Ground Profile Research Articles

Evaluation of approaches to compensate the thermo-mechanical distortion effects during profile grinding

This paper presents the results of a distortion analysis of profiled workpieces after the grinding process and a distortion compensation. Heat treatments and machining processes such as grinding change the residual stress state of steel workpieces in a quite uncontrolled manner. This significantly influences the deformation of the quenched and tempered steel workpieces used in this work. The lack of a detailed representation of profile grinding processes and the stress-related distortions of slim components still poses a great challenge in research. Simulative mapping of the thermo-mechanical effects on profiled components has proved difficult in literature. The influences of different cutting depths and feed speeds of the grinding wheel on the distortion were investigated with developed finite element simulations as well as experimentally. The grinding model, validated by experimental results, expands the understanding of the profile grinding process. This enables a detailed evaluation and an analysis of the occurring effects. As an approximation to linear guide rails, a V-groove was ground to improve the surface quality. Inside the V-groove, the machining parameters and thus the generated three-dimensional heat flux influence the residual stresses. The simulated workpiece distortions match very well to the experiments, especially when varying the defined depths of cut. The results provide a model for distortion prediction. Based on this model, approaches to use a laser-based treatment and deep rolling as distortion compensation strategies can be addressed.

Macroscopic process simulation of surface and profile grinding processes estimating forces for the production of turbine blades

In aerospace industry surface and profile grinding processes are widely used for the manufacturing of turbine blades. Maximizing the productivity is a key factor for industry, which can be achieved by reducing the number of cuts necessary in order to remove the remaining millimeters of stock material after casting. This leads to high depth of cut values and, therefore, considerably high process forces. Turbine blades are highly flexible workpieces resulting in process induced deflections. In order to avoid mechanical damages of the workpiece during the process, a macroscopic grinding simulation is used, which is presented in this paper. For this process simulation a coupled multiphysics FE model was created using the software COMSOL Multiphysics. First, the material removal was evaluated using the deformed geometry module of the software. Based on the current material removal the process forces were calculated using an experimentally calibrated grinding force model. The forces were subsequently applied onto the grinding area allowing for a calculation of the resulting stresses and deflections. Further, the simulation was extended for profile grinding processes using an inclined grinding wheel and a fir tree profile of a turbine blade. The simulated forces were validated based on experimentally conducted grinding processes.

Investigation on Heat Transfer Performance of Rotating Heat Pipe Grinding Wheel in Dry Profile Grinding

Investigation on Heat Transfer Performance of Rotating Heat Pipe Grinding Wheel in Dry Profile Grinding

Development of a method for applying free kinematics for gear profile grinding

The quality and performance of gears, especially in aerospace applications, need to be assured. As the manufacturing process has a significant influence on the performance of the gear, any occurring error during the manufacturing process must be detected. In this paper, a new method is presented that allows the usage of servo traces of the manufacturing machines to simulate the actual process kinematics. This method allows a more precise simulation and therefore more precise prediction of the quality of manufactured gears.

Improved Flux Tracing Method Based on Parametric Curve for Calculating Ion Flow Field of HVDC Transmission Lines

An improved method for tracing electric flux lines in the vicinity of the overhead high voltage direct current (HVDC) transmission lines is developed by using parametric curves. Based on the geometric relation between the electric field vector and the derivative of the parametric curve function, the electric flux lines can be successfully traced without accumulation of the numerical error caused by the differentiation for calculating the electric potential. Considering the structural characteristics of the transmission line, the control points at both ends of the parametric curve can be easily located, and the remaining control points are modified to minimize an error function defined by the electric field vectors at nodes along the curve. Once the electric flux lines are traced, a series of flux tubes is created by bundling adjacent flux lines, and the nodal electric field vectors and space charge densities are iteratively updated. In order to consider the ionized space charge effect, new flux lines of modified electric field are traced, which in turn affects the existing space charge distribution. The process is repeated until the effect of the space charge is fully considered. The performance of the proposed algorithm is verified by comparing the resulting ground profiles of the ion flow field distribution with the result of the analytical model and the long-term measurement of the full-scale transmission line.

