CNC machining process design process - Database & Sql Blog Articles

First, the general process of CNC machining process Figure 2-2-1 Schematic diagram of CNC machining process When machining workpieces on CNC machine tools, firstly, according to the workpiece pattern, analyze the structural shape, size and technical requirements of the workpiece, as the basis for formulating the NC machining process of the workpiece. To formulate the numerical control machining process, first, determine the content and requirements of the CNC machining of the workpiece; then, design the machining process, select the machine tool and the tool, determine the workpiece positioning and clamping, determine the steps and sequence in the numerical control process, and determine the steps of each step. Tool routing, cutting parameters; finally, fill in the process documentation and processing procedures and program verification. The CNC machining process is shown in Figure 2-2-1. Second, the selection of CNC machining content When selecting and determining the CNC machining of a certain part, not all of its processing content is CNC machining, it is advisable to select those suitable and needed content and process for NC machining, pay attention to give full play to the advantages of CNC . 1. Select the NC machining content: (1) Select the complex shaped part structure that ordinary machine tools can't process as the NC machining content. For example, CNC machine tools rely on CNC system to achieve multi-coordinate control and multi-coordinate linkage to form a composite motion, which can process complex profiles. (2) Select the content that is difficult to guarantee the quality of ordinary machine tools as the content of CNC machining. For example, parts with high requirements such as dimensional accuracy, shape and position accuracy, and surface roughness (3) select the contents of the general machine tool with low processing efficiency and labor intensity for manual operation as the NC machining content. For example, the shape is complicated, the size is large, the scribing and the detection are difficult, and the parts that are difficult to observe and control are processed on the ordinary machine tool. (4) Select the parts with good consistency as the NC machining content. In mass production, because the positioning accuracy and repeat positioning accuracy of the CNC machine tool are high, it can avoid various errors caused by human factors in the processing of ordinary machine tools. CNC machine tools can easily ensure the consistency of batch parts and make them processing precision. It is improved and the quality is more stable. 2. It is not advisable to choose CNC machining content: (1) It is necessary to use a long time to adjust the processing content. (2) The machining allowance is extremely unstable, and the machining content of the coordinate position of the part cannot be automatically adjusted on the CNC machine. (3) It is inconvenient to use CNC machining in the scattered parts that can not be processed in one installation. The effect is not obvious, and the ordinary machine tool can be arranged for additional processing. In addition, in the selection of CNC machining content, we must also consider factors such as production batch, production cycle, and process turnover, and use CNC machine tools reasonably. Third, CNC machining requirements analysis analyzes the workpiece drawings suitable for CNC machining to clarify CNC machine tools. Processing requirements for processing content. The analysis of the workpiece drawing is the beginning of its machining process, and the requirements presented by the workpiece drawing are the result and goal of the machining process. (1) Analysis of dimensioning The dimensioning of workpieces to determine part shape, structure size and position requirements is the main basis for a correct understanding of part processing requirements. CNC machining technicians should pay attention to the following points when analyzing the dimensioning of parts: 1 Analyze whether the pattern dimensioning method is suitable for the characteristics of CNC machining. For CNC machining, the dimensions are marked from the same reference, which is convenient for process programming to keep the design, process, and inspection reference consistent with the programming origin setting. The use of locally dispersed dimensional markings with different benchmarks often causes inconvenience to the process design. 2 Analyze whether the geometric elements of the contours in the pattern are sufficient. Due to the inconsistency or negligence of the part designer during the design process, the geometric elements that make up the contour of the part often have insufficient conditions, errors, leaks, contradictions, and ambiguities. When the above defects occur, they should be promptly reflected to the designer or technical management personnel of the drawings, and the programming can be carried out after the solution. 3 Analyze the unified problem between the design basis and the process positioning reference, and analyze the reliability of the positioning reference plane, so as to take measures to reduce the positioning error when designing the clamping scheme. (2) Tolerance requirements Analyze and analyze the tolerance requirements on the part pattern to determine the machining process that controls its dimensional accuracy. The process factors that affect the dimensional processing accuracy include machine tool selection, tool tool setting scheme, workpiece clamping positioning selection, and determination of cutting amount. Dimensional tolerance, from the point of view of the design of the part, is the range of tolerances allowed for the size of the workpiece. Its size affects the performance of the part. From the perspective of the machining process of the workpiece, the tolerance is first, which is one of the commands for production. It is stated that the sum of the error of the machining factor caused by all machining factors in the machining must be within the tolerance range, or all machining factors 'share' the tolerance, which is the common tolerance of all machining factors. For CNC machining, the continuum consisting of machine tools, fixtures, tools and workpieces is called the “process system”. The various errors in the process system are the root cause of the dimensional error of the part. Process system errors include errors in control systems, errors in machine tool servo systems, part positioning errors, tool setting errors, and