At present, the processing precision requirements of aerospace products are constantly improving. The overall structural components in the aerospace industry are increasing. The application of high-precision and thin-wall cavity parts in the aerospace products industry is more and more extensive. In particular, the control accuracy of parts in the servo system is directly related to the requirements of many performance indicators in the weapon system. These parts are generally processed from titanium alloy blanks, and the material removal rate is up to 85%. At the same time, a significant production feature of this type of parts is that it has many varieties, small batches, and even single-piece production. This structural feature and production mode have determined that its manufacturing technology has been in an unstable state. Processing and manufacturing have always had a long processing cycle. The processing cost is high and the machining accuracy is difficult to control.
With the upgrading of the weapon system and the continuous improvement of its performance, the current parts of the servo system are more likely to develop in small size and high precision, and put forward higher requirements on the processing technology. At the same time, in order to pursue a small-volume, high-precision structure, sometimes it is impossible to balance the processing technology of the product. Therefore, a high-precision (dimensional accuracy requirement of 0.01 mm or more) inner cavity cylindrical surface structural parts has appeared in the servo product. This structure has poor machining processability, and it is difficult to meet the accuracy requirements of parts by using conventional casting and machining methods.
1. Process analysis of parts and development of process plans
(1) Part structure and process analysis.
The material of a ball ring frame (see Fig. 1) is titanium alloy TC4-M, which is produced in small batches in small parts. The parts are precision machined parts. The outer dimensions of the parts are Sφ 108 mm and the wall thickness is 4 mm, which belongs to the thin-wall difficult-to-machine materials. Parts, various positional accuracy and dimensional accuracy requirements are high. Because it is a whole material cutting and forming, it is easy to be deformed during the processing, the material cutting performance is poor, and the workability of the part structure is also poor, which brings great difficulty to the processing. Therefore, the selection of reasonable processing methods and the correct tool becomes the key to processing quality assurance.
(2) Formulation of processing technology plan.
Through the analysis of the structure of the ball and ring frame parts, the following process flow is established: blank → roughing (using wire cutting to remove the margin and then machining) → heat treatment aging → finishing → inspection, the processing difficulty is the inner cavity cylindrical surface φ The machining of 22-0.002-0.010 mm and the guarantee of geometrical tolerances are poor in structure and material processing, and can not be machined by standard tools.
Through the analysis of the structure of the ball and ring frame parts, the following processing scheme is adopted for the cylindrical surface of the ball ring frame φ 22-0.002-0.010 mm and the hole of φ 20+0.008+0.002 mm: the roughing is performed by the two sides of the front and back sides. The cylindrical surface of the inner cavity is machined, leaving a margin of 1 mm on one side, and the inner cylindrical surface of φ 22-0.002-0.010 mm is processed by a special boring tool in the five-axis machining center during finishing. The special boring tool can perform high-precision fine adjustment. Therefore, the dimensional accuracy of the φ 22-0.002-0.010 mm cylindrical surface is ensured, and the φ 22-0.002-0.010 mm cylindrical surface and the φ 20+0.008+0.002 mm hole are processed in one clamping by the five-axis machining center to ensure the parts. Coaxiality and perpendicularity tolerance requirements.
2. Design of special boring tool
(1) Analysis of the material properties of the parts.
The material of the ball ring frame is titanium alloy TC4-M, and its specific characteristics are as follows: 1 The poor thermal conductivity of titanium alloy is a poor thermal metal material. When cutting, the contact area between the chip and the rake face is small, which is especially easy to cause thin. Thermal deformation of the wall member. 2 Titanium alloy has low elastic modulus and large elastic deformation. The elastic modulus of titanium alloy is 1 078 MPa (about 1/2 of steel). The amount of springback of the workpiece near the flank during cutting is large, resulting in the processed surface and The contact area of ​​the flank face is particularly large. The geometry and precision of the workpiece are poor, the surface roughness value is increased, and the tool wear is increased. 3 Titanium alloy has high affinity and high cutting temperature. When cutting, the titanium chips and the cut surface layer are engaged with the tool material, which causes a serious sticking phenomenon, which easily causes strong bond wear of the tool. Titanium alloy has high chemical activity at high temperature. When it is above 600 °C, it forms a solid solution with oxygen and nitrogen. After the gas is absorbed, the hardness of the surface of the titanium alloy increases obviously, and the knife has a strong wear effect. Therefore, it is required that the tool for processing titanium alloy has high strength and high toughness, and also has high red hardness.
