LASER MACHINING APPARATUS

Abstract: This invention relates to a laser machining apparatus which outputs a laser beam, irradiates it on a work surface and machines the work. The apparatus includes a power sensor (21) for detecting the level of the reflected laser beam (11) reflected from the surface of the work (10) and fed back into a laser oscillator and abnormality detection means for stopping the operation of the laser oscillator and the movement of the work (10) when the level of the reflected laser beam (11) exceeds a predetermined value, and for displaying an alarm on display (19). The power sensor (21) monitors the level of the reflected laser beam (11) fed back into the laser oscillator. When abnormal reflection occurs, the monitor value rises beyond the set value. Therefore, the abnormality detection means detects this unusual rise, stops the operation of the laser oscillator and the movements of the work and a nozzle and at the same time displays the alarm on the display (19).

Description: The present invention relates to a laser beam machining device for machining a workpiece by irradiating a laser beam thereto, according to the preamble of claim 1 (see JP-A-63-63589). BACKGROUND ART When carrying out a machining operation with a laser beam machining device, a laser beam is focused at a point on a workpiece by a focusing lens or the like, whereby the temperature of the irradiated portion is raised, and accordingly, machining operations such as drilling and cutting, etc. can be effected by evaporating a portion of a workpiece by a laser beam converged onto a very small spot, and welding can be effected by slightly shifting the focal point to thereby maintain the fused state of the workpiece. Therefore, various machining operations can be effected regardless of hardness of the material of the workpieces. Metals such as copper and aluminum, etc., however, have a low capacity for the absorption of a laser beam at normal temperatures, and thus more than 80% of the beam is reflected immediately after the irradiation of a laser beam. Nevertheless, once the metals are heated, the absorption capacity thereof is increased, and thus a required machining operation can be effected. A specific example of this phenomenon is shown in Figs. 4(a) and 4(b). Figure 4(a) is a graph showing a change of the capacity for the absorption by a workpiece of a laser beam, wherein a CO₂ gas laser beam is used as the laser beam and aluminum is used as the workpiece. In the figure, 40 denotes the characteristic of an incident laser beam irradiated to the workpiece, i.e., a pulse waveform having a width of a period of from a time t0 to a time t2 and a peak value of Pi, and 41 denotes the characteristic of a reflected laser beam. The level of the reflected laser beam rises, together with the incident laser beam, from the time t0 up to a level Pr, but as the absorption capacity of the workpiece increases thereafter, abruptly drops when close to the time t1. A change of the reflectivity in the above case is shown in Fig. 4(b). In the figure, the times t0 to t2 correspond respectively to those designated by the same symbols in Fig. 4(a). As illustrated, the reflectivity is close to 1 at the initial stage of the laser beam irradiation, drops approximately to zero at the time t1, and again approaches 1 after the irradiation is ended. The above-mentioned reflected laser beam is allowed to pass through the focusing lens and bender mirrors, etc., and is fed back to an oscillator through an output mirror, and therefore, if the peak level Pr of the reflected laser beam, shown in Fig. 4(a), is high, the optical components can be damaged by an abnormal increase of the laser power in the oscillator. Furthermore, while the power level of the reflected laser beam is high, i.e., while the reflectivity is high, a required machining operation cannot be satisfactorily effected. JP-A-63-63589 discloses monitoring the reflected laser beam using a beam splitter positioned separately from the laser discharge tube or through the rear mirror of the laser discharge tube. The present invention was created in view of the above circumstances, and an object thereof is to provide a laser beam machining device by which damage to optical components and defects in the machining of a workpiece caused by a reflected laser beam are prevented. According to the present invention, there is provided a laser beam machining device for machining a workpiece by irradiating a laser beam onto a surface of the workpiece, the device comprising: reflected light detecting means for detecting a power level of a reflected laser beam fed back to a laser oscillator due to a reflection of the laser beam by the surface of a workpiece; and, abnormality detecting means for stopping an operation of the laser oscillator and a movement of one of a workpiece and a nozzle portion and displaying an alarm at a display device when the power level