Choosing the Right Cutting Parameters for CNC Plasma Cutting Machines

The selection of cutting process parameters for CNC plasma cutting machines is crucial to the quality, speed and efficiency of the cutting results.

To use a CNC plasma machine correctly for high-quality and fast cutting, it is essential to have a profound understanding and mastery of the cutting process parameters.

I. Cutting Current

The cutting current is the most critical parameter in the plasma cutting process, as it directly influences the thickness and speed of the cut, thereby determining the cutting ability. The effects of cutting current are as follows:

1. Increased Cutting Ability and Speed: As the cutting current increases, the arc energy also increases, resulting in higher cutting ability and increased cutting speed.
2. Wider Cuts: An increase in cutting current also increases the diameter of the arc, making the cut wider.
3. Nozzle Overheating: If the cutting current is too high, the nozzle may overheat, leading to premature damage and a decrease in cutting quality. In extreme cases, it can prevent normal cutting from occurring. Therefore, it is essential to select the appropriate cutting current and corresponding nozzle based on the material thickness before cutting.

II. Cutting Speed

The optimal cutting speed range can be determined according to the equipment instructions or through experimentation. Various factors such as material thickness, material type, melting point, thermal conductivity, and surface tension after melting influence the cutting speed. The main effects of cutting speed are as follows:

1. Improved Cut Quality: Moderately increasing the cutting speed can improve the quality of the cut by slightly narrowing the cut, making the cut surface smoother, and reducing deformation.
2. Excessive Speed: If the cutting speed is too fast, the energy of the cutting line will be insufficient, preventing the jet from blowing off the melted material immediately. This results in a larger amount of back drag and slag hanging on the cut, causing a decrease in cut surface quality.
3. Low Speed Issues: When the cutting speed is too low, the cutting position acts as the anode of the plasma arc. To maintain arc stability, the anode spot or area must find a place to conduct current near the nearest cut, transferring more heat radially to the jet. This results in:
   • A wider cut.
   • Molten material gathering and solidifying at the bottom edge, forming hard-to-clean slag.
   • The upper edge of the cut forming a rounded corner due to excessive heating and melting.
   • In extreme cases, the arc may extinguish if the speed is too low.

III. Arc voltage

The arc voltage, typically considered the cutting voltage, is another crucial parameter in plasma cutting. Plasma arc cutting machines usually operate with high no-load voltage and working voltage. The effects of arc voltage are as follows:

1. High Ionization Energy Gases: When using gases with high ionization energy such as nitrogen, hydrogen, or air, the voltage required for a stable plasma arc is higher.
2. Increased Arc Enthalpy and Cutting Ability: When the current is constant, an increase in voltage means an increase in arc enthalpy, which enhances the cutting ability.
3. Improved Cutting Speed and Quality: If the diameter of the jet is reduced while increasing the gas flow rate simultaneously with an increase in enthalpy, it often results in faster cutting speeds and better cutting quality.

IV. Working Gas and Flow Rate

Working Gases in Plasma Arc Cutting

In plasma arc cutting, the selection and management of working gases are crucial for achieving optimal cutting performance. The working gases typically include cutting gas, auxiliary gas, and, in some cases, starting gas. The appropriate working gas should be selected based on the type, thickness, and cutting method of the material being processed.

Role of Cutting Gas

The cutting gas serves several essential functions:

1. Formation of Plasma Jet: The cutting gas is ionized to form the plasma jet, which is the primary cutting tool.
2. Removal of Molten Metal and Oxides: It helps in expelling molten metal and oxides from the cut, ensuring a clean cut edge.

Importance of Gas Flow Rate

The gas flow rate is a critical parameter that must be carefully controlled:

• Excessive Gas Flow: If the gas flow is too high, it can dissipate more arc heat, shorten the plasma jet length, reduce cutting ability, and cause arc instability.
• Insufficient Gas Flow: Conversely, if the gas flow is too low, the plasma arc may lose its necessary straightness, resulting in shallow cuts and increased slag formation.

Therefore, the gas flow rate must be well-coordinated with the cutting current and speed to maintain cutting efficiency and quality.

Controlling Gas Flow Rate

Most modern plasma arc cutting machines control the gas flow rate by adjusting the gas pressure. When the nozzle aperture is fixed, controlling the gas pressure effectively controls the flow rate. The gas pressure required for cutting a specific material thickness is usually provided by the equipment manufacturer. For special applications, the gas pressure may need to be determined through actual cutting trials.

