Freeform Workpiece Research Articles

Robotic measurement system based on cooperative optical profiler integrating fringe projection with photometric stereo for highly reflective workpiece

Robotic measurement has emerged as a prominent method for the quality control of large-scale metal workpieces. However, it faces two significant challenges: optical measurement of homogeneous high-reflective metal surfaces and point cloud registration on local featureless surfaces. To address these challenges, this study presents a cooperative optical profiler that integrates a certain number of light-emitting diodes into a conventional fringe projection profiler. This integration enables the use of photometric stereo, providing comprehensive and accurate surface normals of machined metal surfaces with intricate details such as tool marks. The surface normal was then fused with the initial point cloud from the fringe projection profiler to yield complete local measurement results. In addition, a geometry descriptor incorporating the surface normal was designed to improve data registration accuracy. Experiments based on six small workpieces with the combination of three shapes and two materials and a large freeform workpiece demonstrated that the proposed method achieved measurement accuracy close to that of a coordinate measuring machine.

Compliant polishing of thin-walled freeform workpiece

Compliant polishing using elastic tools can achieve ultraprecision surface quality on freeform workpieces. However, when it comes to thin-walled parts, the challenge arises with the complex polishing mechanics between the tool and freeform surface, without the knowledge of which the surface form can be easily degraded. To address this, an analytical model is established that can predict the interaction mechanics during polishing. On this basis, two adaptive strategies are proposed. Experiments with controllable material removal verify the high accuracy of the model, and compliant polishing of a thin-walled freeform workpiece with highly precise removal consistency below 0.5 µm was achieved.

A Feedrate-Constraint Method for Continuous Small Line Segments in CNC Machining Based on Nominal Acceleration

When the CNC machining of continuous small line segments is performed, the direction of the machine tool movement will change abruptly at the corner of adjacent line segments. Therefore, a reasonable constraint on the feedrate at the corner is the prerequisite for achieving high-speed and high-precision machining. To achieve this goal, a feedrate-constraint method based on the nominal acceleration was proposed. The proposed method obtains the predicted value of acceleration during the machining process by the machining trajectory prediction and acceleration filtering. Then, the feedrate at the corner is constrained, according to the predicted acceleration. Specifically, for any corner of adjacent line segments, the proposed method assumes that the CNC machining of a short path centered on the corner is carried out at a constant feedrate. First, the actual machining trajectory is predicted according to the transfer function of the servo system. Then, the nominal acceleration, when the CNC machining is carried out to the corner, is calculated and processed by a low-pass FIR filter. Last, the feedrate-constraint value at the corner is obtained according to the nominal acceleration and the preset normal acceleration. The advantage of the proposed method is that it can be used for different machining paths consisting of long segments or continuous small segments and it has no special requirement for the accuracy of the machining path. As a result, the feedrate-constraint value obtained is reasonable and the smooth machining process can be ensured. The simulation results in both 2D and 3D machining paths show that the proposed method is insensitive to the length of the line segment and the angle of the corner, and the calculated feedrate-constraint value is close to the theoretical value, which has good stability and versatility. In contrast, the feedrate-constraint values obtained by conventional methods change abruptly along the machining path, especially in the 3D simulation, which will damage the machining quality. The experiment was performed on a three-axis CNC machine tool controlled by a self-developed controller, and a free-form surface workpiece was machined by a conventional feedrate-constraint method and the proposed method, respectively. The experimental results showed that the proposed method can make the feedrate of the machining process higher and more stable. Then, machining defects such as overcutting and undercutting can be avoided and the machining quality can be improved. Therefore, the article proposes a new method to constrain the feedrate at the corner of continuous small line segments, which can improve the machining efficiency and quality of the CNC machining.

Contact force modeling and analysis for robotic tilted-disc polishing of freeform workpieces

This paper investigates the tool-workpiece contact mechanism in the disc polishing process, where the soft polishing disc touches the rigid freeform workpiece with a small tilt angle. The developed contact force model can explain how the contact parameters, including the disc contact depth, the disc tilt angle, the radii of curvature of the disc and the workpiece, affect the normal contact force. Under some reasonable geometric and mechanical assumptions of the disc and the workpiece, an approximated contact force equation is derived in a simple power-law form similar to the Hertzian contact model. The contact force is found to be positively related to the disc contact depth and the disc radius, while negatively related to the disc tilt angle and the workpiece normal curvature that is orthogonal to the feed direction. Both finite element simulations and experiments verify the effectiveness of the proposed contact force model. Furthermore, the material removal process is analyzed and a model-based process parameter planning method is developed to achieve uniform material removal considering the variation of workpiece curvatures.

