Design of clamping and positioning scheme for the

Design of clamping and positioning scheme for machining large workpieces

1 Introduction in order to accurately process a batch of large workpieces used in key projects, it is necessary to design clamping and positioning schemes for workpieces. The workpiece weighs about 200 t (the main structural dimensions are shown in Figure 1), and the material of the machined part of the workpiece is zg55. It is required that the two surfaces to be machined are parallel to each other and perpendicular to the top surface (the maximum machining allowance on one side is only 5mm). The processing equipment is two five axis horizontal CNC milling machines with 300mm end milling cutters. Through careful analysis of the working conditions, we must first solve the following two problems: ① is it necessary to clamp such a heavy workpiece to ensure the stability of the workpiece during the machining process? Do you need special support devices? ② In order to ensure the machining requirements and make the two surfaces to be machined have machining allowance and are perpendicular to the top surface, the workpiece needs to be accurately positioned. Because the workpiece is very heavy, it is difficult to find the workpiece by position adjustment according to the conventional method, so it is necessary to find a simple and fast positioning method, that is, the workpiece can be accurately positioned by measuring and calculating no matter how the workpiece position is. By analyzing and calculating the stress state of workpiece processing and use, this paper puts forward a specific method to judge the support and clamping mode of large workpiece; Three dimensional measurement and coordinate transformation are used to determine the position relationship between the workpiece and the machine tool, so as to achieve the purpose of accurate positioning without adjusting the position of the workpiece. 2. The clamping scheme is designed to make the stress state of the workpiece during processing consistent with that during use. The workpiece is supported at four points according to the design requirements (as shown in Figure 2). In the figure, N1, N2, N3 and N4 are the supporting forces on the workpiece, FZ and FX are the cutting forces on the workpiece during processing, and the workpiece weight is g. FZ ≤ 5428.4n and FX ≤ 2285.6n are estimated according to the spindle power, cutting parameters and milling mode of the machine tool. In order to prevent the stability of the workpiece and the support point from being affected by virtual contact, N1, N2, N3 and N4 are required to be g/4. Under the above conditions, due to the action of cutting forces FZ and FX, the workpiece will mainly produce two movement trends that affect its stability, namely, overturning around the ad axis and moving in the ABCD plane. If the workpiece is only supported but not clamped, the stability of the workpiece during processing depends on the ability of the workpiece to resist overturning and movement by relying on its own weight g and friction F. Because the stress condition of the workpiece when machining on one side is worse than that when machining on both sides at the same time, this paper only analyzes and calculates the stability of the workpiece when machining on one side. Anti overturning ability of workpiece: the overturning torque around the ad axis generated by the cutting force FZ is m =fz × 1.875=5428.4 × 1.875=10178.25nm the anti overturning torque generated by the self weight of the workpiece is m anti =g × 3.98/2= × 1.99=nm shows the anti channeling ability of M anti m turnover parts: according to figure 2, the maximum channeling force caused by cutting force is f=fz+fx= (fz2+fx2) = (5428.42+2285.62) =5889.9n. If the friction coefficient between the workpiece and the support device is 0.3, the friction between the workpiece and the support device is f=g × 0.3= × 0.3=600000n, f f can be seen from the above analysis results: when milling this large workpiece, even if it is not clamped, the stability of the workpiece in the processing process can be reliably guaranteed only by the weight and friction of the workpiece, so the clamping mechanism can be ignored in the tooling design. In order to ensure that the supporting force of the four supporting points of the workpiece is equal, we adopt the CNC synchronous jacking system produced by Enerpac company of the United States. The system is composed of hydraulic jacking components, hydraulic pump station, control system and hydraulic pipelines connecting each unit, information (2) industrial energy efficiency water efficiency continuous improvement cable, power line, etc, The system not only enables us to understand the principle of 1 pulling machine through the following parts: the load on the hydraulic jacking part is used as the feedback signal for force synchronization control, but also the displacement of the jacking part is used as the feedback signal for lifting or lowering synchronization control. The use steps of the hydraulic system are as follows: pre jacking. Start the jacking system to make the hydraulic jacking parts contact the bottom surface of the workpiece. At the same time, use the force of each jacking as the feedback signal. When the force of each jacking reaches 20t, the jacking stops, but the system remains in working state. Synchronous jacking. The jacking displacement of the support is used as a feedback signal to control the synchronous jacking of each top (keeping the four supporting forces on the workpiece equal during the jacking process). After the workpiece is lifted away from the support platform and reaches the appropriate position, the position of the support top is locked with a mechanical self-locking device and the system is closed to ensure the stability of the workpiece position state during the processing process. Synchronous fall back. After processing, start the system and release the system self-locking device, control the support top to think about the problem based on the long-term interests, fall back synchronously, and gently place the workpiece on the support platform. 3. When the positioning scheme is designed to be processed by five axis CNC machine tool, it is difficult to use the traditional method of adjusting the position of the workpiece because the weight and structural size of the workpiece are relatively large. In order to achieve the purpose of accurate positioning without adjusting the position of the workpiece, the position relationship between the workpiece and the machine tool is determined through three-dimensional measurement and coordinate transformation. The specific procedures are as follows: set the measuring points, establish the workpiece coordinate system as shown in Figure 3, and set a, B, C, and D on the workpiece, with a total of four positioning measuring points, which are required to be the same distance from the top surface; Point E and point F are the midpoint of AB line and CD line respectively. The origin of the workpiece coordinate system coincides with point E, the X'axis of the workpiece coordinate system coincides with the EF line, and the Y' axis is perpendicular to the plane CDE, so as to ensure the correctness of the position of the workpiece coordinate system

Figure 3

measurement use a three coordinate measuring head to measure points a, B, C and D, and the machine coordinate values of points E and F can be obtained by calculating the machine coordinate values of points a, B, C and D. Let the measured values of points a, B, C and D in the machine coordinate system be a= (XA, ya, ZA), b= (XB, Yb, ZB), c= (XC, YC, ZC), d= (XD, YD, ZD), then the machine coordinate values of points E and F are e= (a+b)/2= ((xa+xb)/2, (ya+yb)/2, (za+zb)/2) f= (c+d)/2= ((x)