What is plasma cutting?
Plasma cutting is a processing method that uses the heat of a high-temperature plasma arc to partially or partially melt (and evaporate) the metal at the workpiece kerf, and uses the momentum of the high-speed plasma to remove the molten metal to form the kerf.
Plasma cutting machine is a machine that processes metal materials with the help of plasma cutting technology.
Plasma cutting with different working gases can cut various metals that are difficult to cut by oxygen cutting, especially for non-ferrous metals (stainless steel, aluminum, copper, titanium, nickel) the cutting effect is better.
The main advantage is that when cutting metals of small thickness, plasma cutting speed is fast, especially in cutting ordinary carbon steel sheet, the speed can be 5-6 times faster than the oxygen cutting method.
The cutting surface is smooth and clean, with little thermal deformation and less heat-affected zone.
Plasma cutting machine is widely used in various industries such as automobile, locomotive, pressure vessel, chemical machinery, nuclear industry, general machinery, engineering machinery, steel structure, ships, etc.
With the development of plasma cutting, the working gas that can be used (working gas is the conductive medium and heat carrier of plasma arc, but also excluding the molten metal in the incision) has a significant impact on the cutting characteristics, cutting quality and speed of plasma arc.
The common working gases of plasma arc include argon, hydrogen, nitrogen, oxygen, air, water vapor and some mixed gases.
Selection of plasma gas
The gas into the plasma arc is called the ion stream.
Its main functions are as follows:
- A. It plays a certain role of heat insulation and insulation between the arc column and the inner wall of nozzle hole, compresses the arc and ensures the stable combustion of the arc.
- as the thermal conductor of ionization medium and arc, the cut metal is melted rapidly.
- blow out the melted metal at the slit to form a narrow and smooth slit.
- cooling protection for electrodes.
In view of the above effects of ionic gases, the following requirements should be considered in the selection of gases.
- easy to start arc and stable arc. Therefore, the ionization potential of the gas should be as low as possible, the thermal conductivity should be poor, and the cooling effect on the arc should be smaller. From this point of view, it is obvious that monoatomic gas is better than diatomic gas.
- the compression property of arc is better. This requires that the gas has good thermal conductivity, large heat capacity and great cooling effect on the arc.
- good heat carrying capacity, that is, the ability to transfer heat to the workpiece is strong. Generally speaking, diatomic gas with higher ionization potential has better heat carrying capacity than monoatomic gas with low ionization potential.
- the gas should be able to generate large momentum. This requires the gas to have a large mass and density.
- the gas should be cheap, available and non-toxic.
Some of the above requirements are contradictory. Therefore, the main requirements should be met according to the specific situation.
At present, the gas commonly used in plasma arc cutting is nitrogen, argon, hydrogen and their mixture gas, among which nitrogen is the most widely used.
Nitrogen is cheap and easy to obtain, and it is less dangerous to use.
It is a diatomic gas, which is endothermic when it decomposes in high temperature of arc
When the decomposed and ionized nitrogen ions are sprayed on the cold metal surface, they recombine into nitrogen molecules and release the heat absorbed during decomposition.
Although the decomposition and recombination of nitrogen makes the temperature of arc column lower than that of monoatomic gas, it can make the plasma arc obtain high temperature in a wide range of length, which creates favorable conditions for cutting thick materials.
Especially for cutting copper with high thermal conductivity and stainless steel with poor liquid fluidity, nitrogen is economical and can ensure cutting quality.
The purity of nitrogen used in plasma cutting should not be less than 99.5%. If there is more oxygen and water vapor in the nitrogen, it will oxidize the electrode, seriously burn the tungsten and even burn out the nozzle.
Argon is a kind of monoatomic gas. Its ionization potential is lower than that of nitrogen, which is prone to thermoionization, stable arc combustion and high temperature. However, its price is much higher than that of nitrogen.
When cutting the workpiece with large thickness, a certain amount of hydrogen can be added into nitrogen or argon gas. Because of the strong compression effect of hydrogen on the arc, the voltage and power of the plasma arc will rise, and the slit surface will be narrower and smoother than that with pure nitrogen.
In air plasma cutting with compressed air as ion gas, the expensive argon, argon / hydrogen or argon / nitrogen mixture is replaced by compressed air, so the power consumption is less, the management and maintenance are convenient, and the cost is low.
In addition, when using high current, the cutting speed is much higher than that of flame cutting. For carbon steel plate below 25 mm, the cutting speed is 3-5 times higher, and 6-8 times higher for 5-10 mm thin plate.
The cutting quality is also very good, so it has replaced the flame cutting in many occasions.
How to choose the parameters of NC plasma cutting
Various plasma arc cutting process parameters directly affect the stability of the cutting process, cutting quality and effect.
The main cutting specifications are as follows:
1) Cutting current
The most important parameter of cutting speed and cutting is the cutting process.
- When the cutting current increases, the arc energy increases, the cutting ability increases, and the cutting speed increases;
- With the increase of cutting current, the arc diameter increases and the arc becomes thicker, which makes the incision wider;
- If the cutting current is too high, the thermal load of the nozzle will increase, the nozzle will be damaged prematurely, and the cutting quality will naturally decline, even unable to cut normally.