other errors caused by the rigidity of machine tools, workpieces, and tools. In addition to the process system error, it also includes the programmed coordinate data value, tool compensation value, and tool wear compensation value error. For CNC machining, the shape and position error of the part are mainly affected by the geometrical accuracy of the machine motion and the feed motion. If the direction line moving along the X coordinate axis is not perpendicular to its spindle axis, the positional tolerance requirement for verticality cannot be guaranteed. (3) Surface roughness requires surface roughness is an important requirement to ensure the microscopic accuracy of the surface of the part, and is also an important basis for rational selection of machine tools, tools and determining the amount of cutting. There are two main reasons for the formation of surface roughness during machining. One is geometric factor and the other is physical factor. The geometric factor affecting the surface roughness is mainly the residual area of ​​the cutting layer left on the machined surface when the tool is fed relative to the workpiece. The larger the residual area, the rougher the surface. The size of the residual area is related to the feed amount, the radius of the tool nose arc, and the primary and secondary declination of the tool. Physical factors are factors related to the nature of the material being processed and the mechanism of the cutting. For example, when the tool cuts plastic material at medium speed, it will accumulate chip and scale, which makes the surface of the machine rough. The high frequency vibration in the process system leaves the blade on the machined surface and increases the surface roughness. Value (4) Other requirements The material requirements for the parts given on the analysis pattern are important basis for selecting the tool (material, geometry and service life) and selecting the machine model and determining the amount of cutting. The number of parts processed, the setting and positioning of the clamping, the selection of the tool, the arrangement of the process and the determination of the route of the tool are all factors that cannot be ignored. Fourth, the selection of CNC machine tools to select the machine tool in a reasonable manner, in order to obtain the best production results with reasonable input. Considering the characteristics of parts and processing requirements, comprehensive measurement of the characteristics of CNC machine tools is the key to the selection of suitable CNC machine tools. The selection of numerical control equipment should be considered from the following aspects. (1) Considering the machining target of the machine tool, there is always a certain starting point when selecting the CNC machine tool. The purpose is to solve one or several problems in the production. The selection is for the purpose, and the selected CNC machine tool should be able to achieve the predetermined target better. . For example, consider the machining target of CNC machine tools to machine complex parts? Or to improve processing efficiency? Is it to improve accuracy? Or to concentrate the process and shorten the cycle? Or to achieve flexible processing requirements? With a clear goal and targeted selection of machine tools, we can get the best results with reasonable input. (2) The numerical control machine tool or machining center that considers the scope of the machine tool and the type of the machine with different types of machining has certain limitations. Only the workpieces processed in the process range can achieve good results. Various machine tools have typical parts for their optimum machining. For example, vertical CNC milling machines and boring and milling machining centers are suitable for machining complex single-sided machining parts such as plane cams, prototypes, cover covers, housings, etc., as well as internal and external cavities of the mold; horizontal milling machines and machining centers with rotation The workbench is suitable for processing parts of the box, pump body and housing with multi-faceted machining tasks. If the side and top surfaces of the box are required to be processed in one setup, a pentahedron machining center can be used. Most workpieces can be machined with two-axis and half-axis machines, and some workpieces require three-axis, four-axis or even five-axis machining. The redundancy of the machine linkage function is also a waste, and it brings unnecessary trouble to use, maintenance and repair. For example, when the workpiece is drilled or milled, do not use the machining center; if the workpiece can be finished with a CNC lathe, it is not necessary to use the CNC turning center. The choice of the scope and type of machine tool should be sufficient, and it is not appropriate to blindly pursue "advancedness". (3) Considering the machine tool specifications mainly refers to the table size and range of motion of the machine tool. When the workpiece is installed on the workbench, the position of proper calibration and clamping should be left; the stroke of each coordinate should meet the requirements of the advance and retreat of the tool during machining; when the workpiece is heavier, the rated load of the workbench should be considered; For large workpieces, it is necessary to consider not to touch the protective cover during processing, nor to hinder the tool change. For CNC lathes, the chuck diameter, tip pitch, spindle hole size, maximum turning diameter and machining length are mainly considered. (4) When using CNC machine tools for consideration of main motor power and feed drive force, it is often done by roughing and finishing in one setup. Therefore, when selecting, consider whether the main motor power can meet the rough machining requirements, and whether the speed range is suitable; when reaming and tapping, low speed and high torque are required; when drilling, especially for drilling holes with large diameter, it is necessary to check whether the feed force is enough. For machine tools with constant cutting speed control, the main motor power is quite large, in order to achieve real-time speed following, such as f 360mm CNC machine tool, the main motor power is 27kW. (5) Considering the factors of machining precision and precision retention affecting the machining accuracy of CNC machine tools, such as programming accuracy, interpolation accuracy, servo system following precision, mechanical precision, etc., there are many factors affecting