Research on the Algorithm of Tool Trace Automatic Generation in CNC Turning Process
Abstract
Computer numerical control (CNC) turning is an important part of the machining process. The trajectory of the tool is an important factor affecting the quality of machined parts, so the accuracy of the tool trajectory generation is very important. In current work, a trajectory generation method based on shape recognition is proposed to automatically generate tool trajectories for turning process. First, a plane graph-based topology model for describing the shape of the part is established. Secondly, several trajectories are generated and the boundary conditions are used to select appropriate trajectories. Finally, the optimized trajectory is obtained under the requirements of manufacturing conditions, material properties and tool properties.
Keywords: CNC turning process; tool trajectory; automatic generation; shape recognition; topology model
1 Introduction
Computer numerical control (CNC) technology is an important part of modern manufacturing technology, and CNC turning is an important part of CNC machining. The trajectory of the tool is the basic element of CNC machining, which affects the quality, size and geometric shape of the machined part. It is difficult to generate simple straight-line tool trajectories and curved surface arcs by programming. Moreover, it is easy to make mistakes, so it is necessary to develop a method to automatically generate tool trajectories.
At present, the research on automatic tool trajectory generation is mainly divided into two categories: data modeling based on feature point and contour recognition based on contour. The method based on data modeling is to acquire the process data of the machined part through reverse engineering. The tool path is calculated according to the process data, and the trajectory is generated. But this method can only work on geometric models which have a certain degree of symmetry. The contour recognition based on contour is to use special mathematical description of the shape of the machined part. The algorithm selects the appropriate motion path according to the actual shape of the part, and then the tool path is determined by the motion path, which can meet the need of machining parts with complex shape.
Based on the above understanding, a method for the automatic generation of tool trajectories based on shape recognition is proposed in the work. The tool trajectory is generated by using the topology model established by the plane graph of the part. The trajectory is optimized by considering the boundary conditions, such as the requirements of materials properties, manufacturing conditions and tool properties.
2 Tool trajectory automatic generation method
2.1 Shape recognition
The shape recognition step is to identify the geometric features of the workpiece, generate the topological model and establish a plane graph with vertices, edges and faces. The shape recognition algorithm is designed on the basis of vector graphic processing. The idea is to divide the graphic pattern into regions, classify them according to their types and characteristics, recognize each type and calculate the coordinate values, connection relations and special characteristics of the regions.
For convex graphic pattern recognition, the vector processing can be divided into three steps: graphic segmentation, graphic classification and spatial relation recognition. Graphic segmentation separates the entire processing graphic pattern into a number of regions. Graphic classification is to classify the region types, features and characteristics according to the characteristics of the regions. The recognition of the spatial relation determines the geometric relations between the regions.
2.2 Generation of tool trajectory
Tool trajectory generation includes four steps: pre-processing, motion path generation, points selection and path optimization. Pre-processing is to set necessary information for the processing, such as tool shape and feed direction. Motion path generation is to select several possible trajectories from the geometry model based on the boundary condition of the workpiece. Point selection is to determine the boundary points of the trajectory. Path optimization is to select the trajectory with the best machining efficiency from several possible trajectories.
2.2.1 Pre-processing
Before the automatic generation of tool trajectories, the necessary information for processing needs to be set up in advance. The necessary information for processing includes the basic parameters of machining tool, such as tool shape and feed direction.
2.2.2 Motion path generation
The motion path generation is to select several possible trajectories from the geometry model according to the boundary conditions of the machined parts. The motion path generation method is a relatively simple algorithm which is based on the recognition result of the topology model.
First, according to the geometry model, the motion path generation algorithm divides the machined parts into several straight line segments and circular arcs. Then, the length of the line segment and the radius of the circular arc can be calculated accordingly. Last, simplified motion paths are obtained by connecting all the straight line segments and circular arcs.
2.2.3 Points selection
In the automatic tool trajectory generation, it is necessary to determine the boundary points of the trajectory. After the motion path is calculated, it is necessary to select some key points from the path.The key point selection is to set key points on the machined parts with certain fixed distances according to the type of the machined parts and the length of the path.
2.2.4 Path optimization
Path optimization is to select the trajectory with the best machining efficiency from the candidate paths. In path optimization, the requirements of manufacturing conditions, material properties and tool properties are all considered.
3 Conclusion
In this paper, a trajectory generation method based on shape recognition is proposed to automatically generate tool trajectories for turning process. First, a plane graph-based topology model for describing the shape of the part is established. Next, several trajectories are generated and the boundary conditions are used to select appropriate trajectories. Finally, the optimized trajectory is obtained under the requirements of manufacturing conditions, material properties and tool properties. The proposed trajectory generation method can significantly reduce the time and effort spent on manual programming.
