In the development of gas turbine turbine discs, tie rod holes are a key feature for connecting adjacent impellers, and their position accuracy directly affects the assembly quality of the gas turbine rotor. The processing technology of tie rod holes in gas turbine wheel discs is explored, and through high-precision positioning and difficult boring processing, it is expected to provide technical reference for the processing of tie rod holes in gas turbine wheel discs.
#01
As the core component of the gas turbine, the turbine impeller has a complex structure and requires high processing accuracy. Its process development and finished product manufacturing are the key. Among them, the tie rod hole serves as the connecting structure between adjacent impellers, and its importance is self-evident [1-3].
As shown in Figure 1, the turbine impeller is connected by inter-stage arc end tooth meshing, and is assembled into a rotor by inserting tie rods through 12 circumferentially distributed tie rod holes on the disk surface. Because the arc end teeth have a self-centering function, the position of the tie rod hole cannot be adjusted after the adjacent two-stage impellers are meshed. Therefore, extremely high requirements are placed on the position of the tie rod hole.
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Figure 1 Turbine shaft system connection diagram
The diameter of the tie rod hole is 42mm, which is generally processed by drilling and boring. Its maximum aspect ratio exceeds 5 times the diameter, and the surface roughness value requires Ra=1.6μm, which poses a great challenge to the boring process.
In view of the above-mentioned high-difficulty processing requirements, we mainly focus on two aspects: high-precision positioning and high-difficulty boring.
2.1 Tie rod hole position processing technology
The drawing requirements for tie rod holes are: φ42.58mm, position φ0.12mm, and cylindricity 0.012mm. In order to ensure smooth assembly, the process requirements are increased to: φ42.58mm, positioning φ 0.05mm, and cylindricity 0.012mm. Therefore, process test verification is required. This test simulates the processing of impeller tie rod holes, and the specimen structure is shown in Figure 2. A boring and milling machine is used, the clamping method is an angle iron clamping plate, and the specimen is clamped vertically.
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Figure 2: Tie rod hole test piece
(1) The traditional axially distributed hole processing method is to "turn the center" of the lever dial indicator, find the 4 o'clock position of the left and right of the sky and the earth, determine the center of the circle, use a T drill to drill the bottom hole according to the φ420mm pitch circle, and precision boring the tie rod hole .
1) Three-coordinate detection results. The maximum position value is 0.0756mm, and the outer circle D600mm roundness is 0.0056mm (see Table 1).
Table 1: The first three-coordinate detection data picture
2) Data sorting. As shown in Figure 3, the position of the hole is shifted overall to the positive Y direction.
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Figure 3: Three-coordinate detection data diagram
3) Data analysis. Taking the I position in Figure 3 as an example, the X-direction deviation is small and the offset is about 0.01mm; the Y-direction deviation is large and the offset is about 0.035mm. The Y direction is the main influencing factor of position deviation. If the Y-direction offset of 0.03mm is excluded, the positioning situation will be greatly improved, and the overall contour of the measured points is better, which shows that the repeated positioning accuracy of the machine tool itself is better. The angle data after conversion of the three-coordinate detection data is shown in Table 2. It can be clearly found that the main offset is in the Y direction.
Table 2: The first three-coordinate detection conversion data [Unit: (°)] Picture
4) Possible cause analysis. The X-direction alignment method is: the lever dial indicator touches two points on the outer circle of the specimen in the left and right directions to determine the horizontal direction; the Y-direction alignment method is: the lever dial indicator touches two points on the outer circle of the specimen in the up and down directions to determine the vertical direction. direction to determine the center position of the specimen. Here, when the Y-direction dial indicator touches the two points of the upper and lower outer circles, due to the gravity of the dial indicator stylus, it may lag or retract, resulting in a deviation between the actual position and the displayed position. After the hole processing is completed, it may It is the main factor causing Y-direction deviation. In addition, the accuracy of the dial indicator is 0.01mm, and its error will also affect the Y-direction deviation, which is a secondary influencing factor with a small impact. The X direction is in the horizontal direction, and there is no relative position deviation caused by gravity. The influencing factor is the accuracy error of the dial indicator itself, which has a smaller impact and better performance.
(2) Improvement method: Use the above-mentioned lever dial indicator to "turn over the center" to find the center and initially boring two holes at symmetrical positions. The dial indicator measures the size from the outer circle point in each direction to the farthest end of the hole through the movement of the machine tool, and then subtracts the two sets of data in the X and Y directions respectively to obtain the deviation values in the X and Y directions, and It is compensated and 12 holes are finally precision bored.
As shown in Figure 4, X-direction compensation is The data measured by this method are relative values, which can eliminate the influence of gravity on the retraction of the dial indicator. At the same time, because the measurement is the same dial indicator, the influence of the accuracy error of the measuring tool can also be eliminated. After fine boring, recheck the position data using three coordinates. Based on the above method, the second positional processing test was carried out. The processing status is shown in Figure 5.
