1、 Selection of positioning datum
In the process specification design, the correct selection of positioning datum has a vital impact on ensuring the processing requirements of parts and arranging the processing sequence reasonably.
The positioning datum can be divided into fine datum and rough datum. The rough surface on the blank is used as the positioning datum. This positioning datum is called rough datum. The machined surface is used as a positioning reference, and this positioning reference becomes a fine reference.
When selecting the positioning datum, the precise datum is often selected first according to the machining requirements of the parts, and the process route is pushed forward. Finally, which group of surfaces is selected as the rough datum to process the precise datum.
1. Selection principle of fine benchmark
In general, the following principles shall be followed in selecting the precise benchmark:
(1) Principle of datum coincidence: the design datum of the machined surface shall be selected as the precision datum as far as possible, so as to avoid the positioning error caused by the non coincidence of datum.
(2) Unified datum principle: the same group of precision datum shall be selected as much as possible to process as many surfaces on the workpiece, so as to ensure the relative positional accuracy between the processed surfaces.
(3) Mutual reference principle: when the positional accuracy requirements between two machining surfaces on the workpiece are relatively high, the method of repeated machining using two machining surfaces as a reference can be adopted.
(4) Self reference principle: for the finishing process of some surfaces, the machining allowance is required to be small and uniform, and the machined surface itself is often used as the finishing reference.
The above four principles for selecting precise benchmarks can not be met at the same time, and the choice should be made according to the actual conditions.
2. Selection principle of rough datum
(1) The first process of workpiece processing needs to use rough datum. Whether the rough datum is selected correctly or not is not only related to the processing of the first process, but also will have a significant impact on the whole process of workpiece processing.
(2) Principle of reasonable distribution of machining allowance: in consideration of ensuring uniform machining allowance of important surfaces, important surfaces shall be selected as rough reference.
(3) Principle of easy clamping: in order to ensure stable positioning and reliable clamping of workpieces, it is required that the selected rough datum should be as flat and clean as possible, and there should be no forging flash, casting gate and riser notch or other defects, and there should be sufficient bearing area.
(4) The principle that coarse datum shall not be reused in general.
The above four principles for selecting rough benchmarks can not be considered at the same time, and can only be selected according to the primary and secondary.
2、 Selection of surface processing methods
The structural shape of machine parts is composed of some basic geometric surfaces, and the machining process of machine parts is the process of obtaining these geometric surfaces. Different processing methods can be selected for the same surface, but the processing quality, processing time and cost obtained by each processing method are different.
The task of engineering technicians is to select the most appropriate processing method according to the specific processing conditions to ensure that the machine parts meet the requirements of the drawings.
When selecting the processing method, generally, the final process processing method is first selected according to the technical requirements of the main surface of the part and the specific conditions of the factory, and then the processing methods of the relevant previous processes of the surface are selected one by one.
There are certain processing requirements for the processing surface of parts. Generally, it is impossible to meet the requirements through one processing, but the requirements can be gradually met through multiple processing (i.e., multiple processes).
The steps to determine the processing method can be summarized as follows, on the basis of analyzing and studying the part drawing:
(1) First, according to the technical requirements of each processing surface, determine the processing method and processing scheme.
(2) The properties of the processed materials shall be considered when determining the processing method.
(3) To determine the processing method, the production type should be considered, that is, the productivity and economy should be considered. Special and high-efficiency equipment can be used in large quantities, and general equipment and process equipment are usually used for single piece and small batch production.
(4) The existing equipment and technical conditions of the plant (workshop) shall be taken into consideration when determining the processing method, and the existing equipment shall be fully utilized to tap the potential of the enterprise.
3、 Division of processing stages
When the processing quality of parts is required to be high, they generally have to go through different processing stages to gradually meet the processing requirements, which is the so-called “gradual refinement” principle. Generally, rough machining, semi finishing machining and finishing machining are required. If the machining accuracy of the parts is particularly high and the surface roughness is particularly small, the finishing process is also required.
The main tasks of each processing stage are summarized as follows:
(1) Rough machining stage: most of the remaining parts on the machining surface can be cut off efficiently to make the blank close to the finished part in shape and size.
(2) Semi finishing stage: remove the error left after rough machining, so that the workpiece to be processed can reach a certain accuracy, prepare for finishing work, and complete the processing of some secondary surfaces.
(3) Finishing stage: ensure that the main surfaces meet the processing quality requirements specified in the part drawing.
(4) Finishing stage: the main task is to reduce the surface roughness or further improve the dimensional accuracy and shape accuracy, but generally cannot correct the position error between surfaces.
