Early studies on robot machining were reported in the 1990s. Even though there are continuous worldwide researches on robot machining ever since, the potential of robot applications in machining has yet to be realized. In this paper, the authors will first look into recent development of robot machining. Such development can be roughly categorized into researches on robot machining system development, robot machining path planning, vibration/chatter analysis including path tracking and compensation, dynamic, or stiffness modeling. These researches will obviously improve the accuracy and efficiency of robot machining and provide useful references for developing robot machining systems for tasks once thought to only be capable by CNC machines. In order to advance the technology of robot machining to the next level so that more practical and competitive systems could be developed, the authors suggest that future researches on robot machining should also focus on robot machining efficiency analysis, stiffness map-based path planning, robotic arm link optimization, planning, and scheduling for a line of machining robots.

https://link.springer.com/content/pdf/10.1007%2Fs00170-012-4433-4.pdf

Robot machining: recent development and future research issues

The redundancy of a 6R industrial robot is resolved for five-axis milling applications. This paper describes a post-processor focused algorithm that is adequate to numerically solve the redundancy with multiple constraints. Some critical constraints for cutting process, such as singularity, joint limits and collision are considered. The corresponding criterion functions are designed for the optimization procedure. Those constraints are synthesized as the multi-objective function to be minimized. A example of tool path testing the functionality of the optimization algorithm is presented. The optimization process is implemented into a 3D simulation system for the cutting robot. Some test pieces are cut to validate the feasibility of the algorithm.

Redundancy and optimization of a 6R robot for five-axis milling applications: singularity, joint limits and collision

In the era of advanced, intelligent, and flexible manufacturing, machining with industrial robots, it is expected to be set up in the next few years. This is due to the vast progress of these robots in terms of precision and stiffness. Moreover, there is a recent development of off-line programming. Consequently, industrial robots offer a real gain of modularity, flexibility, and access for machining on production lines, and are viable solutions to improve the productivity. At the same time, all the potentials of CAD/CAM solutions available for machining off-line programming are not fully exploited because of old G-code (ISO 6963, 1980) language that only enables the description of elementary actions and tools moves. In the context of Industry 4.0 and the development of smart equipments at all levels of production chain, the manufacturing digital thread has to be profoundly reconsidered in order to guarantee high-level information from the design to the manufacturing. In this paper, a solution is proposed based on the high-level programming STEP-NC standard combined with a suitable CAD/CAM solution for industrial robots machining. A complete platform has been developed to enable advanced and intelligent manufacturing with the possibility to integrate several modules of simulation, optimization, and visualization, as well as in-process fabrication adaptation, cloud manufacturing, and machine learning with database analytics.

A STEP-NC compliant robotic machining platform for advanced manufacturing

Industrial robots are used in a great variety of applications for handling, welding and milling operations. They represent a cost-saving and flexible alternative for machining applications. A reduced pose and path accuracy, especially under process force load due to the high mechanical compliance, restrict the use of industrial robots for further machining applications. Test results showed that these deviations range up to 0.6 mm. In this paper, a method is presented to determine the resulting path deviation of the robot under process force by using a structured light scanner. The obtained data is compared with the CAD (Computer Aided Design) data of the machined part within a developed software module. Additionally, the developed module provides functions for manipulation, registration of STL (Surface Tessellation Language) surface point clouds and a postprocessor for program translation. The comparison is performed using a dexel discretization of each data set. Based on this comparison the robot path is adapted to improve the machining quality. This method can be applied to 3- and 5-axis machining operations. The results show that the deviation can be reduced to 0.1 mm.

Tool path adaption based on optical measurement data for milling with industrial robots

Purpose – The robot offers interesting capabilities, but suffers from a lack of stiffness. The proposed solution is to introduce redundancies for the overall improvement of different capabilities. The management of redundancy associated with the definition of a set of kinematic, mechanical and stiffness criteria enables path planning to be optimized. Design/methodology/approach – The resolution method is based on the projection onto the kernel of the Jacobian matrix of the gradient of an objective function constructed by aggregating kinematic, mechanical and stiffness weighted criteria. Optimized redundancy management is applied to the 11-DoF (degrees of freedom) cells to provide an efficient placement of turntable and track. The final part presents the improvement of the various criteria applied to both 9-DoF and 11-DoF robotic cells. Findings – The first application concerns the optimized placement of a turntable and a linear track using 11-DoF architecture. Improved criteria for two 9-DoF robotic cells...

