Electric heating – Metal heating – By arc
Reexamination Certificate
2002-06-21
2004-01-20
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121700, C219S121670
Reexamination Certificate
active
06680459
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser beam machining apparatus for forming a tearable portion in a workpiece such as automotive instrument panel by irradiating a laser beam and a laser beam machining method therefor.
2. Description of the Related Art
Air bags for passenger seats of automobiles are generally placed on the inside of instrument panels. Moreover, an opening is formed in the instrument panel in a position opposite to the air bag and a cover plate is mounted over the opening; or otherwise there is formed a groove for use in cleaving a predetermined opening portion in the instrument panel though no opening is provided therein. In this case, the air bag is normally covered with the cover plate or the opening portion and when the air bag is activated, the cover plate is broken or the opening portion is cloven out of the groove so that the air bag is inflated outside the instrument panel.
With the above structure of covering the air bag, however, the problem of spoiling the external appearance arises as the outward form of the cover plate or the groove of the opening portion is exposed on the front surface of the instrument panel. A further problem is that the reflection of the outward form of the cover plate or the grooves of the opening portion on the front window also puts a restraint on driving activity.
In order to deal with the foregoing problems, there has been proposed a method of forming a tearable portion by drilling blind holes at predetermined intervals in the rear side of an instrument panel to form a line of perforations. In this method, however, it is needed to form the tearable portion having a predetermined value of breaking strength over the whole periphery while a predetermined remain thickness is secured between the bottom face of each perforation and the front surface side of the panel.
Nevertheless, because instrument panels are curved and of uneven thickness when they are produced by molding, it has been difficult to drill blind holes in the instrument panel so that the remain thickness is secured uniformly.
In order to deal with the problem above, there have heretofore been proposed laser beam machining methods as disclosed in, for example, Japanese Patent Laid-Open No. 85966/1998 (the first prior art) and Japanese Patent Laid-Open No. 2001-38479 (the second prior art).
According to the first prior art, when a tearable portion as a blind hole is drilled by the irradiation of a laser beam to a workpiece, the laser beam transmitted through the aperture is detected whereby to stop the drilling of the aperture when the power transmitted therethrough reaches a predetermined value. According to the second prior art, further, a laser beam is synthesized with a detection beam beforehand and the detection beam transmitted through an aperture is detected when the aperture is drilled by irradiating the laser beam to a workpiece whereby to stop the drilling of the aperture when the power transmitted therethrough reaches a predetermined value.
As set forth above, according to the conventional laser beam machining methods, the depth of apertures to be drilled is controlled so that the remain thickness is secured uniformly by detecting a small quantity of laser beam or detection beam transmitted through the cut part (aperture) of the workpiece. Therefore, a sensor for detecting the laser beam or the detection beam is needed to be provided on a supporting member such as a jig for supporting a workpiece to be subjected to machining and in a position corresponding to a laser nozzle in such a manner that the sensor is movable integrally with the workpiece on the opposite side of the workpiece, and this makes the detection device not only complicated in construction but also expensive. The problem in this case is that the necessity of adjusting the detection level before the workpiece is subjected to machining makes it troublesome to detect slight variation in the thickness of the workpiece. According to the first prior art, moreover, there is the possibility that as the laser beam is transmitted through the aperture of the workpiece, a small hole or a scar is bored or left in the base of the aperture in the form of a tearable portion.
Further, in order to make constant the remain thickness, the output power of the laser beam has to be stabilized. For the stabilization of the laser beam, various factors or conditions such as the voltage supplied to the oscillator of a laser beam machining apparatus, the temperature of cooling water in an optical system and the ambient temperature of the machining apparatus also have to be definite. Actually, these factors and conditions are hardly kept constant and with the change of the optical system with time, moreover, the actual output power of the laser beam tends to vary in most cases.
In case where laser beam machining is carried out while the output power of the laser beam remains variable, the remain thickness of a workpiece also varies and this may result in impeding development of cleavage when the air bag is inflated, thus making small holes or leaving scars in the workpiece because a laser beam is allowed to pass through the workpiece.
In order to compensate for the variable output power of the laser beam, there has been proposed the art described in, for example, Japanese Patent Laid-Open No. 278687/1988 (the third prior art). In this prior art, the output power of a pulsed laser beam following a reference pulse oscillated at a predetermined fundamental frequency as shown in
FIG. 12
is detected pulse-to-pulse and the output power is integrated by an integration circuit. Then the integrated value is compared with a reference value whereby to decide the error therebetween.
However, though discharge time is required for the integration circuit when the output of the pulsed laser beam is integrated, the discharge time is difficult to secure because the output OFF time of the pulsed laser beam oscillated at a high speed of 5 KHz, for example, is too short.
Therefore, it is conceivable to shorten the time constant of the integration circuit but as is obvious from an integral value
30
of
FIG. 12
, the oscillation noise of the pulsed laser beam is detected. In case where the time constant is set longer, on the other hand, the discharge time of the integration circuit would not be secured as stated above. Consequently, a value widely different from the actual output power value is monitored and the laser output power cannot be detected accurately according to the prior art method. When the prior art method is applied to forming a tearable portion in the instrument panel, it becomes impossible to control the laser output power and this results in difficulty in making constant the remain thickness of the panel.
SUMMARY OF THE INVENTION
A first object of the invention made with a special attention directed to the foregoing problems existing in the prior art is to provide a laser beam machining apparatus for ensuring that slight variation in the thickness of a workpiece is made detectable by a detection device which is simple in construction and producible less costly and for accurately drilling perforations in a panel while the remain portion of the panel base secures the predetermined thickness, and a laser beam machining method therefor.
Further, a second object of the invention is also to provide a laser beam machining apparatus for making constant the remain thickness of a panel base by allowing the value of a laser beam by pulse oscillation to be accurately securable and a laser beam machining method therefor.
In order to accomplish the first object, according to a first aspect, there is provided a laser beam machining apparatus for forming a blind hole in a workpiece by the irradiation of a laser beam from a nozzle to a rear surface of the workpiece while moving a machining head having the nozzle and the workpiece relatively, comprising:
a workpiece support member made of conductive material for supporting and fixing the workpiece;
a distance detection unit prov
Kanaya Shuso
Morita Yuichi
Elve M. Alexandra
Nippei Toyama Corporation
Sughrue & Mion, PLLC
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