Workpiece Mass Research Articles

Automatic trajectory adaptation for the control of quality characteristics in a powder compaction process

The manufacturing process of sintered components requires the compaction of powder in a rigid die and the sintering of the resulting green parts. To produce green parts of the same constant quality over a long period of time, regular quality checks and trajectory adjustments are needed to account for changing operating conditions due to e.g. varying temperature, humidity, stroke rate, or powder quality. Also the first setup of the machine requires manual measurements and iterative adjustments of trajectory key points. To overcome these labour-intensive, manual measurements and adjustments, we propose an algorithm for the automatic adaptation of trajectory key-points that for the first time allows bringing and keeping more than one quality characteristic (mass and dimensions) fully automatically within predefined intervals. To safe costs, we further present two real-time capable, model-based solutions by employing grey-box and black-box models for the online estimation of the workpiece mass and height instead of their direct measurement. The grey-box model combines a filling and analytical compaction model and allows estimating mass and height rather than density as typically investigated in literature. The black-box model based on neural networks represents a complete novelty in the field as no similar data-driven approach could be found that aims at estimating mass and dimensions of green compacts. We finally compare the performance of these models to the baseline of sensor measurements as well as study their effect on the overall control performance. Results indicate that for both sensor-based and model-based solutions it is possible to reach and keep aforementioned quality characteristics within desired intervals. While overall the sensor-based approach performed best, also for the model-based approaches a good estimation performance is observed with the black-box model based on neural networks outperforming the grey-box model. While the black-box approach is fully data-driven and thus allows to widely avoid manual modelling activities, it still requires the presence of expensive sensors for capturing training data. The proposed grey-box approach, on the other hand, requires some manual modelling and model calibration activities, but provides a cost-effective solution to the problem and represents an alternative to more expensive, sensor-based implementations.

Tracking and Vibration Control with a Parallel Structure Controller Based on a Flexible Ball Screw Drive System

In this paper, we present a parallel structure controller for flexible ball screw drive systems with dynamic variations mainly caused by variations in table position and workpiece mass. The controller consists of two parts: a linear quadratic regulator (LQR) controller with the aim of tracking reference trajectories with high response and accuracy and an interpolated gain-scheduled controller used to restrain system vibration. To damp out the varied resonant modes, the controller is obtained by a set of μ-synthesis linear time-invariant (LTI) controllers interpolated via Youla parameterization. Comparison experiments are conducted to confirm the performance of the proposed controller with a ball screw drive experimental setup. We demonstrate that the parallel structure controller achieves high performance in tracking, vibration suppression and disturbance rejection.

Mass Loss and Displacement Modeling for Multi-Axis Milling

During the cutting process, material of the workpiece is continuously being removed by the cutting tool, which results in a reduction of mass as well as a displacement in the center of the workpiece mass. When using workpiece sided force sensors, such as table dynamometers, the total mass and the displacement of the center of mass affects the force measurement due to gravitational and inertial effects. The high flexibility of the milling process leads to a complex change of volume and mass and necessitates the consideration of the engagement conditions between tool and workpiece along the tool path in order to estimate changes in mass and center of mass. This paper proposes a method for estimating the mass loss and the displacement of the center of mass during multi-axis milling processes. In this method the tool gets numerically sliced along the tool axis and the workpiece removal for each slice along an arbitrary tool path gets calculated. To validate the mass loss model, experiments in both three-axis milling as well as multi-axis milling processes have been conducted. Since it is difficult to measure the center of mass, validation for the displacement of the center of mass was done by comparison with data extracted from CAD. The results show good agreement between the simulated and measured mass loss using the proposed approach.

Dynamic properties analysis of ball screw feed system based on improved hybrid model

Ball screws are widely used as feed systems for machine tools due to their high transmission efficiency and long service life. However, the large inertia force generated by the ball screw in high-speed motion will excite its flexible mode and cause system vibration. In the actual working condition, the dynamics of the system will also change with the nut position and workpiece mass. In order to further study the modeling method and dynamic properties of the ball screw feed system, this paper firstly improves the traditional hybrid model by introducing an energy loss factor to modify the hysteresis damping of the model. Further, the rationality of the improved model is verified by hammer test. Finally, the model was used to analyze the dynamic properties of the system and to obtain the influence law of workpiece mass and nut position on the system natural frequency.

