Research on NC Programming Technology for Sheet Metal


2.1 Current CNC programming technology
2.1.1 Manual programming
The manual programming process is essentially a comprehensive application process of CNC machine tool processing instructions, process foundations and patterns to be processed. In manual programming, the content that needs to be completed is as follows: ①Analysis and process processing of the part drawing. Through the analysis of the part drawing, the processing plan is determined, the appropriate CNC machine tools, fixtures, and cutting tools are selected, and the reasonable tool path and cutting parameters are determined. ② Mathematical processing. According to the geometrical size and processing route, the tool is calculated, and the tool position data is obtained by the motion trajectory trap of the center. ③Compile parts processing numerical control program, program inspection and make control medium. The general steps are shown in Figure 2.1.

Figure 2.1 Contents and steps of manual programming

Although manual programming can basically meet the requirements of use for the numerical control programming of two-dimensional processing objects, manual programming has a large labor scene, cumbersome calculations, low efficiency, and error-prone. For three-dimensional processing objects, manual programming cannot be completed at all. , So the current CNC machining generally adopts automatic programming technology.

2.1.2 CNC automatic programming
CNC automatic programming refers to the use of a computer to partially or completely complete manual programming. Although the content that needs to be completed remains unchanged, with the development of advanced manufacturing technology, computer technology, and network technology, the realization methods and structure of the CNC automatic programming system are constantly evolving to adapt to the development of its supporting platform. For now, the traditional structure of the CNC automatic programming system is shown in Figure 2.2. In order to realize generalized processing, the NCP system is generally divided into two parts: pre-processing and post-processing, which perform different functions respectively.

Figure 2.2 The traditional structure of CNC automatic programming system

The functions completed by the pre-processing include reading in geometry and process information, determining the path and parameters of the tool, calculating the tool path and generating a tool location file. In order to realize the universalization of the NCP system, the tool position file should be able to adapt to the requirements of various machine tool post-processing, that is, the tool position file format cannot carry any information about the machine tool, and the front-end is completely universal.

After the post-processing is completed, the NC program of the corresponding specific NC machine tool is generated from the tool location file. In realizing the generalization strategy of the NCP system, post-processing combines different machine tool files to generate CNC programs that adapt to different machine tool instruction formats.

Compared with manual programming, although the content of automatic programming remains the same, with the development of advanced manufacturing technology, computer technology, and network technology, the implementation and structure of CNC automatic programming systems are constantly evolving to adapt to its supporting platform. development of. Until today, a wide variety of programming systems with different functions have appeared. Such as: NC language programming system, conversational programming system, NC graphic programming system, etc. CNC language programming system
The CNC language programming system is the first to be developed, it is to use the CNC language to write the source program of the part processing. Compared with other types of automatic programming systems, it is the most widely used, most powerful, versatile and technologically mature system so far.

APT (Automatically Programmed Tool) is the abbreviation of Automatically Programmed Tool. It is a symbolic language close to English used when defining the geometry of the workpiece, the tool's geometry, and the tool's movement relative to the workpiece. The part processing program written in APT language is input into the computer, and the tool location file is compiled by the computer's APT language programming system, and then the CNC post-processing is performed to generate the part CNC processing program that the CNC system can accept is called APT language automatic programming. 2.3 is the principle and process of APT language automatic programming.

Figure 2.3 Principle and process of APT language automatic programming

The APT language has rich vocabulary and many types of definitions. It uses geometric definition statements to describe the shape of the processed object, tool motion statements to describe the movement state of the machine tool when processing the workpiece, and post-processing statements to describe the control of machine functions.

The use of APT language to compile CNC machining programs has the advantages of concise program and flexible tool movement control, which makes CNC machining programming from the "assembly language education" level for machine instructions to a high-level language for geometric elements (points, lines, and edges). level. Because the computer (or programming machine) automatic programming replaces the programmer to complete the tedious numerical calculation work, and saves the workload of writing a program list, the programming efficiency can be increased several to tens of times, and manual programming is solved. Many complex and even programming problems that cannot be solved in the APT series, such as the APT series can handle the milling of two-dimensional contours and three-dimensional curved surfaces, including multi-coordinate machining with tiltable tool axis (currently up to five coordinates), but APT is a huge The system is difficult to master, and the professional requirements for hardware and programmers are very high. However, with the development of advanced manufacturing technology, especially the application of flexible manufacturing system (FMS) and computer integrated manufacturing system (CIMS), higher requirements are put forward on automatic programming technology, and automatic programming using APT language has become very important. The shortcomings are mainly manifested in the following points:

(1) Difficult to apply. Although it uses high-level language and symbols to describe the parts, it still needs to manually complete the compilation of the APT source program, so the professional requirements for the operators are very high, which is difficult and error-prone.

