake Angle

An experimental study on dead metal zone in orthogonal cutting with worn rounded-edge cutting tools

A dead metal zone is formed by the negative rake angle effect of the rounded-edge and it affects the cutting forces during machining operations. Therefore, investigating the size of the dead metal zone and changing it with cutting edge radius and flank wear rate are important. In this study, the dead metal zone was investigated experimentally for unworn and worn rounded-edge cutting tools and it was defined by changing the flank wear rate and cutting edge radius. It was observed that the size of the dead metal zone increased with increase of the cutting edge radius and it became smaller with increasing the flank wear rate. In addition, cutting force and thrust force were investigated. It was proven that both forces increased with increase of the cutting edge radius and the flank wear rate.

A study on the influence of pick geometry on rock cutting based on full-scale cutting test and simulation

In this paper, series of full-scale cutting tests and cutting simulations are carried out to investigate the influence of installation parameter and geometry of the pick on cutting performance. The discrete element method is used to simulate the rock cutting process. A general process to calibrate macro properties of rock including uniaxial compressive strength (UCS), elastic modulus, Poisson’s ratio, cohesion and internal friction angle is proposed and used to complete the calibration of coal model. The cutting simulations are performed using picks with different tip angles and rake angles. The results show that the peak cutting force (PCF) decreases with the increase of rake angle following an inverse proportional function when the rake angle is positive, while it varies following a parabolic curve in the condition of negative rake angle. Moreover, the crack mode changes from primarily shear failure to primarily tensile failure with the increase of rake angle. Finally, a multiple-attribute index is proposed to evaluate the cutting performance and select the optimum cutting parameters.

Effects of Cutting Angles in Ti-6al-4v Milling Process on Surface Integrity: Influence of Roughness and Residual Stresses on Fatigue Limit

□ The influence of the milling process on the fatigue behavior of a titanium alloy was investigated. The effect of cutting conditions such as the cutting angles (axial and radial rake angle) on the surface integrity (roughness and the residual stresses) was observed. The results indicated that the cutting angles have a limited influence on roughness parameters, whereas the effects on residual stresses were greater. A negative axial rake angle induced compressive residual stresses regardless of the radial rake angle. In contrast, a positive axial rake angle combined with negative radial rake angle induced tensile residual stresses. To evaluate the fatigue limit, the four point fatigue tests were carried out. Result showed the fatigue limit is sensitive to the surface integrity. The fatigue limit was also evaluated by analytical method (Arola model). A good correlation was found between the analytical results and the experimental results when cutting angles induced compressive residual stress. However the Arola model was less accurate for tensile residual stress surface condition. To improve the prediction precision, the residual stress was considered as a sensitivity parameter and added to Arola model.

The Study of the CBN Grain Shape on its Microcutting Performance Based on Finite Element Method

Given that the grain geometry has an important impact on grinding process, it’s necessary to investigate the influence of grain shapes on grinding mechanism. In this paper, a 3D FEM model is developed to study the effects of CBN grain geometry on its microcutting performance under various speed levels, which is effective to understand the microcutting process qualitatively and quantitatively. The simulation results indicate that the single grain microcutting force ratio and material removal ratio are both sensitive to the grain shape and those grains with truncated tetrahedron shape of large cross-sectional area and negative rake angle would improve the cutting performance.

Material Behaviour of Armco-Iron and AISI 4140 at High Speed Deformation during Machining

During machining of metals high temperatures and deformations occur at the surface layers leading to changing component states. Depending on the thermal and mechanical set of stress microstructural changes like residual stresses and grain refinement can be found. Grain refinement is influenced by the amount of deformation. In the present investigations high deformations of the surface layers are realized by using the cutting tool in opposed direction resulting in high negative rake angles. This test setup is used to investigate the material behaviour of Armco-Iron and AISI 4140 at different cutting velocities during machining. Furthermore the flow stresses at different strain rates and temperatures were determined by means of high speed tensile tests. The flow behaviour of the investigated materials is used to explain the results of the machining experiments.

Evaluation of Cutting Forces During Single Pass Microtrenching of Poly (Methyl Methacrylate)

Research was conducted to evaluate a microtrenching process to create microchannels on the surface of poly (methyl methacrylate) (PMMA) for applications in tissue engineering. Experiments with a trenching tool included an exaggerated cutting edge radius (48 μm) to study the impact of a highly negative effective rake angle on forces during single pass microtrenching at subradius cutting conditions. During microtrenching, forces were measured by dynamometer and compared to a finite element (FE) model using an elastic-perfectly plastic material model for an undeformed chip thickness from 9 to 64 μm. During experiments, cutting was first observed when the ratio of undeformed chip thickness to cutting edge radius was 0.33. Measured and predicted values of thrust force exceeded cutting force up to an undeformed chip thickness equivalent to the cutting edge radius. The FE model predicted a linear trend in cutting force with feed (r = 0.99) and was substantiated by linear regression of experimental data (r = 0.99). However, at lower values of feed the model overestimated force, with a maximum difference of 42% at a feed of 22 μm. Thrust force was also predicted to be linear (r = 0.99), but at greater feed the experiments indicated a nonlinear decline in thrust force, resulting in a maximum difference of 27% at 64 μm. Finally, an analysis of nodal velocity plots from the FE model revealed a material stagnation zone developed along the cutting edge, rising from the workpiece surface in proportion to feed and then remaining fixed at 63 deg (stagnation angle) for all feeds greater than 35 μm. While the application of an elastic-perfectly plastic material model for PMMA was sufficient to predict microtrenching forces by the FE method, differences between predicted and measured thrust forces at greater undeformed chip thickness implies a more complex rheological model may add value.

Simulation of Chip Formation Behavior during Single Diamond Grains Grinding by CEL Method

The grain orientation plays a great important role during single grain grinding. Single grain grinding simulation could offer an insight into the material removal behavior. However, grinding simulation with FE methods has problems arising from large deformations at the chip root and negative rake angles of the grain. In this work, a coupled Eulerian-Lagrangian method (CEL) is used to model the grinding behavior of single diamond grain in different orientations. The influence of the grain orientations on the grinding forces, grinding process (sliding, ploughing and cutting) and critical undeformed chip thickness are analyzed. The force ratio shows great accordance with the analytical calculation data. The simulation results show that the chip generates quickly and the special grinding force is relatively small when the diamond particle is in octahedral orientation (111)-(100). The workpiece material flows to the side and front owing to the pressing of cutting crystal face. The swelling of the material is generated beside the abrasive grain. The range and height of swelling are increased gradually and kept invariant after chip formation. The deformation of workpiece material contacting with grain edge is very severe. Therefore the biggest residual stress appears at the grain edge of the groove.