Precautions for CNC plasma cutting
The selection of cutting process parameters for CNC plasma machines has a crucial impact on cutting quality, cutting speed, efficiency, and other cutting effects. Proper use of CNC plasma machines for high-quality and rapid cutting requires a profound understanding and mastery of cutting process parameters.
1、 Arc voltage: It is generally believed that the normal output voltage of the power supply is the cutting voltage. Plasma arc cutting machines usually have high no-load voltage and operating voltage. When using gases with high ionization energy such as nitrogen, hydrogen, or air, the voltage required to stabilize the plasma arc will be higher. When the current is constant, an increase in voltage means an increase in arc enthalpy and cutting ability. If the diameter of the jet is reduced and the gas flow rate is increased while increasing the enthalpy value, faster cutting speed and better cutting quality can often be achieved.
2、 Cutting current: It is the most important cutting process parameter, directly determining the cutting thickness and speed, i.e. cutting ability.
Effects: 1. As the cutting current increases, the arc energy increases, the cutting ability improves, and the cutting speed increases accordingly; 2. As the cutting current increases, the arc diameter increases and the arc becomes thicker, resulting in a wider incision; 3. Excessive cutting current increases the thermal load on the nozzle, causing premature damage to the nozzle and a natural decrease in cutting quality, even making it impossible to perform normal cutting. So before cutting, the cutting current and corresponding nozzle should be selected correctly according to the thickness of the material.
3、 Cutting speed: The optimal cutting speed range can be selected according to the equipment instructions or determined through experiments. Due to factors such as material thickness, material differences, melting points, thermal conductivity, and surface tension after melting, the cutting speed also varies accordingly. Main manifestations:
1. Moderately increasing the cutting speed can improve the quality of the incision, that is, the incision becomes slightly narrower, the incision surface is smoother, and deformation can be reduced.
2. The cutting speed is too fast, causing the cutting line energy to be lower than the required amount. The jet in the cutting seam cannot quickly blow away the melted cutting melt immediately, resulting in a large amount of drag. Along with the slag hanging on the cutting edge, the surface quality of the cutting edge decreases.
3. When the cutting speed is too low, due to the fact that the cutting point is the anode of the plasma arc, in order to maintain the stability of the arc itself, the anode spot or anode area must find a place to conduct current near the nearest cutting seam to the arc, and at the same time, it will transfer more heat radially to the jet, thus widening the incision. The melted material on both sides of the incision gathers and solidifies at the bottom edge, forming difficult to clean slag, and the upper edge of the incision forms a rounded corner due to excessive melting caused by heating.
4. When the speed is extremely low, the arc may even extinguish due to the wide incision. It can be seen that good cutting quality and cutting speed are inseparable. 4、 Working gas and flow rate: Working gas includes cutting gas and auxiliary gas. Some equipment also requires arc starting gas, and the appropriate working gas is usually selected based on the type, thickness, and cutting method of the cutting material. Cutting gas requires both the formation of plasma jet and the removal of molten metal and oxides from the incision. Excessive gas flow will take away more arc heat, causing the length of the jet to become shorter, resulting in decreased cutting ability and unstable arc; A too small gas flow rate can cause the plasma arc to lose its proper straightness, resulting in a shallower cutting depth and a higher risk of slag accumulation; So the gas flow rate must be well coordinated with the cutting current and speed. Most plasma arc cutting machines nowadays rely on gas pressure to control flow rate, because when the gun body aperture is constant, controlling the gas pressure also controls the flow rate. The gas pressure used for cutting a certain thickness of material usually needs to be selected according to the data provided by the equipment manufacturer. If there are other special applications, the gas pressure needs to be determined through actual cutting tests. The most commonly used working gases include argon, nitrogen, oxygen, air, as well as H35, argon nitrogen mixture gas, etc.
1. Hydrogen is usually used as an auxiliary gas for mixing with other gases, such as the famous gas H35 (with a volume fraction of 35% hydrogen and the rest argon), which is one of the gases with the strongest plasma arc cutting ability, mainly due to hydrogen. Due to the significant increase in arc voltage caused by hydrogen gas, the hydrogen plasma jet has a high enthalpy value. When mixed with argon gas, the cutting ability of the plasma jet is greatly improved. For metal materials with a thickness of 70mm or more, argon+hydrogen is commonly used as the cutting gas. If water jet is used to further compress the argon+hydrogen plasma arc, higher cutting efficiency can be achieved.
2. Argon hardly reacts with any metal at high temperatures, and the argon plasma arc is very stable. And the nozzle and electrode used have a relatively long service life. However, the voltage of argon plasma arc is lower, the enthalpy value is not high, and the cutting ability is limited. Compared with air cutting, its cutting thickness will be reduced by about 25%. In addition, in an argon protected environment, the surface tension of molten metal is relatively high, about 30% higher than in a nitrogen environment, so there will be more slag hanging problems. Even when using a mixture of argon and other gases for cutting, there is a tendency for slag adhesion. Therefore, it is now rare to use pure argon gas alone for plasma cutting.
