The selection of cutting process parameters for CNC plasma cutting machines is crucial to the quality, speed and efficiency of cutting results.
To correctly use a CNC plasma machine for fast, high-quality cutting, it is essential to have in-depth knowledge and mastery of the parameters of the cutting process.
I. Power cut
It is the most important parameter of the cutting process that directly determines the thickness and cutting speed, that is, the cutting capacity. Its effects are as follows:
1. As the cutting current increases, the arc energy also increases, resulting in higher cutting capacity and higher cutting speed.
2. As the cutting current increases, the arc diameter also increases, making the cut wider.
3. If the cutting current is too high, the nozzle will overheat, causing premature damage and decreased cutting quality, or even preventing normal cutting from occurring. Therefore, it is necessary to choose the suitable cutting current and corresponding nozzle based on the thickness of the material before cutting.
II. Cutting speed
The ideal cutting speed range can be determined according to equipment instructions or by experimentation.
Due to factors such as material thickness, material type, melting point, thermal conductivity and surface tension after melting, the cutting speed also changes accordingly. Its main effects are the following:
Moderately increasing the cutting speed can improve the cutting quality, that is, slightly narrow the cut, make the cutting surface smoother, and reduce deformation.
If the cutting speed is too fast, the energy of the cutting line will be lower than the required value, and the jet cannot blow the molten cutting melt immediately, resulting in a greater amount of drag and slag hanging in the cutting, causing a decrease in the quality of the cutting surface.
When the cutting speed is very low, such as the cutting position is the anode of the plasma arc, to maintain the stability of the arc itself, the anode point or anode area must find a place to conduct the current close to the nearest cut, the which will transfer more heat radially into the jet.
Therefore, the cut becomes wider and the molten material on both sides of the cut accumulates and solidifies at the bottom edge, forming slag that is difficult to clean. Furthermore, the top edge of the cut forms a rounded corner due to excessive heating and melting.
When the speed is extremely low, the arc may even extinguish. Therefore, good cutting quality and cutting speed are inseparable.
III. Arc voltage
The normal output voltage of the power supply is generally regarded as the cut-off voltage.
Plasma arc cutting machines generally have high no-load voltage and working voltage.
When using gases with high ionization energy, such as nitrogen, hydrogen or air, the voltage required for a stable plasma arc is higher. When current is constant, an increase in voltage means an increase in arc enthalpy and cutting capacity.
If the jet diameter is reduced while increasing the gas flow rate while increasing the enthalpy, this generally results in faster cutting speed and better cutting quality.
4. Working gas and flow rate
Working gases include cutting gas, auxiliary gas, and some equipment also requires starting gas. Typically, the appropriate working gas should be selected based on the material type, thickness and cutting method.
The cutting gas needs to ensure the formation of the plasma jet while removing molten metal and oxides from the cut.
Excessive gas flow can remove more heat from the arc, shorten the jet length, lead to a decrease in cutting capacity and an unstable arc; Insufficient gas flow can cause the plasma arc to lose the required straightness, resulting in shallow cuts and also easily causing slag suspension.
Therefore, the gas flow rate must be well coordinated with the cutting current and speed.
Most modern plasma arc cutting machines control the flow rate by gas pressure because when the gun opening is fixed, controlling the gas pressure also controls the flow rate.
The gas pressure used to cut a given thickness of material generally needs to be selected according to data provided by the equipment manufacturer.
If there are other special applications, the gas pressure must be determined through actual cutting tests.
The most commonly used working gases are argon, nitrogen, oxygen, air and H35, argon-nitrogen mixed gas, etc.
1. Argon gas almost does not react with any metals at high temperatures, and the argon gas plasma arc is very stable.
Furthermore, the nozzle and electrode used have a relatively long service life. However, the voltage of argon plasma arc is lower and the enthalpy value is not high, resulting in limited cutting capacity.
Compared with air cutting, the cutting thickness will decrease by approximately 25%.
Furthermore, in a protective argon environment, the surface tension of the molten metal is greater, approximately 30% higher than in a nitrogen environment.
