Freeform Surface Grinding Research Articles

Offline programming method and implementation of industrial robot grinding based on VTK

Purpose The purpose of this paper is to present a robotic off-line programming method for freeform surface grinding based on visualization toolkit (VTK). Nowadays, manual grinding and traditional robot on-line programming are difficult to ensure the surface grinding accuracy, thus off-line programming is gradually used in grinding, however, several problems are needed to be resolved which include: off-programming environment depends on the third-party CAD software, leads to insufficient self-development flexibility; single support for robot type or workpiece model format contributes to lack of versatility; grinding point data depends on external data calculation and import process, causes human-computer interaction deterioration. Design/methodology/approach In this method, the visualization pipeline and observer/command mode of VTK are used to display the 3D model of the robot grinding system and pick up the workpiece surfaces to be grinded respectively. Two groups of cutter planes with equidistant spacing are created to form the grinding nodes on the surface, and the extraction method for the position and posture of the nodes is proposed. Furthermore, the position and posture of discretized points along the grinding curve are obtained by B-spline curve interpolation and quaternion spherical linear interpolation respectively. Finally, the motion simulation is realized by robot inverse kinematics. Findings Through a watch case grinding experiment, the results show that the proposed method based on VTK can achieving high precision grinding effect, which is obviously better than traditional method. Originality/value The proposed method is universal which does not depend on the specific forms of surface, and all calculations in simulation are completed within the system, avoiding tedious external data calculation and import process. The grinding trajectory can be generated only by the mouse picking operation without relying on the other third-party CAD software.

Machine learning–based robust trajectory tracking control for FSGR

Here, a robust adaptive trajectory tracking algorithm is proposed for free-form surface grinding robot (FSGR) in metal surface production line. Machine-learning method is used for robot dynamic approximation which is hard to obtain directly. Adaptive law is proposed to adjust the neural network parameters. Sliding-mode control is employed to deal with the disturbance, joint friction, and approximation error of the adaptive machine learning. The scheme based on machine-learning feedforward compensation can significantly reduce the chattering of sliding mode. The performance of the proposed control scheme is illustrated through simulations.

A Robust Adaptive Trajectory Tracking Algorithm Using SMC and Machine Learning for FFSGRs with Actuator Dead Zones

The actuator dead zone of free-form surface grinding robots (FFSGRs) is very common in the grinding process and has a great impact on the grinding quality of a workpiece. In this paper, an improved trajectory tracking algorithm for an FFSGR with an asymmetric actuator dead zone was proposed with consideration of friction forces, model uncertainties, and external disturbances. The presented control algorithm was based on the machine learning and sliding mode control (SMC) methods. The control compensator used neural networks to estimate the actuator’s dead zone and eliminate its effects. The robust SMC compensator acted as an auxiliary controller to guarantee the system’s stability and robustness under circumstances with model uncertainties, approximation errors, and friction forces. The stability of the closed-loop system and the asymptotic convergence of tracking errors were evaluated using Lyapunov theory. The simulation results showed that the dead zone’s non-linearity can be estimated correctly, and satisfactory trajectory tracking performance can be obtained in this way, since the influences of the actuator’s dead zone were eliminated. The convergence time of the system was reduced from 1.1 to 0.8 s, and the maximum steady-state error was reduced from 0.06 to 0.015 rad. In the grinding experiment, the joint steady-state error decreased by 21%, which proves the feasibility and effectiveness of the proposed control method.

Design of material removal rate to reduce machining time of dental crown

Freeform surface grinding is one of the commonly used dental medical manufacturing processes. In dentistry machining, zirconia is one of main workpieces used for artificial teeth. Machining process for grinding of the ceramic usually takes a long time. In order to reduce this production time, it is critical to apply proper grinding parameters while maintaining high quality of machined artificial teeth. Required material removal rate (MRR) for workpiece can be used to determine the grinding parameters. However, it has been hard to find grinding parameters achieving a proper MRR of complex 3D models like as human teeth. In this study, MRR is predicted by using geometric data in the engagement region between machining bur and its workpiece while the MRR varies all along the tool path. Simulations of the machining time shows that the machining time can be reduced by adjusting the bur’s feed rate to maintain the predicted MRR.

