Flame cutting accuracy refers to the error relationship between the geometry of the cut work and its drawing size.
Flame cutting quality refers to the surface roughness of the cut section of the workpiece, the degree of melting and collapse of the upper edge of the kerf, the presence of slag on the lower edge of the kerf and the uniformity of the width of the cut, etc.
And the accuracy of flame cutting is guaranteed by its process parameters.
The main factors affecting flame cutting are the following.
- The type of combustible gas.
- The type of cutting torch.
- Cutting oxygen purity, pressure, flow rate, shape of oxygen flow.
- Cutting speed, inclination angle.
- Flame adjustment.
- Preheating flame energy rate.
- Tilt angle between cutting nozzle and workpiece, distance of cutting nozzle from workpiece surface, etc.
The cutting oxygen flow plays a leading role.
The cutting oxygen flow both burns the metal and blows the oxides generated by combustion away from the kerf.
Therefore, the purity, flow rate, flow rate and shape of the cutting oxygen flow have an important influence on the flame cutting quality and cutting speed.
Combustible gas type
In flame cutting, the commonly used flammable gases are acetylene, gas, natural gas, propane, etc. Some manufacturers also use MAPP, i.e.: methane + ethane + propane.
Generally speaking, gases with fast burning speed and high burning value are suitable for thin plate cutting.
Combustible gases with low combustion value and slow combustion speed are more suitable for thick plate cutting, especially for steel plates with thickness above 200mm, if gas or natural gas is used for cutting, ideal cutting quality will be obtained, only the cutting speed will be slightly reduced.
In comparison, acetylene is much more expensive than natural gas, but due to the problem of resources, in actual production, acetylene gas is generally used, only when cutting large thick plates while requiring high cutting quality and sufficient resources, natural gas is considered.
The thicker the part to be cut, the higher the torch type, nozzle number and oxygen pressure should be.
The relationship between oxygen pressure and the thickness of the cut piece, torch type and nozzle number is shown in the following table.
|Nozzle spec.||Nozzle throat diameter|
|mm/min||Oxygen||Acetylene||Liquefied petroleum gas|
Cutting oxygen purity, pressure, flow rate, oxygen flow shape
Cutting oxygen purity
The purity of oxygen also has a great impact on the oxygen consumption, the quality of the cut and the speed of gas cutting.
If the purity of oxygen decreases, impurities in oxygen such as nitrogen will absorb heat in the gas cutting process and form a gas film on the surface of the kerf, which will prevent the metal from burning and slow down the oxidation process of the metal, reduce the cutting speed greatly, make the cut wider, rough the cutting surface, slag the lower edge of the kerf, and increase the oxygen consumption.
The following graph shows the influence curve of oxygen purity on gas cutting time and oxygen consumption, 1 indicates the gas cutting time; 2 indicates the oxygen consumption.
In the range of oxygen purity from 97.5% to 99.5%, the gas cutting time will increase by 10% to 15% for each 1% decrease in oxygen purity for a 1m long cut; the oxygen consumption will increase by 25% to 35%.
Therefore, the purity of oxygen for gas cutting should be improved as much as possible, generally requiring more than 99.5%.
If the purity of oxygen drops below 95%, the gas cutting process will be very difficult.
To obtain a slag-free gas cutting kerf, the oxygen purity should reach 99.6%.
Though the one-time investment is large by using liquid oxygen cutting, the comprehensive economic index is much better than imagined in the long run.
Cutting oxygen pressure
When the cut piece is thin, the cutting oxygen pressure can be reduced appropriately.
However, the cutting oxygen pressure should not be too low or too high.
If the cutting oxygen pressure is too high, the cutting seam will be too wide and the cutting speed will be reduced, which will not only waste oxygen, but also make the surface of the incision rough, and will also have a strong cooling effect on the cut parts.
If the oxygen pressure is too low, it will slow down the oxidation reaction in the gas cutting process, the cutting oxide slag can not be blown off, forming a difficult to remove slag bond on the back of the cut, and even can not cut through the workpiece.