So before cutting, you have to choose the correct cutting current and the corresponding nozzle according to the thickness of the material.
When choosing a power supply before plasma cutting, you should not choose a power supply that is too large or too small.
A power supply that is too large is a waste in terms of cutting cost, because it simply does not use that much current.
Also can’t choose too small current when choosing plasma power for saving the cost. In this way, the actual cutting is also unable to achieve their own cutting requirements, which is a great harm to the CNC cutting machine itself!
2) Cutting speed
The high temperature and high energy of plasma arc determine the cutting speed.
The optimum cutting speed range can be selected according to the equipment instructions or determined by experiments
Due to the thickness of the material, different materials, melting point, thermal conductivity and surface tension after melting, the cutting speed changes accordingly.
On the premise of ensuring the cutting quality, the cutting speed should be increased as much as possible.
This can not only improve productivity, but also reduce the deformation of the parts to be cut and the heat affected area of the slotted area.
If the cutting speed is not suitable, the effect will be opposite, and the slag sticking will be increased and the cutting quality will be decreased.
The main manifestations are as follows:
- If the cutting speed is increased moderately, the incision quality can be improved, that is, the incision becomes narrower slightly, the incision surface is smoother, and the deformation can be reduced.
- If the cutting speed is too fast, the linear energy of cutting is lower than the required value, and the jet in the slit can not blow off the molten cutting melt immediately and form a large amount of drag. With the slag hanging on the incision, the surface quality of the incision decreases.
- When the cutting speed is too low, because the cutting position is the anode of the plasma arc, in order to maintain the stability of the arc, the anode spot or anode area must find the conductive current place near the cutting seam nearest to the arc, and at the same time, it will transfer more heat to the radial direction of the jet, so the incision will be widened, and the molten materials on both sides of the incision will gather and solidify at the bottom edge, which is not easy to clean.Moreover, the upper edge of the incision is rounded due to excessive heating and melting.
- When the speed is very low, the arc will even extinguish due to the wide incision. Thus, good cutting quality and cutting speed are inseparable.
3) Arc voltage
It is generally considered that the normal output voltage of the power supply is the cutting voltage.
Plasma plasma machine will usually work with higher voltage of plasma arc, such as high voltage required for air plasma cutting.
When the current is constant, the increase of voltage means the improvement of arc enthalpy and cutting ability.
If the enthalpy increases, the diameter of the jet is reduced and the gas flow rate is increased, the faster cutting speed and better cutting quality can be obtained.
No load voltage and arc column voltage
Plasma cutting power supply, must have enough high no-load voltage, in order to start the arc easily and make the plasma arc stable combustion.
The no-load voltage is generally 120-600v, and the arc column voltage is generally half of the no-load voltage.
The power of plasma arc can be obviously increased by increasing the voltage of arc column, thus the cutting speed and the metal plate with larger thickness can be cut.
The arc column voltage is usually achieved by adjusting the gas flow rate and increasing the internal shrinkage of the electrode, but the arc column voltage should not exceed 65% of the no-load voltage, otherwise the plasma arc will be unstable.
4) Working gas and flow
The working gas includes cutting gas and auxiliary gas, and arc starting gas is required for some equipment
Generally, the appropriate working gas should be selected according to the type of cutting material, thickness and cutting method.
Cutting gas should not only ensure the formation of plasma jet, but also ensure the removal of molten metal and oxide in the incision.
Excessive gas flow will take away more heat from the arc, making the length of the jet shorter, resulting in the decline of cutting ability and arc instability;
If the gas flow rate is too small, the plasma arc will lose its straightness and make the cutting depth shallower, and at the same time, it is easy to produce slag;
The cutting speed and flow rate must match well.
At present, the gas flow rate of plasma cutting machine is controlled by the gas flow rate.
The gas pressure used to cut a certain thickness of material should be selected according to the data provided by the equipment manufacturer. If there are other special applications, the gas pressure should be determined by actual cutting test.
The most commonly used working gases are: argon, nitrogen, oxygen, air and H35, argon nitrogen mixture.
Argon hardly reacts with any metal at high temperature, and the argon plasma arc is stable.
The nozzle and electrode used have a long service life. But the voltage of argon plasma arc is low, the enthalpy is not high, and the cutting ability is limited. Compared with air cutting, the cutting thickness will be reduced by 25%.
In addition, in argon protection environment, the surface tension of molten metal is larger, which is about 30% higher than that in nitrogen environment, so there will be more slag problems.
Even if the mixture of argon and other gases is used, there will be a tendency of slag sticking.
Therefore, it is rare to use pure argon for plasma cutting.
Hydrogen is usually used as auxiliary gas to mix with other gases. For example, the famous gas H35 (volume fraction of hydrogen is 35%, the rest is argon) is one of the most powerful gases in plasma arc cutting, which mainly benefits from hydrogen.
Because hydrogen can significantly increase the arc voltage, the hydrogen plasma jet has a very high enthalpy, when mixed with argon, the cutting ability of the plasma jet is greatly improved.