5-axis grinding path generation for free-form surface based on plane instantaneous grinding engagements

Grinding is an important process to remove the material efficiently. Before the finish machining, it is necessary to remove the cusps by the wider grinding band for obtaining more uniform allowance. In this paper, the grinding wheel hold by the multi-axis machine tool is introduced into the free-form surface grinding. The engagement between the wheel and the to-be-ground surface is established for generating the paths. An instantaneous engagement model is proposed in terms of machine tool kinematic chain, and the spatial engagement is represented on a group of planes. Based on the engagement model, the grinding path generation for enlarging bands is transformed into an optimization. Different with traditional path generation, a slight local overcut is allowed on the wheel locations. However, the envelope of grinding engagements is limited in the required boundaries to satisfy the design tolerance. They are realized by adjusting the contact locations and orientations of the wheel. At last, a compressor blade is used to verify the advantages about the proposed path generation method.

3D distribution of envelope trace heights related to ground surface smoothness in CNC grinding of free-form surface

The dispersed calculation of envelope trace heights was proposed in CNC grinding of free-form surface. The aim is to understand how 3D distribution of envelope trace heights affects ground surface smoothness. First, the 3D tool path surface was constructed to plan 3D tool paths, and then the dispersed envelope trace heights were calculated using the interference section between adjacent wheel profiles. Finally, a CNC grinding experiment was carried out to investigate the relation between envelope trace height distribution and ground surface smoothness. The results show that the dispersed calculation of envelope trace height can be used to display its distribution on 3D ground curve surface. It is confirmed that the envelope trace height depends on the slope of ground curve surface. The smooth curve surface can be gradually generated when the envelope trace height is less than 2 μm.

Form-truing error compensation of diamond grinding wheel in CNC envelope grinding of free-form surface

In computer numerical control (CNC) grinding of free-form surface, an ideal arc profile of trued diamond grinding wheel is generally employed to plan 3D tool paths, whereas its form-truing errors greatly influence the ground form accuracy. A form-truing error compensation approach is proposed by using an approached wheel arc profile to replace the previously designed ideal one. The objective is to directly compensate the trued wheel arc-profile errors. It may avoid the time consumption of traditional approach that compensates the measured coordinate point errors of workpiece to an iterative grinding operation. First, the 3D tool path surface was constructed to plan the 3D tool paths. Second, the CNC arc truing of grinding wheel was conducted to analyze the form-truing error distribution relative to the applied wheel arc profile. Then, the form-truing error compensation was carried out in CNC envelope grinding. Finally, the iterative closest point (ICP) algorithm was used to match the measured coordinate points of workpiece to ideal free-form surface. It is shown that the 3D tool path surface constructed is practicable to plan arbitrary 3D tool paths for the form-truing error compensation. The ICP matching may be used to investigate 3D ground form error distribution. It is confirmed that the form-truing error compensation can directly improve the 3D ground form accuracy. It may decrease the 3D ground form error by about 20% when the 2D form-truing error is reduced by about 58% using the same truing conditions for CNC grinding.

An experimental study on mirror-form grinding of free-form surface by the use of coarse diamond grinding wheel

In this paper, the mirror grinding of free-form surface was conducted by the use of coarse diamond grinding wheel. In this new grinding manner, the wheel moving direction is vertical to the wheel cutting direction, which is different from the traditional plunge grinding and traverse grinding. The aim is to realise mirror-form grinding without the polishing with loose abrasive grain and ultra-fine diamond grinding wheel. First, the mirror grinding experiment for high-speed steel was undertaken by means of plane grinding to investigate the formation of mirror ground surface and identify the mirror grinding parameters. Second, the arc-shaped diamond wheel trued by CNC arc interpolation was employed to realise the form grinding of free-form surface whose tool path is controlled by the equal-envelope height on CNC grinder. Finally, the mirror-form grinding of free-form surface was carried out with reference to the mirror grinding conditions. Experiment results showed that the ductile-mode grinding may be conducted and the mirror-form grinding of free-form surface can be realised when decreasing the wheel-axial-orientation moving speed, whose surface roughness Ra is less than 20 nm.