As the cutting oxygen pressure increases, the oxygen flow rate increases accordingly, so the thickness of the cut plate can be increased accordingly.
But the pressure increases to a certain value, the thickness of the cuttable also reaches a maximum, and then increase the pressure, the thickness of the cuttable instead of decreasing.
The effect of cutting oxygen pressure on cutting speed is approximately the same.
The influence of cutting oxygen pressure on cutting speed
As can be seen from the figure, when gas cutting with ordinary nozzle, the cutting speed increases with the increase of pressure at low pressure, but when the pressure exceeds 0.3MP, the cutting speed decreases instead; if the pressure continues to increase, not only the cutting speed decreases, but also the kerf widens and the cross section of the kerf is rough.
When gas cutting with diffusion-shaped nozzle, if the cutting oxygen pressure conforms to the design pressure of the nozzle, the cutting speed increases when the pressure increases, because the flow rate and momentum of the cutting oxygen flow increases, so the cutting speed also increases than when using ordinary nozzle.
Recommended value of cutting oxygen pressure
|Thk./mm||Cutting oxygen pressure|
In the actual cutting work, the best cutting oxygen pressure can be used to determine the test release of the “wind line” approach.
For the adopted nozzle, when the wind line is the clearest and longest, the cutting pressure at this time is the appropriate value, and the best cutting effect can be obtained.
Cutting oxygen flow
The effect of oxygen flow rate on the cutting speed when cutting a steel plate of 12 mm thickness is shown in the figure.
As can be seen from the figure, with the increase of oxygen flow rate, the cutting speed gradually increases, the cutting speed increases, but beyond a certain limit value but decreases.
Therefore, there is an optimal oxygen flow rate value for a certain steel plate thickness, when not only the cutting quality is the highest, but also the best cutting quality.
The influence of oxygen flow rate on cutting speed (plate thickness 12mm)
Cutting speed, tilt cutting speed, tilt angle
The cutting speed is related to the thickness of the workpiece and the form of the cutting nozzle, and generally slows down as the thickness of the workpiece increases.
Cutting speed must be adapted to the oxidation rate of the metal in the kerf.
Cutting speed directly affects the stability of the cutting process and the quality of the cut section.
If you want to artificially adjust the cutting speed to improve productivity and slow down the cutting speed to best improve the quality of the cut section, it can not be done, but only make the cut section quality worse.
Too slow cutting speed will reduce productivity, so that the upper edge of the kerf melting collapse, the lower edge of the rounded corners, the lower part of the cut section appear water wash deep groove crater, etc.;
Too fast the amount of dragging after too much, so that the cutting section appears depression and hanging slag and other quality defects, serious and even can not cut through, resulting in cutting interruption.
Machine cutting speed can increase 20% on average than manual cutting speed, the following table lists the recommended data of cutting speed in mechanized cutting.
Recommended data for cutting speed during mechanical cutting
|Steel Thk.||Cutting form|
|Semi-product straight cutting||Cutting of organic processing allowance||Cutting with low surface cutting quality requirements||Precise straight cutting||Precise forming cutting|
By observing the characteristics of the slag ejected from the kerf, the cutting speed can be adjusted to the proper cutting speed.
In the normal flame cutting process, the cutting oxygen flow is slightly offset by an angle relative to the vertical torch, and the corresponding offset is called the back drag amount (see figure below).
Cutting speed can be mastered according to the direction of the slag sparks falling in the kerf. When the speed is too low, and there is no back-drag amount, the spark beam at the kerf below the workpiece is offset to the cutting direction.
If you increase the torch running speed, the spark beam will be shifted in the opposite direction, when the spark beam is parallel to the cutting oxygen flow or slightly in front of the discharge, the cutting speed is considered normal.
When the speed is too high, the spark beam is obviously backward.
The cutting inclination angle between the cutting nozzle and the cutting part directly affects the gas cutting speed and the amount of back drag.
