Hou Qinghai, Wang Yongjun, Wang Junbiao, Yang Xudong, Li Zhenqiang, and Zhu Li of the Key Laboratory of Modern Design and Integrated Manufacturing Technology, Ministry of Education, Northwestern Polytechnical University, studied the key technologies for converting ordinary lathes into CNC spinning machines. They proposed a conversion method and, taking the C630 lathe as an example, detailed the technical solutions and implementation steps for its mechanical modification design, hydraulic system design, and control system design, transforming it into a high-precision CNC spinning machine with force and displacement control functions based on upper and lower computer systems. This research provides valuable theoretical and methodological references for converting ordinary lathes into CNC spinning machines. 1 Introduction Spinning, as an important branch of metal plastic processing, has advantages such as good flexibility and low cost, making it suitable for processing various metal materials. It is an economical and rapid method for forming thin-walled rotating parts. Compared with other stamping processes, it can produce products of various shapes and sizes, especially when combined with efficient and precise CNC technology, giving it significant advantages. These characteristics have led to the widespread application of spinning technology in aviation, aerospace, rockets and missiles, weaponry, and other military and civilian industries. Domestic and international scholars have conducted extensive research on spinning technology and equipment. Eamonn QuiSley and John Monaghan analyzed the stress variation curves during simple ordinary spinning and multi-pass ordinary spinning processes. Xia Qinxiang et al. developed the XPD series of CNC spinning machine tools, detailing the characteristics of its mechanical parts, drive unit, and control system. For the spinning of teapot-shaped parts, a conventional lathe was modified into a dedicated ordinary spinning machine tool, and the design of the cutter head and diameter reduction components was analyzed. CNC spinning machine tools can not only reduce labor intensity and improve the production efficiency and yield of spun products, but also form high-precision, complex-shaped spun products. However, due to the high price of CNC spinning equipment, many domestic manufacturers still use conventional lathes and manual control methods for spinning, resulting in outdated spinning equipment and lower forming quality of spun parts. Converting a conventional lathe into a CNC spinning machine offers advantages such as low investment, stable control, and suitability for specialized spinning technologies. However, this conversion involves key technologies including strength and rigidity design, mechanical structure design, hydraulic system design, CNC system design, and its installation and commissioning. Currently, there are few reports in the literature on converting conventional lathes into CNC spinning machines. This paper proposes a method for converting a conventional lathe into a CNC spinning machine and uses the conversion of a C630 lathe as an example to study its key technologies, transforming the C630 conventional lathe into a CNC spinning machine with precise force and displacement control functions. [IMG=Figure 1 Schematic diagram of spinning principle]/uploadpic/THESIS/2007/11/2007111414512649598A.jpg[/IMG] Figure 1 Schematic diagram of spinning principle [IMG=Figure 2 Schematic diagram of the overall concept of converting a C630 lathe into a CNC spinning machine]/uploadpic/THESIS/2007/11/20071114145137157328.jpg[/IMG] Figure 2 Schematic diagram of the overall concept of converting a C630 lathe into a CNC spinning machine 2 Spinning principle and overall concept of CNC conversion Spinning is a plastic processing method that uses spinning tools to apply pressure to a rotating blank, causing it to undergo continuous local plastic deformation to form the desired rotating part. Typically, a flat metal blank or pre-made blank is first clamped onto the spinning die of a spinning machine. The spindle drives the spinning die and the blank to rotate, producing continuous, point-by-point plastic deformation, thereby obtaining hollow rotating parts with various generatrice shapes. The spinning principle is shown in Figure 1. Converting a conventional lathe into a CNC spinning machine requires analysis of the machine bed's strength, design of mechanical components such as the fixing of the spindle and spinning die, and the fixing device for the spinning wheels, as well as research on the closed-loop control of the spindle rotation and spinning wheel transmission device. The overall approach to converting a C630 lathe into a CNC spinning machine (as shown in Figure 2) is as follows: To improve accuracy, the mechanical and electrical components of the C630 lathe are overhauled; the spinning wheel and spinning wheel frame are modified using the lathe body, transverse guide rails, longitudinal guide rails, spindle rotation device, and three-jaw chuck, and the transverse and longitudinal guide rails are converted to hydraulic drives, and the tail section is converted to a hydraulic device; the original main drive system and speed change mechanism are retained, and a frequency converter is added to achieve stepless speed change of the spindle rotation; the control system adopts an advanced proportional servo valve and a CNC system based on upper and lower computer systems. 