Abstract: This paper applies CNC technology to the hemming of stamped parts, studies the processing principle of CNC hemming, analyzes the differences between CNC hemming and traditional spinning hemming, and introduces the composition and application of the developed CNC hemming system. Keywords: CNC; hemming; stepper motor; open-loop control Currently, the hemming of stamped parts is basically completed by dies, such as deep drawing or extrusion dies. With the development of the market economy, rapid product updates and diversification have become key factors for enterprises to stand firm in the market. Users have put forward higher requirements for products. Enterprises can only survive by adapting more to market demands. This requires enterprises to expand the variety of products, improve product quality, and shorten product development and delivery time to achieve multi-variety, small-batch production. To meet the needs of different users, it is difficult to fundamentally solve the problem using mold processing. This is mainly reflected in the following three aspects: (1) The mold processing cycle is long, suitable for the production of large-scale standardized products, and the investment recovery rate of small-batch production is not high; (2) The mold processing has poor adaptability. A set of molds can generally only be used to process one type of product. If the size of the product changes, the mold must be redesigned and processed, which will undoubtedly increase the production cost; (3) The mold processing and maintenance process is complicated. It is necessary to repair the mold frequently to ensure the processing accuracy. The adjustability is poor. CNC machining technology is currently the focus of the development of the manufacturing industry. Using this technology, there is no need to make too many changes to the hardware. Only the software parameters need to be adjusted appropriately for different types of products to obtain satisfactory results. It greatly improves the flexibility of production. Therefore, applying CNC machining technology to the edge forming of stamped parts can overcome the shortcomings of mold processing and meet the difficulty of future single-piece and multi-piece production. 1 Processing Principle Figure 1 shows the outline curve of a workpiece with arbitrary shape. Its curve equation is y=f(x). In order to be suitable for the system's processing control calculation, it needs to be converted into a polar coordinate equation. The origin of the coordinate system is taken at the rotation center of the workpiece. The point farthest from the rotation center (point 1 in the figure) is taken as the starting point. The transformation equation is obtained by using equation (1) to obtain the polar coordinates θ and ρ(θ) of each point on the curve. The workpiece's outer shape development curve is shown in Figure 2. In the figure, ρmax is the polar radius of the point farthest from the rotation center on the workpiece's outer shape curve, and ρmin is the radius of the nearest point. In order to complete the edge rolling of the above-mentioned curved workpiece, the feed amount D(θ) of the rotary wheel starting from (point I) should satisfy the following equation: ρ(θ) is the polar radius of the point on the workpiece's outer shape curve. Generally speaking, it is difficult to accurately represent the outer shape curve of such a workpiece with theoretical formulas. In this paper, cubic spline function fitting is used. The boundary conditions are determined by periodic functions (closed curves). That is, the further obtained rotary wheel feed amount curve is shown in Figure 3. The process of using the rotary wheel feed amount curve to solve the motion trajectory curve of the rotary wheel rotation center is shown in Figure 4. Arbitrarily select the workpiece's outer shape curve, with point O as the workpiece rotation center and the rotary wheel radius as R. From the forming point A, take a point O1 at a distance R from point A along the outer normal direction. Connect O1. The intersection of O, O1 and the outer contour curve is B, which is the feed point. That is, when the workpiece rotates by an angle β to point B, the rotary wheel feeds, and the forming point will be at point A. The solution process for point B is to obtain the difference between the coordinates Δx and Δy of point A and point O1. In the formula: ρ() is the slope of the tangent line at point A on the outer contour curve. The polar angle of point B is ρ(b), which can be obtained from the equation of the outer contour curve. The length of O1. It is obtained from the following formula: In the formula: IOO1max is the distance to the point O1 farthest from point O, at which point B coincides with point A; ρmax is the polar radius of point B corresponding to the farthest O1. The expression for the trajectory curve of the rotating wheel's rotation center is the equation for the rotating wheel's feed curve. Using F(t), f(θ) and f(θ) can be obtained. The speed control curve F(t) of the stepper motor can be obtained from equation (9), where PL is the pulse equivalent of the stepper motor; ω is the rotational speed of the workpiece. Using the curve F(t), the corresponding stepper motor control pulse command can be compiled, and the stepper motor can be driven to rotate by the driver, while the rotating wheel is controlled to complete the forming feed according to the above feed curve f(θ). The curves f(θ) and f(θ) are solved by cubic spline interpolation, and then the F(t) curve is obtained. There must be numerical calculation errors. In this paper, the results calculated by the theoretical curve are compared with the results obtained by this algorithm, and the error is very small. This shows that the algorithm is feasible. For simplicity, 6 sets of data from 0 to 5 are selected for comparison. The results are shown in Table 1 and Table 2. (The major semi-axis of the ellipse is 90 mm, and the minor semi-axis is 70 mm.) 2. Comparison of CNC Hemming Process and Traditional Spin Hemming Process Figure 5 compares the CNC hemming process and the traditional spin hemming process. As shown in Figure 5, the forming mechanism of the two processes is consistent; both employ continuous local plastic forming technology, maintaining a relatively constant plate thickness during the forming process. During operation, the spinning wheel feeds into near-point contact with the blank. When the stress on the blank at the contact point with the spinning wheel exceeds its deformation resistance, local plastic deformation occurs. With the continuous feeding of the spinning wheel and