Although CAM programming is quite common nowadays, manual programming will still have a wide application market for a period of time, so the CNC turning programming skills described in this article are still very practical. The principles, instructions, techniques and precautions listed in the article are all obtained and verified by the author through practice, so they are of practical value.
With the rapid development of science and technology, society has higher and higher requirements for the structure, performance, precision, efficiency and variety of mechanical products, and the proportion of single-piece and medium- and small-sized products is increasing. Traditional general-purpose, special-purpose machine tools and process equipment have not been well adapted to the requirements of high quality, high efficiency, and diverse processing. As a combination of electronic information technology and traditional mechanical processing technology, CNC machine tools integrate modern multi-disciplinary technologies such as precision machinery, computer, communication, hydraulic pneumatics and optoelectronics, effectively solving complex, precise and small batches of variable parts. Processing problems can meet the requirements of high-quality, high-efficiency and multi-variety, small-volume flexible production methods, adapt to the needs of rapid replacement of various mechanical products, and represent the trend and trend of today's machining technology. Among them, CNC lathes have become widely used in the mechanical manufacturing industry due to their high efficiency, high precision and high flexibility. They have become one of the most widely used CNC machine tools. However, in order to give full play to the role of CNC lathes, the key is programming, that is, according to the characteristics and accuracy requirements of different parts, a reasonable and efficient machining program is prepared. Commonly used numerical control programming methods are manual programming and automatic programming. Manual programming refers to the programming process that is mainly done manually from the part pattern analysis process processing, data calculation, writing program list, input program to program verification. It is suitable for the processing of point machining or parts with less complex geometry, as well as the calculation is simple, the program is not much, and the programming is easy to implement. For parts with complex geometries and parts that are not complex in geometry but require a lot of programming, it is difficult to do with manual programming, so automatic programming is required. The author takes the FANUC0-TD system as an example to discuss some of the problems of manual programming techniques in the machining of CNC lathe parts.
First, the correct choice of program origin
In CNC turning programming, first select a point on the workpiece as the origin of the NC program, and use this as the origin to establish a workpiece coordinate system. Reasonable determination of the workpiece coordinate system is important for NC programming and workpiece alignment during machining. The selection of the program origin should be as simple as possible, such as simple programming, small size conversion, and small processing error. In order to improve the machining accuracy of parts and facilitate calculation and programming, we usually set the program origin to the intersection of the workpiece axis and the front end face, the rear end face and the front end face of the workpiece. Try to make the programming reference coincide with the design and assembly standards.
Second, reasonable choice of feed route
The feed route is the trajectory of the tool in the whole machining process, that is, the tool starts from the feed motion to the tool point, and the tool returns to the point and the path that passes after the machining program is finished, which is one of the important basis for writing the program. Reasonable selection of the feed route is important for CNC machining. The following aspects should be considered:
1. Minimize the feed route, reduce the idle travel, and improve production efficiency.
(1) Use the starting point. For example, in the cycle machining, according to the actual machining condition of the workpiece, the starting point and the tool setting point are separated. Under the premise of ensuring safety and meeting the needs of the tool change, the starting point is as close as possible to the workpiece, and the idler stroke is reduced. Shorten the feed route and save execution time during processing.
(2) When programming the complex contour machining program, make the “zero return†route reasonably, so that the distance between the end point of the previous knife and the starting point of the next knife is as short as possible, or zero, to shorten the feed route and improve production efficiency. .
(3) When roughing or semi-finishing, the remaining amount of blank is large, and the appropriate cyclic processing method should be adopted. Under the requirements of the rigidity and processing technology of the machined parts, the shortest cutting feed route should be adopted to reduce the empty space. Travel time, increase production efficiency and reduce tool wear.
2. Ensure the accuracy and surface roughness of the machined parts.
(1) Reasonably select the starting point, the cutting point and the cutting method to ensure that the cutting process is stable and there is no impact. In order to ensure the roughness requirements after the contour surface of the workpiece is machined, the final contour should be arranged continuously in the last pass during finishing. Carefully consider the cutting and cutting path of the tool, and minimize the stopping of the knife at the contour to avoid the elastic deformation of the cutting force and the blade mark. Generally, it should be cut in and out along the tangential direction of the surface of the part. Try to avoid cutting and retracting the workpiece along the vertical direction of the workpiece.
(2) Select the route where the workpiece is less deformed after machining. For slender parts or thin-plate parts, the tool should be machined to the final size in several passes, or the feed path should be arranged with a symmetrical margin. When determining the axial movement dimension, the introduction length and the overrun length of the tool should be considered.
(3) The processing procedure of “pre-fine and rough†is adopted for special parts. In some special cases, the processing steps are not considered according to the principle of “first near and then far†and “first coarse and then fineâ€, but the special treatment of “pre-fine and coarse†can better ensure the dimensional tolerance of the workpiece. .
3. Guarantee the safety of the processing
Avoid interference between the tool and the non-machined surface and avoid collision of the tool with the workpiece. If the workpiece needs to be machined in the groove, it should be noted that the advance and retraction points should be perpendicular to the groove direction during programming, and the feed speed cannot be “G0â€. The “G0†command should avoid “X, Z†moving at the same time when retracting.