3 Research on Key Technologies for Converting a Conventional Lathe into a CNC Spinning Machine 3.1 Analysis and Modification Design of Mechanical Components 3.1.1 Bed and Guide Rails A simplified structural model of the entire bed of the C630 lathe is established, and the structural strength is analyzed using the finite element analysis software ANSYS. First, a simplified model of the machine tool was established in ANSYS, and material properties and constraints were defined. Then, triangular elements were used to mesh the machine tool model. Finally, finite element analysis was performed in Static mode to obtain the stress-strain diagram when a 30kN force was applied in both the transverse and longitudinal directions (as shown in Figure 3). The sum of the displacements in the X, Y, and Z directions was 0.091mm, the maximum stress was 24.26MPa, and the maximum strain was 0.132e-3. The tensile strength of cast iron is around 200MPa, and the stress value is much lower than the bed limit, indicating that the bed and guide rail of the C630 lathe meet the requirements. Considering the safety factor and actual modification needs, 30kN hydraulic cylinders were selected for the transverse and longitudinal directions, and a 15kN hydraulic cylinder was selected for the tail cylinder. 3.1.2 Design of the spinning head, spinning die, and spinning wheel The C630 lathe originally had an annular dovetail groove on the carriage for fixing the base of the lathe tool post. During the C630 lathe modification process, as shown in Figure 4, a spinning wheel base was designed. It is connected to the annular dovetail groove on the slide by four fastening bolts, enabling the spinning wheel base to rotate 360°, facilitating the spinning of complex parts such as conical and flared shapes. The spinning wheel frame is mounted on the spinning wheel base. To prevent the spinning wheel from moving axially on the axle, a pair of tapered roller bearings are installed inside the spinning wheel and secured with a pressure cap. This allows for the replacement of the spinning wheel and spinning wheel frame without needing to replace the spinning wheel base during different spinning processes, making it economical and practical. The shape of the spinning die can be designed according to the shape requirements of the part, and the positioning and clamping of the spinning die and the positioning of the workpiece must be fully considered. The design and manufacturing of the spinning wheel has a significant impact on spinning forming, mainly including the design of working angles such as the guide angle γ and the forming angle α. The spinning wheel is manufactured using materials such as GCrl5, 9CrSi, and CrWMn, and the quenching hardness must reach HRC55～62. [IMG=Figure 3 Finite Element Analysis Results]/uploadpic/THESIS/2007/11/2007111414520234487Z.jpg[/IMG] Figure 3 Finite Element Analysis Results [IMG=Figure 4 Schematic Diagram of Spinner Base Design]/uploadpic/THESIS/2007/11/2007111414521274884J.jpg[/IMG] Figure 4 Schematic Diagram of Spinner Base Design 3.2 Hydraulic System Design As shown in Figure 5, the lateral and longitudinal cylinders drive the lateral and longitudinal movements of the spinner, and the tail cylinder is used to clamp the workpiece. The lateral and longitudinal cylinders are controlled by their respective proportional servo valves, and the tail cylinder is controlled by a solenoid valve. In the hydraulic system design process, through detailed calculations of hydraulic cylinder diameter, proportional valve flow rate, hydraulic pump power, and movement speed, and comprehensively considering the requirements of space size, weight, rigidity, cost, and sealing, appropriate hydraulic components are selected. 3.3 Control System Design The modified CNC spinning machine has two control modes: manual and automatic. The control system consists of a host industrial computer and a slave PLC. The host computer's spinning wheel motion trajectory parameter control program is independently developed using VC++, and data sharing between the host industrial computer and the slave PLC is achieved through an RS232 interface. The PLC mainly consists of a CPU unit, a digital input/output unit, an analog input/output unit, and a high-speed counter unit, as shown in Figure 6. The X-axis and Y-axis displacements are controlled by proportional servo valves, the movement of the tailstock device is controlled by a solenoid valve, and the spindle speed is controlled by a frequency converter, with closed-loop control achieved through an angular displacement sensor. As shown in Figure 7, the closed-loop control process for the X-axis and Y-axis displacements is as follows: Given a target displacement value, the actual displacement value detected by the grating ruler displacement sensor is compared with the feedback information from the counter count to obtain a difference. Through a PID control algorithm, the proportional servo valve opening control voltage is output, which controls the movement of the hydraulic cylinder. X-axis and Y-axis pressure detection is achieved through pressure sensors. The process is the same as displacement control, and the force is closed-loop controlled through proportional servo valves. 