In the machinery manufacturing industry, it is common to machine cylindrical parts with complex cross-sections, such as the rotors used in Roots vacuum pumps. These components are typically challenging to process due to their intricate curved surfaces. However, a conventional planer can be adapted for this purpose if properly upgraded. In this case, we performed a numerical control (NC) transformation on a BC6063 ordinary planer to enable precise machining of these complex curved surfaces.
The NC system was developed using a custom-designed single-chip microcomputer system, featuring an 80C196KC CPU. The mechanical components were upgraded with ball screws and stepper motors for accurate movement. To achieve plane curve processing, two-axis linkage control was implemented. The original longitudinal table movement was replaced with a stepping motor-driven system, while the vertical feed was handled by a modified tool holder. This setup allows for adjustments in the processing range depending on the part size.
One major limitation of traditional planers is the absence of an automatic tool-lifting mechanism. During the return stroke, the planer ram slides over the workpiece, leading to increased tool wear and potential surface damage. To address this issue, we redesigned the tool holder by removing the original lifting mechanism and replacing it with a fixed installation. Instead, the system automatically raises the tool by 0.5mm before each return stroke. A photoelectric encoder disk is connected to the stepper motor shaft to ensure precise step counting and prevent errors caused by missed or excessive steps.
The ram's position is monitored using two proximity switches installed on the ram seat. These switches (ST1 and ST2) are connected to the 80C196KC microcontroller’s P0.0 and P0.1 ports. Their signals are combined via an OR gate and fed into the high-speed input port HSI.0. By adjusting the positions of the metal blocks, the sequence of signals as the tool passes through positions I, II, and III can be controlled. The interrupt handler for HSI.0 detects the P0 port data and updates a flag unit accordingly. The flag unit values change in a predictable pattern, as shown in Table 1, allowing for accurate feed control of both the tool holder and the table.
The feed control process includes: 1) Automatically raising the tool after passing position III; 2) Lowering the tool during the ram’s return stroke and at positions II and I; 3) Completing the feed motion once the tool has been raised. The automatic feed interrupt service routine checks the flag unit value before interpolation. When the value is 12H, the tool feed is complete, and when it is 21H, the tool post and worktable have finished their motion. Additionally, the number of pulses from the encoder is compared with the actual feed amount to compensate for any lost or overshoot errors.
After the NC upgrade, the planer was successfully used to process Roots vacuum pump rotors. After initial machining on other machines, a 3mm allowance was left, which was then completed by the CNC planer. The cutting tools used had a sharp corner angle of approximately 50°, main and auxiliary relief angles between 4° and 8°, and a tip radius of 2mm. The results demonstrated that the machining accuracy met the design specifications. Moreover, the surface finish of the finished rotor reached Ra3.2 after the tool modification, proving the effectiveness of the NC transformation.
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