CNC Knowledge: Method for precision machining of the spherical surface of an aircraft engine piston pump

01

Preface

As a typical friction pair of aircraft parts, spherical fit involves many parts. It is commonly found in parts such as piston shoes, ball block supports, ball block cages, etc. In the traditional process, let it be an outer spherical surface. or inner spherical surface, manual grinding is mainly used as the final finishing process, but manual grinding has disadvantages such as low efficiency and unstable quality, which have become a bottleneck in the finishing process. At the same time, the precision level of fitting the spherical surface of the parts will affect the performance indicators such as flow and pressure, and even the wear between the two will reduce the efficiency and service life of the entire pump, so the treatment method of the spherical surface is crucial. The process flow of spherical surface processing is “rough machining → heat treatment → (semi-finishing) → finishing”. This article mainly analyzes some typical spherical structures of aircraft engine piston pumps and several commonly used finishing methods. Based on several spherical surface processing methods, the processes of turning, milling, grinding and grinding are analyzed, compared and objectively evaluated.

02

turning

When using CNC lathes for processing spherical surfaces, the trajectory method is usually used, that is, the processing trajectory is determined via CNC macro programs or point programming to obtain spherical surface processing, which generally requires higher precision of the machine tool.

Spherical surfaces processed by CNC lathes are generally used for copper parts. The most widely used are ball stop parts, made of QSn7-0.2. For such parts (see Figure 1), the ball diameter tolerance is generally less than 0.015 mm and the surface area. roughness value Ra=0.4 μm, due to the subsequent surface heat treatment of the spherical surface, only pre-coating treatment is necessary.

Figure 1 Parts

Since the tin bronze material is soft, grinding and grinding are generally not used for processing, so it is processed directly to the final size through a CNC lathe.

Tool selection: The diamond blade tip has high strength, good wear resistance and chipping resistance, and excellent cutting effect, achieving stable and long-lasting processing . It is suitable for high-speed processing of aluminum alloys, brass and other non-metallic materials. ferrous metals, plastics and hard materials. Alloy turning. The tool tip adopts a 35° diamond-shaped blade with a rounded corner R (0.1 ~ 0.2) mm.

During finishing turning, the speed is generally controlled at 2000 rpm, and the feed rate is 0.015 mm/r. Usually, semi-finishing turning leaves 0.05-0.10mm machining allowance for the finishing process. the part can generally be controlled within 0.4 μm.

Although the surface roughness of these parts after processing meets the requirements, the turning lines are obvious. The surface roughness Ra value of the parts can be reduced to 0.1 µm through the subsequent finishing process.

03

Milling

Use a milling cutter to mill a spherical surface. The cutter and the workpiece rotate at the same time, the center of rotation of the cutter intersects the center of rotation of the workpiece. on the surface of the part is part of the spherical surface. Generally, end mills and ball mills are used, or using multi-axis linkage equipment such as coordinateless machining centers to mill spherical surfaces. This method requires relatively high precision for milling cutters and machine tools and is rarely used in general production shops.

04

sharpening

Spherical grinding method[1]It is widely used, including trajectory method, development method and forming method.

4.1 Trajectory correction

Path grinding (see Figure 2) is basically a linear contact between the grinding tool and the workpiece, and the contact line moves along the spherical contour to complete the grinding of the entire spherical surface.

Figure 2 Path grinding method

1) Grinding tools: flat grinding wheel or grinding wheel with pointed or semi-circular shape.

2) Installation: The workpiece is clamped on the chuck and the top grinding method is used. The connection line is coaxial with the workpiece chuck and parallel to the axis of the grinding wheel.

3) Movement: The grinding wheel rotates at high speed after starting, and the workpiece and chuck rotate at low speed while rotating back and forth around another vertical center line of the ball. As the grinding wheel advances, a spherical surface is generated and ground.[2]。

4) Features: ① Suitable for grinding larger diameter or wider spherical surfaces. ②Grinding resistance is low. ③The accuracy of the spherical contour is determined by the positioning accuracy, the movement accuracy of the CNC machine tool, the shape accuracy of the grinding wheel and the contour tracking accuracy of the system. ④The same specification grinding wheel can grind spheres of any size, with less abrasive consumption, low investment and good economy. However, the grinding wheel is in direct contact with the workpieces, so the grinding efficiency is low and is not suitable for batch and large-scale sphere production.