Generally, argon and hydrogen are commonly used as cutting gas for metal materials with thickness more than 70mm.
If the argon + hydrogen plasma arc is further compressed by water jet, higher cutting efficiency can be obtained.
Nitrogen is a common working gas. Under the condition of higher power supply voltage, nitrogen plasma arc has better stability and higher jet energy than argon
Even when cutting materials with high liquid metal viscosity, such as stainless steel and nickel base alloy, the amount of slag on the lower edge of the incision is very small.
Nitrogen can be used alone or mixed with other gases. For example, nitrogen or air are often used as working gas in automatic cutting
These two gases have become the standard gases for high-speed cutting of carbon steel.
Sometimes nitrogen is also used as the starting gas of oxygen plasma arc cutting.
Oxygen can increase the cutting speed of low carbon steel.
When oxygen is used for cutting, the cutting mode and flame cutting are very imaginative. High temperature and high energy plasma arc makes the cutting speed faster, but it must be combined with the use of high temperature oxidation resistant electrode, and at the same time, the anti impact protection of the electrode when arcing is carried out to prolong the service life of the electrode.
The air contains about 78% nitrogen by volume, so it is very imaginative to use air cutting to form slag and nitrogen cutting; the air also contains about 21% oxygen
Because of the existence of oxygen, the cutting speed of low carbon steel with air is also very high; at the same time, air is the most economical working gas.
However, when air cutting is used alone, there will be problems such as slag hanging, oxidation of cut and nitrogen increase. Moreover, the low life of electrode and nozzle will also affect the work efficiency and cutting cost.
5) Nozzle height
It refers to the distance between the nozzle end face and the cutting surface, which constitutes a part of the whole arc length.
The distance is generally 4 ~ 10 mm.
It is the same as the reduction in the electrode, the distance should be appropriate to give full play to the cutting efficiency of plasma arc, otherwise the cutting efficiency and cutting quality will be reduced or the cutting nozzle will be burned out.
Because the plasma arc cutting generally uses the power supply with constant current or steep drop external characteristics, the current changes little after the nozzle height increases, but it will increase the arc length and lead to the increase of arc voltage, so as to improve the arc power; but at the same time, the arc length exposed to the environment will increase, and the energy loss of the arc column will increase.
In the case of the combined effect of the two factors, the effect of the former is often completely offset by the latter, which will reduce the effective cutting energy and reduce the cutting ability.
It usually shows that the blowing force of the cutting jet is weakened, the residual slag in the lower part of the incision is increased, and the upper edge is over melted and rounded.
In addition, considering from the shape of plasma jet, the diameter of jet expands outward after leaving the muzzle, and the increase of nozzle height will inevitably lead to the increase of incision width.
Therefore, choosing the nozzle height as small as possible is beneficial to improve the cutting speed and cutting quality. However, when the nozzle height is too low, it may cause double arc phenomenon.
The ceramic external nozzle can set the nozzle height to zero, that is, the nozzle end face directly contacts the cutting surface, which can obtain a good effect.
5) Arc power
In order to obtain high-pressure plasma arc cutting arc, cutting nozzles adopt smaller nozzle diameter, longer channel length and enhance cooling effect, which can increase the current passing through the effective section of the nozzle, that is, the power density of the arc increases.
But at the same time, the power loss of arc is increased.
Therefore, the actual effective energy for cutting is smaller than the output power of the power supply, and the loss rate is generally between 25% and 50%.
Some methods, such as water compression plasma arc cutting, have a higher energy loss rate. This problem should be considered in the design of cutting process parameters or the economic accounting of cutting cost.
Most of the metal plate thickness used in industry is less than 50 mm. In this thickness range, the conventional plasma arc cutting will often form a cut with a large top and a small bottom, and the upper edge of the cutting edge will lead to the decrease of the accuracy of the incision size and increase the subsequent processing amount.
When oxygen and nitrogen plasma arc is used to cut carbon steel, aluminum and stainless steel, when the plate thickness is within 10 ~ 25 mm, the thicker the material is, the better the perpendicularity of the end edge is, and the angle error of the cutting edge is 1 ~ 4 degrees.
When the plate thickness is less than 1 mm, with the decrease of plate thickness, the incision angle error increases from 3 ~ 4 degrees to 15 ~ 25 degrees.
It is generally believed that this phenomenon is caused by the unbalanced heat input of plasma jet on the cutting surface, that is, the energy release of plasma arc in the upper part of the cut is more than that in the lower part.
The imbalance of energy release is closely related to many process parameters, such as plasma arc compression degree, cutting speed and the distance between nozzle and workpiece.
Increasing the compression degree of the arc can make the high temperature plasma jet longer and form a more uniform high temperature area. At the same time, increasing the jet speed can reduce the width difference between the top and bottom of the incision.
However, the over compression of conventional nozzle often leads to double arc phenomenon. The double arc will not only consume the electrode and nozzle, so that the cutting process can not be carried out, but also lead to the decline of the cutting quality.