Modelling the effect of slide table mass on machine tool energy consumption: The role of lightweighting

Much of the environmental footprint of a machine tool is due to the energy consumed during its use phase. A promising strategy for reducing this energy consumption is to minimize the mass of components through changes in the design of the machine tool. This paper explores the lightweighting of the machine slides (or tables) to achieve energy savings. An energy model is developed which considers the table and workpiece mass, the cutting force, and the motor that drives the table. A case study is carried out on a three-axis vertical milling machine to explore the energy savings that can be realized via lightweighting a slide table. The model is used to compare the energy consumption of a solid table and a lightweight table. Results show a potential energy savings of up to 38% in terms of the energy required to move the table. The model is verified using experimental data already found in the literature. It was observed that the model has a lower prediction capability when the table speed and power demand are very low but as the speed and power increase the prediction capability increases significantly.

Research on linear parameter varying modeling method of ball screw feed system

To realize the accurate motion control of the ball screw feed system, it is necessary to establish a reasonable dynamic model. However, the dynamic characteristics of the ball screw feed system are time-varying and nonlinear in practice. Therefore, a Linear Parameter Varying (LPV) modeling method is proposed. Firstly, the rigid body model considering the mechanical and electrical characteristics and the flexible body model considering the change of table position and workpiece mass are established, and the model parameters are further identified. Then, the LPV model is established based on the rigid body model and flexible body model. Finally, the LPV model is proved to be able to well characterize the time-varying dynamic characteristics of the ball screw feed system, which is proved by simulation experiment of modal analysis.

Numerical and experimental study on the ultrasonic-assisted soft abrasive flow polishing characteristics

To resolve the low-polishing efficiency problem caused by the turbulent layer separation in soft abrasive flow (SAF) processing, an ultrasonic-assisted SAF (UA-SAF) method is proposed based on the vibration effect. The coupled computational fluid dynamics (CFD)-discrete element method (DEM) modeling method (CFD-DEM) and the dynamic mesh are used to represent the UA-SAF flow and the periodic moving boundary caused by ultrasonic vibration, while the particle-wall erosion is calculated by the Archard model. Through tracking the abrasive particle motion in the simulation, the particle-wall collision states and the particle cutting effect are revealed, which mainly includes the particle plowing and the particle impact erosion. It has been found from the simulation that the fluid turbulent motion and the fluid dynamic pressure which are the power source for the particles to obtain the anisotropic cutting kinetic energy can be strengthened obviously by the ultrasonic vibrating, which results in the increase of the probability of particle impacting the target surface, and then the particle-wall wear rate and the wear area can be improved. A set of comparative experiments, including the rough and precision polishing procedures, are then conducted to evaluate the polishing performance by using the SAF and UA-SAF, respectively. The variations of the surface roughness and the workpiece mass are obtained, and the surface morphologies are observed. The experimental results prove that the ultrasonic vibration can effectively improve the abrasive flow polishing efficiency and precision without producing cavitation erosion.

Effect of mass of parts on removal rate under vibroabrasive machining

Introduction . It should be noted that the study on the problem of the effect of the mass of parts on the vibration-abrasive processing is insufficient. In the works of A.P. Babichev and M.A. Tamarkin, the fact of such an effect is mentioned, but the degree and mechanism of the effect are not disclosed. In the metal removal formulas, only the number of interactions leading to microcutting is taken into account. The present work objective is to determine the effect of the mass of parts on the metal removal rate under vibroabrasive machining. Materials and Methods . An empirical, i.e., experimental, approach is used. Parts from D16 and 30KhGSA materials which are widely used in the aviation industry were selected as samples. To change the mass, holes were drilled in the blanks; lead was poured into some samples, and plugs made of the same material as the blanks themselves were clogged into the others. Thus, experiments were carried out with solid, hollow, and weighted with lead samples. The working abrasive medium was scrap of grinding wheels of 40 x 80 mm, 25 grain size, and of trihedron prisms of 15 x 15 mm, 16 grain size. The experiments made it possible to clearly demonstrate the effect of grain size on the removal rate of the workpiece. Results . The parameters of the effect of the mass of parts on the removal rate under vibroabrasive processing are determined. The results obtained show the removal per unit area. The data are approximated by the least squares method with a linear function. A version of its distribution is selected using the Fisher statistical criterion. Discussion and Conclusion . It is shown how the workpiece mass determines the specific removal rate under the vibroabrasive machining. In the future, the database which is used to determine the effect of the work material characteristics on the process under consideration should be replenished. This will allow introducing a correction factor for the influence of mass in the metal removal formula, which will provide more accurate prediction of metal removal at the design stage of technological processes of vibration-abrasive machining.

