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Key points of CNC lathe programming

In NC turning, the program runs through the entire part. Because each person's processing methods are different, and the processing procedures are different, but the ultimate goal is to improve the production efficiency of CNC lathes, so it is particularly important to choose the most reasonable processing route. In this paper, starting from the details of determining the cutting path and selecting the appropriate G command, this paper analyzes the programming method in NC turning. I. Analysis of part drawing Analysis of part drawing is the first task in process preparation, which directly affects the preparation and processing results of parts. It mainly includes the following items: Analyze the geometric conditions of the machining contour: the main purpose is to deal with the unclear size and closed dimension chain on the drawing. Analyze the dimensional tolerance requirements on part drawings to determine the processing technology that controls their dimensional accuracy, such as the selection of tools and the determination of cutting consumption. Analysis of shape and position tolerance requirements: For NC cutting, the shape and position errors of parts are mainly affected by the accuracy of the machine's mechanical motion pairs. In turning, if the direction of movement along the Z coordinate axis is not flat with its main axis, the shape tolerance requirement of cylindricity cannot be guaranteed; if the direction of movement along the X coordinate axis is not perpendicular to its main axis, it cannot be guaranteed to be vertical This position tolerance is required. Therefore, it is necessary to consider the relevant schemes for technical processing before programming. Analyze the surface roughness requirements of parts, material and heat treatment requirements, blank requirements, and number of pieces. These are also parameters that cannot be ignored. 2. Reasonably determine the cutting route and make it the shortest. The work of programming is the focus of programming. Because the cutting route of the finishing cutting program is basically performed along the contour of its parts, the main content is to determine Route for roughing and empty stroke. The tool path generally refers to the path that the tool moves from the point where the tool is set until it returns to that point and ends the machining program. Including cutting path and non-cutting idle stroke such as tool introduction and cutting out. Minimizing the cutting path can save the execution time of the entire machining process, and can also reduce unnecessary tool consumption and wear of sliding parts of the machine's feed mechanism. Figure 1 below shows three methods of car cones, which are processed with rectangular cycle commands to analyze the reasonable determination of the cutting path. Figure 1a is the parallel turning cone method. In this method, after each infeed, the turning path of the turning tool is parallel to the cone busbar. With each infeed, the Z-phase size increases by a certain proportion, which is the same as that of the normal turning machining cone. The same body method makes it easy for beginners to understand. The calculation method of the Z direction dimension is obtained according to the formula C = D-d / L. If C is 1:10, it means that 1 mm is removed from the diameter X and 10 mm is added to the length Z. It can be easily programmed according to this ratio, and it can ensure the same allowance for each turning to make the cutting uniform. Figure 1b is the method of changing the taper angle and turning the cone. With each X-direction infeed, keep the Z dimension to the drawing size, and each cutter changes the size of the taper angle. This car cone method does not need to calculate the Z dimension every time, but in the process, because the Z dimension is the same, the processing route is longer, and the cutting allowance is uneven, which affects the surface size and roughness of the workpiece. It is generally suitable In a cone with a short cone surface and a small margin. Figure 1c is the step processing cone method. In this processing method, each small trajectory is parallel to the axis of the workpiece, and many small steps are processed. The last tool is to move the tool along the inclined surface of the cone. This processing method To make a 1: 1 scale drawing, otherwise, the easy-to-turn waste workpiece is step-shaped, so the margin is uneven, which affects the quality of taper surface processing. Obviously, among the above three cutting routes, if the starting point is the same, the parallel French car cone route is the most reasonable, and this method is commonly used in production for processing. 3. Reasonably call the G command to make the program segment at least compile a corresponding processing program according to each individual geometric element (ie, straight line, oblique line, arc, etc.), and each program constituting the processing program is a program segment. In the preparation of processing programs, we always hope that the parts can be processed with the minimum number of program segments, so that the program is simple, the probability of errors is reduced, and the efficiency of programming is improved. As CNC lathe devices generally have the functions of linear and circular interpolation operations, in addition to non-circular curves, the number of blocks can be obtained from the geometric elements constituting the part and each program determined by the process route. Least Principle. Choosing a reasonable G command can reduce the number of program segments, but it also needs to take into account the shortest path. For example, if the parts in Figure 1 are processed, if the blanks are bar stock, you can use linear interpolation command G01 to program, rectangular cycle command G90, and compound cycle command G71 to program the workpiece. . As shown in Figure 2 below, Figure 2a is the path determined by the G01 command, which is the same as the route determined by the G90 command in Figure 2b. However, when G01 is used, programming is complicated and there are many program segments, which are often used in finishing programs. Figure 2c is a G71 type machining route. First, the rectangular cycle feed route is followed. The last two passes of the contour are equidistant and final contour lines. The path is not very long, and the cutting amount is the same. The cutting force is uniform. The combination of commands can also make programming simple and commonly used in programming. If the CNC lathe used does not have this command, the G90 momentary cycle command should be selected for programming first. Therefore, it should be applied flexibly in programming, and reasonable G commands should be used for programming. For the processing of non-curve trajectories, the required number of main blocks must be calculated only after the machining accuracy is guaranteed. At this time, a non-circular curve should be divided into several main program segments (mostly straight lines or arcs) according to the approximation principle. When the accuracy requirements can be met, the number of divided main program segments should be minimal. In this way, not only the calculation workload can be greatly reduced, but also the input time and the number of occupied memory capacity can be reduced. 