In addition, too high cutting speed and too large nozzle height will lead to the increase of the gap width.
6) Electrode shrinkage
The so-called internal shrinkage refers to the distance between the electrode and the end face of the cutting nozzle. The proper distance can make the arc get good compression in the cutting nozzle, and obtain the plasma arc with concentrated energy and high temperature for effective cutting.
If the distance is too large or too small, the electrode will be severely burned, the cutting nozzle will be burnt out and the cutting ability will be reduced.
The internal shrinkage is generally 8-11mm.
Selection of electrode and polarity
The electrode used in plasma arc cutting must have a small burning loss to ensure the stability of the cutting process.
The practice shows that there is still a considerable amount of burning loss when tungsten with high melting point is used as electrode, so it can not guarantee the stable cutting process.
If a small amount of elements with low ionization potential (such as thorium) are added into tungsten, the burning loss of thorium tungsten electrode can be significantly reduced.
This is because Th-W electrode has a strong ability of electron thermal emission, and most of the energy of Th-W electrode end face is used to escape electrons, which makes the temperature of electrode end face lower.
On the other hand, the thorium tungsten electrode has a weak effect on oxygen, which reduces its burning loss at high temperature.
The commonly used thorium tungsten electrode is thorium tungsten rod containing 1.5% ~ 2.5% thorium.
However, thorium is radioactive, so cerium tungsten electrode or lanthanum tungsten electrode are widely used. They are non radioactive and can replace thorium tungsten electrode.
In general, the polarity of plasma arc cutting is directly connected, that is, thorium tungsten rod is connected with negative electrode, and the workpiece is connected with positive electrode.
In this way, it is beneficial to the thermal emission of electrons, to ensure the stable combustion of the isoelectric arc, and to reduce the burning loss of the electrode.
In addition to the general form of plasma cutting methods, derived forms include water compression plasma cutting.
The most commonly used methods are general plasma cutting and air plasma cutting.
General plasma cutting does not need protective gas, working gas and cutting gas are ejected from the same nozzle.
When the arc is started, a small flow of ionic gas is ejected as the ionization medium;
When cutting, air stream gas is ejected at the same time to eliminate molten metal.
Air plasma cutting generally uses compressed air as ion gas.
The cutting cost of this method is low and the source of gas source is convenient.
Compressed air is heated, decomposed and ionized in the arc, and the generated oxygen can cut the metal to produce chemical exothermic reaction and accelerate the cutting speed.
The fully ionized air plasma has high enthalpy, so the arc energy is large and the cutting speed is fast.
Plasma cutting system is mainly composed of gas supply device, water device, power supply and cutting gun.
The water-cooled gun also needs cooling circulating water device.
(1) Air supply unit
The main equipment of air supply device for air plasma arc cutting is an air compressor larger than 1.5KW, and the required gas pressure is 0.3-0.6mpa.
If other gases are selected, bottled gas can be used for cutting after decompression.
(2) Power supply
Plasma cutting adopts DC power supply with steep drop or constant current characteristics.
In order to obtain satisfactory arc starting and stabilizing effect, the no-load voltage of the power supply is generally twice that of the arc voltage.
The no-load voltage of common cutting power supply is 350-400v.
(3) Cutting gun
The specific form of the cutting gun depends on the current level of the cutting gun. Generally, the air-cooled structure is adopted for the cutting gun below 60A, while the water-cooling structure is used for the cutting gun above 60A.
The electrode in the cutting gun can be pure tungsten, thorium tungsten, bell tungsten rod, or inlaid electrode.
Cast tungsten is preferred as electrode material.
It has the advantages of simple cutting and cutting of all kinds of non metal materials.
- The lower part of plasma cutting should be set with a water tank. During the cutting process, the cutting part should be cut under water to avoid the toxicity of smoke to human body.
- Avoid direct visual plasma arc during plasma arc cutting. Wear professional protective glasses and face mask to avoid eye and skin burns caused by arc light.
- A large amount of toxic gas will be produced in the process of plasma arc cutting, which requires ventilation and wear multi-layer filtering dust mask.
- During the plasma arc cutting process, it is necessary to wear towel, gloves, foot sheath and other labor protection appliances to prevent the skin from being burned by sparks.
- The high frequency and electromagnetic radiation generated by the high frequency oscillator in the plasma arc cutting process will cause damage to the body, and some long-term practitioners even have infertility symptoms. Although there is no definite conclusion in the medical field and the industry, it is still necessary to do a good job of protection.
How to choose CNC plasma cutting machine
No matter what kind of plasma is purchased, it is important to consider the factor of budget, because if the budget is ignored, even if the intended effect can be achieved, it will not necessarily be purchased if it exceeds the budget too much.
At present, plasma is mainly divided into domestic and imported two categories, of which domestic price is divided into low, medium and high three specifications, but generally speaking, users should still be acceptable!
The price of imported products is several times higher than that of domestic products, even dozens of times, and consumables like cutting nozzle and electrode are much more expensive!
Usually, compared with the same specification of domestic plasma, imported plasma cutting effect is better, the disadvantage is that the price and the late use of the cost is too high, the general enterprises are difficult to accept!
