At presentspaceproduct processing accuracy requirements are constantly improving, and the overall structure parts in the space field are increasing, and the application of high-precision, thin-walled cavity parts in the space product industry is becoming more and more extensive. Especially the processing accuracy control of parts in the servo system is directly related to the multiple performance index requirements in the weapon system, and such parts are generally processed from titanium alloy billet as a whole, with the highest material removal rate reaching more than 85%. At the same time, a significant production feature of this kind of parts is that there are many varieties, small batches, and even single-piece production, and this structural feature and production mode determine that its manufacturing technology has always been in an unstable state, and there are always difficulties such as long processing cycle, high processing cost and difficult control of processing accuracy.

With the renewal and upgrading of weapon systems and the continuous improvement of performance, the current product parts of the shell-on servo system are more inclined to develop towards small volume and high precision, which puts forward higher requirements for the processing technology. At the same time, in order to pursue small volume and high precision structure, sometimes it is also impossible to take into account the processing technology of the product. Therefore, in servo products, there is a kind of high-precision (the dimensional accuracy requirement is above 0.01 mm) inner cavity cylindrical structure parts, which has poor mechanical processing technology, and it is difficult to meet the accuracy requirements of the parts by using traditional casting, mechanical processing methods.

1. Analysis of the part's processability and the formulation of the process plan.

(1) Analysis of part structure and processibility.

A certain ring frame (see Fig. 1) made of titanium alloy TC4-M, which belongs to single-piece small batch production, is a precision mechanical processing part, with an outer diameter size of Sφ 108 mm, a wall thickness of 4 mm, and is a kind of thin-walled difficult-to-machine material part with high requirements for shape and position accuracy and dimensional accuracy. Since it is cut from a whole piece of material, it is easy to deform during the machining process, the material has poor machinability, and the processability of the part structure is also poor, which brings great difficulty to the machining. Therefore, it is very important to select a reasonable machining method and the correct tool to ensure the machining quality.

(2) The formulation of the processing technology scheme.

By analyzing the structure of the ball ring frame parts, the following process flow is formulated: blank → rough machining (removing the excess material by wire cutting and then performing mechanical machining) → heat treatment aging → fine machining → delivery inspection. The processing difficulties are the machining of the inner cavity cylindrical surface φ 22-0.002-0.010 mm and the guarantee of the shape tolerance. Its structure and material processing technology are poor, and standard cutting tools cannot be used for mechanical processing.

Through the analysis of the structure of the ball ring frame parts， the following processing scheme is adopted for the φ 22－0.002－0.010 mm cylindrical surface and the φ 20＋0.008＋0.002 mm hole of the ball ring frame： during rough machining， the inner cavity cylindrical surface is machined twice by three-dimensional milling through positive and negative two-sided clamping， leaving a 1 mm allowance on each side， and during finish machining， the inner cavity cylindrical surface of φ 22－0.002－0.010 mm is machined by a special boring tool on a five-axis machining center， which can be precisely adjusted， thus ensuring the dimensional accuracy of the φ 22－0.002－0.010 mm cylindrical surface， and at the same time， the five-axis machining center is used for one-time clamping to complete the processing of the φ 22－0.002－0.010 mm cylindrical surface and the φ 20＋0.008＋0.002 mm hole， ensuring the tolerance requirements of the parts’ coaxiality and perpendicularity.

2. Design of Special Boring Tools

(1) Analysis of parts material characteristics.

The material of the ball ring frame parts is titanium alloy TC4-M, and its specific properties are as follows: ① Titanium alloy has poor thermal conductivity and is a poor conductor of heat. Metal materials. The contact area between the chip and the front tool face is very small during cutting, which is especially prone to thermal deformation of thin-walled parts. ② Titanium alloy has a low elastic modulus and large elastic deformation. The elastic modulus of titanium alloy is 1 078 MPa (about half of steel), and the rebound amount of the workpiece at the back tool face is large during cutting, resulting in a particularly large contact area between the machined surface and the back tool face. This causes the geometric shape and accuracy of the machined parts to be poor, the surface roughness value to increase, and the tool wear to increase. ③ Titanium alloy has strong affinity and high cutting temperature. During cutting, titanium chips and the cut surface bite into the tool material, resulting in serious adhesion phenomenon, which is prone to strong adhesion wear of the tool. Titanium alloy has strong chemical activity at high temperature, and above 600 ℃, it generates interstitial solid solution with oxygen and nitrogen, and after absorbing gas, the hardness of the surface of titanium alloy increases significantly, which has a strong wear on the tool. Therefore, it is required that the tool for machining titanium alloy should have high strength, high toughness, and high red hardness at the same time.

