METHOD FOR MONITORING A FUTURE CUTTING PROCESS

Description

[0001] The invention relates to a method for monitoring a flame cutting process according to the preamble of claim 1.

[0002] In a flame cutting process, a cutting torch generates a flame by burning a gas mixture and then applies this flame to a workpiece to be cut. The workpiece is first heated up during a preheating phase before the flame pierces a hole through the workpiece in a piercing phase. Following the piercing phase, the tool is cut in a cutting phase by moving the torch along a cutting line at a distance from the tool. The distance between the cutting torch and the workpiece is adjusted differently in the different phases. While rapid heating is preferred in the preheating phase by setting a short distance, material such as slag often splashes off the workpiece in the piercing phase, so the cutting torch is then preferably moved further away from the workpiece. After the hole has been pierced and during the transition to the cutting phase, the distance to the workpiece is usually reduced again.The progress of the flame cutting process is usually monitored by an operator who identifies which phase the process is in. This is complex and often less reliable.

[0003] A method according to the preamble of claim 1 is known from US 2016 / 0018812 A1. In this method, a current increase due to electrons escaping from the material is detected when high temperatures are reached. Furthermore, US 5,470,047 A discloses a method in which a voltage drop across a measuring resistor is measured during a flame cutting process. This voltage increases sharply at the end of a heating phase, whereby the flame of the cutting torch is always saturated by applying a sufficiently high voltage.

[0004] It is therefore an object of the invention to further develop a method of the type mentioned at the outset in such a way that it can be automated to a greater extent.

[0005] This object is achieved according to the invention by a method having the features of claim 1. Advantageous developments of the invention are the subject of the dependent claims.

[0006] The invention is based on the idea that the flame is electrically conductive and thus an electric current can flow through the flame, provided at least part of the cutting torch and the workpiece are both electrically conductive. The applicant has experimentally discovered the possibility of automatically detecting the ignition point at which the flame begins to pierce the hole through the workpiece due to oxidation of the workpiece, based on a change in the measured current flowing via the cutting torch through the flame to the workpiece. It is thus possible to detect when the hole begins to be pierced through the workpiece by monitoring the measured current, so that the distance control can then be carried out automatically without the need for operator intervention.

[0007] To increase the accuracy of the method, the invention provides that a constant voltage is applied between an electrically conductive part of the cutting torch and the workpiece by means of a voltage source, at least during the measurement period during which the current is measured. The voltage is preferably a direct current, but can also be an alternating current.

[0008] According to the invention, the voltage applied during the measurement period is at most equal to a first predetermined voltage, and the ignition point is detected by a drop in the measured current. The first predetermined voltage is advantageously less than 5 volts and preferably at most 4 volts, more preferably at most 3 volts. It has been experimentally found that at such low voltages at the ignition point, a significant drop in the measured current is detectable.

[0009] It is advisable to keep the distance between the cutting torch and the workpiece constant during the measurement period. This prevents the measured current from changing due to changes in distance. For the same reason, the gas mixture supplied to the flame is kept constant during the measurement period.

[0010] If the ignition point is detected by applying a low voltage, which is at most as high as the first predetermined voltage, through a drop in the measured current, an advantageous development of the invention can provide for the ignition point to be further detected by a rise in the measured current during the preheating phase that precedes the drop. It has been found experimentally that, typically, before the ignition point is reached in the preheating phase, the measured current rises almost continuously. This rise, which precedes the subsequent drop in the measured current, can be considered a further indication that the ignition point has been reached, in order to make the detection of the ignition point more reliable.

[0011] After the ignition point is detected, the distance between the cutting torch and the workpiece and the gas mixture supplied to the flame are expediently controlled by a control device. The distance between the cutting torch and the workpiece is preferably increased by the control device. After the hole has been pierced, the distance between the cutting torch and the workpiece is preferably reduced again by the control device. Furthermore, after the ignition point is detected, the quantity and / or proportion of the combustible gas in the gas mixture is expediently increased by the control device, and additional cutting oxygen is supplied to the flame.

[0012] It is preferred that the workpiece be grounded during the measurement period. Furthermore, it is preferred that the electrically conductive part of the cutting torch be electrically insulated from the workpiece, so that an electrical current can only flow between the cutting torch and the workpiece through the flame. The electrically conductive part of the cutting torch expediently has a torch tip facing the workpiece.

[0013] The invention will be explained in more detail below with reference to two exemplary embodiments shown schematically in the drawing. Fig. 1 shows the current plotted over time, measured in a method for monitoring a flame cutting process according to a first exemplary embodiment, and Fig. 2 shows the current plotted over time, measured in a method for monitoring a flame cutting process according to a second exemplary embodiment.

[0014] The current curves shown in the drawing were each measured during a flame cutting process 10 and are used to monitor the flame cutting process 10. A current I is measured which flows from an electrically conductive torch tip of the cutting torch used in the flame cutting process 10 through a flame generated by the cutting torch to the electrically conductive, grounded workpiece exposed to the flame. During the flame cutting process 10, the workpiece is first heated up by means of the flame in a preheating phase 12. The preheating phase 12 is followed by a piercing phase 14 in which the heated workpiece is oxidized in the area of the flame and the flame pierces a hole through the workpiece. Once a hole has been pierced through the workpiece, it can be cut along a cutting line by moving the flame along the cutting line during a cutting phase 16.

