CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Control Tracking System for Dished Head Cladding
Literature Overview
Dished heads (also called formed heads or pressure vessel heads) are curved components used to close the ends of pressure vessels, columns, and tanks. The curved geometry of dished heads presents significant challenges for welding overlay processes, as the torch must maintain a precise distance and angle relative to the surface throughout the welding path. A numerical control (NC) tracking system automates the torch movement along the curved surface, ensuring consistent weld quality and productivity. This study note examines the design, operation, and performance of NC tracking systems for dished head cladding, including the tracking algorithms, sensor technology, and process integration.
Core Technical Points
Dished Head Geometry and Cladding Challenges
Dished heads come in several standard geometries, each with different curvatures and challenges for cladding:
| Head Type | Geometry | Typical Radius/Thickness Ratio | Cladding Challenge |
|---|---|---|---|
| Ellipsoidal (2:1) | Semi-ellipsoid | 2R/D = 1 | Moderate curvature, uniform thickness |
| Hemispherical | Half sphere | R/D = 0.5 | High curvature, varying surface normal |
| Torispherical | Sphere + torus | R/D = 0.85 | Complex curvature transition |
| Flanged dished | Ellipsoidal + flange | 2R/D = 1 | Geometry change at flange |
The primary challenge in dished head cladding is maintaining a constant torch-to-surface distance and angle as the torch moves along the curved surface. A deviation of even 1–2 mm in torch height can cause significant variations in weld bead profile, dilution, and hardness. The NC tracking system must continuously adjust the torch position and angle to compensate for the changing surface geometry.
NC Tracking System Architecture
A typical NC tracking system for dished head cladding consists of the following components:
| Component | Function | Specification |
|---|---|---|
| CNC controller | Process control, path planning | 3–5 axis control, resolution 0.01 mm |
| Tracking sensor | Real-time surface position measurement | Laser scanner or capacitive sensor, accuracy ±0.1 mm |
| Servo motors | Torch movement along X, Y, Z axes | High-speed response, repeatability ±0.02 mm |
| Torch height controller | Maintains constant torch-to-surface distance | Feedback loop, response time <50 ms |
| Welding power source | SAW, GTAW, or FCAW power supply | Adjustable current, voltage, polarity |
| Software interface | Path programming, parameter setting | CAD/CAM integration, G-code compatible |
The tracking sensor is typically a laser triangulation scanner that measures the distance from the torch to the surface at a frequency of 100–1000 Hz. The sensor data is fed to the CNC controller, which calculates the required torch position and angle adjustments in real time. The servo motors execute the adjustments with a response time of less than 50 ms, ensuring that the torch maintains the correct position throughout the welding path.
Tracking Algorithm and Path Planning
The NC tracking algorithm for dished head cladding involves the following steps:
- Surface scanning: The laser scanner measures the surface geometry along the planned welding path, creating a 3D point cloud of the surface.
- Path planning: The CNC controller calculates the torch position and angle at each point along the path, ensuring a constant torch-to-surface distance and angle.
- Real-time tracking: During welding, the sensor continuously measures the actual torch-to-surface distance, and the controller adjusts the torch position to maintain the target distance.
- Feedback correction: Any deviation from the planned path is detected and corrected in real time, ensuring consistent weld quality.
The path planning algorithm must account for the curvature of the dished head, the weld bead width, and the overlap between adjacent passes. For multi-pass cladding, the path is planned to ensure uniform coverage with 50–70% overlap between adjacent passes, which is critical for achieving a consistent overlay thickness and hardness.
Process Parameters and Performance
| Parameter | Typical Value | Effect on Cladding Quality |
|---|---|---|
| Torch height | 8–12 mm (SAW), 2–3 mm (GTAW) | Controls arc stability and penetration |
| Travel speed | 200–500 mm/min | Controls heat input and bead profile |
| Welding current | 300–500 A (SAW), 80–150 A (GTAW) | Controls penetration and dilution |
| Welding voltage | 28–36 V (SAW), 12–18 V (GTAW) | Controls bead width and profile |
| Number of passes | 2–4 | Controls overlay thickness |
| Interpass temperature | ≤250 °C | Prevents softening and cracking |
| Tracking accuracy | ±0.1 mm | Ensures consistent weld quality |
The NC tracking system enables precise control of the torch position and angle, resulting in consistent weld bead profiles, dilution rates, and hardness across the entire cladded surface. The tracking accuracy of ±0.1 mm is sufficient to ensure that the torch height variation is within ±0.2 mm, which is well within the acceptable range for most cladding processes.
Integration with Engineering Practice
In pressure vessel fabrication, dished heads are commonly cladded with stainless steel or nickel-based alloys to provide corrosion resistance in aggressive chemical environments. The NC tracking system enables the application of uniform overlay layers with consistent thickness and hardness, which is critical for ensuring the long-term integrity of the pressure vessel.
For large dished heads (diameter >2 m), the NC tracking system is integrated with a positioner that rotates the head to allow access to all areas of the surface. The system coordinates the torch movement with the head rotation to ensure continuous coverage without gaps or overlaps. The total cladding time for a large dished head (3 m diameter, 4-pass overlay) is typically 8–12 hours, compared to 24–48 hours for manual welding.
The NC tracking system also improves the quality of the cladded surface by ensuring consistent weld bead profiles and minimizing defects such as porosity, undercut, and lack of fusion. The consistent quality reduces the need for rework and inspection, saving time and cost in the fabrication process.
Key Questions and Reflections
The integration of the NC tracking system with automated inspection systems (e.g., ultrasonic testing, eddy current testing) is an important development direction. Real-time inspection during welding can detect defects such as cracks, porosity, and lack of fusion as they occur, allowing immediate correction and reducing the need for post-weld inspection and rework.
Another consideration is the adaptability of the NC tracking system to different dished head geometries and sizes. The system must be able to quickly reconfigure the path planning and tracking parameters for different head types and dimensions, which requires flexible software algorithms and user-friendly interfaces. The ability to import CAD models of the dished head and automatically generate the welding path would significantly improve productivity and reduce programming time.
Study Insights and Implications
The NC tracking system provides a highly effective solution for the automated cladding of dished heads, enabling consistent weld quality, high productivity, and reduced labor costs. The key to success lies in the precise control of the torch position and angle through real-time sensor feedback and advanced tracking algorithms. The system is particularly valuable for large dished heads and multi-pass cladding operations, where manual welding is difficult to achieve consistent quality. Engineers should ensure that the tracking system is properly calibrated and maintained, and that the process parameters are optimized for the specific substrate material, overlay alloy, and service conditions. The integration of automated inspection and quality monitoring systems will further enhance the reliability and efficiency of the cladding process.