machining accuracy in the process of machine tool use. Occurs, such as the effects of temperature, force, vibration, and the effects of wear and tear. For the user to select the machine tool, the main consideration is the comprehensive machining accuracy, that is, processing a batch of parts, and then measuring, statistical, and analyzing the error distribution. The accuracy level of the selected machine tool should be determined according to the machining accuracy requirements of the key parts of the workpiece. Generally speaking, when mass-produced parts, the actual machining tolerance tolerance value is 1.5 to 2 times the machine positioning accuracy tolerance value. (5) Considering the reliability of equipment operation Equipment failure is the most troublesome problem. Especially when the number of similar equipment is small, equipment failure will directly affect production. The machine tool has high stability and reliability, and it has problems of numerical control system and mechanical parts, especially the numerical control system. One of the selected devices is to have fewer failures, and also to consider troubleshooting in a timely manner. Two indicators that measure device reliability. 1 Mean Time Between Failure (MTBF), the value can be expressed as: MTBF = total working time / total number of failures (hours). 2 Mean Time to Trouble (MTTR), which is the average time from failure until the failure is normal. (6) Considering the machine configuration The machine configuration mainly includes the tool changer, the cooling device, and the chip removal device. The turret of CNC machine tools has 4 to 12 knives, and there are more large machine tools. Some machines have double turrets or three knives. According to the complexity of the machined parts, it is generally sufficient to select 8~12 knives (including spare knives). The machining center has a tool magazine capacity of 10 to 40, 60, 80, and 120. When selecting, use the principle of sufficient use, and consider the tool change time and the matching tool system. Modern CNC machine tools use a large flow of coolant, which not only reduces the temperature in the cutting zone, but also ensures efficient cutting, and can act as a chip. When equipped with a chip evacuation device, it is guaranteed that the machining is carried out automatically and continuously. V. Numerical control processing scheme The contents of the machining scheme of CNC machine tools include: adjusting the selected CNC machine tools to meet the processing precision requirements; rationally dividing the contents of the CNC machining process, determining the number of installations and the number of work steps, and determining the processing sequence. Sequence; determine the clamping scheme, positioning reference, workpiece coordinate system; determine the step, including the selection of the tool of the step, the route of the tool, the cutting amount of the tool, etc.; determine the compensation plan of the tool setting and the tool and pre-tool Tune. (1) Fixing scheme: Select the appropriate fixture to ensure the positioning and clamping requirements, so that the workpiece clamping is fast and effective. (2) Select the appropriate tool according to the workpiece processing requirements, material properties, cutting amount, machine characteristics and other factors, correctly select the tool type, tool material, tool geometry parameters, and make the tool installation and adjustment convenient. (3) Selection of work step division and machining allowance The size of the machining machine surface allowance is equal to the sum of the machining steps of the same surface or the machining allowance of the same surface, and the machining allowance between each step or process of the same surface. The choice can be made according to the following methods: 1 Try to use the smallest sum of machining allowances in order to shorten the processing time and reduce the processing cost of parts. 2 The machining allowance should be sufficient to ensure that the final machining of each step or process of processing the same surface can obtain the accuracy and surface roughness requirements specified on the drawing; 3 the machining allowance should be compatible with the size of the machined part. Generally speaking, the larger the part, the larger the deformation caused by the cutting force and the internal stress, so the machining allowance is correspondingly larger; 4 The determination of the machining allowance should take into account the changes caused by the heat treatment of the parts, so as to avoid waste; 5 When determining the machining allowance, the rigidity of the machining method and processing equipment should be considered to avoid deformation of the parts. (4) Selecting a reasonable tool path The tool path is the trajectory and direction of the tool point relative to the workpiece during machining. Reasonable selection of the tool path must take into account the safety of the tool feed motion, processing quality, processing benefits. (5) Determine the reasonable cutting amount to determine the size of the main motion and feed motion during the cutting motion of the tool, that is, to select the cutting speed, the amount of back-feeding and the feed amount to meet the processing quality requirements and give full play to the processing potential. Strive to reduce processing costs. (6) Reasonable formulation and optimization of CNC machine tool processing plan: In the process of CNC machine tool processing, the processing object is complex and diverse. Due to the shape and size of the workpiece, different technical requirements, different blanks, different materials, different batches and other factors, When formulating a machining plan, in addition to the CNC machining plan of a typical workpiece, it should also be specifically analyzed for specific processing content and requirements, flexible treatment or special treatment, and only make the planned processing plan more reasonable. When determining the processing scheme, there are many processing schemes for the same workpiece. The most economical, most reasonable and most complete processing scheme should be selected to achieve the objectives of excellent quality, high efficiency and low cost, that is, optimization of the processing scheme. Requirements. 