I. Introduction
The feature-based CAPP system lays the foundation for the CAD/CAM information sharing. In the development of CAPP system, the design of machining process is a complex engineering problem, which depends to a large extent on the level and experience of the designer. At the same time, due to the large amount of information involved in the process design, each decision function Due to the different structural artifacts and different precision levels, the difference in properties is large, so different process decision methods and reasoning mechanisms are generated. The technical characteristics of the gear shafts are firstly characterized by its many features. In the feature-based description of the parts, it can be divided into main features: inner and outer cylindrical surfaces, conical surfaces, gear surfaces, etc.; auxiliary features: keyways, facets , splines, threads, etc. In addition, there are many machine tools used in the processing of gear shaft parts, and there are many types of materials and heat treatments. Moreover, its process characteristics such as dimensional accuracy, geometrical tolerance, and surface quality are also required. Based on the description of feature-based parts, this paper discusses the process decision principle of gear shaft parts, and generates the feature processing method according to the principle of decision tree. The reverse processing method is used to form the whole processing process of the feature. Its overall structure is shown in Figure 1.
Figure 1 Process design overall structure
Figure 2 Outer circle feature processing decision tree
Figure 3 gear feature processing decision tree
Second, the generation of feature processing chain
In machining, each part has several processing methods corresponding to it, according to the production scale, the overall shape and contour size of the part, manufacturing resources, etc., the processing precision, surface roughness and different material selection for each feature. Different processing methods. These processing methods have a certain arrangement law and appear as a tree structure. It is a diagram of nodes and branches that describe and deal with the relationship between "conditions" and "actions." A node has a root node, a middle node, and an end node. It represents a test or an action, and the final action to be taken is generally placed on the final node. Each node of the tree should be a certain processing step of a feature, and each branch is a processing method that a feature is likely to take. For example, roughing is located at the root of the tree, and the crown is located in the multi-branch of the tree. Tracing from the finished canopy to the root of the roughed tree is a reverse reasoning process, and the result of each inference process forms a characteristic processing chain from the root to the canopy. Figure 2 shows the outer circle feature processing decision tree. Figure 3 shows the gear feature processing decision tree. Using some nodes in the decision tree as the inference choices in fuzzy reasoning, the reverse design method is adopted, that is, the final processing method of the feature is selected first, and then the corresponding preparatory process is gradually selected from the back to the front, passing through each tree node. Back to the root of the tree, and finally form the processing chain of the feature. The machining feature decision tree is part of the process knowledge and is used for process reasoning. For example, the material is 20CrMnTi, the shaft gear with the accuracy of 5 grades, the diameter of the index circle is 125.475mm, the number of teeth is Z=33, the normal modulus is Mn=3.5, the surface roughness is Ra0.8μm, and the processing chain is as follows:
Rough car blanks → fine car outer circle → hobbing → tooth end face chamfering → cleaning → shaving → surface heat treatment → grinding teeth → strong shot peening.
Third, the process route design of the parts
The previous discussion is the generation of a single feature processing chain. In actual production, different features are combined into one part, that is, the process of finally discharging a complete part.
The process decision of a part is a complex and diverse knowledge accumulation, and the knowledge of different types of workpiece decision making is not the same. Firstly, according to the determined feature processing method, considering the division of the machining stage, the selection of the machine tool, and the determination of the positioning datum, the primary and secondary algorithms of these factors should be considered. The decision-making principle discussed in this paper is based on the processing characteristics and process conditions of the gear shaft parts of the gearbox factory of a typical automobile manufacturer in China, and has certain practical application value.
(1) Decision-making principles
In addition to the machining characteristics of the rotary parts, the gear shafts also have gear machining characteristics. The decision-making principles considered under mass production conditions are as follows:
Process concentration and dispersion Mainly due to the combination of machine tools and semi-automatic machine tools, the process is combined with concentration and dispersion. For example, in the roughing and semi-finishing process, semi-automatic machining machines simultaneously complete the machining of multiple surfaces, while the gear surface and other auxiliary The surface is relatively dispersed.
After the heat is heated, according to the difference of the material and mechanical properties of the parts, the heat treatment process is arranged in the middle, and the processing is divided into two stages after the heat before the heat. The front of the heat includes gear shaft turning and rough machining of the gear surface; after heat, it includes cylindrical grinding and tooth surface finishing.
The processing is carried out in stages on the basis of the heat before the heat. When optimizing step, the principle of coarse and fine should be followed.
The roughing step and the semi-finishing step of the main feature are arranged first, then the processing of the secondary features is arranged, and finally the finishing steps of the main features are arranged.
Guaranteed position accuracy Positional accuracy is mainly for coaxiality, verticality, symmetry, and so on. To ensure positional accuracy of the parts, it is best to machine all relevant surfaces in one installation. Worksteps with positional accuracy requirements are relatively centrally arranged.
(2) Generation of processing routes
In the arrangement of the process route, the processing order of the main features of the parts is arranged first, that is, the decision knowledge of the relationship matrix formed in the inference mechanism is called to form the trunk of the process route, and then the processing steps of the auxiliary shape features are sequentially inserted, and then the heat treatment, inspection, and Additional processes such as cleaning ultimately form a complete process route. This constrained-driven step-by-step expansion method is more suitable for the design ideas of process designers, and is conducive to the optimization design of the process and the development of the principle of the first and second principles.
The choice of positioning scheme in the processing chain first considers the choice of coarse positioning criteria in the machining route. The positioning scheme for the decision-making rough reference is based on the premise that the precision positioning reference can be guaranteed. The fine reference of the gear shaft is generally a two-way top hole, which is relatively easy to determine. However, the selection of the rough reference is relatively complicated. After analyzing a large number of drawings, it is considered that the axial coarse positioning reference has more axial mounting reference or intermediate shaft diameter, so it is processed to interactively specify the rough reference plane.
The feature prioritization is based on the equipment capacity and process level of the production plant under study, and the priority processing order of the discharge features, that is, the typical process with the main features as the main line, supplemented by the interactive auxiliary features, and finally automatically generates the complete process route. The main characteristics of the hot front mainly consider the roughing, semi-finishing and finishing of the outer cylindrical surface on the semi-automatic lathe; the hobbing (pinning) and shaving of the gear surface; the auxiliary surfaces such as the snap ring groove, the undercut groove and the keyway Wait for the interaction to be inserted between the main features. After the heat is mainly the grinding sequence of the outer surface, the main gear surface finishing process is determined.
The determination of the processing parameters of each process includes the machining allowance, process size and tolerance, and blank size of each process. The process size is inverse reasoning, which is calculated by querying the machining allowance in the process database; the blank size is determined by increasing or decreasing the machining allowance. The automatic labeling of process dimensional tolerances is what we are currently studying and is intended to be done interactively through the principles of graph theory.
Fourth, the conclusion
Based on the description of the feature-based gear shaft parts, this paper discusses the process decision principle of gear shaft parts. Using the principle of decision tree as the main line of process knowledge base, different from the traditional production process knowledge, can simplify the decision process, make a more complicated process reasoning process become systematic and concise, and facilitate the development of more complex, multi-process Route CAPP system.
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