Residual stresses of turbine blade root produced by creep-feed profile grinding: Three-dimensional simulation based on workpiece–grain interaction and experimental verification

Grinding-induced residual stresses have an important influence on the antifatigue property of turbine blade roots. The conventional finite element (FE) simulation model based on workpiece-tool interaction cannot accurately predict the grinding-induced residual stresses of turbine blade root of DD6 single-crystal nickel-based superalloy. Therefore, in this work, a new thermal-mechanical FE model considering the intensive workpiece–grain interaction was developed to predict the distribution of grinding-induced residual stresses. Furthermore, the grinding temperature, the instantaneous stresses generated during the creep-feed profile grinding operation, and the residual stresses generated after grinding were simulated and verified experimentally. Compared with the simulated results obtained with the conventional model, those obtained with the new model agreed well with the experimental results. By using the new model, the average errors between the simulation and experiment of less than 16 % and 20 % for the grinding temperature and residual stress were obtained, respectively. Moreover, the grinding-induced residual stresses in the peak regions of the blade root workpiece were generally compressive (e.g., from −150 to −100 MPa along the vertical grinding direction ([010] crystal orientation of DD6 alloy), and from −200 to −150 MPa along the parallel grinding direction ([001] crystal orientation of DD6 alloy), while they were transformed gradually to the tensile stresses in the adjacent valley regions.

Uncertainty analysis of wireless temperature measurement (WTM) in borehole heat exchangers

The reliability of temperature measurements in open and closed geothermal systems is closely related to their design, quality control and performance evaluation. Thus, wireless and miniaturized probes, which provide highly resolved temperature profiles in borehole heat exchangers (BHEs), experience a growing interest in research. To ensure quality assurance and reliability of these emerging technologies, errors and uncertainties relating to wireless temperature measurements (WTMs) must be determined. Thus, we provide a laboratory analysis of random, systematic and dynamic measurement errors, which lead to the measurement uncertainties of WTMs. For the first time, we subsequently transfer the calculated uncertainties to temperature profiles of the undisturbed ground measured at a BHE site in Karlsruhe, Germany. The resulting precision of 0.011 K and accuracy of -0.11 K ensure a high reliability of the WTMs. The largest uncertainty is obtained within the first five meters of descent and results from the thermal time constant of 4 s. The fast and convenient measurement procedure results in substantial advantages over Distributed Temperature Sensing (DTS) measurements using fiber optics, whose recorded temperature profiles at the site serve as qualitative comparison. We additionally provide recommendations for technical implementations of future measurement probes. Our work will contribute to an improved understanding and further development of WTMs.

LOS OJOS DE I’ITOI. EL TELESCOPIO SOLAR DE KITT PEAK (ARIZONA)

Antes de que la observación del universo saltase al espacio para liberarse de la gravedad y de la contaminación lumínica de la superficie terrestre, hubo un rápido y notable desarrollo de esta ciencia durante el siglo XX. Esta evolución pasó por el crecimiento y la afinación de los instrumentos de observación, así como por la detección de lugares que reunieran especiales condiciones de visión y transparencia atmosférica. A este proceso no resultó ajena la participación de ingenieros y arquitectos que contribuyeron a la construcción de originales instalaciones cuya configuración incluía una redefinición del perfil del suelo –la línea de tierra– donde se asentaban. En esta carrera, ocupa un lugar destacado el Observatorio Nacional de Kitt Peak en Arizona, cuya primera instalación –el mayor telescopio solar construido nunca– fue proyectada por el ingeniero civil y arquitecto Myron Goldsmith. La atención a su trayectoria personal servirá para entender el resultado de este conjunto que media entre lo profundo de la tierra y el firmamento de las estrellas.

Design method of rail grinding target profile based on non-uniform rational B-spline

The grinding strategies for worn rails have a significant influence on the service life and maintenance cost of railways. Current grinding strategies, grinding the worn rails to their original profiles, leads to excessive amount of material removed and reduction of rail service life. Based on Non-Uniform Rational B-Spline theory (NURBS), which contributes to the control of curve smoothness, a rail profile reconstruction method was established considering the geometrical characteristics of the rail profile. The design of economical grinding rail profile can be achieved by using optimization method, which regards the amount of material removed and derailment coefficient as objectives, aiming at reducing rail wear. A new rail profile can be designed by integration of Archard wear model, FE contact model and vehicle-track coupling dynamic model. The result indicated that the wear of left and right rail is reduced by 29.26% and 31.06% compared with original CN60 profile, respectively. With the new rail profile, the dynamic performance is improved, and the contact stress, fatigue index and the amount of material removed is reduced. This optimized rail grinding profile contributes to the reduction of rail wear and guarantees the safety of train operation.