3. Nitrogen is a commonly used working gas. Under high power supply voltage conditions, nitrogen plasma arc has good stability and higher jet energy than argon gas. Even when cutting materials with high viscosity of liquid metals such as stainless steel and nickel based alloys, the amount of slag hanging on the lower edge of the incision is very small. Nitrogen can be used alone or mixed with other gases, such as nitrogen or air, which are often used as working gases in automated cutting. These two gases have become standard gases for high-speed cutting of carbon steel. Sometimes nitrogen is also used as the starting gas for oxygen plasma arc cutting.
4. The air contains about 78% nitrogen by volume, so the slag formation during air cutting is very similar to that during nitrogen cutting; The air also contains about 21% oxygen by volume, and due to the presence of oxygen, the cutting speed of low-carbon steel materials using air is also very high; At the same time, air is also the most economical working gas. However, when using air cutting alone, there may be issues such as slag accumulation, oxidation of the cutting edge, and nitrogen addition. Additionally, the low lifespan of the electrodes and nozzles can also affect work efficiency and cutting costs.
5. Oxygen can increase the speed of cutting low-carbon steel materials. When using oxygen for cutting, the cutting mode is very similar to flame cutting. The high-temperature and high-energy plasma arc makes the cutting speed faster, but it must be combined with high-temperature oxidation resistant electrodes, and the electrodes must be protected against impact during arc initiation to extend their lifespan.
5、 Cutting power density: In order to obtain a high compressibility plasma arc cutting arc, the cutting nozzle adopts a smaller nozzle aperture, a longer channel length, and enhances the cooling effect. This can increase the current passing through the effective cross-section of the nozzle, that is, the power density of the arc increases. But at the same time, compression also increases the power loss of the arc. Therefore, the actual effective energy used for cutting is smaller than the power output of the power supply, with a loss rate generally between 25% and 50%. Some methods such as water compression plasma arc cutting may have a higher energy loss rate. This issue should be considered when designing cutting process parameters or conducting economic calculations of cutting costs.
For example, the thickness of metal plates used in industry is mostly below 50MM. Within this thickness range, conventional plasma arc cutting often forms a cut with a larger top and smaller bottom, and the upper edge of the cut will also cause a decrease in the accuracy of the cut size and increase the subsequent processing volume. When using oxygen and nitrogen plasma arc cutting for carbon steel, aluminum, and stainless steel, when the plate thickness is in the range of 10-25mm, the thicker the material, the better the verticality of the end edges, and the angle error of the cutting edge is usually between 1 degree and 4 degrees. When the plate thickness is less than 1MM, as the plate thickness decreases, the cutting angle error increases from 3 to 4 degrees to 15 to 25 degrees.
It is generally believed that the cause of this phenomenon is due to the unbalanced heat input of the plasma jet on the cutting surface, that is, the release of plasma arc energy is greater in the upper part of the cutting than in the lower part. The imbalance of energy release is closely related to many process parameters, such as the degree of plasma arc compression, cutting speed, and the distance between the nozzle and the workpiece. Increasing the compression degree of the arc can prolong the high-temperature plasma jet, forming a more uniform high-temperature area. At the same time, increasing the jet velocity can reduce the width difference between the top and bottom of the incision. However, excessive compression of conventional nozzles often leads to the double arc phenomenon, which not only damages the electrode and nozzle, making the cutting process impossible, but also results in a decrease in the quality of the cut. In addition, excessive cutting speed and nozzle height can cause an increase in the width difference between the upper and lower cuts.
6、 Nozzle height: refers to the distance between the nozzle end face and the cutting surface, which constitutes a part of the entire arc length. Due to the use of constant current or steep drop external characteristic power sources in plasma arc cutting, the current changes very little as the nozzle height increases, but it will increase the arc length and lead to an increase in arc voltage, thereby increasing the arc power; But at the same time, it will also increase the arc length exposed to the environment and the energy loss of the arc column. In the case of the combined effect of two factors, the former's effect is often completely offset by the latter, which in turn reduces the effective cutting energy and leads to a decrease in cutting ability. The usual manifestation is a decrease in the blowing force of the cutting jet, an increase in residual slag at the lower part of the incision, and the appearance of rounded corners due to excessive melting at the upper edge. Furthermore, considering the shape of the plasma jet, the diameter of the jet expands outward after leaving the muzzle, and an increase in nozzle height inevitably leads to an increase in the width of the incision. Therefore, choosing a nozzle height that is as small as possible is beneficial for improving cutting speed and quality. However, when the nozzle height is too low, it may cause double arc phenomenon. By using a ceramic outer nozzle, the nozzle height can be set to zero, meaning that the nozzle end face directly contacts the cutting surface, which can achieve good results.