This paper discusses the structural design process of the feed system, a key component of extended CNC machine tools, and proposes measures to improve transmission rigidity and positioning accuracy. Through theoretical analysis, it points out issues that should be considered during actual assembly, and the actual debugging results of the machine tool are consistent with the theoretical analysis results. This feed system has been successfully applied to CK6163×3000 and CK6180×5000 CNC machine tools. 1 Introduction The mechanical transmission structure of a CNC lathe feed system refers to the entire mechanical motion chain that converts the rotary motion of the electric motor into the reciprocating motion of the worktable or tool post, including the speed reduction transmission pair, the lead screw and nut pair, and their supporting components. To ensure the positioning accuracy and dynamic performance of the CNC machine tool feed system, high transmission rigidity, vibration resistance, low friction, low inertia, and zero backlash are required for its mechanical transmission device. In recent years, some domestic industries, such as valves, military, aerospace, and submarines, require large CNC lathes for machining parts to ensure high dimensional accuracy and curve contour requirements, which can solve the machining problems of large rotating parts. Large CNC lathes involve high cutting forces and long strokes, requiring careful consideration in the design of the feed system. Only by resolving issues related to transmission stiffness and positional accuracy can user requirements be met. The design of the feed system for extended CNC lathes with a headstock distance exceeding 2 meters mainly involves the selection of the feed motor, the selection and calculation of the ball screw, the choice of support type and bearings, and the calculation of positioning accuracy. The design of the feed system for the developed CK6163×3000 and CK6180×5000 CNC lathes incorporated a structure to improve transmission stiffness. During machine tool debugging, theoretical guidance led to the resolution of insufficient transmission stiffness in the Z-axis feed system encountered during assembly. Measures to improve transmission stiffness and ensure positioning accuracy were proposed, and the structure was improved, leading to the successful development of the CK6163×3000 and CK6180×5000 CNC lathes. This provides a theoretical basis and successful practical experience for the structural design of feed systems for extended CNC machine tools. 2. Transmission Stiffness of CNC Machine Tool Feed System The transmission stiffness of the feed system consists of two parts: the axial stiffness of the ball screw and the torsional stiffness of the feed system. The axial stiffness of the ball screw refers to the comprehensive tensile and compressive stiffness of the transmission system, including the ball screw nut and the bearing supporting the screw. It represents the ability of the ball screw and its supporting components to resist axial deformation. This includes the axial stiffness of the ball screw; the axial stiffness of the nut assembly; the axial stiffness of the supporting bearing; and the axial stiffness of the nut bracket and bearing bracket. The deformation of the nut assembly includes the deformation of the nut, the axial deformation caused by the fixing bolts of the nut, and the axial deformation caused by the elastic contact deformation between the balls and the raceway surface. The deformation of the nut is mainly considered in terms of the elastic contact deformation between the balls and the raceway surface; that is, the contact stiffness of the ball screw and the balls and raceway can be approximately regarded as the axial stiffness of the nut assembly. The contact between the balls and the raceway is point contact, and the relationship between deformation and load is non-linear; that is, the stiffness is related to the load and is not a constant value. To eliminate backlash and improve rigidity, a preload should be applied between the ball nut and the lead screw. As can be seen from the above analysis, the structural form of the feed system directly affects its transmission rigidity. 3. Structural Design of Feed System for CK6163×3000 and CK6180×5000 Machine Tools 3.1 Known Parameters of Feed System for CK6163×3000 and CK6180×5000 Machine Tools 3.1.1 Diameter and Accuracy of Lead Screw The working stroke of the CK6163×3000 machine tool is 3m, and the support span L=4280mm. The working stroke of the CK6180×5000 machine tool is 5m, and the support span L=6280mm. The target value of the effective stroke positioning accuracy of both machine tools is 0.03mm. A ball screw pair with P4 lead accuracy is adopted, and the ball screw from Shandong Jining Bote Precision Lead Screw Manufacturing Co., Ltd. is selected. Through the verification of the stability and critical speed of the pressure rod, the diameter of the lead screw for the CK6163×3000 machine tool is determined to be 63mm, and the model is GD6310-4. The lead screw diameter of the CK6180×5000 machine tool is 80mm, and the model is GD8010-4. 