The size of the cutting inclination is mainly determined by the thickness of the workpiece.
When gas cutting steel plate less than 4mm thick, the cutting nozzle should be tilted back 25°~45°.
When gas cutting 4-20mm thick steel plate, the nozzle should be tilted back 20°~30°.
When gas cutting steel plate of 20-30mm thickness, the cutting nozzle should be perpendicular to the workpiece.
When gas cutting the thickness of the workpiece is more than 30mm, the cutting nozzle should be perpendicular to the workpiece at a forward angle of 5°~10° at the beginning of cutting and 5°~10° at the end after cutting through.
For manual curve cutting, the cutting nozzle is perpendicular to the workpiece.
The relationship between the cutting inclination of the cutting nozzle and the cutting thickness is shown in the figure below.
- 1-Thickness < 6mm
- 2- Thickness is 6-30mm
- 3- Thickness > 30mm
The angle of inclination between the cutting nozzle and the workpiece has a direct impact on the speed of gas cutting and the amount of dragging after, if the angle of inclination is not selected properly, not only can not improve the speed of gas cutting, but also will increase the consumption of oxygen, and even cause difficulties in gas cutting.
Three cutting flames can be obtained by adjusting the ratio of oxygen and acetylene: neutral flame (i.e. normal flame), oxidizing flame and reducing flame, see the figure below.
The normal flame is characterized by the absence of free oxygen and reactive carbon in its reduction zone, with three distinct areas and a sharply defined flame core (close to cylindrical).
The composition of the flame core is acetylene and oxygen with a uniformly rounded and shiny shell at its end.
The outer shell consists of red-hot carbonaceous points. The temperature of the flame core reaches 1000°C.
The reduction zone is outside the flame core, and the obvious difference with the flame core is its darker brightness.
The reduction zone consists of the products of incomplete combustion of acetylene – carbon dioxide and hydrogen, the temperature of the reduction zone can reach about 3000 ℃.
The outer flame, the complete combustion zone, is located outside the reduction zone, which consists of carbon dioxide and water vapor, nitrogen, its temperature varies between 1200 ~ 2500 ℃.
The oxidizing flame is produced in the presence of excess oxygen, its flame core is conical, its length is significantly shortened and its outline is unclear, and its brightness is dull.
Similarly, the reduction zone and the outer flame are also shortened, the flame is violet-blue, burning with a loud sound, the size of the sound is related to the pressure of oxygen, and the temperature of the oxidation flame is higher than the normal flame.
If the oxidation flame is used for cutting, it will deteriorate the cutting quality significantly.
Reduction flame is produced in the case of excess acetylene, its flame core does not have a clear outline, the end of its flame core has a green edge, according to this green edge to determine the presence of excess acetylene.
The reduction zone is unusually bright and almost blends in with the flame core.
The outer flame is yellow in color.
When there is too much acetylene excess, it starts to smoke black, which is caused by the lack of oxygen necessary for acetylene combustion in the flame.
The size of the energy of the preheating flame is quite closely related to the cutting speed and the quality of the kerf.
With the increase in the thickness of the workpiece to be cut and cutting speed, the flame energy should be enhanced, but not too strong, especially when cutting thick plate, metal combustion reaction heat increased, strengthening the preheating ability of the cutting point front.
At this time, too strong preheating flame will make the upper edge of the incision serious melting collapse.
Too weak preheating flame, and the steel plate will not get enough energy, forcing to reduce the cutting speed, and even cause interruptions in the cutting process.
Therefore, the relationship between the strength of the preheating flame and the cutting speed is mutually dependent.
Generally speaking, cutting steel plate below 200mm using neutral flame can get better cutting quality.
In cutting large thickness steel plate should use reduction flame preheating cutting, because the flame of reduction flame is longer, the length of the flame should be at least 1.2 times more than the plate thickness.