4. Conducive to simplify numerical calculation, reduce the number of blocks and the amount of programming work
In the actual production operation, a certain fixed machining operation is often repeated, and this part of the operation can be written into a subroutine, which is stored in the memory in advance, and can be called at any time as needed to make the programming easier and faster. For those series of parts with the same graphics, different sizes or the same process path, but different position data, you can use macro programming to reduce or even eliminate the cumbersome numerical calculation during programming and reduce the amount of program.
Third, accurately grasp the processing characteristics of various cycle cutting instructions and its impact on the machining accuracy of the workpiece, and make reasonable selection.
In the FANUC0-TD numerical control system, there are more than ten kinds of cutting cycle machining instructions for CNC lathes. Each type of command has its own processing characteristics. The machining accuracy after machining is different. The programming methods are different. We are choosing When you want to carefully analyze, use it reasonably, and strive to process parts with high precision. There are two machining instructions for thread cutting cycle machining: G92 straight cutting and G76 diagonal cutting. Due to the difference in the cutting mode of the cutting tool, the two machining methods are different, and the respective programming methods are different, resulting in different machining errors, and the machining accuracy of the thread segments after the workpiece machining is also different. The G92 thread cutting cycle uses a straight feed method for thread cutting. The thread diameter error is large. However, the accuracy of the tooth shape is high, and it is generally used for the processing of high-precision thread with small pitch. The machining program is long and should be measured frequently during machining; the G76 thread cutting cycle uses the oblique feed method for thread cutting. The shape accuracy is poor. However, the process is reasonable and the programming efficiency is high. This machining method is generally suitable for machining large pitch and low precision threads. This processing method is simpler and more convenient when the thread precision is not high. Therefore, we must master the respective processing characteristics and scope of application, and select these cutting cycle instructions according to the machining characteristics of the workpiece and the precision required by the workpiece. For example, if you want to machine high-precision, large-pitch threads, you can use G92 and G76 mixed methods, that is, use G76 for roughing the thread and then G92 for finishing. It should be noted that the starting point for rough finishing should be the same to prevent the occurrence of thread buckles.
Fourth, flexible use of special G code to ensure the processing quality and accuracy of parts
1. Return to reference point G28, G29 command
The reference point is a fixed point on the machine that can be easily moved to this position by the reference point return function. The reference point is mainly used as an automatic tool change or setting coordinate system. Whether the tool can accurately return to the reference point is an important indicator to measure the accuracy of its repeated positioning, and it is also a prerequisite for CNC machining to ensure its dimensional consistency. In actual machining, the use of the return reference point command can be used to improve the accuracy of the product. For machines with high repeatability, in order to ensure the machining accuracy of the main dimensions, the tool can return to the reference point and re-run to the machining position before machining the main dimensions. The purpose of this approach is actually to re-check the benchmark to determine the dimensional accuracy of the process.
2. Delay G04 command
Delay G04 command, its role is to artificially limit the running of the machining program, in addition to the common general use, in the actual CNC machining, the delay G04 command can also make some special use:
(1) In the processing of parts with short processing time of large batches, the start button is frequently used. In order to reduce the operator's malfunction due to fatigue or frequent buttons, the G04 command is used instead of the start of the first part. The delay time is set according to the loading and unloading time of one part. After the operator skillfully masters the NC machining program, the delay command time can be gradually shortened, but it must be guaranteed a certain safety time. The part machining program is designed as a cyclic subroutine. The G04 command is designed in the main program that calls the cyclic subroutine. If necessary, the design selection plan stops the M01 command as the end or check of the program.
(2) When tapping the center thread with a tap, it is necessary to use the elastic collet chuck to ensure that the tap will not break when tapping to the bottom of the thread, and set the G04 delay command at the bottom of the thread to make the tap for non-feed cutting. The time of delay should ensure that the spindle stops completely. After the spindle stops completely, it will reverse according to the original forward rotation speed, and the tap will retreat according to the original lead.
(3) The G04 command can be set when there is a large change in the spindle speed. The purpose is to stabilize the spindle speed and then machine the part to improve the surface quality of the part.
3. Relative programming G91 and absolute programming G90 instructions
Relative programming is based on the position of the tool tip as the coordinate origin, and the tool tip is programmed to be displaced relative to the coordinate origin. That is to say, the relative programmed coordinate origin is often transformed, and the operation is to control the displacement based on the current tool tip point. Then, when the continuous displacement is performed, a cumulative error is inevitably generated. Absolute programming has a relatively uniform reference point, that is, the origin of coordinates, in the whole process of machining, so its cumulative error is smaller than relative programming. In CNC turning, the accuracy of the radial dimension of the workpiece is higher than the axial dimension. Therefore, when programming, the radial dimension is preferably absolutely programmed. Considering the convenience of machining, the axial dimension is relatively programmed, but for important Axial dimensions are also available in absolute programming. In addition, in order to ensure some relative position of the parts, according to the requirements of the process, flexible use of relative programming and absolute programming.
In short, with the rapid development of science and technology, CNC lathes have more and more applications in the mechanical manufacturing industry due to their superior processing characteristics. In order to give full play to the role of CNC lathes, we need to master certain skills in programming. A reasonable and efficient processing procedure is ensured to ensure the production of qualified workpieces that meet the requirements of the drawings. At the same time, the functions of the CNC lathe can be reasonably applied and fully utilized, so that the CNC lathe can work safely, reliably and efficiently.
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