The content and steps of the design of machining process regulations

       (1) When analyzing the part drawing and product assembly drawing design process specification, first analyze the part drawing and the assembly drawing of the part or assembly where the part is located, and understand the position and function of the part in the part or assembly, as well as the part or assembly For the technical requirements of the part, analyze the main technical key and the corresponding process measures.

(2) Perform process review on the part drawing and assembly drawing. Check whether the views, dimensional tolerances and technical requirements on the drawing are correct, uniform and complete, and evaluate the structural manufacturability of the part design. , If any unreasonable points are found, they should be raised in time, and the drawing modification plan should be discussed with the relevant designers, and reported to the competent leader for approval.

　　 (3) The production type of parts is determined by the annual production program of the product and the study of the product's own characteristics (see Chapter 1, Section 3).

　　 (4) Determine the blank to improve the quality of blank manufacturing, which can reduce the amount of machining labor and reduce the machining cost, but at the same time it may increase the manufacturing cost of the blank. The blank manufacturing method must be determined according to the production type of the part and the production conditions of the blank manufacturing. It should be pointed out that the utilization rate of materials in my country's machinery manufacturing plants is low. Whenever possible, the use of advanced blank manufacturing methods such as precision casting, precision forging, cold rolling, cold extrusion, and powder metallurgy should be promoted. Material utilization factor is an important parameter to measure whether the process design is reasonable.

　　 (5) The main content of the process route is to select the positioning reference, determine the processing method of each processed surface, divide the processing stage, determine the concentration and dispersion degree of the process, and determine the sequence of the process. When drawing up a process route, several possible processing schemes must be proposed at the same time, and then through a comparative analysis of technology and economy, the most reasonable process scheme is finally determined.

　　 (6) Determine the machine tool equipment and process equipment (including tools, fixtures, measuring tools, auxiliary tools, etc.) used in each process, and propose design tasks for special process equipment that needs to be modified or redesigned.

　　 (7) Determine the machining allowance of each process, and calculate the size and tolerance of the process.

　　 (8) Determine the technical requirements and inspection methods of each process.

　　(9) Determine the cutting amount and man-hour quota for each process.

　　 (10) Compile process documents.

　　 2. Drafting of the process route

　　 Drawing up the process route is the most critical step in the design of the process specification, and the following tasks need to be completed in sequence.

　　 (1) Select the positioning datum

　　 In the process design, the correct selection of the positioning datum has a vital influence on ensuring the technical requirements of the parts and determining the processing sequence. The positioning datum is divided into fine datum and coarse datum. Use the unprocessed surface on the blank as a positioning datum, this positioning datum is called a rough datum; using a processed surface as a positioning datum, this positioning datum is called a fine datum. When selecting the positioning datum, it is generally based on the machining requirements of the part to select the fine datum, and then consider which group of surfaces can be used as the rough datum to process the fine datum.

  1. Selection principle of fine benchmark

　　 The selection of fine benchmarks should generally follow the following principles:

　　 (1) Principle of benchmark coincidence

　　 (2) Uniform Standard Principle

　　 (3) The principle of mutual reference

　　 (4) Self-standard principle

  2. Selection principle of rough benchmark

　　The datums used in the first process of workpiece processing are all rough datums. Whether the rough datum is selected correctly or not is not only related to the processing of the first process, but also has a significant impact on the whole process of the workpiece processing. When choosing a rough benchmark, the following principles should generally be followed:

　　 (1) The principle of ensuring that the machined surface of the part has a certain position accuracy relative to the unmachined surface

　　 (2) The principle of reasonable allocation of machining allowance

　　 (3) The principle of easy clamping

　　 (4) The principle that rough datums in the same dimension direction generally cannot be reused

　　 (two) the selection of the surface processing method of the part

　　 (3) Division of processing stages

When the machining quality requirements of the parts are high, they generally have to go through three stages of rough machining, semi-finishing and finishing; if the machining accuracy requirements of the parts are particularly high, and the surface roughness requirements are particularly small, they must be smoothed. stage. The main tasks of each processing stage are:

　　 (1) roughing stage

　　 (2) Semi-finishing stage

　　 (3) Finishing stage

　　 (4) Finishing processing stage

　　 The main purpose of dividing the processing of parts into several processing stages is:

　　 (1) Guarantee the quality of parts processing

　　(2) It is conducive to early detection of blank defects and timely treatment

　　 (3) Conducive to the rational use of machine tools

　　When selecting a processing method, generally first select the final processing method of the surface according to the technical requirements of the main surface of the part and the specific conditions of the factory, and then select the processing method of each related leading process of the surface one by one. For example, to process the outer surface of a steel part with a precision of 1T6 and a surface roughness of 0.2, the final machining process is fine grinding, and the leading process can be selected as rough turning, semi-finishing turning and rough grinding (see Figure 5). -5). After the processing plan and processing method of the main surface are selected, the processing plan and processing method of the secondary surface are selected.