4.2 Grinding generation method

Generation method Grinding (see Figure 3) of spherical surfaces is also called generation method. This method requires that the machine tool has at least one linear feed X axis and one C axis which drives the rotation of the part.

Figure 3 Grinding generation method

1) Grinding tools: cup-shaped grinding wheel or bowl-shaped sanding disc (hereinafter collectively referred to as grinding wheel).

2) Installation: The rotation axis of the grinding wheel and the rotation axis of the workpiece form an angle α, and the intersection point is the center of the sphere to be processed.

3) Movement: The grinding wheel rotates around the axis of the grinding head spindle at high speed and the workpiece rotates around the axis at low speed in one direction. As the grinding head advances, the entire spherical surface is ground.

4) Features: High grinding efficiency, high precision and good surface roughness are the significant advantages of this grinding method. If the spherical surface specifications change, the wheel size must be changed. This method is suitable for mass production of spherical parts and is less economical. When grinding by the generation method, the diameter of the grinding wheel is related to the size of the spherical surface of the workpiece, as shown in Figure 4, L≤D sand≤(D2-d2)1/2, where D is the diameter of the spherical surface (mm) and d is the diameter of the chuck (mm), L is the thickness of the ball (mm) and D is the diameter of the cup-shaped grinding wheel (mm).

Figure 4 Grinding scheme

During the whole grinding process, the center of the grinding wheel axis must pass through the center of the workpiece sphere. In addition, the geometric relationship between the size of the final feed position H, the inner diameter of the cup-shaped grinding wheel D and the spherical diameter D of the workpiece is H = (D2 – D sand 2) 1/2 /2. By adjusting the H value, grinding can be achieved by cutting spherical workpieces of different diameters. In the same way, the value of the feed quantity ΔH can be deduced based on the value of the ball diameter after processing the first workpiece, i.e. ΔH =[（D12－D砂2）1/2－（D2－D砂2）1/2]/2。

If the workpiece is 1/2 spherical or larger than 1/2 spherical, the workpiece and grinding wheel must be adjusted to a certain angle, as shown in Figure 5.

a) The first situation

b) Second case

Figure 5 Analysis of grinding conditions

Cup wheel diameter D sand =[D（D/2＋K）]1/2, when the spherical diameter of the workpiece is larger than the semicircle, K is a positive value, and when it is smaller than the semicircle, K is a negative value. Maximum inner diameter of the cup-shaped grinding wheel D sand = {D[D＋（D2－d2）1/2]/2}1/2. The corresponding angle between the axis of the grinding wheel and the spherical axis of the part is α=arcsin (D sand/D). The selection range of α is arcsin (1/2+K/D) 1/2≤α≤arcsin{.[D＋（D2－d2）1/2]/(2D)}1/2.

As can be seen from the above, when the grinding wheel grinds a spherical surface, the diameter of the grinding wheel must be within a certain range. The linear speed of the grinding wheel can grind the workpieces normally and cannot interfere with the machine tool. Pieces. Select the grinding wheel inner diameter D to the maximum, the cutting fluid can enter the inner hole of the grinding wheel from the workpiece side, which can maximize the cooling effect. In order to extend the life of the grinding wheel, it is generally necessary to make the inner hole of the grinding wheel into a bevel cut shape.

The inner hole of the grinding wheel is dressed with diamond. When grinding the outer spherical surface, the hole diameter of the grinding wheel can be selected to be slightly smaller to accurately adjust it to the required size. The grinding wheel should be softer grinding wheel (K grade, L grade), vs should be 15~20 m/s, v W should be 1~5 m/min, ap=0.02~0.04mm for coarse grinding, ap=0.002~0.005 for fine grinding mm. The processing parameters for grinding the spherical surface of the piston using the generation method are shown in Table 1.