No matter what kind of plasma is used, it has its own most suitable cutting size range, usually low power plasma is suitable for cutting thin plates, which is well known.
If the plasma power is too high, it is not suitable for cutting thin plates!
At present, domestic 40-60a plasma cutting is widely used for domestic steel plates below 2mm, and imported ones are occasionally used, but the quantity is not large, because the price is relatively expensive.
It is hereby seriously stated that for steel plates below 2mm, it is important to remember that any other models, such as portable and gantry type, are not suitable for cutting except high-speed desktop plasma cutting machine.
If it is more than 2mm, it is not subject to this restriction and can be used.
For thicker steel plate, such as 2-16mm steel plate, portable and gantry type machines can be cut. If the thickness is 25 mm or even thicker, only the gantry type is suitable!
This general engaged in metal processing technical personnel all know, using plasma to cut stainless steel and other alloy materials is its strength, carbon steel plate mainly depends on flame cutting method, so as long as there is stainless steel, it must be equipped with plasma cutting machine!
Otherwise, it can’t be cut with flame!
Advantages of NC plasma cutting machine
1.Better cutting quality
Scum, heat affected zone, top fillet and cutting angle are the main factors affecting the cutting quality.
Especially in scum and heat affected zone, plasma cutting is much better than flame cutting. There is no residual scum on the edge of plasma cutting, and the heat affected zone is much smaller.
Plasma process uses high temperature charged gas to melt metal and blow the melted metal material off the cutting surface.
Flame cutting is to use the chemical reaction between oxygen and steel for cutting, which will produce iron red slag or scum.
Because of the difference of this technology, the scum produced by plasma cutting is less, and the attached scum is easier to remove. These scum can be easily knocked down without grinding or removing, greatly reducing the time required for secondary processing.
Fewer grinding operations lead to higher productivity.
3.Heat affected zone
The size of the heat affected zone (HAZ) is one of the important issues in metal cutting.
High temperature will change the chemical structure of the metal, making the heated edge dark (tempering color) and warping. If the heated edge is not removed, the workpiece may not be suitable for secondary welding.
No matter what process is adopted, the faster the torch moves, the smaller the heat affected zone will be.
Because of the rapid cutting characteristics of plasma, the heat affected zone is smaller, which shortens the time spent in the secondary processing to remove the heated edge.
Some flame cutting users may also pay attention to the fire color.
The heat affected zone cannot be seen from the outside, but the tempering color is not. It will change the color of the metal.
In the same way, the rapid cutting of plasma makes the area of fire color smaller.
With CNC plasma cutting and piercing speeds up to 8.5 times faster than flame cutting, the productivity gains can be significant, not counting the time saved in preheating and secondary processing.
5.Lower cost per part
When analyzing costs, it is important to understand the difference between operating costs and operating costs per part or per meter.
So, how to determine the actual cost of cutting a part?
The running cost per meter is the total cost of cutting per hour divided by the total length (meters) that can be cut in one hour. The costs involved in cutting include vulnerable parts, electricity, gas, manpower and sustainability costs.
Cost per part is the total length of cutting required to produce a part multiplied by the cost per meter of operation.
The cutting speed of plasma system is faster, and the number of parts produced in a certain time is more, so the cutting cost of each part is much lower.
For hand-held cutting, calculating the cost of each job or task can better evaluate the cost savings.
The running cost per hour multiplied by the total time required to complete the job equals the cost of the job.
For flame cutting, the preheating time and long secondary processing time must be calculated in the required time.
The lower cost per part of plasma system directly leads to the increase of profit.
Cutting every part can save money and thus increase profit margin.
The more parts are cut per hour, the more total profit will be increased.
7.Easier to use
For users who use flame cutting, it takes time to learn and practice to master the setting of flame chemical parameters and maintain the chemistry of flame.
The powermax series of Hypertherm products operate through compressed air, and there is no gas to be mixed or regulated.
If you are using a mechanical plasma system with CNC or automatic gas console, all parameters have been set for the operator when leaving the factory, and there is no need to manually adjust the gas.
In addition, the powermax handheld plasma system can drag the cutting torch, which means that the operator can drag the torch along the metal surface.
There is no need to maintain any distance (a key element of good flame cutting).
Moreover, it is easier to cut according to the template and pattern, especially the template and pattern with curve.
No need to adjust the gas, drag cutting ability and easy-to-use control device, making the plasma system easier to learn.
Plasma system can cut any conductive metal, including stainless steel, aluminum, copper and brass.
In contrast, flame cutting machine uses chemical reaction between oxygen and iron in low carbon steel to cut, so it can only be used to cut low carbon steel.
In addition, the plasma system can also be used for planing, marking or cutting rusty, painted or even stacked metal.
Moreover, you can also use plasma system for groove cutting or steel mesh cutting, which is very difficult for flame cutting.
The fuel used in flame cutting is a mixture of oxygen and gas.
The most commonly used fuel gases are acetylene, propane, MAPP, propylene and natural gas.