　　 (3) Division of processing stages

When the machining quality requirements of the parts are high, they generally have to go through three stages of rough machining, semi-finishing and finishing; if the machining accuracy requirements of the parts are particularly high, and the surface roughness requirements are particularly small, they must be smoothed. stage. The main tasks of each processing stage are:

　　 (1) The rough machining stage efficiently cuts off most of the margin on the machined surface, so that the blank is close to the finished part in shape and size.

(2) In the semi-finishing stage, the errors and defects left after roughing are removed, so that the processed workpiece can reach a certain accuracy, prepare for the finishing work, and complete some secondary surface processing, such as drilling, tapping, and milling keyway Wait.

　　 (3) In the finishing stage, ensure that each main surface meets the processing quality requirements specified in the part drawing.

(4) For surfaces with high precision requirements (above IT5) and a small surface roughness value (0.2μm) in the finishing stage, the finishing stage needs to be set up. The main task is to reduce the surface roughness and further improve it. Dimensional accuracy and shape accuracy, but generally do not improve the position accuracy between surfaces.

　　 The main purpose of dividing the processing of parts into several processing stages is:

(1) To ensure the quality of parts processing, most of the margins on the processed surface should be removed during the rough processing stage. The cutting force and cutting heat are relatively large, and the clamping force required to clamp the workpiece is also relatively large. Deformation under force and heat; in addition, after most of the margin is removed from the workpiece in the roughing stage, the residual internal stress in the workpiece will be redistributed, which will also cause the workpiece to deform. If the machining process is not divided into stages, and the roughing and finishing processes of each surface are mixed together and interlaced, then the machining accuracy obtained through the finishing process in the early stage of the process will inevitably be destroyed by the subsequent roughing process, which is unreasonable of. After the machining process is divided into several stages, the errors and defects generated in the rough machining stage can be gradually corrected through the semi-finishing and finishing stages, and the machining quality of the parts can be guaranteed.

(2) It is conducive to early detection of blank defects and timely treatment of each surface after rough machining, because most of the machining allowance of each machined surface is removed, the defects of the blank can be found early (pores, sand holes, cracks and insufficient machining allowance) , So that it can be scrapped or repaired in time, without wasting the manufacturing cost of the subsequent finishing process.

　　(3) It is conducive to the rational use of machine tools and equipment. The rough machining process needs to use high-power and low-precision machine tools, and the finishing process should use high-precision machine tools. Arrange rough machining work on a high-precision machine tool, and the accuracy of the machine tool will drop rapidly. It is unreasonable to arrange the rough and finishing work of a certain surface on the same machine tool.

It should be pointed out that the process of dividing the process into several different processing stages is for the entire processing process of the part, and cannot be restricted to the processing of a certain surface, for example, the positioning base surface of the workpiece, in the semi-finishing stage (sometimes even In the roughing stage), it needs to be processed very accurately; and in the finishing stage, it is also common to arrange certain roughing processes such as drilling and tapping holes.

　　Of course, the division of processing stages is not absolute. On high-rigidity, high-precision machine tools and equipment, it is not necessary to divide the processing stages for workpieces with good rigidity, low machining accuracy requirements or not too large machining allowances; some heavy parts with low accuracy requirements are due to the transport of workpieces and clamping The workpiece is time-consuming and labor-intensive, and generally does not divide the processing stages, but completes all roughing and finishing work in one process; in this type of processing, in order to reduce the impact of clamping deformation on the processing accuracy of the workpiece, rough processing is generally performed Then loosen the clamping device, and then re-clamp the workpiece with a smaller clamping force to continue finishing, which is beneficial to improving the machining accuracy of the workpiece.

　　 (4) Concentration and dispersion of processes

　　 After determining the processing method, it is necessary to determine the number of processes in the process according to the production type of the part processing and the production conditions of the factory (workshop). There are two very different principles for determining the number of processes in a part processing process, one is the principle of process concentration, and the other is the principle of process dispersion. Organizing the process according to the principle of process concentration is to make the processing content included in each process as much as possible to form a centralized process; the maximum process concentration is to complete the processing of all surfaces of the workpiece in one process. Organizing the process according to the principle of process dispersion is to make the processing content included in each process as few as possible; the maximum process dispersion is to make each process only include a simple step.

　　The characteristics of organizing the process according to the principle of process concentration are:

　　1) Facilitate the use of high-efficiency machine tools and process equipment with a high degree of automation for processing, and high production efficiency;

　　 2) The number of processes is small, and the number of equipment is small, which can reduce the number of operators and the production area accordingly;

　　3) The number of clamping times of the workpiece is small, not only can shorten the auxiliary time, but also because many surfaces are processed in one clamping, it is helpful to ensure the mutual position accuracy requirements between the processed surfaces.