of the reflected laser beam becomes higher than a predetermined value; characterised in that: the predetermined value is a maximum output value estimated to be within a range of a normal machining operation, and in that the device comprises calculating means arranged for calculating the maximum output value according to the following formulae: Pu = Pc x Ov x D + α (when ( Pc x Ov x D + α ) < Pl) Pu = Pl (when ( Pc x Ov x D + α ) ≧ Pl) where Pu = maximum output value; Pc = command value for continuous oscillation; Pl = breakdown threshold value; Ov = output override value; D = value of pulse duty factor; α = parameter set in accordance with a material and condition of the surface of the workpiece. The power level of the reflected laser beam fed back to the laser oscillator is monitored by the reflected light detecting means and, when an abnormal reflection occurs, the monitored value becomes higher than the predetermined value, and this increase is detected by the abnormality detecting means, which can then stop the operation of the laser oscillator, close a mechanical shutter and the movement of the workpiece and nozzle and displays an alarm at the display device. In the drawings: Figure 1 is a block diagram showing a laser beam machining device according to the present invention; Fig. 2 is a graph showing a laser output of a laser beam machining device according to the invention, and a threshold value; Fig. 3(a) is a graph showing a change in the capacity of a workpiece for the absorption of a laser power; and, Fig. 3(b) is a graph showing a change of reflectivity with time. Figure 1 is a block diagram showing a laser beam machining device using a CO₂ gas laser oscillator, according to a first embodiment of the invention. In the figure, a processor (CPU) 1 reads out a machining program stored in a memory 12, in accordance with a control program stored in a ROM, not shown, and globally controls the laser beam machining device. An output control circuit 2 converts an output command value supplied from the CPU 1 into a current command value, and has a DA converter incorporated therein for converting a digital value into an analog output. A power supply 3 for the laser device rectifies the commercial power, and outputs a high-frequency voltage in accordance with a command from the output control circuit 2. A laser gas is circulated within a discharge tube 4, and when a high-frequency voltage is applied from the power supply 3, an electrical discharge is produced and thus the laser gas is pumped. A rear mirror 5 is made of germanium (Ge) and has a reflectivity of 99.5%, and an output mirror 6 is made of zinc selenide (ZnSe) and has a reflectivity of 65%. These mirrors constitute a Fabry-Perot resonator by which 10.6µm light emitted from the molecules of the pumped laser gas is amplified and partly output to the outside through the output mirror 6 as a laser beam 7. The direction of the laser beam 7 is changed by a bender mirror 8, converged into a spot having a diameter of 0.2 mm or less by a focusing lens 9, and irradiated onto a surface of a workpiece 10; 11 denotes a laser beam reflected by the surface of the workpiece 10. The reflected laser beam 11 is returned to the discharge tube 4 through the focusing lens 9, bender mirror 8, and output mirror 6. The memory 12 stores the machining program and parameters, etc., and is formed by a CMOS backed up by a battery, or the like. A position control circuit 13 decodes a position command value output from the CPU 1, controls the rotation of a servomotor 15 through a servo amplifier 14, and controls the movement of a table 18 through a ball screw 16 and a nut 17 to thereby control the position of the workpiece 10. A CRT or a liquid-crystal display is used as a display device 19. Numeral 20 denotes a monitoring laser power, output from the rear mirror 5 at a 0.5% transmittance. The output level of the monitoring laser power 20, i.e., a monitored value, corresponds to a value obtained by superposing the reflected laser beam 11 on the laser beam 7, and a fluctuation of the reflected laser beam 11 can be detected by measuring this value. The monitored value is measured by a power sensor 21 formed by a thermoelectric or photoelectric conversion element of the like. The output of the power sensor 21 is amplified and digitally converted, and then supplied to the CPU 1 for comparison with a threshold value mentioned hereinafter. Figure 2 is a graph showing a laser output and a threshold value, in which a pulse duty factor is indicated along the axis of the abscissa and a laser power level is indicated along the axis of the ordinate; Pc denotes a basic command value for a continuous oscillation output by the machining program. During a pulse oscillation, a pulse duty factor, in addition to the basic command value Pc, is such that a pulse oscillation command 30 proportional to the value of the pulse duty factor is output. In this