Characterizing vibration responses of a handheld workpiece and the hand–arm system

The objective of this study is to characterize the vibration responses of a handheld workpiece and the hand–arm system, which is an important step toward identifying and developing effective methods and technologies for controlling the vibration exposures to workers performing the grinding of handheld workpieces. This study established a method for measuring the vibration responses of the entire workpiece–hand–arm system; the vibration exposure of a worker holding and pressing a typical workpiece against a sanding belt or grinding wheel in order to shape the workpiece was simulated. This method was applied to measure the apparent mass and vibration transmissibility of the system under two different feed forces (15 N and 30 N) and six simulated grinding interfaces with different stiffness values. A major resonance was observed in each transmissibility spectrum of the workpiece, which was correlated with the major resonance of the impedance of the entire system. This resonant frequency depended primarily on the workpiece mass and the grinding interface stiffness, but the hand–arm system could substantially affect the resonance magnitude. The feed force also significantly affected the resonance frequency and magnitude. While increasing the feed force increased the overall vibration transmissibility on the hand–arm system, the transmissibility with respect to the workpiece was not significantly affected by the interface conditions. The implications of the results are discussed.

Aluminium Machining Chips Formation, Treatment & Recycling: A Review

The recycling of alumium alloys has been growing in interest and applications during the last fourthy years and has become a cost effective, ecological and reliable way to produce aluminium parts. The aluminium scraps that can be recycled include cans and machining chips. The machining processes produce chips of various sizes and shapes, wet or dry, oxidised or not, depending on type of process and the machining conditions, parameters and tools used. Some processes produce metallic dusts and fine chips while other produce large or medium size chips. In some industries such as mould making and aeronautic industries, the chip removal can easily represent 80% of the initial workpiece mass. The type of chips produced during machining can have a great impact on chip management, on part quality, on machine and tool reliability and on part manufacturing costs. The machining chips can be recycled using casting, sintering or pressing and extrusion processes depending on the goal targeted. The selection of the recycling process must take into account the targeted applications, the chip (composition, sizes and cleanliness) and its mechanical properties. Depending on the nature of process to be used and the machining chip generation conditions, some treatments might be necessary prior to transportation and recycling. Parts made with recycled chips can either be bi-phase metal matrix composites materials or usual one phase material with mechanical properties and wear properties comparable or not to the parent alloys. Over the last decades, several chip recycling processes have been proposed for aluminium alloys. This article review the aluminium chips formation, treatement methods, the recycling processes and their impact on recomposed parts’ performance: strength, ductility, corosion and wear resistances.

Process optimization for design of duplex universal joint fork using unequal thickness flash

To avoid defect formation, a two-step forging process for duplex fork was developed and optimized using an unequal thickness flash. The large cross-sectional difference between the different parts of the complex shape of the duplex fork made it difficult to forge. Crack defects formed during initial forging were observed and analyzed by finite element simulation, and the metal flow lines and velocity distributions on the main cross section were studied. A non-uniform velocity distribution with a large difference in the dangerous area and large amount of oxide scale most likely caused the crack defects. An optimized two-step forging process was developed, preupsetting was used to remove the oxide scale, and final forging using a die with an unequal flash gutter depth was used to obtain a reasonable velocity distribution. The effect of different process parameters on the optimized forging process was determined. The parameters included the pre-upsetting stroke, the friction factor, the workpiece mass, and the offset of the preform, and the results were used to improve the process stability. A duplex fork of acceptable quality and without any cracks was forged in successful trial production using the optimized process in the forging plant.