(2) Poor integration. The use of drawings to transfer data between part design and processing cannot meet the requirements of CAD/CAPP/CAM integration. The computer only participates in calculations and cannot use process information. It cannot realize the automatic integration of process and NCP, which hinders the improvement of work efficiency.

(3) Poor versatility. The APT source program needs to be compiled by a dedicated programming machine into numerical control codes that can be used by the machine tool. Since the programming systems (code formats) used by the CNC machine tools produced by various manufacturers are not completely consistent, different types of machine tools need to be equipped with different types of CNC programming machines, and the CNC programming machines are expensive, which makes users not to consider when choosing CNC machine tools Choosing the same type of programming system makes the user's choice range too small and does not meet the requirements of openness (generalization). If a universal computer numerical control automatic programming system is used, one machine can be used for multiple purposes.

(4) The APT language lacks intuitionistic graphic display of stripping shapes, tool motion trajectories, and tool trajectory verification methods. Conversational automatic programming system
The conversational automatic programming system is developed on the basis of the numerical control language programming system. Figure 2.4 shows the conversational programming flowchart. Take Japan’s FAPT as an example. Its conversational programming system is basically the same as the original APT except for geometric definition statements and tool movement statements. Due to the addition of conversational commands, it can not only process the original APT part source program, but also It has the following functions: you can execute or suspend any statement or statement group in the program at any time; you can change the part source program at any time, such as deleting some statements, modifying or inserting some statements; Data such as straight lines can be used without being defined in future part source programs; the program sheet or an intermediate processing result can be printed or not printed at any time, such as the data of points, winning lines, circles, etc.: can be printed out and modified at any time The following part source program list. However, the conversational programming system has its shortcomings. The main reason is that there must be a process of converting the drawing information when inputting the part information. This conversion process is completed by the programmer, so this system is prone to human error and reduces Improve the efficiency of programming.

Figure 2.4 Flow chart of conversational processing Numerical control automatic programming system based on graphics
Numerical control graphics programming system is a kind of computer-aided programming technology, which is realized by special computer software. This kind of software is usually based on mechanical computer-aided design (CAD) software, and uses the graphic editing function of CAD software to draw the geometry of the part to the computer to form the graphic file of the part; then call the numerical control programming module and adopt human-computer interaction Specify the part to be processed on the computer screen, and then input the corresponding processing parameters, the computer can automatically perform the necessary mathematical processing and compile the CNC processing program, and dynamically display the tool's processing path on the computer screen. Because the programmer basically "copy" the part drawing to the computer without converting the graphic information, this method minimizes human error and maximizes the efficiency and quality of programming, which is called the most efficient Programming method.

More importantly, because the graphic programming system generates the CNC machining instruction sheet from the processed part drawing, the CAD system can be used to design the workpiece, and then the process used on the CNC machine tool can be generated by the CAPP because the result of the computer-aided design is full of graphics. Card, you can generate a CNC machining instruction sheet. Obviously, this programming method has the advantages of fast speed, high accuracy, intuitive use, easy to use, and easy to check. Therefore, "graphic interactive automatic programming" has become the universal numerical control used by advanced CAD/CAM software at home and abroad. Programming method.

Graphical interactive automatic programming is a brand-new programming method. Compared with manual programming and APT language programming, it has the following characteristics:
① This programming method does not require complicated mathematical calculations like manual programming to calculate the coordinate data of each node, nor does it like a numerical control programming language to write source programs that describe geometric shapes, machining passes, and post-processing. It is the geometric figure that faces the part directly on the computer. The result of its programming is also displayed on the computer graphically
on. This method has the advantages of simplicity, intuition, accuracy and convenience for inspection.