Therefore, there may be more slag suspension problems.
Even when cutting with a mixture of argon and other gases, there will be a tendency for sticky slag to form. Thus, pure argon gas is rarely used alone for plasma cutting.
2. Hydrogen gas is generally used as an auxiliary gas mixed with other gases.
For example, the well-known gas H35 (hydrogen volume fraction 35%, the rest is argon) is one of the strongest gases in plasma arc cutting ability, which is mainly due to hydrogen gas.
Because hydrogen gas can significantly increase the arc voltage, causing the hydrogen plasma jet to have a high enthalpy value. When mixed with argon gas, the cutting ability of the plasma jet is greatly improved.
Generally, for metal materials with a thickness of more than 70mm, argon + hydrogen is commonly used as cutting gas. If using water jet to further compress the argon + hydrogen plasma arc, even higher cutting efficiency can be achieved.
3. Nitrogen is a commonly used working gas.
Under high supply voltage conditions, nitrogen plasma arc has better stability and higher jet energy than argon gas.
Even when cutting materials with high viscosity such as stainless steel and nickel-based alloys, the amount of slag hanging under the incision is also very small. Nitrogen can be used alone or mixed with other gases.
In automated cutting, nitrogen or air is often used as the working gas, and these two gases have become standard gases for high-speed cutting of carbon steel. Nitrogen is also sometimes used as the arc starting gas in oxygen plasma cutting.
4. Oxygen can increase the cutting speed of low carbon steel materials.
When using oxygen for cutting, the cutting mode is similar to flame cutting. The high-temperature and high-energy plasma arc makes the cutting speed faster, but it must be used in conjunction with high-temperature oxidation-resistant electrodes.
At the same time, the electrode must be protected from impacts during arc initiation to extend its service life.
5. Air contains about 78% volume fraction of nitrogen, so the slag suspension situation formed when using air for cutting is similar to that when using nitrogen for cutting.
Air also contains about 21% volume fraction of oxygen, and due to the presence of oxygen, the cutting speed of low carbon steel material using air is also high. At the same time, air is also the most economical working gas.
However, when air is used only for cutting, there are problems with slag suspension as well as increased oxidation and nitrogen in the incision. Low lifespan of electrodes and nozzles can also affect work efficiency and reduce costs.
V. Nozzle height
refers to the distance between the end face of the nozzle and the cutting surface, which constitutes a portion of the entire length of the arc. Because plasma arc cutting generally uses constant current or power sources with steep drop characteristics, the current changes very little after the nozzle height increases.
However, it will increase the arc length and cause the arc voltage to increase, thus increasing the arc power. But at the same time, it will also increase the energy loss of the arc column exposed to the environment.
Under the combined effect of these two factors, the effect of the former is often completely compensated by the latter, which can reduce the effective cutting energy and decrease the cutting capacity.
This usually manifests as weakening of the cutting jet force, increased residual slag at the bottom of the incision, and rounding of the upper edge.
Furthermore, considering the shape of the plasma jet, the diameter of the jet expands outward after exiting the gun, and the increase in nozzle height will inevitably cause an increase in incision width.
Therefore, choosing the lowest possible nozzle height is beneficial to improve cutting speed and quality.
However, when the nozzle height is too low, double arches can occur. The use of external ceramic nozzles can set the nozzle height to zero, that is, the end face of the nozzle comes into direct contact with the cutting surface, which can achieve good results.
SAW. Reducing power density
In order to obtain a high-compression plasma arc for cutting, the cutting nozzle uses a small nozzle opening, a longer hole length and a reinforced cooling effect. This can increase the current passing through the effective cross-sectional area of the nozzle, i.e., increase the power density of the arc.
However, compression also increases the power loss of the arc, so the actual energy used for cutting is less than the power output of the power source, and its loss rate is generally between 25% and 50%.
Some methods, such as water compression plasma cutting, may have a higher energy loss rate. This issue must be considered when designing the parameters of the cutting process or in the economic accounting of cutting costs.
For example, the thickness of metal plate commonly used in industry is generally less than 50mm.