During processing, the axis of the grinding wheel should be at the same height as the axis of the workpiece to ensure the roundness of the processed spherical surface, and the processed texture will be a cross mesh. When the surface of the workpiece has a concave-convex pattern, it indicates that the center of the wheel is higher or lower than the center of the workpiece.

Table 1 Processing parameters of piston spherical surface grinding by generation method

The methods of internal cylindrical grinding and cylindrical grinding are similar, and the calculation methods are also the same.

4.3 Shape rectification

Shape grinding[3]That is to say, the grinding wheel is cut into an arc shape, the grinding wheel is cut into a concave spherical shape, the workpieces are convex spherical, and the spherical diameter of the grinding wheel is equal to the spherical diameter of the workpiece, as shown . in figure 6.

1) Grinding tools: shaped grinding wheels.

2) Movement: During the grinding process, the workpiece rotates in the C direction, the grinding wheel rotates at high speed, advances along the X axis, and cannot move along the Z axis.

3) Characteristics: ① The width of the grinding wheel should be greater than the thickness of the workpiece. ②The larger the grinding width of the grinding wheel, the greater the grinding resistance of the workpieces. ③The accuracy of the workpiece contour depends on the accuracy of the shape of the grinding wheel and the accuracy of the feed position of the machine tool. ④Normally, one type of part corresponds to one type of forming wheel, which is dedicated. ⑤ During the grinding process, neither the grinding wheel nor the parts can move in the Z direction. After the processing is completed, the surface roughness value of the parts will be relatively high.

Figure 6 Schematic of form grinding

For shape grinding, the spherical diameter of the workpiece should not be too large, which is more suitable for grinding small spherical surfaces.[4]the X-axis feed accuracy of the machine tool is high, this method has high grinding efficiency and is suitable for mass production. Since form grinding generally requires the workpiece profile to be processed in a single step, the grinding wheel is usually made of CBN. The material is harder, the grinding wheel wear is low, and the surface quality after grinding is high. , the surface roughness value Ra can generally be guaranteed within 0.1 μm. The processing parameters of the spherical surface of the forming grinding piston are shown in Table 2.

Table 2 Processing parameters of spherical surface of shape grinding piston

05

Grind

Grinding is mainly a method used for processing spherical surfaces with high precision. Generally, spherical surfaces require better contours and dimensional tolerances. General grinding methods can be divided into manual grinding and mechanical grinding. Manual grinding of spherical surfaces requires a large amount of labor, has low efficiency and unstable processing precision. The contour of the workpiece is determined by the mutual movement of the grinding ball and the workpiece. If higher accuracy requirements are required, frequent adjustments are necessary. When the surface of the grinding ball is larger than the machined surface of the workpiece, the contour of the workpiece depends on the grinding ball itself, and the grinding ball has a lower hardness than the workpiece and consumes a lot of money. Generally, one or more grinding balls are required for each part, and the cost is relatively high.

The principle of mechanical grinding is similar to grinding. The difference is that the equipment used for grinding changes from a grinding wheel to a cast iron grinding tool or a grinding head equipped with a soft grinding bar. It has the following characteristics: ① Since the grinding balls are generally made of cast iron and are manually ground, the grinding force is small, so the deformation produced during processing is small, and higher precision can be obtained. ②The surface of parts processed by grinding method has good corrosion resistance and wear resistance. ③Parts of different shapes can be processed depending on the shape of the grinding tool. ④Easy to use and does not require complex machine tools. ⑤ Can process parts of various materials, and may also have higher precision requirements.

06

Conclusion

There are many spherical structures in the piston pump parts of aviation engines, such as the outer spherical surface of the cage, the inner spherical surface of the cage, the spherical steel surface of the ball block, the surface copper spherical surface of the ball block, the inner spherical surface of the sliding pad, outer spherical surface of the piston and inner spherical surface of the bearing. This paper conducts experiments on turning, milling, grinding and grinding processes on typical workpieces to discover a suitable spherical surface processing method and take advantage of grinding processing, using mechanical grinding instead of grinding manual to improve the efficiency of spherical surface processing and shorten the service life. processing time. cycle to ensure the stability of the quality of the final product.