its relative rotation with the workpiece, the deformation zone expands from point to line, and from line to surface, ultimately achieving the desired shape. This continuous local plastic forming deformation zone is very small; therefore, compared to ordinary forming processes, the forming force is very small, only a fraction of the overall stamping forming force, or even a hundredth. There are also significant differences between the two processes: in traditional spin hemming, the spinning wheel only performs a unidirectional continuous feed motion, while in CNC hemming, the spinning wheel must coordinate with the rotation of the workpiece to perform a rapid reciprocating feed motion. Therefore, the traditional spinning process is only suitable for processing axisymmetric rotating parts. For parts such as ovals, plum blossoms, and square parts with rounded corners, it is impossible to process them. CNC edge rolling is not only suitable for edge rolling of the above-mentioned shapes, but also suitable for edge rolling of many stamped parts with more complex shapes. 3 System architecture of CNC edge rolling system The system architecture of the developed CNC edge rolling forming system is shown in Figure 6. It mainly consists of three parts: (1) Edge rolling forming mechanism, which is driven by a stepper motor through a single ball screw to feed the mechanism, and driven by an AC motor through a worm gear to rotate the workpiece; (2) PC486 microcomputer control system, which adopts a general microcomputer interface circuit; (3) Online position detection component, pressure sensor and its signal processing circuit. The system adopts a single-axis open-loop control mode. In the forming process, the process parameters are first calculated: the shape requirements and the amount of curling of the formed part are determined to determine the outer dimensions of the blank. Then, the shape, size, amount of curling, and single-piece feed of the blank are input into the computer using a disk file or manually. The computer sends control signals to the stepper motor driver through the interface circuit to drive the stepper motor to rotate according to the required direction and speed. The ball screw connected to the stepper motor drives the rotation feed simultaneously. The workpiece mounted on the reducer output shaft rotates at a constant speed; the photoelectric circuit breaker on the reducer output shaft detects the start signal of each feed. It feeds back to the microcomputer to determine the initial feed of the rotating wheel and compensate for accumulated errors. In this way, by controlling the stepper motor to rotate according to the predetermined speed curve, the forming mechanism (rotating wheel) is driven to feed, and the workpiece rotates to complete the curling forming process. 4. Control Software Interface To simplify the control operation of the entire CNC curling forming process in practical applications, the author developed a Windows-like interface control software in C++ under the DOS environment. It uses a color graphic Chinese character drop-down menu and provides both mouse and keyboard operation modes. Its main interface is shown in Figure 7, and its functional block diagram is shown in Figure 8. In addition to the main control module, this software also provides many auxiliary function modules: help, data preparation, data lookup, forming simulation, etc. Data preparation offers three interactive methods to input machining curves. Data lookup can output the workpiece's shape curve and corresponding rotary feed speed and acceleration curves on the screen or printer, facilitating verification of the data preparation results. Forming simulation offers static and dynamic modes. Static simulation provides a direct comparison between the theoretical feed speed curve and the actual realized curve. Dynamic simulation vividly displays the interaction between the rotary wheel and the workpiece. The help section is essentially a user manual for the software. It can be used to find solutions to problems encountered online. The entire control software is simple, easy to learn, easy to operate, runs stably and reliably, has a user-friendly interface, strong fault tolerance, and performs well in practical applications. 5. Application Scope of the CNC Hemming System This system can not only process workpieces with shape curves that have theoretical expressions, but also process other workpieces that cannot be accurately represented by theoretical formulas using the above methods. Furthermore, the modification process is very convenient; only some characteristic parameters of the workpiece shape need to be input during the initial data preparation stage. This function, which can be controlled by a microcomputer to change the shape of the workpiece being processed, greatly expands the application range of this system and makes the system highly flexible. In addition, by making appropriate adjustments to the structure of the rotary wheel, it can also complete the forming process of stamped parts that can be completed by other processes such as flanging, necking, flaring, and shaping. Figure 9 shows several typical workpiece curves designed and processed by the system: (a) standard ellipse, (b) pseudo-ellipse, (c) pseudo-plum blossom, (d) polygon with rounded corner transition. These curves have been made into standard drawings and stored in the system. During processing, only their characteristic values ​​need to be input. 6 Conclusion The CNC forming system can perform forming of flat stamped parts with arbitrary curves using a low-precision forming rotary wheel. If the rotary wheel wears, it can be automatically compensated by direct online detection of the workpiece. The significant advantages of this technology are simple processing equipment structure, low investment, low processing cost, and good flexibility. Based on the CNC edge forming technology, further improving the system developed in this paper, enhancing the system's accuracy, reliability, and adaptability, and striving to apply it to production practice, developing a CNC edge forming machine with commercial value will be the focus of the next step. References [1] Liu Yadong. Development and accuracy analysis of microcomputer CNC edge forming equipment [D]. Xi'an: Xi'an Jiaotong University. 1999 [2] Ma Ze'en. Computer-aided plastic forming. [M] Xi'an: Northwestern Polytechnical University Press, 1995 [3] Zhang Jianmin. Mechatronics system design [M]. Beijing: Beijing Institute of Technology Press, 1996 [4] Takadhi. Stepping Motors and their Microprocess or Controis [M]. Oxford: CLEADON Press