Machining of micro spherical parts

2021/08/19


This paper proposes a new processing method using optical surface grinder with self-made special tooling, which has the advantages of high qualification rate, simple operation and controllable processing accuracy, and solves the problem of processing tiny ball head parts.



1 Preface
The ball can be seen as a circular arc rotating around its axis to become a surface. Ball head class parts due to the existence of complex surfaces, so usually use CNC machining to ensure the size, position accuracy and surface quality requirements. With the continuous improvement of machine tool performance and tool material and performance, the ball head class parts with ball head diameter ≥8mm and material hardness ≤64HRC can be processed by traditional methods, including: ① using omnidirectional turning tool or cycloid tool for CNC lathe processing. ② using the rotary milling processing with high requirements for tool position adjustment. ③Processing by profile grinding with special grinding wheel dressing device. ④Spreading method grinding processing with customized cup-shaped grinding wheels. ⑤ Mass production filing processing using a special grinding block (only hemispherical shaft ends with regular shapes can be processed).
However, the analysis of the structural characteristics of the tiny spherical parts suggests that it is difficult to realize the precision forming processing of the tiny spherical parts by using the traditional processing method.

2 Micro ball head parts characteristics
The characteristics of micro ball head parts: ① in the shaft end has a curved ball head structure, divided into small semi-spherical, hemispherical and large semi-spherical structure, as shown in Figure 1, the hardness of about 60HRC. ② ball diameter is small, the general ball diameter ≤ 3.5mm. ③ processing ball transition section by radial force section easy to break. ④The size of the part is small, and the tool structure has a greater impact on the workpiece processing. ⑤ forming grinding wheel dressing is more difficult.



From the characteristics of the tiny ball head class parts, it can be seen that the traditional processing method for forming the tiny ball head at the end of the shaft appears to be incompetent.

For a small number of general precision requirements of the tiny ball head class parts, can be used under the soft state carving and milling rough machining, heat treatment after polishing to achieve. But for the high precision requirements of the tiny ball head class parts, still need to find a more suitable processing method.

Because of the difficulty of precision hard machining of tiny ball-like parts, it is not easy to achieve batch processing, so according to the formation mechanism of the ball end of the shaft (let the bar material high-speed rotation, in the form of circular arc envelope to form a ball head) and the processing principle of the equipment, proposed a tiny shaft end of the large half ball-like ball parts processing method: the production of special tooling, let the rod high-speed rotation, with the optical curve grinder set point into line processing The principle is to process the ball head in the form of an envelope for precision forming.

3 Optical curve grinding machine processing method
The optical curve grinder is mainly composed of a bed, a coordinate table, a grinding wheel table and a projection system. The table can move in longitudinal, transverse and vertical directions; the grinding wheel can do the main motion of rotation and linear motion on the guide rail of the grinding wheel table; the grinding wheel table can do longitudinal and transverse feeding motion and rotation around the X/Y/Z axis. The grinding of multi-circular surfaces, logarithmic surfaces or Archimedean spiral surfaces can be realized.
Optical curve grinder processing uses an optical projection magnification system to project the workpiece magnification onto the screen against the magnified image of the workpiece on the screen, and operates the grinding wheel to grind the workpiece to grind off the excess material beyond the contour line of the magnified image until the contour of the object image fully coincides with the magnified image. It is possible to compare 25 times or 50 times magnification. The types of diamond grinding wheel structures used in optical curve grinders are flat grinding wheels, sharp grinding wheels, etc., as shown in Figure 2.


According to the structure of the grinding wheel, the sharp grinding wheel shown in Fig. 2b can be used with the digital control system to grind surfaces with a diameter of ≥0.2 mm. If combined with the rotation of the workpiece, the corresponding curved rotary surface can be shaped and processed.