Modeling of curved diamond wheel errors for improvement of freeform grinding accuracy

A superhard diamond grinding can machine the hard and brittle freeform workpiece, but it is difficult to control the form-truing errors of curved wheel-profile in multiaxial grinding. Generally, it needs multiple compensation grindings through the on-machine measurement of the ground surface, leading to inefficiency. Hence, the model of 2D wheel-profile errors is proposed to directly compensate for 3D curved wheel errors in automatic grinding. This is because the wheel-profile form-truing errors mainly influence the ground freeform errors. The objective is to improve the freeform accuracy without inefficient measurement-to-compensation in the grinding system. First, the on-machine form-truing of the torus-shaped diamond wheel was employed in freeform grinding; then, the form-truing errors were systematically analyzed in connection to the ground freeform errors. It is shown that the curved form-truing errors may be parameterized by the dresser width, the wheel-profile width, and the wheel-profile in the case of large wheel positioning error. The simulated results also display that the freeform form errors decrease by increasing the wheel-profile curvature and decreasing the workpiece curvature in the case of same form-truing errors. Moreover, the simulated wheel-profile errors may directly be compensated to freeform grinding without any on-machine measurement.

Optimizing Sampling Parameters of CMM Data Acquisition for Machining Error Correction of Freeform Surfaces

Abstract An optimization study using the design of experiment technique is described, in which the surface profile height of a freeform surface, determined in coordinate measurements, is the response variable. The control factors are coordinate sampling parameters, i.e. the sampling grid size and the measuring tip diameter. As a result of the research, an optimal combination of these parameters was found for surface mapping with acceptable measurement uncertainty. The presented study is the first stage of optimization of machining error correction for the freeform surface and was intended to take into account mechanical-geometric filtration of surface irregularities caused by these geometrical parameters. The tests were carried out on a freeform workpiece milled with specific machining parameters, Ra of the surface roughness was 1.62 μm. The search for the optimal combination of parameters was conducted using Statistica software.

On the inverse design of discontinuous abrasive surface to lower friction-induced temperature in grinding: An example of engineered abrasive tools

In order to lower temperature, abrasive tools with passive-grinding, e.g. textured, areas (PGA) have been suggested. However, most of the reported PGA geometries (e.g. slots, holes) have been determined based on the engineering intuition (i.e. trial and error) rather than in-depth phenomenological analysis. To fill this gap, this paper proposes a method to design the PGA geometry according to the desired temperature, i.e. the inverse design method. In the method, the analytical model of grinding temperature for tools with PGA is established and treated as the primary constraint in the inverse problem, while the models of the ground surface roughness and grinding continuity as the subsidiary constraints. The method accuracy is validated by conducting grinding trials with tools with the calculated PGA geometries and comparing their performances (temperature, roughness and force fluctuation) to the required ones. In comparison with conventional tools, our tools designed by the method have been found effective to reduce harmful, or even destructive, thermal effects on the ground surfaces. This work might lay foundation for designing discontinuous abrasive tools, and future work can be probably extended to the tools or the workpiece with more complex shapes (e.g. ball end/cup tools, and free-form workpiece).

A practical approach for automated polishing system of free-form surface path generation based on industrial arm robot

In manufacturing environment where a robot arm is programmed to follow a specified path such as in polishing, the geometric coordinate transformation of an automated polishing system frames is needed for polishing workpiece surface. As the presence of kinematic singularities can extremely affect the robot’s performance, singularity regions in the task space for robot are clearly identified using determinant of the robot’s Jacobian matrix. Based on the free-form surface polishing requirements, programing an industrial robot with Teach Pendant manually is skill dependent and time consuming. Therefore, a scheme of an automatic polishing system is proposed, which includes a 6DOF arm robot. An automatic planning and programming system based on data from a CAD system is described to create robot paths. The robot program which contains the polishing path is generated in certain order using RoboGuide software in virtual environment. A Human Machine Interface has been used to control the entire polishing system online. Finally, the generated path is successfully applied in robotic polishing system. The experimental results prove that the proposed method is effective and feasible. The outcomes of this work contribute to enhancing the use of arm robot for polish free-form workpiece surfaces.

Optimal Workpiece Setup for Time-Efficient and Energy-Saving Five-Axis Machining of Freeform Surfaces

Energy consumption in five-axis machining of freeform surfaces can be considerably large for large-size parts. This paper presents a study on how to setup the workpiece in order to minimize the energy consumption without modifying the toolpath itself. For an arbitrary freeform workpiece, the way how it is setup on the working table highly affects the machine's kinematic behavior, which dominates the overall processing time and energy consumption. Taking into account the speed and acceleration limit of each axis of the machine, we first establish the energy consumption model as a function of the workpiece setup. However, this original model involves certain critical physically pertinent coefficients (such as the moment of inertial of a rotary table) which are usually unavailable in practice. Instead, by exploring insightful geometric characteristics of the five-axis machine, an alternative energy consumption model is established which is independent of those hard-to-obtain coefficients. A simple algorithm is then designed to optimize this model. Both computer simulations and physical cutting experiments demonstrate that, when compared with an arbitrary setup, the optimized workpiece setup is able to achieve a significant saving (as much as 50%) in both energy consumption and total machining time, both using a same tool path.

