**Laser Cutting and Its Processing System Fundamentals – Laser Cutting Equipment**

**II. Composition of Laser Cutting Equipment**

**2.1 Components and Working Principle of Laser Cutting Machines**

A laser cutting machine consists of a laser generator, cutting head, beam transmission components, machine tool worktable, CNC system, computer (hardware, software), chiller, shielding gas cylinders, dust extractor, and air dryer, among other parts.

1. **Laser Generator:** This is the device that produces the laser source. For laser cutting applications, except in a few cases where YAG solid-state lasers are used, the majority use CO2 gas lasers due to their higher electro-optical conversion efficiency and ability to output higher power. Since laser cutting has very high requirements for beam quality, not all lasers are suitable for cutting.

2. **Cutting Head:** Mainly includes components such as the nozzle, focusing lens, and focus tracking system. The cutting head drive device is used to drive the cutting head along the Z-axis according to the program, consisting of components like a servo motor and lead screw or gears.

* (1) **Nozzle:** Nozzle types mainly include three forms: parallel, convergent, and conical.

* (2) **Focusing Lens:** To utilize the energy of the laser beam for cutting, the raw beam emitted from the laser must be focused by a lens to form a spot with high energy density. Medium and long focal length lenses are suitable for thick plate cutting and have lower requirements for the distance stability of the tracking system. Short focal length lenses are only suitable for thin plates below 3mm; they have strict requirements for the distance stability of the tracking system but can significantly reduce the required laser output power.

* (3) **Tracking System:** The laser cutting machine's focus tracking system generally consists of the focusing cutting head and the tracking sensor system. The cutting head includes parts for light guidance and focusing, water cooling, gas blowing, and mechanical adjustment. The sensor consists of the sensing element and amplification control parts. Depending on the sensing element, tracking systems are completely different; primarily, there are two forms: capacitive sensor tracking systems (non-contact) and inductive sensor tracking systems (contact).

3. **Beam Transmission Components (External Light Path):** Refractive and reflective mirrors are used to direct the laser to the required direction. To prevent beam path failure, all mirrors are protected by covers and supplied with clean, positive-pressure shielding gas to prevent contamination. A high-quality lens set will focus a beam with no divergence angle into an infinitely small spot. A 5.0-inch focal length lens is commonly used. A 7.5-inch lens is only used for materials >12mm thick.

4. **Machine Tool Worktable (Machine Host):** The mechanical part of the laser cutting machine that enables movement along the X, Y, and Z axes, including the cutting work platform.

5. **CNC System:** Controls the machine tool to achieve X, Y, Z axis movement and also controls the output power of the laser.

6. **Cooling System (Chiller Unit):** Used to cool the laser generator. A laser is a device that converts electrical energy into light energy. For example, the conversion rate of a CO2 gas laser is typically 20%, with the remaining energy converted into heat. The cooling water carries away the excess heat to maintain normal operation. The chiller also cools the external light path mirrors and focusing lens to ensure stable beam transmission quality and effectively prevent lens deformation or cracking due to excessive temperature.

7. **Gas Cylinders:** Include the laser cutting machine's working medium gas cylinders and auxiliary gas cylinders, used to replenish the industrial gases for laser oscillation and supply auxiliary gases to the cutting head.

8. **Dust Extraction System:** Extracts smoke and dust generated during processing and filters them so that exhaust emissions meet environmental standards.

9. **Air Cooling Dryer and Filter:** Supplies clean, dry air to the laser generator and beam path to maintain normal operation of the path and mirrors.

**2.2 Laser Cutting Torch**

The schematic structure of a laser cutting torch is shown in the figure below, mainly composed of the torch body, focusing lens, mirror, and auxiliary gas nozzle. During laser cutting, the torch must meet the following requirements:

① The torch must be able to eject a sufficient gas flow.

② The gas ejection direction inside the torch must be coaxial with the optical axis of the mirror.

③ The focal length of the torch should be easily adjustable.

④ During cutting, ensure that metal vapor and spatter from the cutting process do not damage the mirror.

The movement of the torch is adjusted through the CNC motion system. The relative movement between the torch and the workpiece can occur in three situations:

① The torch is stationary, and the workpiece moves via the worktable (mainly for smaller workpieces).