Commonly Used Working Gases

The most commonly used working gases in plasma arc cutting include:

• Argon (Ar)
• Nitrogen (N₂)
• Oxygen (O₂)
• Air
• H35 (a mixture of 35% hydrogen and 65% argon)
• Argon-Nitrogen Mixed Gas

Each gas or gas mixture has specific properties that make it suitable for different materials and cutting conditions. For instance:

• Argon: Provides a stable arc and is often used for cutting non-ferrous metals.
• Nitrogen: Offers high cutting speeds and is suitable for stainless steel and aluminum.
• Oxygen: Enhances cutting speed and quality for mild steel.
• Air: A cost-effective option for cutting various materials, though it may require additional filtration.
• H35: Used for cutting thick stainless steel and aluminum due to its high energy density.

Gas Types in Plasma Cutting: Properties and Applications

1. Argon Gas

Argon gas exhibits minimal reactivity with metals at high temperatures, contributing to a highly stable plasma arc. The longevity of the nozzle and electrode is also enhanced when using argon. However, the argon plasma arc operates at a lower voltage and has a relatively low enthalpy value, which limits its cutting capability. Compared to air cutting, the cutting thickness achievable with argon decreases by approximately 25%. Additionally, in an argon-protected environment, the surface tension of the molten metal is about 30% higher than in a nitrogen environment, potentially leading to more slag formation. Even when mixed with other gases, argon tends to produce sticky slag, making pure argon gas less favorable for plasma cutting.

2. Hydrogen Gas

Hydrogen gas is typically used as an auxiliary gas in combination with others. A notable example is H35 gas, which consists of 35% hydrogen and 65% argon. This mixture is highly effective in plasma arc cutting due to the significant increase in arc voltage provided by hydrogen, resulting in a high enthalpy plasma jet. When combined with argon, the cutting efficiency is markedly improved. For cutting metal materials thicker than 70 mm, an argon-hydrogen mixture is commonly employed. The cutting efficiency can be further enhanced by using a water jet to compress the argon-hydrogen plasma arc.

3. Nitrogen Gas

Nitrogen is a widely used working gas in plasma cutting. Under high power supply voltages, nitrogen plasma arcs offer better stability and higher jet energy compared to argon. This makes nitrogen particularly effective for cutting high-viscosity materials such as stainless steel and nickel-based alloys, with minimal slag formation. Nitrogen can be used alone or mixed with other gases. In automated cutting processes, nitrogen or air is often used, making them standard gases for high-speed cutting of carbon steel. Nitrogen is also used as an arc-starting gas in oxygen plasma cutting.

4. Oxygen Gas

Oxygen can significantly increase the cutting speed of low-carbon steel. The cutting mechanism with oxygen is similar to flame cutting, where the high-temperature, high-energy plasma arc accelerates the cutting process. However, oxygen must be used with electrodes that are resistant to high-temperature oxidation and protected against impact during arc initiation to prolong their lifespan.

5. Air

Air, containing approximately 78% nitrogen and 21% oxygen, produces slag formation similar to nitrogen when used for cutting. The presence of oxygen in air enhances the cutting speed of low-carbon steel. Air is also the most economical working gas. However, using air alone for cutting can lead to issues such as slag formation, oxidation, and nitrogen increase at the cut edges. The reduced lifespan of electrodes and nozzles can also impact work efficiency and increase cutting costs.

V. Nozzle Height in Plasma Arc Cutting

Definition and Importance

Nozzle height refers to the distance between the nozzle end face and the cutting surface. This distance is a critical parameter in plasma arc cutting as it influences the overall arc length and, consequently, the cutting performance.

Impact on Arc Characteristics

Plasma arc cutting typically employs power sources with constant current or steep drop characteristics. When the nozzle height increases, the current remains relatively stable. However, the arc length increases, leading to a rise in arc voltage and, thus, arc power. This increase in arc power is counterbalanced by the energy loss of the arc column exposed to the environment.