②Usually the automatic graphics programming software and the corresponding CAD software are organically linked together to become an integrated software system, which can be used for computer-aided design, and can also be used to call the designed part graphics for interactive programming to realize CAD/ CAM integration is extremely advantageous.

③The entire programming process of this programming method is interactive, instead of programming the source program in numerical control language in advance like APT language programming, and then the computer is carried out in batch processing to generate numerical control processing programs. This programming method is simple and easy to learn, and problems can be found and modified at any time during the programming process.

④In the process of programming, the extraction of graphic data, the calculation of nodes, the preparation and output of the program are all carried out automatically by the computer, so the programming speed is high, the efficiency is high, and the accuracy is good.

⑤ This kind of software is all carried out on general-purpose computers and does not require a dedicated programming machine, so it is very convenient for popularization. This kind of automatic programming method can compile the processing program of another part while the machine tool is processing one part. It is convenient and intuitive to use. It can be said that graphical interactive automatic programming is an advanced automatic programming technology and the development direction of automatic programming software. At present, advanced programming software at home and abroad has generally adopted this programming technology. Physical programming system
Physical programming is also called digital processing of non-size graphics. When there are models or physical objects but no size parts to be processed by CNC, a coordinate measuring machine can be equipped to measure the size of the model first, and the control computer of the measuring machine itself The data can be processed and the required NC machining program list can be output. There are also some CNC machine tools with a function called Teach and Playback. Each machine tool has a measuring sensor. According to the actual processing route, the size of the object or model is measured, stored in the CNC system, and processed after processing. Program, the subsequent parts can be processed according to the same route, which is physical programming. The characteristic of this system is that its solid model is generally not designed for CNC programming, or even designed for CNC programming. In order to be used for CNC programming, it is often necessary to analyze the machinability of the solid model and identify machining features (machining features). feature) (processing surface or processing area), and process planning for processing features, and finally NC programming. Each step may be very complicated and needs to be performed in a human-computer interaction mode. Voice programming system
Voice programming is a method of using human voice as input and direct dialogue with the computer and display device to make the computer output part processing programs. There are two main methods for this kind of automatic programming: one is to decompose the automatic programming language into each isolated vocabulary. Therefore, each isolated vocabulary can be input by voice. This method is simple and reliable, but the efficiency is low. The other is to replace the sentences of automatic programming language with a string of voices (that is, one sentence), input them into the computer, and use voice commands to execute the entire operation process. This method is highly efficient, but it is difficult to identify the voice.

Before programming, the system must be familiar with the voice of the programmer, that is, when the system is used for the first time, the programmer should speak the vocabulary and numbers specified by the system through the microphone, so that the system can record it and convert it into what the computer can accept Digital instructions. When programming, the programmer sends out various instructions to the microphone or answers the questions raised on the display screen, and then the instructions from the programmer are displayed on the screen, and the answers to the questions are checked or requested Send instructions, etc., as shown in Figure 2.5.

From the current speech recognition technology, when the operator first touches the speech system, he should train the system to be familiar with the operator's voice. For this reason, the operation skewer must input the words of the vocabulary into the computer through the microphone in advance to create a sample. The voice is mainly compared with the characteristics of the sample, and the voice characteristics and different vocabulary of different operators on the computer can be quickly changed according to needs.

Figure 2.5 System flow chart of voice programming Automatic programming system based on visual angle system
Vision system programming is to use a computer vision system to read and understand the drawings. The programmer gives out the starting point, the lowering point and the retreating point in real time during the editing process, and then automatically calculates the relevant coordinate values ​​of the tool position point, and Post-processing, an automatic programming method that finally outputs the numerical control processing program or perforated paper tape.

Figure 2.6 shows the system block diagram of vision system programming. First, scan the drawing with a drawing scanner (commonly used CCD sensor type scanner and scanning drum) to obtain an image, and preprocess the image to correct the geometric distortion and grayscale distortion of the image, and convert it into Easy-to-handle two-dimensional image, at the same time do fracture correction, geometric intersection part detection, and thin line processing to eliminate the impact of input resolution. Then separate and identify the text, symbols, lines and other elements on the drawing, and remember the relationship between them. The lines need to be vectorized and fitted with straight lines or curves to obtain end points and branch points. Then comprehensively process this information, determine the meaning of each line in the drawing and its size, and finally do the editing process and the calculation of the tool position point coordinate, and then connect the appropriate post-processing to output the CNC machining program list.