One of the most commonly used is acetylene, because acetylene produces a higher flame temperature and faster perforation speed than other gases.
However, acetylene is an unstable and highly flammable gas, which is extremely sensitive to high pressure, temperature and even static electricity.
Acetylene explosion can cause property damage of tens of thousands of dollars and cause serious injury to nearby people.
Some plasma systems, such as the powermax series, usually operate with compressed air and do not require the use of flammable gases.
Hypertherm’s HPR and HSD plasma systems can use a variety of gases, including air, most commonly oxygen and nitrogen.
These gases are more stable than acetylene and do not require much special treatment.
All types of thermal cutting will produce some odor and noise. However, water cutting machine can be selected for thermal cutting of cutting machine and CNC plasma cutting machine system, which can greatly reduce odor and noise.
Most flame cutting should not be carried out underwater because of the possibility of explosion.
Safety guarantee of plasma cutting machine operation
(1) Operators must wear protective masks, welding gloves, hats, filter dust masks and sound insulation earmuffs. It is strictly forbidden to observe the plasma arc directly without wearing protective glasses, and the naked skin is forbidden to approach the plasma arc.
(2) When cutting, the operator should stand on the windward side. It should be able to reduce the air suction area from the lower part of the table.
(3) When cutting, when the no-load voltage is too high, check the electrical grounding, zero connection and insulation of torch handle, insulate the workbench from the ground, or install no-load circuit breaker in the electrical control system.
(4) High frequency generator should be equipped with shielding shield. After high frequency arc striking, high frequency circuit should be cut off immediately.
(5) The use of thorium and tungsten electrodes should comply with article JGJ33-2001 of 12.7.8.
(6) Cutting operation and cooperation personnel must wear labor protection articles according to regulations. Safety measures must be taken to prevent electric shock, falling from high altitude, gas poisoning and fire.
(7) Electric welding machines used on site shall be equipped with rain proof, moisture-proof and sun proof shed, and corresponding fire-fighting equipment shall be installed.
(8) When welding or cutting at high altitude, safety belt must be fastened. Fire prevention measures shall be taken around and under welding and cutting, and special personnel shall be assigned for supervision.
Daily maintenance of plasma cutting machine
1.Assemble the cutting torch correctly
Install the cutting torch correctly and carefully to ensure that all parts match well, and ensure the gas and cooling air flow. Install all components on a clean flannelette to avoid dirt sticking to the components.
2.The consumable parts should be replaced in time before they are completely damaged
The consumable parts should not be replaced after being completely damaged, because the seriously worn electrode, nozzle and vortex ring will produce uncontrollable plasma arc, which is very easy to cause serious damage to the cutting torch.
Therefore, when the first time found cutting quality decline, it should be timely inspection of consumable parts.
3.Clean the connecting thread of cutting torch
When replacing consumable parts or daily maintenance and inspection, it is necessary to ensure that the internal and external threads of the cutting torch are clean. If necessary, the connecting threads should be cleaned or repaired.
4.Clean the contact surface between electrode and nozzle
In many cutting torches, the contact surface between nozzle and electrode is charged contact surface. If there is dirt on these contact surfaces, the cutting torch can not work normally.
5.Check the gas and cooling gas every day
Check the flow and pressure of gas and cooling air flow every day. If the flow is insufficient or there is leakage, shut down the machine immediately to eliminate the fault.
6.Avoid cutting torch collision damage
In order to avoid collision damage of cutting torch, it is necessary to program correctly to avoid system overrun. Installing anti-collision device can effectively avoid cutting torch damage during collision.
7.The most common causes of cutting torch damage
① Cutting torch collision.
② Destructive plasma arc due to consumable damage.
③ A destructive plasma arc caused by dirt.
④ Destructive plasma arc caused by loose parts.
① Do not grease the cutting torch.
② Do not overuse O-ring lubricant.
③ Do not spray splash proof chemicals while the protective sleeve is still on the cutting torch.
④ Do not use manual cutting torch as hammer.
Operation precautions of plasma cutting machine
1.1 in order to reduce energy consumption and improve the service life of nozzle and electrode, “low grade” cutting should be adopted when cutting thinner workpiece.
1.2 when the “cutting thickness selection” switch is set to “high grade”, the non-contact cutting type cutting (except for special cases) should be adopted, and the water cutting torch should be preferred.
1.3 when the “thickness selection” switch must be changed, the power switch of the host must be turned off first to prevent damage to the parts.
1.4 when installing, disassembling or moving the host, the power supply must be turned off first to prevent danger.
1.5 the power switch of the host machine should be turned off before the accessories and components on the host can be installed and removed (such as cutting torch, cutting ground wire, electrode, nozzle, distributor, pressure cap, protective sleeve, etc.). Avoid repeatedly and quickly opening the torch switch, so as to avoid damaging the arc striking system or related components.