Redundancy-based optimization approach to optimize robotic cell behaviour: application to robotic machining

For a 6R milling robot, it is necessary to convert the postprocessing cutter locations (CL) into the robot’s revolute joint variables. This paper introduces an algorithm for calculating the forward and inverse kinematics of a 6R robot according to the CL data generated by conventional CAD/CAM systems. A redundant mechanism is analyzed to avoid the singular configurations and joint limits. The Denavit–Hartenberg (D–H) convention is referred to for developing the forward kinematics, and a closed-form solution of the inverse kinematics is presented by means of kinematic decoupling. A fundamental approach with modifying factor for avoiding singularity are developed with regard to three-axis and five-axis CL data. A gap bridging strategy is applied to reduce the jerk motion caused by tool retraction and cut paths connection. Finally, the result is conducted to simulation and machining test to verify the algorithms.

Closed-form inverse kinematics of 6R milling robot with singularity avoidance

Drilling of wood is besides milling, sawing and grinding one of the most important production processes in industrial furniture manufacturing. For the body assembly and the fixing of fittings, boreholes are essential. They have immediate effect on quality and production costs. The process step drilling limits the output of stationary and throughfeed machines, because the workpieces have to be stopped to create the drillings at a relatively low feed speed. An essentially increased performance cannot be reached by the improvement of conventional drilling devices. The aim of the work presented in this paper is the development of an alternative production process in which blind holes can be punched. Thereby thin-walled, cylinder-shaped tools are pressed into different solid and derived timber workpieces by a simple translative movement. For it special experimental setups have been designed. The new production process opens possibilities for an optimization and acceleration of process cycles in industrial wood machining.

Punching in industrial wood machining: an alternative production process to drilling

With rising feed rates of wood working machines, it is important to realize a process-integrated quality assurance. The quality control of produced workpieces is mainly carried out by the operator. As his capabilities are limited, an automated quality control system can help to assure a constant quality level. Due to the high production speeds, only an optical measurement can be used. While CCD-camera-based control systems are generally applied in wood working, no approach has been made so far to measure the surface roughness which is caused by the machining of the workpiece. This paper presents two different quality control systems for an assessment of the surface quality of a workpiece. In planing of solid wood, the size of the waves on the surface is most significant for workpiece quality. An image processing system has been developed which measures the width of the waves by means of a special illumination. In machining chipboards, the size and number of cracks in the edge are very important. Here, a camera-based system is presented which is able to measure the complete workpiece edge even at high feed rates with a sufficient resolution.

Process-integrated quality assurance in wood machining centers with the help of image processing

In recent years parallel kinematic machines for wood machining have come into use more frequently. Despite first promising prototypes, these machines are single solutions for specific applications. To meet the requirements of shorter product life cycles and higher product diversity, high flexibility is demanded of the machining system. This paper presents a new wood machining center obtaining both, the reduction of the primary and secondary processing times. The machine concept, based on a parallel kinematic structure, allows high operating speeds and accelerations not only for workpiece machining but also for handling. Thus, the machine can be used without any external handling devices. The kinematic structure originates from a plane closed five-bar chain with two linear drives and additional drive axes for stroke and rotation. In order to increase the useable workspace a continuous motion between different assembly modes is realized. To guarantee a high feed rate and to minimize set-up times, an optimized dust exhaustion is included.

Torsten Loohß

Papers

Closed-form inverse kinematics of 6R milling robot with singularity avoidance

Punching in industrial wood machining: an alternative production process to drilling

Process-integrated quality assurance in wood machining centers with the help of image processing

Parallel kinematic concept for stationary high performance cutting in wood machining centers