　　The characteristics of organizing the process according to the principle of process dispersion are:

　　1) The machine tool and process equipment used are simple and easy to adjust;

　　2) The requirements for the technical level of the operators are not high;

　　3) There are many processes, many equipments and many operators, which occupy a large production area.

　　 According to the principle of process concentration and the principle of process dispersion, the organization of the process has its own characteristics and is applied in production. The traditional assembly line and automatic production line with special machine tools and modular machine tools as the main body basically organize the process according to the principle of process dispersion, which can achieve efficient production, but the adaptability to product modification is poor and it is difficult to switch to production. The use of CNC machine tools and machining centers to process parts is organized in accordance with the principle of process concentration. Although the one-time investment in equipment is high, it is still more and more affected due to the strong ability to restructure production, good production adaptability, and relatively easy conversion. Pay more attention.

　　 (5) Arrangement of the sequence of processes

  1. Arrangement of machining processes

　　 The sequence of machining processes should generally follow the following principles:

　　1) Process the positioning base surface first, and then process other surfaces;

　　 2) Process the main surface first, and then process the secondary surface;

　　3) Arrange the rough machining process first, and then arrange the finishing process;

　　4) The plane is processed first, and then the hole is processed.

When arranging the CNC machining sequence, the following conditions must be considered: (1) When the tool change time is greater than the table index time, in order to reduce the number of tool changes and the tool change time, it is advisable to change all the tools without affecting the machining accuracy. The surfaces that can be machined with the same tool are concentrated and completed in sequence; (2) In order to reduce the table indexing error and the time loss caused by the table indexing, when the tool change time is less than the table indexing time , The processing sequence is arranged according to the principle of centralized processing of stations, that is, all the surfaces that can be processed in one station are processed in the same station through constant tool replacement; (3) For the hole system that requires high position accuracy, it is better to work in the same station. Arrange the processing of each relevant surface of the hole system in the same station to eliminate the influence of the repetitive positioning error of the worktable's indexing on the position accuracy requirements of the hole system.

  2. Arrangement of heat treatment process and surface treatment process

　　 The heat treatment process arranged to improve the cutting performance of the workpiece material, such as annealing, normalizing, quenching and tempering, etc., should be carried out before the cutting process.

　　 The so-called artificial aging is to heat the blank to 500-550℃ at a rate of 50-100℃/h, keep it for 3-5h, and then cool it with the furnace at a rate of 20-50℃/h. The so-called natural aging is to place the blanks in the open air for several months to several years, so that the blanks can withstand the sun and rain in nature, so that the internal stress of the material structure relaxes and gradually stabilizes.

　　3. Arrangement of other processes

In order to ensure the quality of parts manufacturing and prevent the production of waste products, inspection procedures should be arranged in the following situations: 1) after the rough machining is completed; 2) before and after sending to the outside workshop for processing; 3) before and after the long working process and important processes; 4) After the final processing, in addition to arranging geometric size inspection procedures, some parts also need to arrange inspection procedures such as flaw detection, sealing, weighing, and balance.

　　The burrs on the surface or the inner cavity of the part have a great influence on the assembly quality of the machine. After cutting, a deburring process should be arranged.

　　The cleaning process should generally be arranged before the parts enter the assembly. Chips are easily stored in the inner hole of the workpiece and the inner cavity of the box. After the finishing process such as grinding and honing, the tiny abrasive particles are easy to adhere to the surface of the workpiece, so pay attention to cleaning.

　　After the process of magnetic clamping, the demagnetization process should be arranged to prevent the workpiece with residual magnetism from entering the assembly line.

　　(6) Selection of machine tool equipment and process equipment

　　 (seven) examples

　　 3. Machining allowance

  (I. Overview

　　 When using the material removal method to manufacture machine parts, it is generally necessary to remove layers of material from the blank before finally making parts that meet the requirements of the drawings. The material layer left on the blank for processing is called the machining allowance. Machining allowance is divided into total allowance and process allowance. The difference between the size of a certain surface blank and the design size of the part is the total margin value, represented by Z0. The difference between the basic dimensions of the previous process and this process is the process margin Zi of this process.

　　The process allowance is divided into unilateral allowance and bilateral allowance.

　　 Due to the deviation of the process size, the actual removal margin value of each process is changed. The process margin has the nominal margin (referred to as the margin), the maximum margin and the minimum margin.

The process size deviation is generally marked according to the "into-body principle". For contained dimensions (such as shaft diameter), the upper deviation is 0, and its maximum size is the basic size; for contained dimensions (such as aperture, groove width), the lower deviation is 0, The minimum size is the basic size; the size deviation of the hole pitch type process is configured according to the "symmetric deviation".

　　 (2) Factors affecting machining allowance

　　In order to reasonably determine the machining allowance, it is necessary to have a thorough understanding of the factors that affect the machining allowance