1.6 when it is necessary to start arc cutting from the middle of the workpiece, the stainless steel cutting thickness ≤ 20 mm can be directly pierced and cut. The method is as follows: put the cutting torch on the starting point of the cutting seam, and make the axis of the cutting torch nozzle form an angle of about 75 ° with the workpiece plane, then turn on the cutting torch switch to start arc piercing; at the same time, slowly adjust the angle between the nozzle axis and the workpiece surface until the cutting through workpiece should be adjusted to 90 °. After cutting through the workpiece, it can be cut normally along the cutting seam direction. However, if the thickness is more than the above, it is necessary to drill a small hole (diameter is not limited) on the starting point of cutting, so as to start arc cutting from the small hole. Otherwise, it is easy to damage the cutting torch nozzle.
1.7 the mainframe continuous working rate is 70% (the “cut thickness selection” switch is placed at low level, and the continuous work can be close to 100%). If the continuous working time is too long and the temperature of the main engine is too high, the temperature protection system will automatically shut down, and it must be cooled for about 20 minutes before it can continue to work.
1.8 when the compressed air pressure is lower than 0.22mpa, the equipment shall be in the protection shutdown state immediately. At this time, the gas supply system shall be overhauled. After troubleshooting, the pressure can be resumed to 0.45MPa.
1.9 if the three-phase input power supply is out of phase, the main engine will not work normally, and the “phase loss indication” red light of some models will be on. It is necessary to eliminate the fault before normal cutting.
1.10 for water-cooled machines, the water tank must be filled with tap water, and the power plug of water pump must be plugged in.
1.11 turn the power switch to the “start” position. If the “insufficient air pressure” indicator is on, it should be adjusted to 0.45MPa as required. So the “insufficient air pressure” indicator is off. The direction of fan rotation should be in accordance with the sign direction. The water pump direction of the water cooler should meet the requirements, otherwise the “insufficient water pressure” indicator light will be on, and the input power phase should be adjusted.
1.12 according to the thickness of the workpiece, turn the “cutting thickness selection” switch to the corresponding position, and select the appropriate cutting torch. The cutting torch has various specifications from small to large according to the use range. Do not exceed the rated current range, otherwise it will be damaged. Place the cutting torch at the cutting starting point of the workpiece and press the torch switch. If the torch is not ignited for one time, press the torch switch again. The arc striking is successful and the cutting is started.
1.13 every four to eight hours of operation (the interval time depends on the dryness of compressed air), the drain screw of “air filter pressure reducer” should be loosened to drain the accumulated water, so as to prevent excessive water from entering the machine or cutting torch and causing failure.
1.14 when the water cooling system is in poor circulation, the main engine will be in the protection shutdown state. At this time, it should be checked and solved according to the methods described in the relevant chapters. After the water pressure returns to normal, the water-cooled cutting torch can continue to be used.
1.15 when working in cold environment, attention must be paid: when the ambient temperature is lower than freezing point, water cooling method is not allowed to be used for cutting, otherwise, the circulating water cooling system will not work normally and the water cooling cutting torch may be damaged. Two
Preparation before operation
2.1 after connecting the equipment (please pay special attention to the safety grounding wire), check carefully, and if everything is normal, the next operation can be carried out.
2.2 close the power supply switch to supply power to the host. Note: the input AC current is about 65A, which should not be too small, otherwise the host can not work normally. At the same time, check that the fan in the main engine should meet the requirements, otherwise, adjust the input power phase until the steering direction is consistent.
2.3 set the “power switch” of the host to the “on” position. At this time, the “power indicator” light is on. However, the “lack of phase indicator” light should not be on, otherwise, the three-phase power supply has the phenomenon of phase loss, which should be checked and solved.
Note: if the main engine shell is not properly connected to the safety ground wire, the phase loss indicator may show wrong results.
2.4 supply air to the main engine, and put the “gas test” and “cutting” switches at the “gas test” position. At this time, compressed air should be ejected from the cutting torch nozzle. After three minutes of test, the red light of “insufficient air pressure” should not be on. Check that the indication value of pressure gauge on “air filter reducer” should not be lower than 0.42mpa, otherwise, it indicates that the air source pressure is less than 0.45MPa or the flow is less than 300L / min. It may also be that the air supply pipeline is too small and the air pressure drop is too large. If there are above problems, check and solve them. In addition, please pay attention to whether the “air filter pressure reducer” is out of balance, if not, it should be readjusted.
The adjustment method is as follows:
Rotate the handle clockwise to increase the pressure, otherwise it will decrease. Adjust the indication value on the pressure gauge to 0.42mpa. If the air supply is normal, the “insufficient air pressure” indicator light will go out. At this time, please put the “cutting” and “gas testing” switch to the “cutting” position.
3.1 Manual non-contact cutting:
3.1.1 contact the cutting torch roller with the workpiece, and adjust the distance between the nozzle and the workpiece plane to 3-5mm. (when the host machine is cutting, set the “thickness selection” switch to high level).
3.1.2 turn on the cutting torch switch to ignite the plasma arc. After cutting through the workpiece, move towards the cutting direction at an average speed. The cutting speed is: cutting through is the premise, which should be fast rather than slow. Too slow will affect the quality of incision, and even break the arc.
