High-precision knuckle pin CNC turning

Figure 1 pin shaft parts Figure 1 The proposed problem is shown in Figure 1 Linde - Xiamen Forklift Co., Ltd. Static pressure transmission forklift and the steering knuckle supporting the use of the key parts pin. The part was originally purchased in Germany and was required for localization after a joint venture in China. The processing of this part has the following difficulties: (1) The used blank 42CrMoS4 tempered bar stock has a strength and hardness of 1.2GPa and 55HRC, respectively, belonging to the high-strength, high-hardness materials; (2) high requirements for machining accuracy, in the pin shaft The machining accuracy requirements in the middle two stages are f33.33-0.013mm between the 5th and 6th grades; (2) The production requires one turning to complete the machining without adding the grinding process. Therefore, in order to meet the above processing requirements, we took certain measures in the tooling, equipment and technology, and obtained a feasible machining program after repeated CNC turning experiments, and successfully achieved high-precision batch NC turning of the part. Processing.

Fig. 2 Schematic diagram of pre-turning tooling process 2 Process plan, tooling and equipment The workpieces in f33.33-0.013 have higher requirements for machining precision on the outer surfaces of the two parts, and they are between 5 and 6 grades of precision. Processing is more difficult and at the same time The concentricity on the four rotating surfaces of f40n6 (this station's note: +0.033+0.017), f33.33-0.013mm, and f32t6 (this station's note: +0.064+0.048) mm is 0.02mm. Obviously, in order to ensure the coaxiality and meet the accuracy requirements, it is necessary to complete the lathe machining in one setup. For this reason, the inner hole with the curved surface on the left end of the part must be processed before finishing. We use the device shown in Fig. 2 to complete the pre-rotation machining on the left-hand inner hole and a section of the outer surface with a diameter of f39mm on a CNC lathe. Then, both ends of the workpiece are positioned with the tip and clamped with a hydraulic chuck for finish turning (Figure 3). Since the central hole is inevitably eccentric when the center hole is punched, a special hydraulic power compensation chuck is used to ensure reliable clamping of the workpiece. Each chuck of the chuck may not have the same moving distance when clamping the workpiece, and evenly applies a clamping force to each jaw, thereby ensuring reliable clamping when the center hole of the workpiece is eccentric.

1. Claw 2. Workpiece 3. Tailstock tip 4. Dead tip 5. Hydraulic compensator chuck Figure 3 Fine turning positioning clamp

Turning speed
(m/min) Cutting amount
(mm/r) Knife quantity (mm) Rough car 120 0.35 3.0 Semi-rough car 160 0.20 0.3 Finishing car 160 0.12 0.05
Figure 4 Error distribution map on the f33.33-0.013mm outer circle To achieve machining accuracy of the two outer surfaces of f33.33-0.013mm, the positioning accuracy and repeatability of the CNC lathe must be ±0.01mm and ±0.003, respectively. Below mm, at the same time, due to the high strength and hardness of the workpiece, the cutting force is large, and the vibration of the machine tool is easily caused during turning, thereby affecting the turning accuracy, ensuring the stability during turning and having a large clamping force and a tail on the workpiece. The top of the rack is tight and a larger size CNC lathe must be used. We chose the PU-MA12-LB CNC lathe produced by Daewoo Heavy Industries of Korea. The main technical parameters are: the maximum rotary diameter of the bed f570mm; the maximum power of the spindle motor 26kW; speed range 20 to 2500r/min; positioning accuracy, X direction ± 0.01mm, ±0.02mm in Z direction; repeatability of positioning, ±0.002mm in X direction, ±0.003mm in Z direction. The turning process includes roughing, semi-finishing and finishing. The blade for turning is Sandik: DNMG150608-PM 4025 for roughing, VNMG160402-PF 4015 for semi-finishing, and TNMG160408-PF 5015 for finishing. Turning parameters are shown in the table. 3 Key Measures to Ensure Parts Processing Accuracy We found that in the turning parts, the machining accuracy of the parts is required to be higher. The dimensional fluctuations in the two parts are larger, and the overshoot is more serious, and high-precision batch processing cannot be guaranteed. To this end, we have adopted the following measures: (1) Before each official turning, the machine tool is left to run for about half an hour. After the machine tool reaches the thermal equilibrium as a whole, three to four scrapped workpieces are used to cut several times to make the tool tip. The temperature field also approximates the temperature field during the formal cutting. Empty operation of the machine can be achieved by programming an empty cycle program. (2) After the official start of turning, the turning process shall be continuous and a batch of workpieces shall be turned for turning. Because of the interruption of the turning process, even if the machine tool is kept running, the temperature field of the turning tool tip will change, and the turning accuracy will be affected due to the thermal expansion and contraction effect of the tool tip. Our turning practice has proved that this effect has a significant effect on the turning accuracy of two locations, such as f33.33-0.013mm. (3) In the turning process, the accuracy of turning the workpiece should be continuously detected. According to the trend of dimensional change after turning the workpiece, the compensation value corresponding to the finishing tool in the tool compensation parameters of the CNC system is timely corrected and adjusted. Using the above process scheme, tooling, equipment and tools, and corresponding measures, we successfully achieved high-precision batch NC turning of the part and achieved the accuracy requirements specified in the drawings. In the turning process, only a few parts due to accidental factors, the machining accuracy exceeds the requirements of the drawings. Figure 4 shows the distribution of error on a circle with a diameter of f33.33-0.013mm after a batch of parts are machined at a time. The dashed line indicates the upper and lower limits of the diameter. As can be seen from the figure, the error distribution of the size is better. By adopting this processing scheme, while ensuring high-precision batch processing of the part, the number of processing steps is reduced and the processing efficiency is improved, and the investment of the cylindrical grinder is also saved, thereby reducing the processing cost of the parts.