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Figure 4: Coordinate compensation diagram
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a) State 1 b) State 2
Figure 5 Processing status
1) Three-coordinate detection results. The maximum position degree is 0.0501mm (see Table 3).
Table 3: The second three-coordinate detection data picture
2) Data sorting. The conversion data of the second three-coordinate detection are shown in Table 4.
Table 4: Second three-coordinate detection conversion data [Unit: (°)] Picture
3) Data analysis. The main offset directions of the hole are the negative X direction and the positive Y direction, and the pitch circle has good roundness. Based on this processing method, the positioning degree has been improved to a certain extent.
(3) Second improvement method Based on the above method, the third positional processing test was carried out, and the feeding method was changed to table feeding.
1) Three-coordinate detection results. The maximum position value is 0.0269mm, and the pitch roundness of 12 holes is 0.0106mm (see Table 5).
Table 5: The third three-coordinate detection data picture
2) Data sorting. The conversion data of the third three-coordinate detection are shown in Table 6.
Table 6: The third three-coordinate detection conversion data [Unit: (°)] Picture
3) Data analysis. The pitch roundness of the hole is better. This processing method significantly improves the position of the tie rod hole.
2.2 Research on difficult hole processing
The material of the fourth stage impeller of the turbine is 21501-5 (factory grade), which has high Cr and Ni content, poor material cutting performance, and the length-to-diameter ratio of the tie rod hole exceeds 5 times the diameter, making boring processing difficult.
This test is based on a homogeneous material test block of the impeller, and the same boring and milling machine is used as the equipment.
1) Walter modular boring tool bar. When using conventional boring tools to process holes with a large aspect ratio, the surface vibration patterns are obvious and the processing is not ideal. The cutting parameters are shown in Table 7.
Table 7 Cutting parameters 1
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2) Choose an anti-vibration damping boring tool bar. The cutting parameters are shown in Table 8. Kyocera inserts wear out quickly during use, so we use a combination of anti-vibration damping boring toolbar + Taguk TCMT 110204 FG CT3000 (or Sandvik Coromant TCGT 110204L-K1125) to process such deep holes. Comparison of processing quality good. The surface quality comparison is shown in Figures 6 and 7.
Table 8 Cutting parameters 2
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Figure 6: Poor surface quality
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Figure 7: Better surface quality
#03
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Application of high-precision tie rod hole machining technology on turbine impeller
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3.1 Clamping plan
The impeller adopts angle iron mounting plate method and is clamped vertically. Before clamping, ensure that the vertical surface of the angle iron is perpendicular to the Z-axis of the machine tool. Calculate and determine the position of the pad block based on the height of the V-shaped block and the overall dimensions of the impeller. All positions in contact with the workpiece are protected by copper-clad iron (see Figure 8).
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Figure 8 Turbine impeller clamping method
After clamping, align the position of the impeller pressure plate at 4 points on the left and right sides of the end face, with an error of ≤0.01mm.
3.2 Processing route
The processing route of the turbine impeller tie rod hole is drilling the bottom hole → rough boring → fine boring. First, traditional alignment methods are used to complete the processing and semi-finishing of the tie rod holes. Secondly, use the lever dial indicator and the X-axis and Y-axis of the machine tool to measure the length data of A, B, C, and D respectively (see Figure 9). Use the inner diameter micrometer to measure the two hole diameters D1 (φ42.3mm) and D2 (φ42.3mm). 4mm).
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Figure 9 Coordinate compensation data
The X-axis offset is [(A-D1/2)-(B-D2/2)]/2, and the Y-axis offset is [(C-D1/2)-(D-D2/2)]/ 2.
The data in Figure 9 is the data measured during actual processing. /2= 0.025 (mm), that is, the machine tool X-axis zero position is adjusted by 0.025mm in the positive direction. Y axis: 321.25 ‒ (42.4/2) = 300.05 (mm), 321.33 ‒ (42.3/2) = 300.18 (mm), Y axis offset (300.18 ‒ 300.05) / 2 = 0.065 (mm), that is, machine tool Y The axis zero position is adjusted 0.065mm in the negative direction.
Finally, through the above calculation, it is concluded that the workpiece circle center X is adjusted by 0.025mm in the positive direction and Y in the negative direction by 0.065mm. Carry out finishing machining of 12 holes with the new circle center obtained.
3.3 Application effects
For the impeller with tie rod holes machined through this process, after the end face teeth self-centering mesh, the tie rod holes of the adjacent two-stage impellers have a high coaxiality, the tie rods can be freely inserted, and the gas turbine rotor is successfully assembled.
#04
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Conclusion
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Research on high-precision tie rod hole processing technology for gas turbine impellers provides guarantee for gas turbine rotor assembly. At the same time, this process can be widely used in the processing of various types of axial array holes, which can effectively improve the processing quality of parts and improve the assembly accuracy of products.