3.1.2 Lead Screw Pitch Although the motor and ball screw are directly connected by a coupling, which simplifies the structure, reduces noise, eliminates clearance and improves transmission rigidity, the CK6163×3000 and CK6180×5000 machine tools are limited by the maximum Z-axis travel speed, so the motor and lead screw cannot be directly connected. Instead, they are driven by a pair of toothed pulleys with a speed reduction ratio of 2, and the lead screw pitch is 10mm. 3.1.3 Servo Motor Model The feed system of the extended CNC machine tool is mainly determined based on the principle of matching the rotor inertia of the motor with the load inertia. The Z-axis motor of the CK6163×3000 and CK6180×5000 machine tools is model SIEMENS802D1FK6103, with a maximum output torque of 36 N·m, a maximum power of 11.3 kW, and a rotational inertia Jm = 121.5 × 10⁻⁴ kg·m². 3.2 Structural Features of the Feed System of the CK6163×3000 and CK6180×5000 Machine Tools To improve axial stiffness, the newly designed CK6163×3000 and CK6180×5000 machine tools adopt the structure shown in Figure 1. [align=center] Figure 1 Structure of the feed system of machine tool CK6163×3000 and CK6180×5000 1-Ball screw 2-Anti-loosening nut 3, 7, 10, 15-Spacer 4, 16-Radial ball bearing 5-Bearing seat 6-Feed box 8, 9, 18-Thrust bearing 11, 12-Sealing ring 13-Hanging angle 14-Disc spring 17-Bearing pad 19-Nut[/align] (1) Flexible coupling is used. Since the motor of the selected SIEMENS system is a medium inertia motor and the load inertia is large, a toothed pulley is used for speed reduction. The speed reduction ratio is 1/2, which improves the characteristics of the motor and amplifies the output torque of the motor. A flexible coupling is used to connect the motor shaft and the toothed pulley shaft. The toothed belt speed reduction avoids the dead zone error of the semi-closed loop servo feed system caused by the tooth side clearance when using gear speed reduction, and reduces noise. (2) The axial support method with fixed ends is selected. Compared with the support method with one end fixed and the other end free, this method can increase the axial stiffness by more than 1 times and the stiffness change is small throughout the entire working stroke, which is conducive to reducing the positioning error caused by the change of transmission stiffness. The axial pre-tension design structure is adopted. The pre-tension of the ball screw can compensate for the thermal deformation of the screw and improve the tensile and compressive stiffness of the screw. (3) Double support is adopted at both ends, namely, thrust bearing plus radial ball bearing to improve the axial stiffness of the bearing, so that the screw has the maximum stiffness and applies pre-tightening force. This structure can convert the thermal deformation of the screw into the pre-tightening force of the thrust bearing. However, the design requires to improve the load-bearing capacity of the thrust bearing and the stiffness of the support. The front and rear supports are arranged with reinforcing ribs in the direction of force to increase the stiffness of the support seat itself. (4) The double nut screw pre-tightening structure with large contact stiffness is selected to improve the axial stiffness of the nut assembly. When the axial load exceeds 3 times the pre-tightening force, the nut raceway and ball screw raceway on the unloaded side will disengage. The preload of the ball screw nut assembly should be greater than 1/3 of the maximum axial force. The screw is preloaded at the factory according to the user's requirements and does not require adjustment. 4. Measures to improve transmission rigidity and ensure positioning accuracy during assembly 4.1 Adjustment of the bearings at both ends of the ball screw The Z-axis feed screws of the CK6163×3000 and CK6180×5000 machine tools adopt a pre-tensioned structure. The adjustment process of the bearings at both ends is as follows: brackets 6 and 13 are fixed to the left and right ends of the lathe bed with screws, and the bearing seats are fixed to the feed housing 6 with screws. First, turn the anti-loosening nut 2, and the radial ball bearing and thrust bearing move to the right, while the thrust bearing 9 moves to the left. Continue to turn the nut 2 until the clearance between the thrust bearings 8 and 9 is completely eliminated. At this time, the ball screw 1 cannot move axially, which is called axial positioning. After the clearance of the left end support is adjusted, the right end is adjusted. The right end support has two features: automatic bearing clearance compensation and the ability to pre-tension the ball screw. Tightening nut 19 compresses the disc spring 14 axially. When the preload is less than the spring tension, the ball screw stretches to the right. The purpose of preload is to improve the axial stiffness of the ball screw, thereby ensuring the machining accuracy of the machine tool. When the ball screw elongates due to thermal expansion during operation or the thrust bearing experiences increased clearance due to wear, the spring automatically compensates, keeping the bearing preload constant. 