The precision of the coupling hole shown in the drawing is very strict. The technical requirements are: 16 coupling holes φ 186 mm and chord pitch tolerance ≤ 0.02 mm. The surface roughness values ​​of the hole φ 186 mm and the hole φ 195 mm were R a = 1.6 μm and Ra = 3.2 μm, respectively. The drawings also require that the fittings in the 16 holes be freely interchangeable during assembly. If a hole machining accuracy and surface roughness do not meet the requirements during the machining process, it will affect the operation of the generator set, so the machining of the shaft hole is a key link.
Coupling hole structure diagram
1. The original process of the shaft hole
(1) The center body of the rotor is placed upright, and the reference surface A faces the main shaft of the boring machine, and the square box is placed back against it.
(2) Find the center of the die hole and punch the box.
(3) Use a 75° machine clamp to machine the φ 195mm counterbore, and then use a 90° knife to clear the root.
(4) Machining φ 186 mm hole, machining with single-blade high-speed steel boring tool, limiting cutting parameters and adding cutting fluid.
According to the process, the test was first carried out. Due to the large body shape of the rotor and the complex geometry, the position of the hole is far from the trampoline. Therefore, the ram and mast must be extended over a long distance during processing.
Several problems found during the test: When processing φ 195mm counterbore, the surface roughness of the test holes for two consecutive runs did not meet the requirements; when machining φ 186+0.04+0 mm holes, surface roughness also appeared. The problem of degree, verticality, and size is out of tolerance; after roughing the hole, the center position of the hole is offset, causing the φ 195mm counterbore to be different from the φ 186+0.04+0 mm hole.
2. Analysis of the cause of the error
The reasons for the hole size, accuracy, surface roughness and perpendicularity appearing in the test processing are as follows:
(1) Thermal deformation of the workpiece: When the coupling hole is rough, the temperature of the coupling hole has started to rise. Although the cutting fluid is used for cooling, the front and rear cooling of the hole is not uniform, resulting in the hole being oversized and taper.
(2) Insufficient rigidity: Because the ram and mast are extended for a long time, and the long arbor is installed, the rigidity of the mast is not enough. There are crepe lines in rough and fine boring, resulting in poor precision of the holes. At that time, after reducing the number of cutting revolutions and the amount of cutting, it was found that the dimensional accuracy and surface roughness barely met the process requirements and took a long time. According to the above method, the processing is reduced by reducing the amount of cutting, but the schedule is not allowed.
(3) Insufficient compensation: When the trampoline ram is extended, the oil pressure compensation is used to keep the ram horizontal and clamped. When the oil pressure compensation is not enough, under the action of the cutting force, if the mast and the ram are slightly offset, the verticality of the hole is excessively poor, resulting in the difference of the φ 186 mm hole and the φ 195 mm counterbore.
(4) The surface roughness is poor due to tool wear: the actual size of the hole after fine boring is 0.05 mm. For the front small and large, if the tool wears, the hole should be large and small. Explain that after the mast is extended, the mast swings large, causing the hole in the fine hole to be tapered.
3. Process improvement
The problems that occurred during the test must be improved on the basis of the original process:
(1) Unification and unification. In this way, each of the coupling holes is sufficiently cooled to avoid thermal deformation and to ensure the accuracy of the boring.
(2) When processing the φ 195mm counterbore, the main deflection angle of the original process is 75°, and the machine is clamped into a hole and added with a 90° knife to clear the root, and directly changed to 90° partial knife processing. Manual sharpening, increase the front angle of the knife to facilitate chip removal, to ensure maximum reduction of cutting force. Reduce the back angle of the tool to improve the stability of the cutting. Achieve the effect of reducing vibration. The specific parameters are: increase the rake angle of the tool to a range of 10° to 15°, ensure the tool strength when rough, select a smaller back angle of 4° to 6°, and select a fine angle of 8° to 12°. Reduce the friction between the flank and the workpiece, avoid cutting heat and reduce the surface roughness value.
(3) Cooperate with the mechanic to adjust the hydraulic pressure compensation of the ram to ensure the level and clamping effect of the ram. In this way, the mast and the ram are not excessively offset, ensuring the perpendicularity of the hole and the coaxiality between the φ 195 mm counterbore and the φ 186 mm hole.
(4) In the fine φ 186mm hole, the single-blade high-speed steel boring tool required by the original process is changed to a double-edged high-speed steel boring tool, and the boring tool is adjusted to be equal to the machined hole by the dial gauge, and the two blades are Symmetrical with the center of rotation of the mast, this reduces the cutting force and also avoids vibration, so that the swing does not occur when the mast rotates.
The shaft holes are machined in accordance with the improved process described above. The result is: after the fine boring, the difference between the front and back diameter of the hole is within ±0.01mm, the taper meets the tolerance requirement, and the surface roughness values ​​of the hole reach R a=3.2μm and R a=1.6μm, respectively. After the improved process, the dimensional tolerances and position tolerances of the holes fully meet the technical requirements of the drawings and are qualified once.
4. Conclusion
Machining the shaft hole of the Three Gorges rotor center body involves the thermal deformation, vibration and deformation of the workpiece, tooling, workpiece, operator, and workpiece during processing. Therefore, the process method and the selected tool used in the actual machining must be combined with the actual conditions of the equipment, tooling, tools and workpieces. Only when they are considered together, will the processing technology requirements specified by the drawings be met to the greatest extent.
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