The main purposes of dividing processing stages are:
(1) Ensure the processing quality of parts: after the stages are divided, the processing error caused by the rough processing stage can be gradually corrected through the semi finishing and finishing stages, and the processing quality of parts can be easily guaranteed.
(2) It is helpful to find the defects of the blank as early as possible and deal with them in time.
(3) It is conducive to the rational use of machine tools and equipment.
(4) In order to insert the necessary heat treatment process in the machining process and make the heat treatment play a full role, the machining process is naturally divided into several stages, and each stage has its own characteristics and objectives.
In addition, the workpiece processing is divided into several stages, which is also conducive to protecting the finished surface from collision, chip slip and other damage.
It is necessary to divide the process into several stages for the whole processing process, and the processing of a certain surface cannot be rigidly adhered to. The division of processing stages is not absolute.
4、 Concentration and dispersion of processes
After the processing method is determined, the number of processes in the process shall be determined according to the production type of the parts and the specific conditions of the factory (workshop).
There are two different principles for determining the number of processes in the part processing process, one is the principle of process concentration, and the other is the principle of process dispersion.
The characteristics of organizing the process according to the principle of process concentration are as follows:
(1) It is conducive to the use of high-efficiency machine tools and process equipment with high automation and high production efficiency;
(2) The number of processes and equipment is small, and the number of operators and production area can be reduced accordingly;
(3) The number of clamping of the workpiece is small, which not only can shorten the auxiliary time, but also because many surfaces are processed in one clamping, which is conducive to ensuring the mutual position accuracy requirements between the processing surfaces.
To organize the process according to the principle of process decentralization is to minimize the processing content included in each process. The extreme situation is that each process only includes one simple step.
The characteristics of organizing the process according to the principle of process dispersion are as follows:
(1) The machine tool and process equipment used are simple and easy to adjust and set the tool;
(2) The technical level of operators is not high;
(3) There are many processes, equipment, operators and large production area.
5、 Arrangement of process sequence
1. Arrangement of machining process
The following principles are generally followed:
(1) First process the positioning reference surface, and then process other surfaces;
(2) Process the main surface first and then the secondary surface;
(3) The rough machining process shall be arranged first, and then the finishing process shall be arranged;
(4) Machining plane first, then machining hole.
2. Arrangement of heat treatment process and surface treatment process
Heat treatment procedures arranged to improve the cutting performance of workpiece materials, such as annealing, normalizing, tempering, etc., shall be carried out before cutting.
The heat treatment process arranged to eliminate the internal stress of the workpiece, such as manual aging and annealing, is preferably arranged after the rough machining stage.
The heat treatment processes to improve the mechanical properties of workpiece materials, such as quenching, carburizing and quenching, are generally arranged between semi finishing and finishing.
The heat treatment process arranged for improving the wear resistance and corrosion resistance of the workpiece surface and the heat treatment process arranged for decoration purposes, such as chromium plating, zinc plating, hair blue, etc., are generally arranged at the last stage of the process.
3. Arrangement of other processes
In order to ensure the manufacturing quality of parts and prevent waste products, inspection procedures shall be arranged in the following situations:
(1) After all rough machining is completed;
(2) Before and after sending to the external workshop for processing;
(3) Long working hours and before and after important processes;
(4) After final processing.
6、 Selection of machine tool equipment and process equipment
It is very important to select the machine tool equipment correctly. It not only directly affects the processing quality of the workpiece, but also affects the processing efficiency and manufacturing cost of the workpiece. The size and specification of the selected machine tool equipment shall be compatible with the shape and size of the workpiece, the accuracy level shall be compatible with the processing requirements of this process, the motor power shall be compatible with the power required for the processing of this process, and the automation degree and production efficiency of the machine tool equipment shall be compatible with the production type of the workpiece.
The selection of machine tools and equipment should be based on the domestic market. The machine tools and equipment that must be imported must be fully demonstrated, compared with each other, and their economy should be reasonably analyzed. Blind introduction is not allowed.
If the workpiece size is too large (or too small) or the machining accuracy of the workpiece is too high, and there is no ready-made equipment to choose from, the self-made special machine tool can be considered. Special machine tool design specification can be put forward according to process processing requirements; The machine tool design specification shall be attached with all necessary data related to the processing of the process, including the dimensional tolerance and technical conditions of the process, the clamping method of the workpiece, the cutting amount, man hour quota, cutting force, cutting power and the general layout of the machine tool.
The selection of process equipment will directly affect the processing accuracy, production efficiency and manufacturing cost of the workpiece, and should be appropriately selected according to different conditions. Under the condition of medium and small batch production, general process equipment shall be selected first; In mass production, special process equipment can be designed and manufactured according to processing requirements.