Preheating flame energy rate
The role of the preheating flame is to heat the metal workpiece to the temperature at which the metal burns in oxygen and to maintain this temperature at all times, while also stripping and melting the oxide on the steel surface to facilitate contact between the cutting oxygen stream and the metal.
Preheating flame is an important process parameter that affects the quality of gas cutting.
Gas cutting generally choose neutral flame or slight oxidation flame.
Carbonized flame can not be used because of the presence of free carbon, which will make the edge of the cut carbonization.
At the same time, the intensity of the flame should be moderate.
The preheating flame should be selected according to the thickness of the workpiece, the type of cutting nozzle and the quality requirements.
As in the gas cutting of thick steel plate, due to the slow gas cutting speed, in order to prevent the upper edge of the cut from melting, the flame energy rate should be reduced accordingly.
If the flame energy rate is too large at this time, the upper edge of the cut will produce continuous bead-like steel particles, and even melt into rounded corners, while also causing more slag adhering to the back of the cut, and affect the quality of gas cutting.
Such as in gas cutting thin steel plate, because of the gas cutting speed, can increase the flame energy rate, but the cutting nozzle should be far from the workpiece, and maintain a certain tilt angle;.
If the flame energy rate is too small at this time, the workpiece does not get enough heat, it will slow down the gas cutting speed, and even interrupt the gas cutting process.
- The power of the preheating flame should be increased with the increase of the plate thickness, the thicker the cut piece, the greater the power of the preheating flame.
The relationship between oxygen-acetylene preheating flame power and plate thickness
- When cutting thicker steel plates, a light carbide flame should be used to avoid collapse of the upper edge of the cut, and also to make the outer flame longer.
- When using diffusion type nozzle and oxygen curtain nozzle to cut steel plate below 200mm in thickness, the flame power should be larger to accelerate the heating of the leading edge of the kerf to the ignition point, so as to obtain a higher cutting speed.
- when cutting steel with higher carbon content or alloying elements, because they have higher ignition point, the power of preheating flame should be larger.
- When cutting bevels with a single cutting nozzle, because the slag is blown to the outside of the kerf, to increase the flame power for additional energy.
Gas flame cutting preheating time should be based on the thickness of the cut piece, the following listed the flame cutting selected preheating time of empirical data.
Empirical data of selected preheating time for gas flame cutting
|Thk./mm||Preheat time/S||Thk./mm||Preheat time/S|
Inclination angle between cutting nozzle and workpiece and distance from workpiece surface
The distance from the cutting nozzle to the surface of the workpiece is determined according to the thickness of the workpiece and the length of the preheating flame.
Too low height of the cutting nozzle will cause the melt collapse on the top line of the cut, and it is easy to block the cutting nozzle when splashing, and even cause tempering.
The height of the cutting nozzle is too large, the heat loss increases, and the preheating flame on the front edge of the incision of the heating effect is weakened, preheating is not sufficient, cutting oxygen flow energy decreases, making it difficult to remove slag, affecting the quality of cutting.
At the same time, the oxygen purity into the kerf is also reduced, resulting in an increase in the amount of back drag and kerf width, in cutting thin plates will also reduce the cutting speed.
Usually the flame core should be kept within 3-5mm from the surface of the workpiece, so that the heating conditions are the best, and the possibility of carburization is also minimal.
If the flame core touches the surface of the workpiece, it will not only cause the upper edge of the cut to melt, but also increase the possibility of carburization of the cut.
Generally speaking, when cutting thin plates, due to the faster cutting speed, the flame can be longer and the distance between the cutting nozzle and the surface of the workpiece can be larger.
When cutting thick plates, due to the slow cutting speed, in order to prevent the upper edge of the cut from melting, the preheating flame should be shorter, and the distance between the cutting nozzle and the workpiece surface should be appropriately smaller.
In this way, the straightness of the cutting oxygen flow and the purity of oxygen can be maintained, so that the cutting quality can be improved.
The best parameters of CNC flame cutting machine for cutting low carbon steel plate (GK1 fast cutting nozzle)
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