[0015] By means of a DC voltage source, a constant DC voltage is applied between the torch tip and the workpiece during a measuring period 18 comprising the preheating phase 12 and the beginning of the piercing phase 14, which Fig. 1 The first embodiment shown is 3 volts, and the distance between the burner tip and the workpiece is kept constant, as is the composition of the gas mixture supplied to the flame. Using an ammeter, the current I flowing from the burner tip through the flame to the workpiece is measured, which is Fig. 1 is plotted against time t. The beginning of the piercing phase 14, referred to as the ignition point 20, is characterized by an abrupt drop in the measured current I. A further characteristic of the ignition point 20 is a preceding, almost continuous rise 22 in the measured current I during the preheating phase 12.

[0016] The second embodiment according to Fig. 2 differs from the first exemplary embodiment only in that a higher DC voltage of constant 6 volts is applied during the measuring period 18, which results in a fundamentally different course of the measured current I over time t. The ignition point 20, which marks the end of the preheating phase 12 and the beginning of the piercing phase 14 of the flame cutting process 10, is characterized in this exemplary embodiment by a characteristic abrupt increase in the measured current I. The continuous increase 22 in the measured current I during the preheating phase 12, which occurs in the first exemplary embodiment, is also not present here.

[0017] The described detection of the ignition point 20 based on the change in the measured current I is used to control the flame cutting process 10. While maximum heat input into the workpiece is desired during the preheating phase 12, for which purpose the distance between the torch tip and the workpiece is kept small, increasing this distance during the piercing phase 14 is advantageous to prevent damage to the cutting torch from splashing slag. If the characteristic change in the measured current I at the ignition point 20 is detected and the ignition point 20 is thus automatically detected, the distance between the torch tip and the workpiece is automatically increased by means of a control device. Furthermore, the proportion of combustible gas in the gas mixture supplied to the flame is increased, and additional cutting oxygen is supplied to the flame.After the end of the piercing phase 14 and at the beginning of the cutting phase 16, the distance between the torch tip and the workpiece is reduced again. In the exemplary embodiments shown here, the DC voltage is kept constant throughout the entire flame cutting process 10, i.e., beyond the measurement period 18. The measured current I is then influenced not only by the conductivity of the flame, but also by the changing distance between the torch tip and the workpiece.

[0018] In summary, the following can be stated: The invention relates to a method for monitoring a flame cutting process 10, which has a preheating phase 12, a piercing phase 14 following the preheating phase 12 and a cutting phase 16 following the piercing phase 14, wherein a cutting torch applies a flame generated by the combustion of a gas mixture to an electrically conductive workpiece and wherein an electrical current I flowing between an electrically conductive part of the cutting torch and the workpiece is measured by means of an ammeter during a measuring period 18. According to the invention, it is provided that an ignition point 20, at which the piercing of a hole through the workpiece by means of the flame begins due to oxidation of the workpiece, is detected by a change in the measured current I.

Claims

1. Method for monitoring a flame cutting process (10), which comprises a preheating phase (12), a piercing phase (14) following the preheating phase (12) and a cutting phase (16) following the piercing phase (14), wherein a cutting torch applies a flame generated by combustion of a gas mixture to an electrically conductive workpiece and wherein an electric current (I) flowing between an electrically conductive part of the cutting torch and the workpiece is measured by means of a current measuring device during a measuring period (18), characterized in that an ignition point (20) at which piercing of a hole through the workpiece by means of the flame starts due to oxidation of the workpiece is detected by a change in the measured current (I), that a constant voltage is applied by means of a voltage source between the electrically conductive part of the cutting torch and the workpiece at least during the measuring period (18), that the voltage is at most as large as a first predetermined voltage and that the ignition point (20) is detected by a drop in the measured current (I).

2. Method according to claim 1, characterized in that the voltage is a DC voltage.

3. Method according to claim 1, characterized in that the voltage is an alternating voltage.

4. Method according to any one of the preceding claims, characterized in that the distance between the cutting torch and the workpiece is kept constant during the measuring period (18).

5. Method according to any one of the preceding claims, characterized in that the gas mixture supplied to the flame is kept constant during the measuring period (18).

6. Method according to any one of the preceding claims, characterized in that the ignition point (20) is further detected by an increase (22) in the measured current (I) during the preheating phase (12) preceding the drop.

7. Method according to any one of the preceding claims, characterized in that, after detection of the ignition point (20), the distance of the cutting torch from the workpiece and the gas mixture supplied to the flame are controlled by means of a control device.

8. Method according to claim 7, characterized in that, after detection of the ignition point (20), the distance of the cutting torch from the workpiece is increased by means of the control device.

9. Method according to claim 8, characterized in that, after completion of the piercing of the hole, the distance of the cutting torch from the workpiece is reduced again by means of the control device.

10. Method according to any one of the claims 7 to 9, characterized in that, after detection of the ignition point (20) by means of the control device, the quantity and / or proportion of the combustible gas in the gas mixture is increased and additional cutting oxygen is supplied to the flame.

11. Method according to any one of the preceding claims, characterized in that the first predetermined voltage is less than 5 volts and preferably at most 4 volts, further preferably at most 3 volts.

12. Method according to any one of the preceding claims, characterized in that the workpiece is grounded during the measurement period.

13. Method according to any one of the preceding claims, characterized in that the electrically conductive part of the cutting torch is electrically insulated from the workpiece.

14. Method according to any one of the preceding claims, characterized in that the electrically conductive part of the cutting torch has a torch tip facing the workpiece.