4 Machining example analysis
The structure of a ball head rod for a certain type of product is shown in Figure 3. The surface roughness of the spherical surface is Ra=0.4μm, and the spherical surface should not have obvious cutter patterns such as joint marks; the size, position accuracy and hardness of the spherical part are high, the diameter of the transition section between the spherical head and the shaft is only 1.2mm, and the machining quantity is 500. The following difficulties exist in the processing of this part: ①Higher dimensional accuracy requirements for the ball head and smaller shaft diameter of the part. ②The diameter of the transition section of the ball head is only 1.2mm, the processing stiffness is low, and the force is easy to break. ③ processing allowance, the processing technology, clamping convenience, stability and bar rotation radial runout and other aspects of the higher requirements. ④Higher requirements for the surface roughness quality of the ball head.

If the optical curve grinder is used to realize the precision forming processing of the ball head bar, special tooling also needs to be made to ensure that when the grinding wheel moves along the contour of the workpiece, the ball head bar rotates around the axis to form the ball head structure. Due to the small size of the ball-head rod, if the grinding wheel moves up and down along the grinding wheel holder, it will reduce the effective contact time between the grinding wheel and the rotating workpiece. Therefore, in order to improve the processing efficiency, the height position of the grinding wheel is adjusted and fixed during the workpiece processing, and the reciprocating motion of the grinding wheel along the grinding wheel holder is canceled, so that the grinding wheel is always in effective contact with the surface of the workpiece when rotating, and the grinding wheel only moves along the forming contour line when rotating at high speed.
5 Tooling design
Adopt small power motor (speed 1400r/min) and small lathe chuck, fix the chuck on the working table, use the spring chuck (can clamp various workpieces within 10mm diameter) to clamp the workpiece. Adjust the table height so that the projection focal distance of the workpiece is within the normal moving range of the grinding wheel. Through the belt pulley drive, the chucking speed reaches 2000 r/min, and the direction of rotation is opposite to the direction of rotation of the grinding wheel. After debugging, the radial circular runout of the workpiece is ≤0.005mm, and the structure of the workpiece is shown in Figure 4. 

6 Process design
Considering the number of workpieces, ball head rod and grinding wheel structure, due to the small round angle of the sharp grinding wheel, processing loss is large, in order to reduce the loss of the sharp grinding wheel and improve the processing efficiency, choose to use flat grinding wheel processing when removing the large residual, step-by-step processing process is shown in Figure 5. Develop the ball head rod processing process as follows.

(1) Rough machining of ball head rod with flat grinding wheel, leaving a margin of 0.02mm for A and B sections and 0.1mm for C section, with a grinding wheel translation speed of 3mm/min and a grinding wheel speed of 12000r/min.

2)Processing the contour of the ball-head rod with the sharp grinding wheel, processing the A and B sections to control the dimensional accuracy, leaving a margin of 0.01mm in the C section, the grinding wheel translation speed of 1.5mm/min, and the grinding wheel rotation speed of 12000r/min.

(3) Finishing the ball head of C section with a sharp grinding wheel, with a grinding wheel translation speed of 0.5mm/min and a grinding wheel rotation speed of 12000 r/min. 

7 Machining effect inspection
The ball head rod is processed in steps, and the surface quality of the machined cylinder is good because the flat grinding wheel has automatic regrinding function; the sharp grinding wheel is mainly responsible for finishing, with small machining allowance and small grinding wheel loss. During the process of machining 500 ball-ended rods, the sharp grinding wheel was resharpened 4 times. The machined parts are shown in Fig. 6, and the test data of the parts are shown in Table 1. it can be seen from the test results that this method of machining ball-head rods meets the design requirements.



8 Conclusion
Due to its own structural characteristics, the micro ball-tip parts cannot be processed by the traditional shaft-end ball-tip parts. By using the optical curve grinder with the homemade special tooling and designing the step-by-step processing process, the radial force on the ball head rod is small and the transition section is not easy to break. The grinding method has solved the problem of processing small-sized ball-end parts with the advantages of high qualification rate, simple operation, controllable processing accuracy and good product consistency.