First, the analysis of the product's parts drawings and assembly drawings Before compiling the parts machining process specification, we should first study the parts of the work drawings and product assembly drawings, familiar with the product's use, performance and working conditions, clearly the parts in the product Location and role; Understand and study the basis for the development of various technical conditions, identify its main technical requirements and technical key, in order to use the appropriate measures to ensure the preparation of process regulations.

The purpose of process analysis is to examine whether the structural shape and dimensional accuracy, mutual position accuracy, surface roughness, material and heat treatment, etc. of the parts are reasonable, whether it is convenient for processing and assembly, and secondly, the process of the parts through process analysis. Requires further understanding in order to formulate a reasonable process specification.

As shown in Figure 3-8, the automobile leaf spring lug is used. When used, the leaf spring does not contact with both sides of the lug, so the roughness of the inner lug can be changed from the original design requirement R a 3.2 μ m R a12.5 μ m. In this way, rough milling can be used without milling to reduce the milling time.

Then, as shown in Figure 3-9, the square head pin requires a quenching hardness of 55-60HRC and the selected material is T8A. There is a hole φ2H7 on the part and it is required to be used when assembling. Since the length of the part is only 15mm, the length of the square head is only 4mm. If the material is partially quenched with T8A, it will inevitably be quenched in its full length. When equipped, the φ2H7 hole cannot be processed. If the proposed material is carburized and quenched with 20Cr, the problem can be solved.

Second, the structural process analysis

The structural technics of the parts refers to the feasibility and economy of the designed parts under the premise of satisfying the requirements for use. The following will analyze the structure and manufacturability of parts from the aspects of machining and assembly of parts.

(I) Requirements for part structure of machining

1 . The structure of the easy-to-clamp parts should be convenient for positioning and clamping during processing and the number of mountings should be small. The parts shown in Fig. 3 -10a are intended to be clamped with the top and center chucks, but the structure is not easy to set up. If you change to Figure b, you can easily install the chuck.

2 . The structure of the easy-to-machine parts should be as standardized as possible in order to use standardized tools and gauges. At the same time also pay attention to retreat knife and feed, easy to ensure the processing accuracy requirements, reduce the processing area and difficult to machine the surface. Table 3-8b shows an example of a part structure that is easy to machine.

3 . Easy CNC machining

The numerical control process problems of the processed parts involve a wide range of problems. The following is a combination of the possibility and convenience of programming for process analysis.

The convenience of programming is often an indicator of the quality of CNC technology. For example, an abstract dimensioning method is used for a part shown in Figure 3-11. If you write the source program of the part in APT language, you will encounter problems when describing the part shape with a geometry definition statement because the B point and its straight line OB are difficult to definition. To solve this problem, it is necessary to make a roundabout, that is, make a straight line L3 parallel to L1 at point B and define it, and also define the straight line AB. Then we can find the intersection B between L3 and line AB, and then define OB. Otherwise, manual calculations outside the aircraft should be performed. This should be avoided as much as possible. From this, it can be seen that the dimensioning method on the part drawing has a great influence on the craftsmanship. For this reason, different requirements shall be put forward for the design drawings of the parts. For the parts processed by CNC, the size data given on the drawings shall conform to the principle of convenient programming.

The shape and size of the parts are preferably of a uniform geometry or size, which can reduce the number of tool changes. It is also possible to apply control programs or special programs to shorten the program length. For example, as shown in Figure 3-12a, the size of the fillet is determined

With the diameter of the cutter, it is easy to see how good the process is. Therefore, we should recommend standardized design structures and dimensions for some major CNC machining parts. Fig. 3-12b shows that ball-end knives should be avoided as much as possible (in this case R=r), which is generally considered to be d=2(Rr). In addition, some CNC machine tools have the function of symmetry processing. When programming some symmetry parts, as shown in Figure 3-13, only one half of the program can be programmed, which can save a lot of programming time.

4 . Easy to measure

When designing a part structure, the possibility and convenience of measurement should also be considered. As shown in Figure 3-14, the parallelism between the center line of the measurement hole and the reference plane A is required. In the structure shown in Fig. 3-14a, the parallelism is difficult to measure due to the offset of the bottom boss. Add a symmetric process boss in Figure 3-14b and center the boss. In this case, the measurement is very convenient.

(B) Assembly and maintenance of the requirements of the part structure process

The structure of the parts should be easy to assemble and disassemble during maintenance and assembly. As shown in Fig. 3-15a, the structure on the left does not have a vent, and the air in the pin hole is difficult to discharge, so the pin is not easily loaded. The improved structure is shown in Figure 3-15a right. In Fig. 3-15b, in order to ensure that the shoulder is in close contact with the bearing surface, chamfering can be made at the shoulder of the shaft or at the opening. Figures 3 - 15c are for the fitting of two parts. Since there can only be one positioning base in the same direction, the left figure in Figures 3 - 15c is not reasonable, and the right figure is a reasonable structure. In Figure 3-15d, the screw assembly space on the left is too small for the screws to fit in. The improved structure is shown in Figure 3-15d at the right.

Figure 3-16 shows an example of a part structure for easy assembly and disassembly. In the left picture of Fig. 3-16a, the inner ring of the bearing cannot be removed because the shoulder exceeds the bearing inner ring. Figure 3-16b shows a press-in bushing. If several screw holes are designed on the end face of the housing, as shown in the right figure in Figure 3-16b, the bushing can be ejected with screws.

Third, technical requirements analysis

The technical requirements for parts are:

1 . Shape accuracy of the machined surface (including shape dimensional accuracy and shape tolerances);

2 . Mutual positional accuracy between major machining surfaces (including distance dimensional accuracy and positional tolerances);

3 . Roughness of the machined surface and other surface quality requirements;

4 . Heat treatment and other requirements.

By analyzing the technical requirements of the parts, it is possible to distinguish the major surface from the minor surface. The above four aspects all require a relatively high surface, that is, the main surface, and a variety of technological measures must be adopted to ensure the focus. After analyzing the structural and technical requirements of the parts, a preliminary outline is formed for the processing route and processing method of the parts, so as to make preparations for the next step of formulating the technological regulations.

If during the process analysis, the structural features of the parts are found to be not good, the technical requirements are unreasonable or there are other problems, the modification design can be proposed for the part design, and after the designer has agreed and fulfilled the required approval procedures, the designer will modify.

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