6. Machining and Verification of Machining Procedures 1. Mathematical Processing In order to describe the tool path (tool path data) to the machine tool computer, it is necessary to set the appropriate workpiece coordinate system and calculate the starting point and end point coordinate data of the various line segments that make up the tool motion path. For simple part machining consisting of straight lines and arcs, calculate the coordinates of the tangent or intersection points (collectively referred to as base points) of the adjacent geometric elements of the part, and obtain the starting point, end point, and center of the arc of each geometric element. Coordinates, as the basis for the description of the feed motion trajectory. For the machining of parts with complex shapes (such as special curves and curved parts), if the CNC system does not have the interpolation function of such curves, it is necessary to use a small straight line segment or a circular segment fitting approximation method, according to the machining accuracy. It is required to calculate the coordinate values ​​of the nodes (the intersection or the tangent point of the approaching line segment), which often needs to be done by means of computer aid. 2. After filling in the machining program tool path, process parameters (such as cutting amount, etc.) and the tool position data, write the part machining program list according to the function command code and block format specified by the CNC system, and input the machining program to the CNC. 3. The program calibration and adjustment program must be verified and tested before it can be used for formal processing. On the numerical control system with the dynamic simulation display function of the tool path, switch to the program of the simulation operation under the CRT graphic display state, and debug and modify according to the automatic alarm content and the displayed tool path or part graphic. It is also possible to check and judge whether the machine tool movement in the execution of the program meets the requirements without using tools, workpieces, and running air. The above method can only check whether the machine tool movement is correct, and can not verify the actual processing quality of the machined part, so the first part of the part needs to be cut. For more complex parts, the first piece of trial cutting can be done with free-cutting materials such as plastic or aluminum. When there is an error in the first test, the cause should be analyzed and modified. After the part program passes the checksum and the first piece is passed, it can be officially processed in batches. During the production process, due to tool wear and other reasons, it is necessary to timely measure the size of the machined parts and perform tool compensation. The operator should also pay attention to the operation to avoid accidents. The practiced inspection process is organized and revised and finalized. Seven, CNC machining technology file filing and writing CNC machining special technical documents is one of the contents of CNC machining process design. These special technical documents are the basis for CNC machining and product acceptance, and are also procedures that require operators to follow and implement. Some are specific instructions or additional instructions for the machining program. The purpose is to make the operator more clear about the contents of the program and the way of clamping. The tools and other problems selected for each processing part. The following describes several special technical documents for CNC machining for reference. 1. CNC machining process card CNC machining process card succinctly explains the machining process of the numerical control process. Including: the number of installations, the number of steps, the processing sequence; the main processing content and requirements of each step; the tools and tool numbers and cutting parameters used in each step. Other process information such as the machine model used, tool compensation, program number and other information. See the CNC machining process card in Table 8-10-2 in Chapter 8. 2, CNC tool adjustment Single CNC machining, the tool management is very strict, generally the tool assembly, numbering, and in the tool set or machine outside the machine, the tool diameter and length are adjusted in advance. The NC tool adjustment list mainly includes two parts: the CNC tool list (referred to as the tool table) and the CNC tool card (referred to as the tool card). The CNC tool schedule shows the tool number, specification and use of the tool used in the CNC machining process, which is the main basis for the operator to adjust the tool. See the tool list in Table 8-10-1 in Chapter 8. The tool card mainly reflects the tool number, tool structure, tail handle specifications, assembly name code, blade type and material, etc. It is the basis for assembling the tool. The format of the CNC tool card is shown in Table 2-2-1. Table 2-2-1 Example of CNC Tool Card

Part drawing number ISO102—4 CNC tool card use equipment

Tool name file TC_30 tool number T13003 tool change mode automatic program number tool composition serial number tool name specification / mm quantity remarks 17013960 pull pin l2390.140-5063050 tool holder 13391.35-4063110M boring bar l4448S-405628-ll boring tool Body l52148C-371103 fine 镗50φφ72l6TRMRll0304-2lSIP blade l3, CNC machining program and description practice proves that there are still many deficiencies in actual machining based on machining program list and CNC machining process card. Because the operator's content of the program is not clear enough, the programmer's intention is not enough to understand, often requires programmers to explain, explain and guide on the spot. This method can cope with the single-piece processing, and for long-term batch production. , will encounter a lot of trouble. Therefore, it is useful to make the necessary detailed instructions for the machining program, especially for programs that require long retention and use. In CNC machining, it is often necessary to pay attention to and prevent accidental collision of the tool with the fixture, workpiece, etc. during the movement. Therefore, it is necessary to try to tell the operator about the movement path of the tool in the programming so that the operator can understand before processing. At the same time, the clamping position should be planned and the height of the clamping element should be controlled, which can reduce the occurrence of accidents. In addition, for some of the machined parts that have to be moved in the machining position due to technical problems, it is also necessary to tell the operator in advance to prevent safety problems.