A controllable material removal strategy considering force-geometry model of belt grinding processes

Belt grinding is commonly used in the process of machining complex surface. However, due to the elasticity of the grinding belt, it needs repeated or longer dwell-time grinding in order to meet the required machining precision, which is inefficient, time-consuming, and always ended up with poor surface quality. So, this paper focuses on a machining method so as to improve machining efficiency and accuracy. First, considering the elastic deformation of the contact wheel and characteristics of the workpiece, the global and local material removal processes of belt grinding are modeled to calculate the acting force. Then, based on the analysis of rigid-flexible coupling, a controlling strategy is proposed to control the acting force and grinding dwell time. The variable feed grinding experiments were carried out on the developed five-axis CNC belt grinding machine integrated with measuring and machining. The ladder type workpiece surface and free-form workpiece surface were employed to validate the proposed controllable material removal strategy. The results verify that the proposed strategy is feasible and efficient.

A Study of Fiducial Aided Precision Positioning of Ultra-Precision Freeform Surfaces

The fabrication of ultra-precision freeform surfaces possessing non-rotationally symmetric geometry and sub-micrometre form accuracy requires an efficient positioning method for precisely locating the position of the workpiece on the machine with high repeatability during the manufacturing cycle. This paper presents an initial attempt to develop a fiducial aided positioning (FAP) method for precisely locating the freeform workpiece on multi-axis machine tools in order to improve the accuracy and efficiency in the manufacturing of ultra-precision freeform surfaces. The FAP method makes use of fiducials to establish intrinsic surface features of the freeform workpiece which are used to determine the coordinate transformation between the coordinate frame of the workpiece and the machine in the machining process and the measurement process. A preliminary simulation experiment was conducted to verify the technical feasibility of the proposed FAP method. The results show that the FAP method is technically feasible to establish a link between the machining and measurement process so as to improve machining accuracy.

Optimal conditions of SU-8 mask for micro-abrasive jet machining of 3-D freeform brittle materials

Micro-abrasive jet machining (AJM), also called micro-blasting, is a mainstream machining process that uses abrasive particles for difficult-to-cut workpieces such as glass, carbides, and ceramics. During the micro-blasting process, non-machined areas are covered by a protective mask. Today, either mask fabrication practice or micro-blasting process is well suited and optimized for producing micro-features on planar workpieces. However, the demand for micro-features on three-dimensional (3-D) freeform substrates in micro electro-mechanical systems (MEMS) and lab-on-a-chip devices requires more refined non-planar micro-manufacturing techniques. We focused on devising an appropriate photoresist mask required by micro-AJM processes on the surface of a 3-D freeform workpiece. Fundamental erosion mechanisms based on SU-8 mask properties (hardness, surface roughness, and thickness) were investigated. The optimal conditions were found at an ultraviolet (UV) energy of 12.0752 μJ/μm, focus ratio of 4.8341, and hard baking time of 8.4974 min. Under these settings, the mask hardness and surface roughness were 25.04 HV and 1.14 μm, respectively. The reliability of the fabricated mask was verified through a micro-AJM process. With existing plant conditions, the engraved microfeature dimensions on the surface of a 3-D freeform workpiece were 535.3 μm (width) and 11.6 μm (depth).

A Virtual Sculpting System Based on Triple Dexel Models with Haptics

AbstractThis paper presents the development of a Virtual Sculpting system and addresses the issues of interactive freeform solid modeling with haptic interface. The workpiece and the sculpting tool are both represented by triple-dexel models and a sculpting tool is controlled by a force-reflective input device. Virtual sculpting is implemented via a series of Boolean operations that computationally subtract/unite successive tool volumes from the workpiece. The haptic device PHANToM Omni is used as an input device to provide the position and orientation data of the sculpting tool, and it is also used as an output device to provide force feedback during sculpting. The freeform workpiece can be converted into triangular mesh models using a novel triple-dexel surface reconstruction method.

Experimental investigation of feed rate limitations on high speed milling aimed at industrial applications

This paper presents an experimental investigation on feed rate [mm/min] limitations when milling free form surfaces, commonly found in dies and moulds, applying the high speed cutting technology-HSC. The results appoint feed rate as the bottleneck to achieve the real benefits of HSC in terms of machining time, surface quality and process stability. That is due to unwanted large variations on the initially programmed feed rate, mainly when milling free form surfaces. These “ups” and “downs” on feed rate result in several setbacks for the machining process itself and it can be found even when using a suitable HSC milling machine available in the market. This paper addresses some of the causes for feed rate variations and evaluates an alternative approach to describe a tool path using spline polynomial technique. In order to focus industrial applications, all equipment, materials and software used are accessible in the market. The milling experiments were accomplished on a high-speed milling machine controlled by an open architecture CNC and a high-end CAD/CAM software was used. A 3D free form workpiece was designed and a real-time monitoring system was developed to investigate the feed rate variations during milling operation. The surface quality after milling and the machine tool/CNC performance were also assessed.