Interpolating gain-scheduled H∞ loop shaping design for high speed ball screw feed drives

This paper presents a method to design servo controllers for flexible ball screw drives with time-varying dynamics, which are mainly due to the time-varying table position and the workpiece mass. A gain-scheduled H∞ loop shaping controller is designed to achieve high tracking performance against the dynamic variations. H∞ loop shaping design procedure incorporates open loop shaping by a set of compensators to obtain performance/robust stability tradeoffs. The interpolating gain-scheduled controller is obtained by interpolating the state space model of the linear time-invariant (LTI) controllers estimated for fixed values of the scheduling parameters and a linear least squares problem can be solved. The proposed controller has been compared with P/PI with velocity and acceleration feedforward and adaptive backstepping sliding mode control experimentally. The experimental results indicate that the tracking performance has been improved and the robustness for time-varying dynamics has been achieved with the proposed scheme.

Axis Position Dependent Dynamics of Multi-axis Milling Machines

Numerous analytical methods are available to predict the stability of milling processes. Most of these methods base on the assumption, that the dynamics of the machine tool are time invariant. This assumption seems to be valid in many cases. However, in case of huge translational or rotatory axes movements or process-induced changes in the workpiece's mass and elasticity a time variant dynamic model might be needed. This paper presents a method to model the axis position dependent dynamics of a multi-axis milling machine. According to this method, the modal parameters of the machine tool are predetermined in different discrete axis positions. An interpolation strategy allows calculatingthe modal parameters in arbitrary resolution along arbitrary toolpaths. Here, an exemplary 2.5-dimensional milling process serves as an example. The conventional step-by-step time domain simulation procedure is complemented by the modal interpolation strategy to account for changing machine dynamics. The effect of changing dynamics on the process is determined and a comparison to a cutting test is performed.

Highly Integrated High Precision Fluidic Feed Axis

Current machine tools for micro manufacturing feature unfavorable ratios of installation space to work space as well as moving masses compared to the work piece mass. Therefore, these machines show potential for optimization [1]. Machine tool components leading to these drawbacks are the currently used feed axes. The required features of such feed axes for micro manufacturing comprise high functional integration, high damping and high precision. Compared to electromechanically build up feed axes incorporating high installation space as well as small damping ratios due to linear guide rails, hydraulic feed axes feature the potential of a high force density and high damping ratios. However, a drawback is the poor precision. At the wbk Institute of Production Science Karlsruhe a novel approach has been realized: A highly integrated hydraulic feed axis with high compactness due to the hydraulic force density. Additionally, it features high functional integration by piezoelectric seat valves built into the piston rod and an integrated hydrostatic guidance system. Within this paper, the approach as well as the measurement results of the prototype of the feed axis by the Institute of Production Science are shown. Furthermore, it addresses the feed axis's properties such as positioning errors, performance and inaccuracies. In the end, an outlook will be given on further ongoing research activities regarding an upgrade enabling an intelligent, active manipulation within the hydrostatic guidance system to compensate the errors of the feed axis.

Switching Gain-Scheduled Control Design for Flexible Ball-Screw Drives

This paper proposes an application of the switching gain-scheduled control technique to the flexible ball-screw drive servo system with a wide range of operating conditions. The wide operating range is caused by the change of the table position and the workpiece mass during the machining operation, and leads to plant dynamics variations. To achieve high tracking performance of the table position against the dynamics variations and the cutting force disturbance, a set of gain-scheduled controllers is designed so that each controller damps out the resonance of the ball-screw system and increases the closed-loop bandwidth for a local operating range, and tracking performance is guaranteed under the switching between these controllers. Experimental results with a laboratory-scale ball-screw drive setup demonstrate that the switching gain-scheduled controller outperforms the nonswitching one by up to 52% in tracking accuracy.

Tracking Control of Flexible Ball Screw Drives With Runout Effect and Mass Variation

Most machine tools rely on precision ball screw drives to accurately position the workpiece relative to the tool. The quality of the machining outcome depends significantly on the tracking performance of the workpiece position over a desired trajectory. This paper addresses the minimization of the tracking error in a ball screw drive system in the presence of dynamic variations. Three sources of dynamic variations are considered: mechanical flexibility, runout of the ball screw shaft, and workpiece mass change during operations. Dynamic variations due to flexibility and runout are related to the workpiece position which is continuously measurable, while the variation caused by the workpiece mass change is uncertain. The ball screw drive system affected by dynamic variations is expressed as an uncertain linear model with time-varying parameters. Based on this model, servo controllers are designed such that their parameters are adjusted in real time by the measurable workpiece position to improve the tracking performance and that their performance is maintained robustly over uncertain mass variation. The importance of taking into account flexibility and runout of the shaft, as well as mass variation, explicitly in controller design is demonstrated through a ball screw drive experimental setup.