Grinding burn limits: Generation of surface layer modification charts for discontinuous profile grinding with analogy trials

By means of surface layer modification charts, the occurrence of thermal impact and thus the risk of grinding burn can be detected. With the present work it was shown that surface layer modification charts for discontinuous profile grinding can be generated using analogy trials carried out on a surface grinding machine for investigation of different grinding conditions and varied tilt angles in profile grinding. The experimental and the analysis effort for the analogy trials prove to be significantly more time and cost saving. The results also suggest that system parameters such as the grinding wheel specification or the cooling strategy are taken into account by the specific grinding power. Thus, the experimental results of this work allow the assumption that the determined process limit is not or just to a low extent depending on the system parameters. This assumption is supported by the thermal process limit determined by Malkin and converted in surface layer modification charts. Malkin's limit, identified for different ground steels with surface and cylindrical grinding processes and thus different system and process parameters, corresponds to the results generated in this work at workpieces ground with a kinematically more complex process. The results reveal that there is at least a group of steel materials which seems to share a similar thermal process limit.

Design and Control of a Compliant Wheel-on-Leg Rover Which Conforms to Uneven Terrain

This article presents the design, control, and implementation of a new compliant wheel-on-leg robot, named Complios. This robot is able to spontaneously adapt its configuration while it is rolling on unknown uneven terrain. The constant contact of all wheels with the ground is ensured by a series elastic actuation at the top of each leg. It provides a force control that allows the robot to freely balance its posture, while the load can be distributed as evenly as possible on each wheel for any relative height differences of the ground. The compliant structure of the Complios also gives access to the measurement of horizontal forces applied on the legs, which constitutes a valuable information when it comes to choose an appropriate response during obstacle crossing. Finally, the robot is tested on different ground shapes and proves its capability to adapt, in real time, to the ground profile.

Concept design of jacket foundations for offshore wind turbines in 10 steps

Simplified design procedures for offshore wind turbine (OWT) support structures are necessary in the concept and tender design stages for assessing the financial viability. Building upon the previous research on simplified monopile design, this paper presents a similar method to design jacket supported OWTs founded on piles or suction caissons. The method is based on the minimum and necessary data, namely the site characteristics (wind speed at reference height, wind turbulence intensity, water depth, wave height and wave period), turbine characteristics (rated power, rated wind speed, rotor diameter, cut-in and cut-out speed, mass of the rotor-nacelle-assembly) and ground profile (soil stiffness variation with depth and soil strength). A flowchart of the design process is also presented for visualisation of the rather complex multi-disciplinary analysis. Insights are given for 3 and 4 legged jackets. The calculation procedures that are required can be easily carried out either through a series of spreadsheets or simple hand calculations. An example problem emulating the design of a jacket on piles is taken from literature to demonstrate the proposed calculation procedure. The data used for the calculations are obtained from publicly available sources and the example shows that the simplified method arrives at a similar foundation to the one reported.

Investigation on cooling behavior of axially rotating heat pipe in profile grinding of turbine blade slots

During profile grinding of fir-tree blade slots, a key issue is usually the excessive heat caused by the complex contact zone. Most studies have focused on the cooling efficiency of the coolant; however only few have investigated the heat transfer potency of the grinding wheel matrix. In this study, a new cooling method that incorporates an axially rotating heat pipe (RHP) in the profile grinding wheel has been proposed. The cooling behavior of the new method was analyzed by both simulation and experimental grinding of titanium alloys. The temperature distributions along the RHP and the workpiece surface were monitored using embedded thermocouples. The effects of input heat flux, filling ratio and rotational speed on heat transfer performance were discussed. Comparative profile grinding experiments among RHP cooling, coolant cooling and no cooling demonstrated a great cooling advantage of RHP cooling with the lowest grinding temperature and better workpiece quality. Considering the research gaps of the previous studies, this work is not only deepens the understanding of the cooling behavior in the RHP during profile grinding of turbine blade slots, but also is helpful to provide guidance on the green machining of industrial products with complex profiles.

