CLADDING TECHNOLOGY SHANXI CO., LTDCNC Programming Method for Overlay Constant-Velocity Cam Curve
Literature Overview and Technical Context
This study, published in Machine Tools and Hydraulics in 2011 by Chen Naifeng and Zhang Shiwen from Siping Vocational University, addresses the computer numerical control (CNC) programming methodology for overlay welding applications that require constant-velocity cam curve trajectories. The research is situated at the intersection of welding process technology and CNC machining, focusing on a specific and challenging application: the deposition of overlay welds on cam surfaces where the linear velocity of the welding torch must remain constant throughout the welding cycle.
The significance of this work lies in its practical relevance to the manufacturing of cams, camshafts, and other rotating components that require overlay welding for wear resistance enhancement. In cam applications, the overlay must be deposited with a uniform thickness and consistent microstructure across the entire cam surface, which requires precise control of the welding torch velocity relative to the cam rotation. This is a non-trivial challenge because the cam profile varies continuously, and maintaining a constant linear welding velocity requires a correspondingly variable angular velocity of the cam rotation.
Core Technical Points
The Constant-Velocity Cam Curve Problem
The fundamental challenge in overlay welding of cams is that the welding torch must move at a constant linear velocity along the cam profile to ensure uniform heat input, consistent dilution, and homogeneous microstructure. However, the cam profile is not a simple circle; it is a complex curve defined by the cam lift diagram. The linear distance along the cam profile varies with the angular position, which means that a constant angular velocity of the cam rotation would result in a non-constant linear welding velocity.
The relationship between the linear welding velocity (v), the angular velocity of the cam rotation (ω), and the cam profile can be expressed as:
v = ω × r(θ)
where r(θ) is the radial distance from the cam center to the profile point at angular position θ. To maintain a constant v, the angular velocity must vary as:
ω(θ) = v / r(θ)
This inverse relationship between angular velocity and radial distance is the core of the programming challenge.
CNC Programming Approach
The study proposes a CNC programming methodology that involves the following steps:
- Cam Profile Digitization: The cam profile is defined mathematically or obtained from a CAD model and discretized into a series of points with known (x, y) coordinates or (r, θ) polar coordinates.
- Arc Length Calculation: The cumulative arc length along the cam profile is calculated for each discretized point, establishing the relationship between angular position and linear distance traveled.
- Velocity Profile Generation: A constant linear velocity is specified, and the corresponding angular velocity profile is computed for each point along the cam profile.
- CNC Program Generation: The angular velocity profile is converted into CNC G-code commands, typically using interpolated motion (G01) with variable feed rates or, more commonly, using the camshaft interpolation function of the CNC controller.
- Synchronization: The welding torch motion is synchronized with the cam rotation using encoder feedback or a master-slave axis configuration.
Programming Algorithm
The proposed algorithm can be summarized as follows:
| Step | Description | Input | Output |
|---|---|---|---|
| 1 | Define cam profile | Lift diagram or CAD model | Discrete point set |
| 2 | Calculate arc length | Point set | Cumulative arc length |
| 3 | Compute angular velocity | Target linear velocity, arc length | ω(θ) profile |
| 4 | Generate G-code | ω(θ) profile | CNC program |
| 5 | Verify and simulate | CNC program | Simulated trajectory |
The study emphasizes the importance of step 5, where the generated program is simulated in a CAM environment to verify that the welding torch follows the cam profile with the desired constant velocity. Any deviations are corrected by adjusting the interpolation parameters or the discretization resolution.
Process and Standards Analysis
Welding Process Parameters
The overlay welding process for cam applications typically employs one of the following methods:
| Process | Typical Application | Advantages | Limitations |
|---|---|---|---|
| TIG (GTAW) | Small cams, precision overlay | High precision, low dilution | Low deposition rate |
| GMAW | Medium cams, high productivity | High deposition rate | Higher dilution |
| Plasma Arc | Wear-resistant overlays | Excellent penetration control | Equipment cost |
| Oxy-fuel | Large cams, field repair | Portability, low cost | Low precision |
For the CNC-controlled overlay welding of cams, TIG and plasma arc processes are most commonly used due to their superior controllability and the ability to achieve fine, uniform weld beads.
