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Basic operation of plasma cutting

Plasma cutting is divided into two types: transfer type and non-transfer type. The operation method of non-transferred plasma cutting is similar to that of oxyacetylene gas flame cutting. When transferring plasma cutting, the cutting torch and the workpiece form an electrical circuit, and the workpiece is an indispensable pole of the plasma arc. During the cutting process, if the distance between the cutting torch and the workpiece is too large (greater than 10 mm), it may break arc.

Due to the large structure of the plasma cutting torch, the visibility during cutting is poor, and the operation is not convenient, so its operation is more difficult than oxyacetylene gas cutting.

Preparation before plasma cutting

Determination of tungsten electrode material, diameter and shape

n the plasma cutting process, if the burning loss of the tungsten electrode is small, the cutting process is stable. Generally, WTh-15 and WTh-30 thorium tungsten rods with a mass fraction of 1.5% -2.5% thorium are used as electrodes, and the burning loss is smaller than that of pure tungsten electrodes, and the arc is relatively stable.

When plasma arc cutting, the cutting current is usually selected according to the thickness of the workpiece, and then the diameter of the tungsten electrode is determined according to the cutting current. It is more appropriate to control the current density of thorium tungsten electrode within 20 A/mm2. If the current density is too high, the tungsten electrode will generate a lot of heat and serious burnout; if the current density is too small, the arc will be unstable and affect the cutting quality. The relationship between the diameter of the thorium tungsten electrode and the maximum working current is shown in Table Relationship between Thorium Tungsten Electrode Diameter and Maximum Operating Current.

Relationship between Thorium Tungsten Electrode Diameter and Maximum Operating Current

The shape of the tip of the tungsten electrode affects the stability of the arc. When in use, the flame produced by grinding the top of the tungsten electrode at a certain angle is more stable than the plane shape, as shown in Figure The shape of the tip of the tungsten electrode.

The shape of the tip of the tungsten electrode

Choice of power supply polarity

In order to make the plasma arc burn stably and reduce the burning loss of the tungsten electrode, DC positive connection is used for plasma arc cutting, that is, the tungsten electrode is connected to the negative electrode, and the workpiece is connected to the positive electrode.

Adjust the concentricity of the tungsten electrode and the nozzle

Poor concentricity between the tungsten electrode and the nozzle will seriously affect the cutting ability, cutting quality and nozzle life. In order to ensure the concentricity of the tungsten electrode and the nozzle during cutting, careful adjustment should be made before cutting, and then the arc is ignited. The method to check the concentricity is to observe whether the spark generated by the high-frequency oscillation between the tungsten electrode and the nozzle is evenly distributed around the nozzle hole. Concentricity is best when sparks are all around the nozzle. If the electric spark only accounts for less than 1/2 of the circumference of the nozzle, it means that the concentricity is not good, and it should be adjusted before continuing to use.

Plasma cutting start point

Before cutting, the starting point of the cutting workpiece surface should be cleaned to make it conduct well. For thick and large workpieces or workpieces with unclean surfaces, it is best to preheat the starting point with a small arc, and then close the high current switch to make the arc turn smoothly.

When cutting, start from the edge of the workpiece and wait until the edge of the workpiece is cut through before moving the torch. If the workpiece to be cut does not allow this, you can first drill a small hole with a diameter of about 15mm on the workpiece to be cut as the starting point of cutting, so as to avoid the slag splashing around under the strong blowing force of the plasma arc, which is easy to operate . Especially in the case of severe splashing, the slag blocks the nozzle, or accumulates on the nozzle to form a double arc with the workpiece, which will cause the nozzle to burn out. When the thickness of the workpiece is not large, it is not necessary to pre-drill the hole. When cutting, the cutting torch is inclined backward in the vertical plane of the cutting seam by an angle, or the cutting piece is placed in an inclined or vertical position, so that the slag can be easily discharged. When cutting, restore the normal cutting posture and position.

Plasma cutting speed

After the cutting process starts, the speed of the torch movement has a great influence on the cutting quality. If the speed is too large or too small, the incision with satisfactory quality cannot be obtained. The torch moves too fast, and the molten metal turns up at the front end of the incision, so it cannot be cut through; if the speed is too slow, in addition to the uneven width of the incision and the increased heat-affected zone. In addition, often because the workpiece has been cut through, the arc is pulled too long and extinguished, which interrupts the cutting process. On the premise of ensuring the cut through, the torch moving speed should be as large as possible. In addition, the moving speed when switching from a small arc to a cutting arc is particularly important, because on the one hand, the arc switching process itself is not good for the continuous combustion of the arc; Before the arc, a small arc should be used to pause a little at the starting point of the tangent, and when the arc has been burning steadily and started to cut through, move forward immediately.

Plasma torch angle

During the entire cutting process, the cutting torch should be kept perpendicular to the incision plane, otherwise the incision will be deflected, the incision will not be smooth, and the bottom surface of the incision will be melted. In order to increase the cutting speed and increase the productivity, the cutting torch can usually be inclined at an angle (0° ~ 45°) in the opposite direction to the cutting direction in the plane where the incision is located, as shown in Figure The back angle of the torch during plasma cutting. When cutting thick plates and using high power, the back inclination angle should be smaller; when cutting thin plates and using low power, the back inclination angle should be larger.