Ultra-precision raster grinding of monocrystalline silicon biconical free-form optics using arc-shaped diamond grinding wheels

Free-form optics are becoming increasingly indispensible in a series of industries, such as aviation, aerospace, medical, digital video and audio optoelectronics, owing to its enhanced optical performance, fewer surfaces, lower mass, lower cost, smaller package-size and reduced stray-light, etc. However, the ultra-precision machining of free-form surface optics, especially the non-rotational asymmetric free-form optics is still faces a great challenge. In this study, ultra-precision grinding of the non-rotational asymmetric biconical free-form optics with raster grinding path was studied. Firstly, a novel feeding compensation truing strategy for truing the arc-shape grinding wheels with high-precision was proposed. A D46 μm metal-bonded grinding wheel and a D7μm resin-bonded diamond wheel, were successfully trued with high profile accuracy and desired surface topography. Subsequently, the influence of the significant grinding factors like grinding path, scallop height, contact arc length and the grinding wheel wear on the ground profile accuracy and surface quality of the biconical free-form optics was theoretically and and experimentally analyzed. Eventually, a larger size monocrystalline silicon biconical free-form optics with profile accuracy 6.0 μm and nanometer surface roughness was successfully manufactured by raster grinding combined the technology introduced in this paper.

3D modeling and simulation of thermal effects during profile grinding

A new heat transfer model for profile grinding was developed to analyze distortions caused by residual tensile stresses in linear guide rails. The simulative analysis of the thermal effects caused by a non-uniform heat source on the surface using the finite element method depends on an accurate representation of the locally variable contact area. The complexity of the V-groove profile disqualifies a 2-dimensional simulation approches so far used in the literature. This paper focuses on the redefinition of these mathematical relationships of the process parameters and the resulting heat flux. The heat flux model is adapted to the geometry of the workpiece depending on the grinding parameters and approximating the V-groove of a linear guide rail. This 3-dimensional modeling allows a better understanding of the thermo-metallurgical effects that occur during the grinding process. Furthermore, the calculation of the internal stresses induced into the workpiece material through the grinding process is possible. The simulation model results in a generally valid model for the analysis of distortions. In order to confirm the validity of the new heat flux profile, a comparison of the different finite element simulation results was made and experiments under wet grinding conditions were conducted. The results show that the newly developed grinding process model allows a more accurate prediction of workpiece distortion caused by grinding forces and temperatures. This research also offers a new approach to a method based on a 2-dimensional implementation developed in the literature for predicting the distortions of linear guide rails and a derivation of possible simulation-based compensation strategies.

Geometric error identification and compensation for rotary worktable of gear profile grinding machines based on single-axis motion measurement and actual inverse kinematic model

Gear profile grinding accuracy is often restricted by geometric errors of the rotary worktable. This paper proposes a novel approach to improve the geometric accuracy of the worktable based on single-axis motion measurement and the actual inverse kinematic model (IKM). The volumetric error model of gear profile grinding machines is first established based on homogeneous transformation matrices (HTMs) and machine kinematic chains, and the actual IKM based compensation method is then proposed to offset the position-dependent geometric errors (PDGEs) of the worktable. It directly gives explicit analytical expressions of compensated motion commands for efficient compensation. Besides, a novel PDGEs identification approach based on single-axis motion measurement with a double ball bar (DBB) is presented for the worktable. This approach has improved identification accuracy and stronger robustness as the interference error of non-targeted axes is avoided. The influence of the installation errors of the workpiece ball on measurement results and the PDGEs on gear grinding errors were discussed. The experiments verified the effectiveness of the identification approach, and the geometric accuracy tests of the worktable validated the compensation method.

Assessment of Practical Methods to Predict Accumulated Rotations of Monopile-Supported Offshore Wind Turbines in Cohesionless Ground Profiles