4.2 Methods to ensure positioning accuracy during assembly The lead accuracy of the ball screw is the positioning accuracy located at the center of the ball screw axis, directly affecting the positioning accuracy of the feed system. Where positioning accuracy is required, the "posture changes during travel" will affect positioning accuracy due to the difference in height and width from the center of the ball screw. Among the "posture changes during travel," the pitch phenomenon (different in the height direction) and the yaw phenomenon (different in the width direction) have the greatest impact on positioning accuracy. A=lsinθ Where: A is the positioning accuracy caused by pitch and roll; l is the distance from the center of the lead screw in the height (width) direction; θ is the pitch (roll) angle. Therefore, in the actual assembly process, it is necessary to ensure the parallelism of the lead screw to the machine tool guide rail, that is, to ensure the parallelism of the lead screw to the machine tool guide rail in the horizontal plane and the vertical plane respectively. In the machine tool acceptance items, the national standard does not specify this item, but in order to avoid the positioning error caused by pitch and roll in the assembly process, we list the three points of the lead screw, two of which are the two ends of the lead screw support, namely the feed box end and the hanging corner end, and the other point is the position of the slide opening to the center of the lead screw (that is, the lead screw is parallel to the bed guide rail through the nut and the slide box at this point) as accuracy inspection items to ensure accuracy. The actual assembly process is as follows: (1) Fix the feed box component on the bed and make a table of its assembly datum upper generatrix and side generatrix. (2) Fix the lead screw to the slide plate via the nut and then the slide box. Similarly, perform a rough mapping of the upper generatrix and side generatrix of the lead screw. By scraping the positioning plane of the lead screw nut and the slide box, ensure that the height difference of the upper generatrix of the lead screw at both ends of the slide box is within 0.005mm. (3) Move the slide plate to the headstock and fix the lead screw to the feed box. Using the upper generatrix of the slide box as a reference, map the upper generatrix of the lead screw on the right side of the feed box. (4) Using the side generatrix of the lead screw on the feed box as a reference, move the slide plate to the middle position of the bed guide rail and adjust the position of the slide box relative to the slide plate to ensure that the side generatrix of the lead screw at the feed box and the slide box are consistent. After rotating the lead screw to ensure the runout of the lead screw, install a positioning pin at the connection between the feed box and the bed to ensure that the assembly accuracy remains unchanged. (5) Fix the lead screw at the hanging corner to ensure the runout of the upper generatrix, side generatrix and lead screw. Install a positioning pin between the hanging corner and the bed to ensure that the assembly accuracy remains unchanged. In order to improve the assembly accuracy and solve the problem of excessive lead screw length and difficult assembly, a high-precision standard rod is made to replace the lead screw for assembly according to the size of the positioning hole of the feed box and the lead screw seat hole. 4.3 Measures to improve transmission stiffness during assembly In the actual assembly and debugging process, when checking various accuracy items such as Z-axis positioning accuracy, repeatability positioning accuracy, and reverse deviation, it was found that the accuracy was out of tolerance, and the lead screw vibrated during rapid feed. When the dial indicator was set at the assembly reference of the front and rear supports of the lead screw, a runout of 0.005mm was found at the feed box. All of these indicate that the rigidity of the Z-axis feed system is insufficient. The support stiffness during the assembly process is also an important factor. The following measures were taken: (1) Reduce the thickness of the empty tool and avoid using narrow and thin pads. (2) The diameter of the four M12 fixing bolts of the fixed support remains unchanged, while the ordinary bolts are replaced with high-strength bolts; the two 10mm diameter taper pins of the positioning cone pins are changed to two 12mm diameter taper pins; at the same time, the fixing bolts of the nut seat and the slide are changed from four M10 to four M12. (3) The feed box and the mounting angle with the bed, the screw nut seat and the slide mating surfaces are re-scraped to achieve a certain number of scraping points, increase the contact area of ​​the mating surfaces, and improve the contact stiffness of the support bearing seat. 5 Conclusion For the Z-axis feed system of the extended CNC lathe, it can be seen from the actual assembly and machine tool debugging that the transmission stiffness of the Z-axis feed system has been greatly improved by improving the structure and improving the assembly quality, and the required positioning accuracy has been achieved. However, no matter how much it is stretched, the sag of the screw cannot be avoided. This causes the screw to vibrate during rapid movement, which prevents the specified rapid movement speed from being reached. The rapid traverse speed of the CK6163×3000 machine tool is 8 m/min, and that of the CK6180×5000 machine tool is 5 m/min. To increase the rapid traverse speed, a lead screw support device must be installed, especially for machine tools with a Z-axis travel exceeding 5 m.