3.1.3 after cutting, turn off the torch switch and the plasma arc will be extinguished. At this time, compressed air will be ejected to cool the cutting torch. After a few seconds, the spray will stop automatically. Remove the cutting torch and complete the whole cutting process.
3.2 Manual contact cutting
3.2.1 when the “thickness selection” switch is set at low gear, it can be used when cutting thin plate by single machine.
3.2.2 place the torch nozzle at the starting point of the workpiece to be cut, turn on the torch switch, ignite the plasma arc, cut through the workpiece, and then move uniformly along the cutting seam direction.
3.2.3 after cutting, open and close the torch switch. At this time, the compressed air is still ejecting. After a few seconds, the spraying will stop automatically. Remove the cutting torch and complete the whole cutting process.
3.3 Automatic cutting
3.3.1 automatic cutting is mainly suitable for cutting thick workpieces. Select the “thickness selection” switch position.
3.3.2 after the cutting torch roller is removed, the cutting torch is firmly connected with the semi-automatic cutting machine, and there are connecting pieces in the attached accessories.
3.3.3 connect the power supply of the semi-automatic cutting machine, and install the guide rail or radius rod according to the shape of the workpiece (if the guide rail for linear cutting is used, if the circular or circular arc is cut, the radius rod should be selected).
3.3.4 if the torch switch plug is removed, replace the remote control switch plug (provided in the attached accessories).
3.3.5 adjust the walking speed according to the thickness of the workpiece. The “reverse” and “forward” switches on the semi-automatic cutting machine are placed in the cutting direction.
3.3.6 adjust the distance between the nozzle and the workpiece to 3 ~ 8mm, and adjust the nozzle center position to the starting strip of the workpiece cutting seam.
3.3.7 turn on the remote control switch. After cutting through the workpiece, turn on the power switch of the semi-automatic cutting machine to cut. In the initial stage of cutting, attention should be paid to the cutting seam at any time and adjusted to the appropriate cutting speed. And pay attention to whether the two machines work normally at any time.
3.3.8 after cutting, turn off the remote control switch and the power switch of semi-automatic cutting machine. So far, the whole cutting process is completed.
3.4 Manual circle cutting
According to the material and thickness of the workpiece, select single machine or parallel machine cutting mode, and select the corresponding cutting method, and tighten the cross bar in the screw hole on the cutting torch holder
If the length of one piece is not enough, it can be connected to the required radius length one by one, and then, according to the radius length of the workpiece, the distance between the tip and the cutting torch nozzle can be adjusted (the slotting width must be considered).
After adjustment, tighten the top fastening screw to prevent loosening, and loosen the knurled screw of cage.
At this point, the workpiece can be cut round.
Key points of plasma cutting stainless steel
- Chromium stainless steel has certain corrosion resistance (oxidizing acid, organic acid, cavitation), heat resistance and wear resistance. It is usually used in power plant, chemical industry, petroleum and other equipment and materials. The weldability of chromium stainless steel is poor. Attention should be paid to the welding process, heat treatment conditions and the selection of suitable electrodes.
- CR 13 stainless steel is easy to crack due to its high hardenability after welding. If the same type of chromium stainless steel electrode (g202, G207) is used for welding, preheating above 300 ℃ and slow cooling treatment about 700 ℃ after welding must be carried out. If post weld heat treatment is not available, chromium nickel stainless steel electrodes (A107, A207) should be used.
- The weldability of Cr 17 stainless steel is better than that of Cr 13 stainless steel by adding appropriate amount of stability elements Ti, Nb, Mo to improve corrosion resistance and weldability. When the same type of chromium stainless steel electrode (G302, G307) is used, preheating above 200 ℃ and tempering at 800 ℃ after welding shall be carried out. If the weldment cannot be heat treated, chromium nickel stainless steel electrode (A107, A207) should be used.
- When chromium nickel stainless steel is welded, carbides are precipitated by repeated heating, which reduces the corrosion resistance and mechanical properties.
- Chromium nickel stainless steel electrode has good corrosion resistance and oxidation resistance, which is widely used in chemical industry, chemical fertilizer, petroleum and medical machinery manufacturing.
- The coating of Cr Ni stainless steel has Ti CA type and low H type. The titanium calcium type can be used for AC and DC welding, but the penetration is shallow and easy to turn red during AC welding, so DC power supply should be used as much as possible. Diameter 4.0 and below can be used for all position weldment, 5.0 and above can be used for flat welding and flat fillet welding.
- The welding rod should be kept dry when using. The titanium calcium type should be dried at 150 ℃ for 1 hour, and the low hydrogen type should be dried at 200-250 ℃ for 1 hour (repeated drying is not allowed, otherwise the coating is easy to crack and peel), so as to prevent the coating from sticking oil and other dirt, so as to increase the carbon content in the weld and affect the quality of the weldment.
- In order to prevent the corrosion between the eyes caused by heating, the welding current should not be too large, about 20% less than that of carbon steel electrode, the arc should not be too long, and the interlayer should be cooled quickly, and the narrow welding pass is suitable. Plasma arc welding should pay attention to the protective measures before cutting, and the smoke and dust are big and the damage is great.
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