Abstract
This paper focuses on the processing technology of cast steel anchorages for self-anchored suspension bridges. As one of the core components of such bridges, cast steel anchorages integrate multiple functions including main cable anchorage, ballast, and restraint. These components feature complex external contour angles and intricate relative positions of anchor holes. Taking a domestic suspension bridge cast steel anchorage as an example, this paper proposes efficient processing techniques for complex slopes and anchor space holes in cast steel anchorages, with the aim of meeting design requirements, enhancing production efficiency, and reducing labor intensity. The technology significantly improves production efficiency and reduces labor intensity during the mechanical processing of products, providing valuable insights into the processing of complex slopes and spatial holes.
1. Introduction
Self-anchored suspension bridges, with their unique structural design and excellent mechanical performance, occupy an important position in modern bridge construction. In domestic self-anchored suspension bridges, cast steel anchorages are commonly used in the main cable anchorage system. As a key component of self-anchored suspension bridges, the manufacturing quality of cast steel anchorages directly affects the overall performance and service life of the bridge. However, the processing of cast steel anchorages faces numerous challenges, such as large structural dimensions, numerous external contour angles, and complex relative positions of anchor holes.
This paper takes a domestic single-tower spatial cable self-anchored suspension bridge as an example, analyzes the processing difficulties of its cast steel anchorages, selects appropriate processing equipment, formulates reasonable processing techniques, and adopts corresponding technological measures. Under the premise of ensuring product quality, an efficient processing technology for cast steel anchorages is proposed to provide strong support for the construction of self-anchored suspension bridges.
2. Overview of Cast Steel Anchorage
2.1 Structural Characteristics
Cast steel anchorages serve as one of the core components of self-anchored suspension bridges, integrating multiple functions such as main cable anchorage, ballast, and restraint. They are characterized by numerous external contour angles and complex relative positions of anchor holes.
The cast steel anchorage is located at the end of the bridge beam. Its primary function is to anchor the发散的主缆索股, which are spatially linear. After passing through the anchor holes, the cables are anchored using anchor nuts.
2.2 Application and Function
The cast steel anchorage is positioned at the end of the bridge beam. Its main role is to anchor the dispersed main cable strands, which exhibit a spatial linear form. The main cable发散s into individual strands through a fan-out saddle or fan-out sleeve, assuming a near-conical shape. These strands pass through anchor holes and are anchored using anchor nuts, as shown in their usage state.
3. Difficulties in Processing Cast Steel Anchorage
3.1 Large Dimensions and Mass
Compared to cast steel anchorages used in previous self-anchored suspension bridges, the one studied in this research exhibits larger structural dimensions and mass. Its dimensions are 4425 mm × 3945 mm × 1200 mm, with a product mass of 54 t and a blank mass of 60 t, Depicts a conventionally sized cast steel anchorage used in a domestic self-anchored suspension bridge, with dimensions of 2690 mm × 2930 mm × 804 mm, a product mass of 14.2 t, and a blank mass of approximately 16 t.
3.2 Complex Shape and High Precision Requirements
The processing datum plane of the cast steel anchorage is the anchor face. The complexity of its shape is mainly manifested in the non-90° angles between the top and bottom plates and the anchor face. Its shape resembles an irregular cuboid, with both side webs perpendicular to the anchor face and the bottom plate. However, the bottom plate has a longitudinal slope α1, while the top plate has both a longitudinal slope α2 and a lateral slope α3.
3.3 Spatial Structure of Anchor Holes
After the cast steel anchorage anchors the main cable of the bridge, each strand assumes a specific angle, and the anchor holes must adapt to the linear shape of the strands. Otherwise, interference will occur between the strand anchors and the anchor holes, preventing strand anchorage. Therefore, the accuracy of the centerline of the anchor holes is crucial.
The angle of the anchor hole can be represented by the angle α4 between the projection line of the anchor hole centerline on the anchor face and the X-axis, as well as the angle α5 between the projection line of the anchor hole centerline on the anchor face and the anchor hole centerline itself, forming a composite angle. Each of the 61 anchor holes in a single cast steel anchorage has unique α4 and α5 angles.
4. Solution to Processing Difficulties
4.1 Dimensional and Mass Solutions
To address the large dimensions and mass of the cast steel anchorage, and to ensure processing precision and efficiency, a CNC floor-type boring and milling machine equipped with a rotary table was selected. The machine’s X-axis and Y-axis strokes should be no less than 5000 mm, the Z-axis stroke should exceed the anchor face thickness by 200 mm, and the rotary table’s load-bearing capacity should be above 60 t, with a surface area of no less than 5000 mm × 5000 mm.
Based on the above analysis, the TK6926 CNC floor-type boring and milling machine was selected for actual processing. It features a spindle diameter of 260 mm, X-axis stroke of 18 m, Y-axis stroke of 6 m, Z-axis stroke of 1.6 m, and positioning accuracy of 0.02 mm/1000 mm for the X, Y, and Z axes. The machine is equipped with a 140 t CNC rotary table, a working surface of 5000 mm × 5000 mm, a load-bearing capacity of 140 t, a stroke of 3 m, and a B-axis rotational positioning accuracy of 0.003°. The machine’s数控 system boasts rapid response, superior linkage performance, and high control accuracy. Additionally, the workshop is equipped with a 120 t crane to meet the processing needs of the cast steel anchorage.
4.2 Exterior Processing Technique
The cast steel anchorage has a total of six exterior processing surfaces, including the anchor face, grid face, and both side web faces, which are mutually perpendicular or parallel planes. However, the bottom and top plates are not perpendicular to the anchor face, which serves as the processing datum.