4. Reasonably arrange the "return to zero" route. When compiling processing programs for more complex contours, in order to make the calculation process as simple as possible, it is not easy to make mistakes and is easy to verify. The programmer sometimes passes the end point of the tool after each tool is processed. "Zero" instruction (that is, return to the tool setting point), make it all return to the tool setting point position, and then execute the subsequent program. This will increase the cutting distance and reduce production efficiency. Therefore, when reasonably arranging the "return to zero" route, the distance between the end point of the previous knife and the start point of the next knife should be made as short as possible, or zero, that is, the shortest path of the knife is met. Fifth, reasonable selection of cutting amount The cutting amount in NC turning is an important parameter that indicates the main motion and feed motion of the machine tool body, including cutting depth, spindle speed, and feed speed. Their choices are basically the same as those required by ordinary lathes, but the parts processed by CNC lathes are often more complicated. After the cutting amount is initially determined according to certain principles, it should also be adjusted at any time in accordance with the actual machining conditions of the parts. Various override switches on the operation panel can be adjusted at any time to achieve a reasonable configuration of cutting amount, which should have certain actual production and processing experience for the operator. 6. Handling of details in programming 1. Note the reasonable use of G04 G04 is a pause command, and its role is to temporarily stop machining within a command time. This instruction is often ignored because it does not perform the actual cutting motion. However, it has good benefits in terms of ensuring machining accuracy and changing the movement of grooving and drilling. It is often used in the following situations: (1) In order to ensure the groove bottom and hole bottom when grooving and drilling, The size and roughness should be set with G04 commands. (2) When the running direction changes greatly, G04 command should be set between the commands for changing the running direction. (3) When the running speed changes greatly, the G04 command should be set when its running command changes. (4) G04 is used for cutting. According to the cutting requirements of roughing, it can be arranged in segments with continuous motion trajectories, and each adjacent processing segment is separated by G04. During processing, after each step of the tool feed, a short delay time (0.5 seconds) is set to implement a pause, followed by a step of feed until the end of processing. The number of segments depends on the cutting requirements. When the cutting is not ideal, increase the number of segments. 2. Separate programming for roughing and finishing In order to improve the accuracy of the part and ensure production efficiency, the last tool for turning the contour of the workpiece is usually completed by the finishing turning tool. Therefore, the roughing and finishing should be programmed separately. In addition, the advance and retreat positions of the tools must be considered properly. Try not to cut in or out or change the tool and stop in a continuous contour, so as to avoid elastic deformation due to sudden changes in cutting force, resulting in scratches on smooth connected contours. Defects such as abrupt shapes or stuck knife marks. 3. When programming, often take the median value of the required size of the part as the basis for the programming size. If you encounter a value smaller than the minimum programming unit specified by the machine tool, you should try to move closer to its largest solid size and round it. If the drawing size is 80 + 00, 026, write X80.013 during programming. 4. When programming, try to comply with the principle of overlapping points. In other words, the origin of the programming should be coincided with the design reference and the position of the tool setting point as much as possible to reduce the processing error caused by the misalignment of the reference. In many cases, if the size reference on the drawing is not the same as the size reference required for programming, you should first convert each reference size on the drawing to a size in the programming coordinate system. When it is necessary to master the allowable variation of some important dimensions, it must be obtained through the solution of the dimensional chain, and then the next programming can be performed. 5, using a cutting knife to chamfer. For parts with a chamfer on the cutting surface, it is more common in batch turning. In order to facilitate cutting and avoid U-turn chamfering, it is possible to use a cutter to complete both chamfering and cutting processes at the same time. At the same time, the cutter has two tool tips. In programming, we must pay attention to which tool tip and tool width to prevent errors during tool setting. In short, the general principles of programming for CNC lathes are rough first, fine first, advanced after far, internal and external, minimum program segments, and shortest tool path. This requires us to pay special attention to the combination of theory and practice when programming. In practice, verify or revise what you have learned to make the procedures most practical

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