Effects on Cutting Performance

The interplay between increased arc power and energy loss can result in a reduction of effective cutting energy. This reduction manifests in several ways:

• Weakening of Cutting Jet Force: The force of the cutting jet diminishes, reducing the cutting efficiency.
• Increase in Residual Slag: More slag is left at the bottom of the cut, indicating a less clean cut.
• Rounding of Upper Edge: The upper edge of the cut becomes rounded, which is undesirable for precision cutting.
• Wider Incision: As the plasma jet expands outward upon leaving the nozzle, an increased nozzle height results in a wider cut, which can affect the precision and quality of the cut.

Optimal Nozzle Height

To improve cutting speed and quality, it is generally beneficial to maintain the smallest possible nozzle height. However, if the nozzle height is too low, it can lead to the formation of double arcs, which are detrimental to the cutting process.

Use of Ceramic Outer Nozzles

Using ceramic outer nozzles can mitigate the issues associated with low nozzle heights. These nozzles allow the nozzle end face to directly contact the cut surface, effectively setting the nozzle height to zero. This configuration can achieve excellent cutting results by minimizing the arc length and maximizing the effective cutting energy.

VI. Cutting Power Density

High-Compression Plasma Arc for Cutting

To achieve a high-compression plasma arc for cutting, the cutting nozzle employs a small nozzle aperture, an extended bore length, and enhanced cooling mechanisms. These features collectively increase the current passing through the nozzle’s effective cross-sectional area, thereby boosting the arc power density. However, this compression also results in increased power loss of the arc. Consequently, the actual energy utilized for cutting is less than the power output from the power source, with a typical loss rate ranging between 25% and 50%.

Energy Loss Considerations

Certain methods, such as water compression plasma arc cutting, may exhibit higher energy loss rates. This factor should be taken into account when designing cutting process parameters or performing economic assessments of cutting costs.

Cutting Metal Plates

In industrial applications, metal plate thicknesses are commonly below 50mm. Within this range, conventional plasma arc cutting often produces cuts with a larger upper edge and a smaller lower edge. This discrepancy can reduce the size accuracy of the incision and necessitate additional processing work.

Material-Specific Cutting Characteristics

When using oxygen and nitrogen plasma arc cutting for materials like carbon steel, aluminum, and stainless steel, the following observations can be made:

• Plate Thickness 10-25mm: For plates within this thickness range, the material is thicker, resulting in better verticality of the edge. The angle error of the cutting edge typically ranges from 1 degree to 4 degrees.
• Plate Thickness <1mm: As the plate thickness decreases, the angle error of the incision increases significantly, ranging from 3-4 degrees to 15-25 degrees.

Causes of Angle Error

The primary cause of angle error is attributed to the uneven heat input of the plasma jet on the cutting surface. The plasma arc energy release is more concentrated in the upper part of the incision compared to the lower part. This energy release imbalance is influenced by several process parameters, including the degree of plasma arc compression, cutting speed, and nozzle-to-workpiece distance.

Optimizing Cutting Parameters

Increasing the compression degree of the arc can extend the high-temperature plasma jet, forming a more uniform high-temperature area. This also increases the jet speed, which can reduce the width difference between the upper and lower edges of the incision. However, excessive compression of conventional nozzles can lead to double arcs, which not only consume electrodes and nozzles but also degrade the quality of the incision and potentially halt the cutting process.

Additionally, excessive cutting speed and nozzle height can exacerbate the width difference between the upper and lower edges of the incision. Therefore, careful optimization of these parameters is crucial to achieve high-quality cuts with minimal angle error and width discrepancy.

VII. Plasma Cutting Process Parameter Table

The process involves creating an electrical channel of superheated, electrically ionized gas (plasma) from the plasma cutter through the workpiece, thereby cutting it. The parameters for plasma cutting can vary based on the type of plasma gas and the cutting current used. Below are the optimized parameters for cutting low-carbon steel using different plasma gases:

Low-carbon steel air plasma/air protection cutting current 130A

Low-carbon steel oxygen plasma/air protection cutting current 130A

Practical Tips

• Ensure Proper Ventilation: Plasma cutting generates fumes and gases that need to be properly ventilated to ensure a safe working environment.
• Regular Maintenance: Regularly check and maintain the plasma cutter and its consumables to ensure optimal performance and cut quality.
• Safety Precautions: Always wear appropriate personal protective equipment (PPE), including gloves, eye protection, and flame-resistant clothing, when performing plasma cutting operations.

By adhering to these parameters and considerations, you can achieve efficient and high-quality cuts when working with low-carbon steel using plasma cutting technology.