Monopiles supporting offshore wind turbines can experience permanent non-recoverable rotations (or displacements) during their lifetime due to the cyclic nature of hydrodynamic and aerodynamic loading exerted on them. Recent studies in the literature have demonstrated that conventional cyclic p–y curves recommended in different codes of practice (API-RP-2GEO and DNVGL-RP-C212) may not capture the effects of long-term cyclic loads as they are independent of the loading profile and the number of applied cycles. Several published methodologies based on laboratory scaled model tests (on sands) exist to determine the effect of cyclic lateral loads on the long-term behaviour of piles. The tests vary in terms of the pile behaviour (rigid or flexible pile), number of applied loading cycles, and the load profile (one-way or two-way loading). The best-fit curves provided by these tests offer practical and cost-efficient methods to quantify the accumulated rotations when compared to Finite Element Method. It is therefore desirable that such methods are further developed to take into account different soil types and the complex nature of the loading. The objective of this paper is to compare the performance of the available formulations under the actions of a typical 35-h (hour) storm as per the Bundesamt für Seeschifffahrt und Hydrographie (BSH) recommendations. Using classical rain flow counting, the loading time-history is discretized into load packets where each packet has a loading profile and number of cycles, which then enables the computation of an equivalent number of cycles of the largest load packet. The results show that the loading profile plays a detrimental role in the result of the accumulated rotation. Furthermore, flexibility of the pile also has an important effect on the response of the pile where predictions obtained from formulations based on flexible piles resulted in a much lower accumulated rotation than tests based on rigid piles. Finally, the findings of this paper are expected to contribute in the design and interpretation of future experimental frameworks for Offshore Wind Turbine (OWT) monopiles in sands, which will include a more realistic loading profile, number of cycles, and relative soil to pile stiffness.

Study on micro structure and composition distribution of concrete surface zone based on fractal theory and XCT technology

Based on the fractal theory, the fractal dimension model of coarse aggregate gradation in concrete was proposed, and the fractal dimension of coarse aggregate gradation in concrete was calculated based on the measured data. The variations of the micro structure, composition distribution and porosity to paste ratio of concrete surface layer were investigated by the methods of X-ray computed tomography (XCT) and profile grinding technology (PGT). Moreover, the content variations of coarse aggregate and paste with depth in concrete surface were also studied. The results show that the coarse aggregate content in concrete surface layer increases but the porosity decreases with increase of depth from concrete surface. Finally, they all tend to be a constant when the depth is larger than a certain value. The porosity to paste ratio of concrete surface decreases with increase of concrete strength. The major composition is cement paste within 0.1 mm of concrete surface, which turns into mortar within 0.1 mm ~ 0.4 mm. The volume fractal dimension of coarse aggregate and the fractal dimension of coarse aggregate gradation in concrete is about 1.35 and 2.68, respectively.

Study on surface generation mechanism and roughness distribution in gear profile grinding

The non-uniform grinding conditions due to the tool and workpiece geometries lead to the complexities of the chip geometry and topography characteristic in gear profile grinding. As a result, the previous surface prediction models that have been extensively studied for surface grinding are not able to reveal the surface generation mechanism of gear profile grinding. In this work, a comprehensive model is presented to calculate the surface topography and the chip geometry considering the non-uniformity of geometric contact and grain-workpiece interaction along the tooth profile. In this process, the geometric conditions are characterized by the radius of local tool circumference and the included angle between the plane of grain rotation and the surface normal. Experimental results show reasonable consistency with numerical simulations that validate the proposed model. Based on the model, the effect of geometric conditions on the texture pattern and roughness distribution is studied to give an in-depth analysis of the process characteristics by comparing the simulated results under actual and ideal conditions. Besides, the optimization method is proposed to obtain a more uniform tooth surface. This work provides new insight into the fundamental understanding of profile grinding of gears and can be utilized to guide the machining process for higher tooth surface integrity.

Influence of gear hobbing feed marks on the resulting gear quality after discontinuous profile grinding

Gear hobbing is one of the most common soft-machining processes for pre-toothing. The process kinematics result in a characteristic tooth-flank topography, which is mainly determined by so-called feed marks. For an economical finishing process by gear grinding in automotive applications, the feed-mark depths should not exceed a maximum value of 35μm. In the present study the validity of this limit has been investigated in view of the development of increasingly powerful grinding machines and grinding wheels. For this purpose, gears with feed-mark depths δx below and above 35μm were machined and ground by means of discontinuous profile gear grinding afterwards. The influence of the feed marks on the grinding process with roughing parameters was systematically evaluated on the basis of various process variables such as the increase in spindle power Ps or the degree of grinding-wheel clogging Zs, while the resulting gear quality was mainly analyzed by various parameters to describe macro- and micro-geometry deviations of the ground tooth flanks. With increasing feed-mark depth, an increase in spindle power was found due to the additional machined volume. An influence of increasing feed-mark depths on the clogging degree, the grinding-wheel wear and the gear quality could not be proven. Therefore, economical finishing of gears by gear grinding is also possible with feed-mark depths of more than 35μm. A new definition of this limit should be sought.