(2) The design principle of special boring tool.

By analyzing the performance of the material of the parts and combining the structural characteristics of the parts， a boring tool specially designed for the processing of φ 22－0.002－0.010 mm cylindrical surface is designed， and the structure of the boring tool is shown in Fig. 2， which includes a standard adjustable boring head 1， a tool bar 2， a tightening screw 4， and a tool plate 3. The tool plate is installed in the square hole at one end of the tool bar， and the tightening screw tightens the tool plate on the tool bar， connecting the standard adjustable boring head with the main shaft of the machine tool. The workpiece to be processed is clamped on the auxiliary device of the CNC machining center， so that the processing position is perpendicular to the main shaft. The boring tool described above is used to bore the inner cavity cylindrical surface of the parts， and the actual measured size can be adjusted by regulating the standard adjustable boring head during the processing， so as to ensure the machining accuracy of the parts. The adjustment range of the standard adjustable boring head is within 0.06 mm， and the accuracy of the diameter micro-adjustment is above 0.002 mm. The state of the parts during machining is shown in Fig. 3.

(3) The design principle of the boring tool bar.

According to the material characteristics of the ball ring frame parts, the tool bar should have high strength and good toughness, so the tool bar material is selected as alloy tool steel CrWMn quenched and tempered material (32～35 HRC), and the diameter of the tool bar should be less than 19 mm due to the size limit of the inner hole of the ball ring frame parts, and at the same time, it is matched with the standard adjustable boring head interface, and the clearance is controlled within 0.01 mm.

(4) The design principle of the boring tool blade.

For the machining of titanium alloy materials, the blade material is selected as YL10.2 fine grain hard alloy scrapped tool as raw material, and the front and back angles are machined on the tool grinding machine after being formed by wire cutting. This kind of material has good thermal conductivity, which is conducive to the dissipation of heat and the reduction of cutting temperature, and it also has good toughness and high red hardness.

When titanium alloy is machined, the tool's back angle α0 is the most sensitive of all tool parameters because of the large elastic recovery of the metal under the machined layer and the large work hardening. A large back angle is generally used to facilitate the tool's cutting into the metal layer and reduce wear on the back tool face, but a very small back angle (less than 15°) will cause adhesion of the metal; on the other hand, a large back angle will weaken the tool and make the tool blade prone to chipping. Therefore, most tools for machining titanium alloys use a 15° back angle. From the perspective of tool life, both α0 less than or greater than 15° will reduce the life of the lathe tool. In addition, the tool blade with α0 = 15° is sharper and can reduce the cutting temperature.

Since titanium alloy forms hard and brittle compounds with oxygen, hydrogen, nitrogen, etc. in the air during cutting, resulting in tool wear (mainly on the front face of the tool), a small value of the front angle should be used; in addition, titanium alloy has low plasticity, and the contact area between the chip and the front face is small, so a small value of the front angle should also be selected, which can increase the contact area between the chip and the front face, so that the cutting heat and cutting pressure are not concentrated near the edge, which is conducive to heat dissipation and strengthens the edge to avoid chipping due to concentrated cutting force. Therefore, when cutting titanium alloy with a hard alloy tool, the front angle γ0 is about 5° and is ground with a chamfer f (width of 0.05 to 0.1 mm), γf = 0° to 10°, and the tool tip is ground into a small value of r = 0.5 mm arc, and the edge inclination angle λ = +3°.

3. Conclusion

The processing of titanium alloy parts occupies an important position in the machinery manufacturing industry, and the turning machining of titanium alloy materials has always been the difficulty of the current processing technology. In order to meet the increasing demand for titanium alloy workpieces in the aerospace industry, titanium alloy turning machining in China must make great progress. On the basis of domestic materials, machine tools and management conditions, further strengthening the optimization of the processing route of titanium alloy materials, the selection of processing parameters, and improving the processing efficiency and product quality are important factors to promote the development of domestic titanium alloy industry and aerospace industry. The internal cavity cylindrical surface fine processing boring tool designed in this paper has a simple structure and is convenient to manufacture and use, which solves the processing technology problem of spherical ring frame parts.