The back angle of the torch during plasma cutting

Features of large thickness workpiece cutting

Large thickness workpiece cutting has the following characteristics:

  • Large-thickness workpieces have strong heat dissipation capacity and increase heat loss, so the plasma arc is required to have greater power, and the diameter of the nozzle and tungsten electrode used should be correspondingly increased.

  • The plasma arc should have greater blowing force, and the arc column should be drawn longer. The main method is to adjust the gas flow to make the white and bright part of the plasma arc long, straight and powerful.

  • When cutting thick workpieces, the instability of the arc increases, so a larger gas flow and a power supply with a higher no-load voltage must be used. The no-load voltage of the dedicated plasma arc cutting power source can reach 400 V, which can meet the requirements of cutting thick workpieces. When using DC arc welding machines in series, more than 3 welding machines are often required to be connected in series.

  • When cutting thick workpieces, the power of the plasma arc is relatively large, and the current suddenly changes from the small arc to the cutting arc, which often causes the arc to be interrupted and the nozzle to burn out. In this regard, the method of graded arc switching can be adopted, a current limiting resistor (about 0.42Ω) is connected in the cutting circuit to reduce the current value during arc switching, and then the resistor is short-circuited to make the plasma arc switch to normal cutting specifications. The circuit diagram is shown in Figure Classified arc ignition circuit.

Classified arc ignition circuit

Plasma cutting operation practice

The exercise piece is made of stainless steel, and the length x width x thickness is 500 mm x 200 mm x 12 mm.

Carefully clean the surface of the cutting piece for practice according to the above cutting points, and draw a secant line every 25 mm along the 500 mm direction, and punch the sample along the line. The specific parameters during cutting can be referred to as follows:

plasma cutting

During operation, practice cutting and normal plasma arc cutting repeatedly. Through practice, the cutting can be achieved accurately, and the transition from the non-transfer arc to the transfer arc should be stable, no arc extinguishing, no undercutting and melting, the kerf is straight along the line, and the surface of the kerf is smooth.

Operation example ------ Cutting of 1Gr18Ni9Ti stainless steel flange

The outer diameter of the flange is 219mm, the inner diameter is 60mm, and the thickness is 20mm. Cutting machine model LG-400-1, the worktable adopts a multi-column bracket, as shown in Figure Multi-column bracket, to prevent the bracket from being cut off during cutting, and to prevent the workpiece from being cut to the end due to the gravity of the cut part. The workpiece sags and the misalignment occurs.

Multi-column bracket

Cutting method Manual cutting.

Cutting process parameters

Cutting current 320 A; cutting voltage 160 v; gas flow 2 400 L/h; cutting speed 25~30 m/h; diameter of cerium tungsten rod 5.8 mm;

Steps

  • Install the IC-400-1 plasma arc cutting machine. Since manual cutting is used, disconnect the multi-core plug “Z” of the control cable connected to the trolley, and connect the multi-core plug “S” of the manually cut control cable (Fig. shown in dashed lines in LG-400-1 plasma arc cutting machine external installation wiring diagram).

LG-400-1 plasma arc cutting machine external installation wiring diagram
  • Check that the installation and wiring of the cutting machine are correct, and then test for water, electricity, gas and high-frequency arc ignition. After the inspection is completed, it is ready to be cut.

  • According to the drawing design process size of the part to be cut, draw a line on the stainless steel plate first, as shown in Figure 3-19. When drawing a line, leave a margin for the incision, and the margin is calculated according to the following empirical formula:

    b=δ/5+8

    where b is the width of the incision (mm);

    Δ——The thickness of the workpiece to be cut (mm).

    Drill a hole of φ12mm at a certain distance from the cutting line of the inner circle of the flange as the arc starting hole when cutting the inner circle.

  • Put the drawn steel plate on the multi-column support, and pay attention to lay it flat.

  • Cut the inner circle of the flange first. Turn on the power, hold the cutting torch, make the torch nozzle 8~10mm away from the workpiece, pull the switch on the cutting torch forward, then the circuit is turned on, and the action procedure of each part of the cutting machine is the same as that of automatic cutting. The arc start starts from the tangent point, from the small arc to the large arc, and then enters the normal cutting.

    If the arc cannot be cut for some reason after ignition, and the arc needs to be disconnected, just move the manual torch away from the cutting workpiece, pull the toggle switch back from the front position, then push it forward, and then pull it back, the arc is cut off. Note that in the process of disconnecting and striking the arc, the time the toggle button of the switch is pulled back for the first time must be short, otherwise the nozzle of the torch and the water-cooling resistor will be burned. To stop cutting, push the toggle switch forward, and then dial it back to stop cutting.

  • After cutting the inner circle of the flange and then cutting the outer circle, the cutting method is the same as before, but it is not necessary to drill the hole when striking the arc, but cut from the edge of the workpiece at a certain distance, as shown in Figure Schematic diagram of cutting flange outer circle.

  • After cutting, turn off the power switch and air source, turn off the cold water and the main power switch.

Schematic diagram of cutting flange outer circle