case, since the pulse enhancement rate is high in a region where the pulse duty factor is small, the laser beam exhibits an output characteristic as indicated at 31. In contrast, when a machining operation is actually carried out, the monitored value rises even during a normal machining, due to the reflected laser beam and, accordingly, the monitored value shows a characteristic as indicated at 35. A characteristic monitored in a region where the pulse duty factor is relatively small is indicated; for example, when the laser beam is oscillated with a pulse duty factor D2, the monitored value is Pm35. Numeral 36 indicates a characteristic monitored when abnormal reflection of the workpiece occurs, in which the monitored value is Pm36. As the value of the pulse duty factor D2 is small, the monitored value Pm36 is lower than the breakdown threshold value Pℓ, even when an abnormal reflection occurs, but in practice, the laser beam is not sufficiently absorbed by the workpiece due to the abnormal reflection, and thus a required machining operation cannot be satisfactorily effected. Therefore, the detection of an abnormal reflection is based on a threshold value obtained from an allowable laser output characteristic indicated at 37. The allowable laser output characteristic 37 is obtained by calculating maximum output levels estimated to be within the range of a normal machining operation, in accordance with the command values, and is given by the following formulae: Pu = Pc x Ov x D + α (when ( Pc x Ov x D + α ) < Pℓ) Pu = Pℓ (when ( Pc x Ov x D + α ) ≧ Pℓ) where Pu: allowable maximum laser output; Ov: output override value; D: value of pulse duty factor; α: parameter. The parameter α is set to different values in accordance with a material and a condition of the surface and the like of the workpiece. Specifically, the allowable maximum laser output at the pulse duty factor D2 is Pu37. When the CPU 1 detects that the monitored value Pm36 is higher than the allowable maximum laser output Pu37, the CPU 1 stops the laser oscillation and movement of the workpiece, whereby a defect in the machining of the workpiece is avoided. Simultaneously, an alarm is displayed at the display device, to inform the operator that an abnormal state exists. As described above, according to the present invention, when the level of the reflected laser beam becomes higher than the previously set threshold value, it is determined that an abnormal reflection has occurred, and accordingly, the laser oscillation and movement of the workpiece are automatically stopped. The threshold value used for the detection is set to a breakdown threshold value of the optical components, whereby damage to the optical components is avoided. Further, since an alarm is simultaneously displayed at the display screen, the operator is quickly made aware of the existence of an abnormal state. By setting the threshold value to a maximum output value estimated to be within the range of a normal machining operation, defects in the machining of the workpiece are avoided. Therefore, according to the present invention, workpiece wastage can be minimized, the machining efficiency improved, and the operating costs lowered.

Claims: A laser beam machining device for machining a workpiece by irradiating a laser beam onto a surface of the workpiece, the device comprising: reflected light detecting means (20,21) for detecting a power level of a reflected laser beam (11) fed back to a laser oscillator (4) due to a reflection of the laser beam by the surface of a workpiece; and, abnormality detecting means (1) for stopping an operation of the laser oscillator and a movement of one of a workpiece and a nozzle portion and displaying an alarm at a display device (19) when the power level of the reflected laser beam becomes higher than a predetermined value; characterised in that: the predetermined value is a maximum output value (37) estimated to be within a range of a normal machining operation, and in that the device comprises calculating means arranged for calculating the maximum output value according to the following formulae: Pu = Pc x Ov x D + α (when ( Pc x Ov x D + α ) < Pl) Pu = Pl (when ( Pc x Ov x D + α ) ≧ Pl) where Pu = maximum output value; Pc = command value for continuous oscillation; Pl = breakdown threshold value; Ov = output override value; D = value of pulse duty factor; α = parameter set in accordance with a material and condition of the surface of the workpiece. A device according to claim 1, wherein the reflected light detecting means detects the power level of the reflected laser beam by monitoring the laser beam transmitted thereto through a rear mirror (5) of the laser oscillator (4).

Inventors: IEHISA NOBUAKI FANUC MANSION H, YAMAZAKI ETSUO, IEHISA, NOBUAKI, FANUC MANSION HARIMONI 7-304, YAMAZAKI, ETSUO
Applicants: FANUC LTD, FANUC LTD.
Application year: 1989
Main classification: B23K26
IPC codes: B23K26