4. Modification steps and effects The modification steps are as follows: (1) Determine the modification scheme and propose a modification plan; (2) Analyze and calculate the structural strength of the lathe, and determine the maximum turning force and stroke; (3) Overhaul the lathe to improve the accuracy of the spindle, guide rail and slide; (4) Design the hydraulic system and select hydraulic components; (5) Measure and calculate the mechanical parts of the machine tool, and design and manufacture parts; (6) Install and debug the mechanical parts; (7) Design the control system and select components; (8) Install the control system hardware and write the control software; (9) System integration and debugging; (10) Acceptance inspection. [IMG=Figure 5 Hydraulic Design Schematic Diagram]/uploadpic/THESIS/2007/11/20071114145227858482A.jpg[/IMG] Figure 5 Hydraulic Design Schematic Diagram [IMG=Figure 6 CNC Spinning Machine Control System Logic Diagram]/uploadpic/THESIS/2007/11/2007111414524811702N.jpg[/IMG] Figure 6 CNC Spinning Machine Control System Logic Diagram [IMG=Figure 7 CNC Spinning Machine X-axis and Y-axis Displacement Closed-Loop Control Logic Diagram]/uploadpic/THESIS/2007/11/2007111414525614811X.jpg[/IMG] Figure 7 CNC Spinning Machine X-axis and Y-axis Displacement Closed-Loop Control Logic Diagram During the modification process, the following matters should be noted: (1) Under the condition of meeting the accuracy requirements, the original mechanical and transmission systems of the lathe should be utilized as much as possible; (2) When overhauling the lathe, it should be dismantled in accordance with the CNC modification design drawings, and the drawings should be marked in time to prevent omissions and over-dismantling; (3) During the installation and debugging process, simulation should be performed first and actual operation should be performed later, and no load should be applied first and then load applied. The debugging results should be recorded at any time so as to find and solve problems; (4) After the spinning wheel is installed, the runout of the spinning wheel should be measured with a dial indicator. If the runout is greater than 0.03mm, the screws of the bearing cover should be adjusted to adjust the runout to within the allowable range; (5) The transmission error of the machine tool should be eliminated as much as possible according to the recorded installation and debugging results; (6) Trial processing can be carried out only after all debugging is normal. If there are no errors, formal processing can be carried out. Through the above steps, the CNC spinning machine modified from the C630 lathe operates stably with high control precision, achieving a relatively ideal modification effect. Its performance parameters are as follows: maximum machining blank diameter is 600mm; the lateral travel of the spinning head is 300mm, the longitudinal travel is 500mm, and it can rotate 360° in the plane; the maximum lateral spinning force is 30kN, and the longitudinal spinning force is 30kN; the tailstock travel is 400mm, and the maximum tailstock force is 15kN; the spindle speed can be continuously variable between 26 and 1600 r/min. The spindle motor power is 10kW, the coolant pump power is 500W, and the hydraulic pump power is 4kW; the displacement control precision can reach 5/1000mm, and the force control precision can reach 1/1000N. 5. Conclusion This paper analyzes the principles and steps of converting a conventional lathe into a CNC spinning machine, studies in detail the key technologies involved in the conversion process, and takes the C630 lathe as an example to convert it into a high-precision CNC spinning machine with force and displacement control based on a master-slave control mode. This has been applied to teaching and research, creating favorable conditions for the study of spinning mechanism and achieving good practical results. The conversion process comprehensively considered the requirements of control accuracy and economic practicality, saving funds. The conversion method has promotional value and provides valuable theoretical and methodological references for converting conventional lathes into CNC spinning machines. (Proceedings of the 2nd and 3rd Servo and Motion Control Forums)