Figure 2.6 Flow chart of system programming from perspective

Visual system programming does not require part source programs. The programmer only needs to input the process parameters in advance. The operation is simple, and it can be directly connected with CAD data to achieve a high degree of automation.

2.2 The presentation of visualized interactive automatic programming technology 

Looking at the above-mentioned numerical control programming method, it can be seen that:

  1. Manual programming: its calculations are complex, error-prone, and only suitable for simple parts processing, which is an obsolete programming method.

  2. Automatic programming system based on APT language: APT uses language to describe geometric figure information, so in order to accurately describe geometric figures and processing processes, APT sentences have added many modifiers or restrictive words; at the same time, APT language is not Structured languages ​​make it difficult to write, debug, and maintain programs. The programmers who use this programming system must be specially trained. Although it has a very complete technology and powerful functions so far, it has occupied a place in many departments at home and abroad, especially in the automotive and aerospace industries. However, due to the high requirements for hardware and software matching, and the integration of various new technologies such as CAD/CAM With the development of chemical technology, it has also become an "expired star".
  3. Graphics-based CNC automatic programming system: graphics operations are intuitive, fast and accurate, and not easy to make mistakes. The user interface is friendly, the interactive function is strong, it avoids cumbersome and complicated calculations, the input of CNC language with strict format requirements, and reduces the level of factory programmers. In the CAD/CAM system, the resources of CAD can be shared, and the graphics generated by CAD can be used as the input of NC programming, which is the best way to realize the integration of CAD/CAM.
  4. Although the voice programming system is simple to input information, it reduces the efficiency of programming due to the difficulty of voice recognition. Although the physical programming can be interactively operated, the operation of each step is more complicated. It should be said that these two types of systems have their own advantages and disadvantages at the same time.
  5. Although the CNC automatic programming system based on the vision system does not need to write the source program, and does not need to manually calculate or input the computer for the own part drawings, but the processing system of this method is more complicated, and some technologies need to be perfected.
  Through the analysis and comparison of these systems, manual programming and APT programming should be regarded as the past methods, while the other methods have their own advantages and disadvantages. At the same time, they are also the development trend of numerical control programming. Therefore, this system is absorbing the advantages of these systems. Based on this, a technical model of visualization and interactive automatic programming based on graphic parameterization for sheet metal blanking parts is proposed.
  Its main idea and principle are: design a CAD aided design drawing module at the head end of the system, in the module structure design of all parts in the form of parametric input structure parameters to complete the drawing, the process information of each structure in the drawing Input the structural graphics together and store the geometry and process information of the structure in a specific database; since the CNC machining of sheet metal blanking is basically carried out on the unfolded sheet, the sheet metal parts need to generate unfolded drawings, but during the unfolding process All the structural parameters of the parts have been expanded calculations and re-stored, and future programming will be based on these data.

  After the unfolding diagram is generated, the CNC program generation module can be called, and the parts to be processed can be directly and interactively clicked on the graphics. The system will automatically perform mathematical processing, compile the program and store it in the database. In this operation, when you click the structure to be processed, the interface will display the machining tool parameters selected by the system, and the tool path can be previewed. This is the technical idea of ​​visualization and interactive automatic programming of sheet metal parts based on graphical parameterization.
  The advantage of this automatic programming technology is that it avoids the difficulty of data integration and data interface between the CAD subsystem and the CAM subsystem in the graphics-based automatic programming system, and at the same time has the characteristics of integration: the programming process is actually It is to use the mouse to directly and interactively click on the processing part on the unfolded drawing. At the same time, the graphics are visible on the interface, which reflects the visual and interactive characteristics of programming; during operation, you can see the tool parameters and parts processing from the top of the world. The geometric parameters of the structure and the tool simulation trajectory make the operation more confident, and the results of the operation can be seen in real time; the operation only needs to be clicked by the mouse, which is very convenient.