② The workpiece is stationary, and the torch moves.

③ Both the torch and the worktable move simultaneously.

**2.2.1 Cutting Head**

The laser cutting head is located at the end of the beam transmission system and includes the focusing lens and cutting nozzle.

Focusing lenses are primarily distinguished by their focal length. Most laser cutting equipment is equipped with several cutting heads of different focal lengths. Taking CO2 laser cutting as an example, common focal lengths are 127mm (5in) and 190mm (7.5in). A short focal length lens yields a small focal spot and short depth of focus, which is beneficial for reducing kerf width and obtaining a finer cut. A long focal length lens yields a larger focal spot and longer depth of focus. Compared to a short focal length lens, a long focal length lens can meet the requirements for focused beam energy density near the focal point over a larger range of material thickness. Therefore, short focal length lenses are mostly used for fine cutting of thin sheets, while thicker materials require long focal length lenses to obtain sufficient depth of focus, ensuring that the spot diameter change is minimal within the cutting thickness range and adequate power density is maintained.

The focusing lens is used to focus the parallel laser beam entering the torch to obtain a small spot and high power density. Lenses are made of materials that transmit the laser wavelength. Solid-state lasers commonly use optical glass, while CO2 gas lasers, which cannot transmit through ordinary glass, use materials like ZnSe, GaAs, and Ge, with ZnSe being the most common.

For laser cutting, a spot diameter as small as possible is desired, as this increases power density, facilitating high-speed cutting. However, when the lens focal length decreases, the depth of focus also becomes smaller, making it difficult to obtain good perpendicular cut surfaces when cutting thicker plates. Additionally, with a smaller lens focal length, the distance between the lens and the workpiece is reduced, making the lens susceptible to contamination by spatter and other molten material during cutting, affecting normal operation. Therefore, the appropriate focal length must be determined by comprehensive consideration of factors such as cutting thickness and quality requirements.

**2.2.2 Mirror**

The function of the mirror is to change the direction of the beam coming from the laser. For beams from solid-state lasers, mirrors made of optical glass can be used, while mirrors in CO2 gas laser cutting devices are often made of copper or metals with high reflectivity. During use, mirrors are typically water-cooled to avoid damage caused by overheating from light exposure.

**2.2.3 Nozzle**

The nozzle is used to jet auxiliary gas into the cutting zone. Its structural shape has a certain influence on cutting efficiency and quality. Figure 4.11 shows the common nozzle shapes used in laser cutting; the jet orifice shapes include cylindrical, conical, and convergent-divergent (de Laval) types.

The selection of the nozzle is generally determined after testing based on the workpiece material, thickness, auxiliary gas pressure, etc. Laser cutting generally uses coaxial nozzles (gas flow coaxial with the optical axis). If the gas flow is not coaxial with the beam, it can easily generate significant spatter during cutting. The orifice wall of the nozzle should be smooth to ensure smooth gas flow and avoid affecting cut quality due to turbulence. To ensure cutting process stability, the distance from the nozzle face to the workpiece surface should be minimized, often taken as 0.5~2.0mm. The nozzle aperture must allow the laser beam to pass through unimpeded, avoiding the beam touching the inner wall of the nozzle. The smaller the aperture, the more difficult beam collimation becomes. At a certain auxiliary gas pressure, there is an optimal range of nozzle aperture diameters. If the aperture is too small or too large, it will affect the removal of molten products from the kerf and also affect the cutting speed.

The influence of nozzle aperture on cutting speed under certain laser power and auxiliary gas pressure is shown in Figures 4.12 and 4.13. It can be seen that there is an optimal nozzle aperture that yields the maximum cutting speed. Whether oxygen or argon is used as the auxiliary gas, this optimal value is approximately 1.5mm.

Laser cutting tests on difficult-to-cut materials like cemented carbide show that the optimal nozzle aperture is very close to the above result, as shown in Figure 4.14. The nozzle aperture also affects the kerf width and the width of the heat-affected zone (HAZ). As shown in Figure 4.15, as the nozzle aperture increases, the kerf widens while the HAZ narrows. The main reason for the narrowing HAZ is the enhanced cooling effect of the auxiliary gas flow on the base material in the cutting zone.

