Electric heating – Metal heating – By arc
Reexamination Certificate
1999-12-17
2001-11-06
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121720, C219S121840
Reexamination Certificate
active
06313432
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a laser cutting method and a laser cutter.
BACKGROUND ART
When mild steel is cut with a laser having a small power relative to the thickness of the mild steel, sufficient cutting laser energy can be obtained at the vicinity of the surface of the mild steel at which the laser is applied. However, there are many cases in which sufficient cutting laser energy cannot arrive at the inside of the mild steel. Therefore, in recent years, high concentration (purity) oxygen (having a concentration (purity) of 99.5% or more) is supplied to the vicinity of the cutting point by use a co-axial double nozzle
1
a
and
1
b
as shown in
FIGS. 10 and 11
, a co-axial triple nozzle (not shown in Figures), or an auxiliary nozzle
2
as shown in FIG.
12
. Thereby, a sufficient oxygen concentration for cutting the inside of the mild steel is maintained. Heat is generated due to the oxidation reaction between steel and the supplied oxygen, and the heat compensates for the deficiency of laser energy of the laser cutter.
In the laser cutting method using the co-axial double nozzle
1
or the auxiliary nozzle
2
, only the oxygen quantity is adjusted, and the oxygen concentration is not adjusted. Therefore, it is difficult to maintain a suitable cutting quality when the thickness or the kinds of the material to be cut is changed. An excess combustion heat is generated due to the small change in conditions when the material is cut, and easily adversely effects to a quality of the cut surface.
That is, when the mild steel is cut at a high speed (1.5 m/min or greater), a high temperature area
3
(represented by a two-dot dashed line in
FIGS. 13 and 14
) suitable for combustion between oxygen and steel is generated at the cutting surface behind the cutting point in a cutting direction, rather than the forward to the cutting point in a cutting direction by the influence of the heat conductivity and the thermal diffusivity, as shown in
FIGS. 13 and 14
. Then excess combustion energy adversely effects to the cutting surface and the cutting quality. Consequently, for example, as shown in
FIG. 15
, a normal cutting quality can be obtained at the vicinity of the surface
4
of the material to be cut; however, an abnormal cutting quality is often obtained on the inside of the material to be cut. In addition, a self burning phenomenon, dross adhesion, or cutting notch generation easily occurs on the inside of the material. Therefore, it is impossible to sufficiently improve the cutting quality. When the laser beam pulse is output continuously in order to cut the material at a high speed, the frequency of occurrence of the self burning phenomenon increases. Therefore, in order to prevent the occurrence of the self burning phenomenon, usually the laser beam pulse is intermittently output. However, when the laser beam pulse is intermittently output, the cutting speed decreases.
When the laser cutter applying a laser beam
5
and a cutting gas (high concentration oxygen)
6
and
6
from the same nozzle shown in
FIGS. 13 and 14
is used, the cutting gas
6
and
6
diffuses more widely than the diameter of the laser beam
5
. The gas stream of the cutting gas
6
consequently generates behind the cutting point in a cutting direction. When the diameter of the nozzle is set so as to equal the diameter of the laser beam, the combustion energy at the high temperature distributional area formed behind the cutting point in the cutting direction, increases due to oxygen. Therefore, it is impossible to solve the problem. The cutting quality depends on the position of the center of the laser beam
5
. In order to adjust the proper diameter of the nozzle against laser beam diameter 5, it is necessary to frequently change the nozzle.
DISCLOSURE OF INVENTION
Therefore, the object of the present invention is to provide a laser cutting method having the following effects:
(a) the oxygen concentration distribution can be easily changed in an area within several millimeters of the cutting point by changing the oxygen concentration of the gas ejecting from a plurality of nozzles; thereby the cutting quality can be improved,
(b) the adverse effects to the cutting surface such as self burning phenomenon, can be prevent by decreasing the oxygen concentration behind the cutting point in a cutting direction; thereby a high cutting quality can be obtained, and
(c) these effects can be obtained; thereby a high cutting quality can be obtained even when the material is cut at a high speed.
In addition, the object of the present invention is to provide a laser cutter having the following effects:
(d) gas supplying means having an oxygen concentration changing function can respectively eject a gas containing a required amount of oxygen, and the oxygen concentration distribution can be easily changed in an area within several millimeters of the cutting point; thereby a high cutting quality can be easily obtained, and
(e) an oxygen concentration distribution can be easily and exactly changed by a control device for controlling the mass flow rate of the ejecting gas; thereby the cutting quality can be improved.
The laser cutting method of the present invention comprises the step of applying a cutting laser beam to a material to be cut at a cutting point while ejecting gas to the cutting point or its surrounding area from a plurality of nozzles arranged in a ring or in a line, wherein the oxygen concentration of the gas ejecting from at least one of the nozzles is changed to adjust the oxygen concentration distribution in an area within several millimeters of the cutting point. It is possible to obtain an oxygen concentration suitable for laser cutting at the cutting point and another oxygen concentration suitable for preventing the disadvantages such as the self burning phenomenon in the area surrounding the cutting point; therefore, a high cutting quality can be obtained by the laser cutting method of the present invention.
Many oxygen concentration distributions can be obtained in the laser cutting method of the present invention. In order to obtain a high cutting quality, the most suitable oxygen concentration distribution is formed depending on the cutting conditions. Moreover, even while the material is being cut, it is possible to change the oxygen concentration distribution.
The oxygen concentration distribution preferably comprises a high oxygen concentration at the vicinity of the cutting point and a low oxygen concentration behind the cutting point in a cutting direction. The oxygen concentration distribution can be formed by ejecting the gas containing oxygen at a high concentration from the at least one nozzle, and ejecting the other gas containing oxygen at a low concentration. Thereby, the high oxygen concentration (having a concentration of 99.5% or more) suitable for a laser cutting can be maintained in the vicinity of the cutting point. Moreover, the other oxygen concentration suitable for preventing the combustion between oxygen and steel can be maintained behind the cutting point in a cutting direction, and the generation of the self burning phenomenon, dross adhesion, or the like can be prevented.
In order to prevent the generation of the self burning phenomenon and the like, it is preferable to set the oxygen concentration at the downstream in an ejecting direction of the gas stream low. However, it is more preferable to maintain the oxygen concentration ensuring the laser cutting at the downstream in the ejecting direction of the gas stream.
In addition, the laser cutting method of the present invention has the following effects:
(1) a high cutting quality can be obtained even when a laser beam comprising a continuous pulse is used; therefore, a cutting speed can be improved,
(2) it is possible to adjust the variation of the cutting conditions such as a thickness of the material to be cut by changing the oxygen concentration or the flow rate of the gas containing oxygen at a low concentration and the gas containing oxygen at a high concentration; therefore, an exchanging
Nagahori Masayuki
Nagata Yozo
Burns Doane , Swecker, Mathis LLP
Heinrich Samuel M.
Tanaka Engineering Works, Ltd.
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