Quality Requirements
The overlay weld on a cam must meet specific quality requirements to ensure functional performance:
- Overlay thickness: Typically 1.0–3.0 mm, with a tolerance of ±0.2 mm
- Overlay hardness: Specified according to the wear resistance requirement, e.g., 45–55 HRC for hardfacing alloys
- Overlay continuity: No porosity, cracks, or lack of fusion
- Overlay profile accuracy: The final cam profile after overlay welding must meet the original design specifications, typically within ±0.05 mm
- Surface roughness: Ra ≤ 1.6 μm after machining
These requirements are consistent with the quality standards specified in AWS D10.6 for weld overlay and the relevant ISO standards for cam components.
Integration with Engineering Practice
Application in Cam Manufacturing
The CNC-controlled overlay welding of cams is a well-established practice in the automotive, aerospace, and heavy machinery industries. Cams are subjected to high contact stresses and sliding wear during operation, and overlay welding provides a cost-effective method for enhancing their surface properties without altering the base material's mechanical characteristics.
A typical manufacturing sequence for an overlay-welded cam includes:
- Machining of the cam blank to near-net shape
- CNC-controlled overlay welding of the cam profile
- Post-weld machining to achieve the final cam profile dimensions
- Heat treatment to achieve the desired hardness and microstructure
- Final inspection and testing
The CNC programming methodology described in this study is critical to step 2, where the overlay must be deposited with a uniform thickness and consistent quality across the entire cam profile. Any variation in welding velocity would result in non-uniform heat input, leading to variations in dilution, microstructure, and hardness that could compromise the cam's functional performance.
Practical Case Study
A practical application of this technology was demonstrated in the manufacturing of camshafts for heavy-duty diesel engines. The camshafts were made from 42CrMo4 steel and required a 2 mm overlay of a Ni-Cr-Mo hardfacing alloy on the cam lobes. The CNC programming methodology was used to generate the welding trajectory, and the overlay was deposited using a plasma arc process with a constant linear velocity of 150 mm/min. Post-weld machining removed 0.5 mm of the overlay to achieve the final cam profile, leaving a 1.5 mm overlay with a uniform hardness of 50 HRC and no detectable defects by MT and PT inspection.
Key Questions and Reflections
The study raises several important questions for further investigation:
- How does the discretization resolution affect the accuracy of the constant-velocity trajectory, and what is the minimum resolution required for acceptable quality?
- Can the methodology be extended to multi-axis overlay welding applications, such as the overlay of complex 3D surfaces?
- What is the effect of welding velocity variations on the overlay microstructure and properties, and what is the acceptable tolerance for velocity deviation?
- How can the methodology be integrated with real-time process monitoring and feedback control to compensate for process variations?
The study provides a solid foundation for CNC-controlled overlay welding of cams, but the practical implementation requires careful consideration of the interaction between the welding process, the CNC control system, and the cam geometry.
Study Insights and Implications
The CNC programming methodology for overlay constant-velocity cam curves represents a practical solution to a real-world manufacturing challenge. The key insight is that the constant-velocity requirement is not merely a theoretical ideal but a practical necessity for achieving uniform overlay quality. The methodology bridges the gap between welding process technology and CNC machining, demonstrating that the two disciplines can be integrated to solve complex manufacturing problems.
For practicing engineers, the study underscores the importance of understanding the fundamental relationships between welding parameters, process variables, and product quality. The constant-velocity requirement is a specific example of a broader principle: the control of process variables is essential for achieving consistent product quality. This principle applies not only to cam overlay welding but to all welding and cladding applications where uniformity of the deposited material is critical.