Highly Integrated Piezo-Hydraulic Feed Axis

Currently used micro-machines are mostly based on macro machine tool principles. This leads to an unfavorable ratio of build and work space and also to high moving masses in comparison to the workpiece mass. To address these disadvantages it is necessary to develop new approaches for micro machines, micro machine components and new kinematic chains especially adapted to the requirements of micromachining. Therefore a new feed unit is designed featuring a highly integrated piezo-hydraulic feed axis, a parallel kinematic and a millimeter based radar measurement system. In this paper the design approach and the characterization of the compact highly integrated piezo-hydraulic feed axis is presented.

PARAMETER SENSITIVITY ANALYSIS OF AXIAL VIBRATION FOR LEAD-SCREW FEED DRIVES WITH TIME-VARYING FRAMEWORK

This paper establishes a dynamic model of leadscrew feed drives by means of the finite element and lumpedparameter and analyzes the effects of changeable table position and workpiece mass on the first three axial modes of the free vibration. The results indicate that the natural frequency of the first mode focusing on table vibration is sensitive to both changeable table position and workpiece mass to a certain extent; the natural frequency of the second mode focusing on rotor vibration is unsensible to changeable table position and workpiece mass; the natural frequency of the third mode focusing on screw vibration is sensitive to the table position whereas unsensible to the workpiece mass. In the second stage of research, the results display that the enhancement of the axial stiffness of the back screw bearing can effectively reduce the sensitivity of the first nature frequency on the changeable table position. http://dx.doi.org/10.5755/j01.mech.17.5.730

Discrete-time model predictive contouring control for biaxial feed drive systems and experimental verification

In this paper we present a novel algorithm to model predictive contouring control for biaxial feed drive systems. model predictive control (MPC) refers to a class of model-based controllers that uses an explicit process model and tracking error dynamics to predict the future behavior of a plant, making it effective for machine tool feed drive systems that must achieve high-precision motion and are severely affected by friction, cutting force and changes in the workpiece mass. To improve contouring performance, we propose a new performance index in which error components orthogonal to the desired contour curve are given more importance than tracking errors with respect to each feed drive axis. Controller parameters are calculated in real time by solving an optimization problem. The parameters depend on the instantaneous slope of the reference trajectory and thus vary with time for curved reference trajectories, resulting in a time-varying controller. Weighting factors for the error components in orthogonal and tangential directions are used to adjust the error importance in each direction. In addition, to consider the required feed drive energy, the control inputs in both directions are included in the performance index. The effectiveness of the proposed control approach is demonstrated with an experimental biaxial feed drive system for circular and non-circular trajectories. The proposed contouring controller allows the feed drive to follow smooth curves and reduces contouring error.

Model Predictive Approach to Precision Contouring Control for Feed Drive Systems

Problem statement: High precision machining requires high capability of multi-axis feed drive systems to follow specified contour accuratel y. Although each feed drive axis is controlled independently in many industrial applications such as X-Y tables and Computer Numerical Control (CNC) machines, machining precision is evaluated by error components orthogonal to desired contour curve. Contouring controller design is required for precision machining, which should consider disturbance and dynamics variation such as friction , cutting force and workpiece mass change. Approach: This study applied model predictive design to cont ouring control systems. Model predictive control utilized an explicit process mod el and tracking error dynamics to predict the futur e behavior of a plant and hence it is effective for p recision machining in machine tool feed drives. To improve the contouring performance, a new performance index was proposed in which error components orthogonal to the desired contour curve are more important than tracking errors with respect to each feed drive axis. Controller paramet ers were calculated in real time by solving an optimization problem. Results: The proposed controller was evaluated by computer simulation for circular and non-circular trajectories. Weighting f actors of performance index terms were used as tuning factors of the proposed controller. Simulati on results showed that a better contouring performance can be obtained by choosing of the weighting factors in performance index items appropriately. Conclusion/Recommendations: A model predictive contouring controller for biaxi al feed drive systems was presented. Simulation result s demonstrated that the proposed approach can significantly improve the contouring accuracy.