(2) Design principle of special boring tool.
Through the analysis of the material properties of the parts, combined with the structural characteristics of the parts, the boring tool designed for the cylindrical surface of φ 22-0.002-0.010 mm is designed. The structure of the boring tool is shown in Figure 2. It includes the standard adjustable boring head 1. The rod 2, the fastening screw 4, and the blade 3 are mounted in a square hole at one end of the cutter bar, and the fastening screw fastens the blade to the cutter bar to connect the standard adjustable hammer head to the machine tool spindle. The machined parts are fastened on the auxiliary parts of the CNC machining center, so that the machining part is perpendicular to the main shaft. The cylindrical surface of the inner cavity of the parts is boring by the above-mentioned boring tool. The machining process can be adjusted according to the actual measurement size. The boring head fine-tunes the blade to ensure the machining accuracy of the part. The standard adjustable boring head can be adjusted within 0.06 mm, and the diameter fine adjustment accuracy is above 0.002 mm. The state of processing is shown in Figure 3.
(3) The design principle of the boring bar.
According to the material characteristics of the ball and ring frame parts, the tool bar should have high strength and good toughness. Therefore, the tool bar material is made of alloy tool steel CrWMn material (32~35 HRC), which is limited by the inner hole size of the ball ring frame part. The diameter of the shank should be less than 19 mm, and it can be combined with the standard adjustable boring interface to control the gap within 0.01 mm.
(4) The design principle of the file.
For the processing of titanium alloy materials, the blade material is selected from YL10.2 fine-grained carbide scrapping tool as the raw material. After forming by wire cutting, the front and rear corners are processed on the tool grinding machine. These materials have good thermal conductivity, which is good for heat dissipation and lower cutting temperature, while having good toughness and high red hardness.
When cutting titanium alloy, the tool back angle α 0 is the most sensitive of all tool parameters, because the metal elastic recovery under the cutting layer is large and the processing hardness is large. Generally, the large relief angle can make the cutting edge easy to cut into the metal layer and reduce the back knife. The wear of the surface, but the back angle is too small (less than 15 °) will appear metal adhesion; and the back angle is too large, the tool will be weakened, the blade is easy to collapse. Therefore, most cutting tools for titanium alloys use a 15° relief angle. In terms of tool durability, α 0 is less than or greater than 15°, which reduces the durability of the turning tool. In addition, the tool edge with α 0 of 15° is sharper and reduces the cutting temperature.
Since the titanium alloy forms a hard and brittle compound with oxygen, hydrogen, nitrogen, etc. in the air during the cutting process, causing tool wear (mainly on the rake face of the tool), a small value of the rake angle should be used; The plasticity is low, the contact area between the chip and the rake face is small, and a small value of the rake angle should also be used for this purpose, which can increase the contact area between the chip and the rake face, so that the cutting heat and the cutting pressure are not excessively concentrated on the cutting edge. In the vicinity, it is beneficial to heat dissipation and strengthen the cutting edge to avoid chipping due to the concentration of cutting force. Therefore, when machining a titanium alloy with a cemented carbide tool, take the rake angle γ 0=5° and grind the chamfer f (width 0.05 to 0.1 mm), γ f=0° to 10°, and the tool tip is ground into r =0.5 mm small arc, edge angle λ = +3°.
3. Conclusion
The processing of titanium alloy parts plays an important role in the mechanical manufacturing industry. The cutting of titanium alloy materials has always been a difficult point in the current processing technology. In order to meet the growing demand for titanium alloy workpieces in aerospace, China's titanium alloy cutting must make significant progress. On the basis of domestic materials, machine tools and management, further strengthening the optimization of processing route of titanium alloy materials, optimization of processing parameters, improvement of processing efficiency and product quality are important for promoting the development of domestic titanium alloy industry and aerospace industry. factor. The inner cylindrical surface finishing boring tool designed in the paper has the advantages of simple structure, convenient manufacture and use, and solves the processing problem of the ball ring frame parts.
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