1 Development Status at Home and Abroad

In a five-axis machine tool, three linear axes and two rotational axes are responsible for controlling the position and orientation of the cutting tool relative to the workpiece. By connecting these five axes in different sequences, various configurations can be achieved. Among them, the layout where the two rotating axes are directly connected offers distinct advantages: 1) it aligns with the principles of kinematic thinking; 2) compared to non-straight structures, this configuration minimizes the offset of the tool tip relative to the workpiece when the tool rotates, thereby reducing the need for compensation on the linear axes; 3) post-processing is simpler, often similar to ball-end mill radius compensation. This structure is typically divided into two types: 1) the dual-rotary table design, where both rotating axes drive the workpiece; and 2) the robotic arm-like structure, where both rotating axes drive the tool. The former provides better rigidity and ease of installation, making it suitable for machining, while the latter allows higher rotational speed and flexibility, commonly used in laser cutting applications and known as a guide head.

The typical structure of a guide head is shown in Figure 1. The rotation axes C and A intersect at the same point as the tool axis T. The laser beam undergoes four 90° reflections before exiting from the geometric intersection point. Axis C controls the horizontal rotation angle, while axis A controls the pitch angle α. The optical path resembles the shape of the Greek letter Ω, and this configuration is tentatively called the Ω structure for easier reference.

Figure 2 shows a simplified version of the Ω structure, which is also widely used. Due to the shorter optical path, the three axes do not intersect at a single point. While this simplifies the structure and reduces manufacturing complexity and optical losses, the post-processing becomes more complex compared to the full Ω structure. During curved surface machining, the linear axes must compensate for the eccentricity LCA shown in the figure, which also reduces the effective travel of the moving axes.

In recent years, leading companies such as NTC (Nisshin Toyama) and Mitsubishi have introduced a new type of guide head in their five-axis laser processing systems (Figure 3). The C-axis is arranged conventionally, while a φ-axis intersects the C-axis at a 45° angle. The tool axis T rotates about the φ-axis and maintains a fixed 45° orientation. Its optical path resembles the shape of the symbol ∑, hence the name "∑ structure." This design is still relatively rare in domestic applications but offers significant advantages that are worth exploring.

Figure 3 Beam guide head structure

2 Kinematics Characteristics of the Crucible Structure

As shown in Figures 3 and 4, in the crucible structure, the C-axis, φ-axis, and tool axis T intersect at one point, and the tool tip (or laser focus) is precisely positioned at the geometric intersection during installation. This setup ensures that the tool tip remains stationary even when the C-axis and φ-axis rotate freely. As a result, during five-axis linkage operations, the accuracy of the machining path depends solely on the three linear axes, while the two rotational axes provide additional degrees of freedom.

Compared to the traditional Ω structure, the crucible structure offers several key advantages:

1. Position control in the crucible structure is independent of angular control, resulting in more reliable position accuracy. This is especially important for laser cutting, where the accuracy directly affects the trajectory generation, the positioning of the laser focus, and the auxiliary air nozzle, ultimately influencing cutting quality, including slit width uniformity and dross formation.
2. The stroke of each linear axis in the ∑ structure is fully utilized, whereas in the Ω structure, the usable range of the linear axis is usually smaller than its total travel.
3. The crucible structure is more energy-efficient and faster for the same machining task compared to the Ω structure. For small-radius arcs, the moving axes in the Ω structure must operate at high speeds, which can exceed the capabilities of standard machine tools and increase hardware costs. In contrast, the crucible structure avoids this issue by maintaining stable motion without excessive speed demands.
4. The crucible structure simplifies teaching programming in multi-axis machines. Teaching programming has become an essential feature in modern multi-axis systems, allowing quick processing without a 3D model. In this structure, the coordinates of the rotating axes can be derived directly from the linear axes, making the process much more efficient.
5. Unlike the Ω structure, which requires inverse calculations for tool radius compensation, the crucible structure enables direct calculation of linear axis positions. This significantly reduces the complexity of manual adjustments during teaching, improving overall usability and reducing labor intensity.

From the above analysis, it's clear that the crucible structure eliminates the need for special post-processing in the three linear axes. CAM software can directly use the axis data generated from the part’s 3D model for subsequent machining. However, the crucible structure does require additional compensation for the horizontal angle when dealing with the two rotating axes. This is because the φ-axis rotation affects both the pitch angle α and the horizontal angle θ, creating an additional offset Δc that must be compensated by the C-axis. This relationship is illustrated in Figure 5, showing how the angles interact and influence each other during motion.

Figure 5

When the C-axis and φ-axis rotate within 2π, the motion of the ∑ structure remains continuous. For example, at point II in Figure 4, the mirror image of the guide head relative to the cylinder end face can align the tool axis T with the normal vector of that point. The specific motion decomposition is customized by the CAM software based on the machining requirements, enhancing efficiency. For instance, in horizontal plane machining, the C-axis may rotate within a 2° range, while the φ-axis is limited to 0–π. In curved surface machining, the φ-axis may rotate multiple times, while the C-axis moves only slightly in the direction of the cylinder axis.

3 Conclusion

This paper introduces a novel five-axis machining guide head and analyzes its structural advantages in practical applications. In fields such as laser processing, high-speed precision, and coordinate measurement where cutting forces are minimal or zero, the guide head structure demonstrates excellent performance and applicability.

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