Within this thickness range, conventional plasma arc cutting generally forms a cut with a larger upper edge and a smaller lower edge, and the upper edge of the cut may cause a decrease in incision size accuracy and increase processing work. subsequent.
When using oxygen and nitrogen plasma arc cutting of carbon steel, aluminum and stainless steel, when the thickness of the plate is in the range of 10-25mm, the material is thicker, the edge verticality is better, and the error of cutting edge angle is 1 degree to 4 degrees.
When the plate thickness is less than 1mm, as the plate thickness decreases, the incision angular error increases from 3-4 degrees to 15-25 degrees.
It is generally believed that the cause of this phenomenon is due to the uneven heat input of the plasma jet onto the cutting surface, that is, the energy release of the plasma arc at the top of the incision is greater than that at the bottom.
This imbalance in energy release is closely related to many process parameters, such as the degree of compression of the plasma arc, cutting speed and distance from the nozzle to the part.
Increasing the arc compression degree can extend the high-temperature plasma jet to form a more uniform high-temperature area, and at the same time increase the speed of the jet, which can reduce the width difference of the upper and lower edges of the incision.
However, excessive compression of conventional nozzles often causes double arcs, which not only consume electrodes and nozzles, making it impossible to continue the cutting process, but also lead to a decrease in the quality of the incision.
In addition, excessive cutting speed and nozzle height can also increase the difference in width between the upper and lower edges of the incision.
VII. Plasma cutting process parameter table
Low Carbon Steel Air Plasma/Air Protection Cutting Current 130A
| Select gas | Set cutting airflow | Material thickness | Arc voltage | Distance from cutting torch to workpiece | Cutting speed | Initial drilling height | Drilling delay | |||
| Plasma | Protective gas | Plasma | Protective gas. | mm | Voltage | mm | mm/min | mm | Coefficient % |
Second |
| Air | Air | 72 | 35 | 3 | 136 | 3.1 | 6,000 | 6.2 | 200 | 0.1 |
| 4 | 137 | 3.1 | 4930 | 6.2 | 200 | 0.2 | ||||
| 6 | 138 | 3.6 | 3850 | 7.2 | 200 | 0.3 | ||||
| 10 | 142 | 4.1 | 2450 | 8.2 | 200 | 0.5 | ||||
| 12 | 144 | 4.1 | 2050 | 8.2 | 200 | 0.5 | ||||
| 15 | 150 | 4.6 | 1450 | 9.2 | 200 | 0.8 | ||||
| 20 | 153 | 4.6 | 810 | 10.5 | 230 | 1.2 | ||||
| 25 | 163 | 4.6 | 410 | Start at the edge | ||||||
| 32 | 170 | 5.1 | 250 | |||||||
Low carbon steel oxygen plasma/air protection cutting current 130A.
| Select gas | Set cutting airflow | Material thickness | Arc voltage | Distance from cutting torch to workpiece | Cutting speed | Initial drilling height | Drilling delay | |||
| Plasma | Protective gas | Plasma | Protective gas | mm | Voltage | mm | mm/min | mm | Coefficient% | Second |
| Oxygen | Air | 65 | 48 | 3 | 128 | 2.5 | 6500 | 5.0 | 200 | 0.1 |
| 4 | 129 | 2.8 | 5420 | 5.6 | 200 | 0.2 | ||||
| 6 | 130 | 2.8 | 4000 | 5.6 | 200 | 0.3 | ||||
| 10 | 134 | 3.0 | 2650 | 6.0 | 200 | 0.3 | ||||
| 12 | 136 | 3.0 | 2200 | 6.0 | 200 | 0.5 | ||||
| 15 | 141 | 3.8 | 1650 | 7.6 | 200 | 0.7 | ||||
| 43 | 20 | 142 | 3.8 | 1130 | 7.6 | 200 | 1.0 | |||
| 25 | 152 | 4.0 | 675 | 8.0 | 200 | 1.5 | ||||
| 32 | 155 | 4.5 | 480 | Start at the edge | ||||||
| 38 | 160 | 4.5 | 305 | |||||||