**2.3 Laser Cutting Equipment Parameters**

**2.3.1 Torch-Driven Cutting Equipment**

In torch-driven cutting equipment, the cutting torch is installed on a movable gantry and moves transversely (Y-direction) along the gantry beam. The gantry drives the torch along the X-direction, and the workpiece is fixed on the cutting table. Since the laser source is separated from the torch, during cutting, the laser transmission characteristics, parallelism along the beam scanning direction, and the stability of the reflective mirrors can be affected.

Torch-driven equipment can process larger parts, requires relatively less floor space for the cutting production area, and can be easily integrated into production lines with other equipment. However, the positioning accuracy is typically around ±0.04mm.

The typical structure of torch-driven cutting equipment is shown in Figure 4.19. This example uses a CO2 continuous wave laser cutter, with a beam transmission distance of 18m from the laser source to the torch. To ensure that changes in beam shape over this distance do not hinder cutting, the combination of oscillator mirrors must be carefully designed.

Main technical parameters for torch-driven equipment:

* Laser Output Power: 1.5kW (single mode), 3kW (multimode).

* Torch Travel: X-axis 6.2m, Y-axis 2.6m.

* Drive Speed: 0~10m/min (adjustable).

* Torch Height (Z-axis) Floating Travel: 150mm.

* Torch Height Adjustment Speed: 300mm/min.

* Maximum Processable Steel Plate Size: 12mm thick * 2400mm * 6000mm.

* Control Equipment: Integrated CNC control method.

**2.3.2 XY Coordinate Cutting Table-Driven Equipment**

In XY coordinate cutting table-driven equipment, the torch is fixed on the machine frame, and the workpiece is placed on the cutting table. The cutting table moves according to NC instructions along the X and Y directions. The drive speed is generally 0~1m/min (adjustable) or 0~5m/min (adjustable). Since the torch is fixed relative to the workpiece, the alignment of the laser beam is less affected during cutting, allowing for uniform and stable cuts. When the cutting table size is small and mechanical accuracy is high, the positioning accuracy can be ±0.01mm, resulting in excellent cutting precision, making it particularly suitable for precision cutting of small parts. There are also cutting tables with X-axis travel of 2300~2400mm and Y-axis travel of 1200~1300mm for processing larger parts.

Main technical parameters for XY table-driven equipment:

* Laser: CO2 gas laser (semi-sealed straight tube type).

* Laser Power Supply: Input voltage 200V AC, Output voltage 0~30kV, Max output current 100mA.

* Laser Output Power: 550W.

* Cutting Table Travel: X-axis 2300mm, Y-axis 1300mm.

* Cutting Table Drive Speed (step adjustable): 0.4~5.0 m/min, 0.2~2.5 m/min, 0.1~1.3 m/min, 0.05~0.6 m/min.

* Torch Height (Z-axis) Floating Travel: 180mm.

* Maximum Processable Sheet Size: 6mm thick * 1300mm * 2300mm.

* Control Equipment: NC method.

**2.3.3 Torch-Table Dual-Driven Cutting Equipment**

Torch-table dual-driven cutting equipment is a hybrid between torch-driven and XY table-driven types. The torch is installed on a gantry and moves transversely (Y-direction) along the gantry beam, while the cutting table drives longitudinally (X-direction). It combines the advantages of high cutting accuracy and space saving. The positioning accuracy is ±0.01mm, the cutting speed adjustment range is 0~20m/min, making it a widely used type of cutting equipment. Larger models of this type can have Y-axis travel of 2000mm and X-axis travel of 6000mm, capable of cutting large parts.

In some designs, the laser oscillator is also mounted on the gantry along with the torch. The accuracy for cutting circular holes with dual-driven equipment is quite good. The production efficiency is also high; for example, it can cut 46 holes of 10mm diameter per minute in 1mm thick steel plate.

**2.3.4 Integrated Cutting Equipment**

In integrated cutting equipment, the laser source is installed on the machine frame and moves longitudinally with it, while the torch and its drive mechanism form a unit that moves transversely on the frame's beam. Using CNC, various shaped parts can be cut. To compensate for the change in optical path length caused by the transverse movement of the torch, an optical path length adjustment component is usually equipped, ensuring a homogeneous beam across the cutting area and maintaining consistent cut surface quality.