If the bottom and top plates were processed using conventional methods, it would be necessary to first mill alignment benchmarks at suitable positions and then process the top and bottom plates separately in a direction perpendicular to the spindle, based on these benchmarks. This method would require multiple clampings and precise alignment according to the angles specified in the drawings, increasing workload and alignment difficulty.
Combining the existing processing machine, it was determined that the exterior processing of the cast steel anchorage would involve placing the workpiece on the rotary table with the anchor face perpendicular to the machine spindle. By fully utilizing the rotation function of the rotary table, all exterior surfaces could be processed with only two clampings.
During the first clamping, the bottom plate is placed on the rotary table, with clamping pads arranged to form an angle α1 between the bottom plate and the rotary table surface, ensuring the anchor face is perpendicular to the spindle. In this state, the anchor face is processed. Since both side web faces are perpendicular to the anchor face and perpendicular to the XZ plane of the machine in this clamping position, after processing the anchor face, the rotary table is rotated clockwise by 90° to process one side web face, and then counterclockwise by 180° to process the other side web face. The actual processing during the first clamping.
During the second clamping, one side web is placed on the rotary table with the grid face perpendicular to the spindle. In this state, the grid face, which represents the thickness dimension of the cast steel anchorage, is processed. In this clamping position, the bottom plate is perpendicular to the XZ plane of the machine and forms an angle of 90° + α1 with the grid face. Therefore, rotating the rotary table clockwise by 270° + α1 aligns the bottom plate for processing.
According to the angles, in the clamping position, rotating the rotary table clockwise by 270° + α2 aligns the top plate for processing. However, due to the lateral slope α3 of the top plate, it is necessary to employ a sloping tool path in the YZ plane during spindle operation, processing the top plate from top to bottom, as illustrated. This results in a slightly wavy surface profile on the processed face. To ensure the flatness and surface roughness of the processed face, the step distance of the tool path is adjusted to control the maximum difference between wave peaks and troughs, meeting the technical requirements for surface quality.
Compared to conventional processing methods, this approach reduces the number of workpiece clampings by four, saving approximately 16 hours of clamping time, as shown in Table 1. It eliminates the need for alignment benchmarks, reducing cumulative errors from multiple clampings, saving alignment time, and lowering alignment difficulty. This method offers advantages in ensuring exterior dimensional accuracy, improving production efficiency, and reducing labor intensity.
Table 1: Comparison of Clamping Times for Anchorage Exterior Processing
| Method | Number of Clampings | Clamping Time per Operation (h) | Total Clamping Time (h) |
|---|---|---|---|
| Conventional Method (without Rotary Table) | 6 | 4 | 24 |
| Proposed Method (with Rotary Table) | 2 | 4 | 8 |
4.3 Anchor Hole Processing Technique
Each cast steel anchorage in this project has 61 anchor holes, with unique α4 and α5 angles for each hole’s centerline. In conventional mechanical hole processing, the tool rotation plane must be perpendicular to the hole centerline. However, the centerline of each anchor hole is not perpendicular to the anchor face.
If conventional methods were used for anchor hole processing, there would be two approaches:
- Method One: Adjust the workpiece angle for each anchor hole to align the hole centerline with the tool rotation plane. Due to the large dimensions and weight of the cast steel anchorage, this method, though theoretically feasible, would involve numerous workpiece adjustments, high alignment difficulty, and significant workload, leading to extremely low processing efficiency.
- Method Two: Use a universal milling head to adjust the tool angle for each anchor hole, aligning the tool rotation plane with the hole centerline. While this method allows precise hole positioning, it requires multiple adjustments of the angle milling head for each hole, increasing workload and reducing efficiency.
Both methods suffer from low efficiency, high workload, intense labor intensity, and long processing times. Analyzing the position and angle relationships between the anchor holes and the anchor face, the following technique was determined:
The composite angle of the anchor hole centerline is decomposed into projection angles in the XZ and YZ planes. With the anchor face perpendicular to the spindle during clamping, X, Y, and Z three-axis linked sloping tool path boring is employed for processing. Although the actual hole shape may appear elliptical, the roundness meets drawing requirements. This method aligns the workpiece clamping state with that for anchor face processing, eliminating the need for additional clamping after anchor face processing and allowing for the completion of all anchor hole processing in this state, significantly improving production efficiency and reducing labor intensity.
Compared to conventional processing methods, this technique reduces the number of workpiece clampings or universal milling head adjustments by 60, saving approximately 300 hours of clamping time compared to Method One and 120 hours compared to Method Two, as shown in Table 2. It reduces alignment difficulty and labor intensity, greatly enhances anchor hole processing efficiency, and avoids cumulative errors from multiple clampings, facilitating the accurate positioning of anchor holes.
Table 2: Comparison of Clamping Times for Anchor Hole Processing
| Method | Number of Clampings | Clamping Time per Operation (h) | Total Clamping Time (h) |
|---|---|---|---|
| Method One | 60 | 5 | 300 |
| Method Two | 60 | 2 | 120 |
| Proposed Method | 0 (same clamping as anchor face) | – | 0 |
5. Conclusion
By analyzing the structural characteristics of the cast steel anchorage and selecting the TK6926 CNC floor-type boring and milling machine (with a rotary table load-bearing capacity of 140 t) as the processing equipment, this paper formulates a reasonable processing technique that significantly improves production efficiency while ensuring product quality. This study provides a reference for the efficient processing of large-scale and complexly structured anchorages in the future and offers insights into the processing of workpieces with complex slope structures or multiple spatial holes.