Machining Head and Laser Machining Apparatus

1. Field of the Invention

The invention relates to a laser machining head for cutting work and a laser machining apparatus for controlling the laser machining head.

2. Description of the Prior Art

The cutting performance of a laser machining apparatus is affected by operating conditions such as the location of the focal point of the laser beam against the work surface, the assist gas pressure and the distance between the work surface and the leading edge of the laser beam nozzle. It also depends on conditions such as the kinds of material, surface condition, quality, composition, and thickness of the work. Regarding the laser machining apparatus, it is expected to work under a wide range of conditions and for a wide range of materials as described above and to obtain a stable machining quality. Especially, in case of cutting steel and so on, the metal which is melted by direct laser beam heating is not only blown off by an oxygen jet flow used as an assist gas, but is also sublimated or melted with a strong oxidized burning reaction, which improves cutting efficiency. Therefore, the cross sectional quality of the work surface very much depends upon oxidized burning speed on it.

Namely, the higher the velocity of oxidized burning, the less the roughness on the cross sectional surface. Since supplied oxygen is consumed continuously, self-burning (the phenomenon where strong self-burning happens explosively in the absent of a laser beam so that cross-sectional quality is damaged) due to excessive oxygen seldom happens. Also, since the cutting speed is increased, high speed cutting can be conducted. Therefore, it is very important to optimize the gas supplying condition and to increase oxidized burning speed. Based on the above situations, many kinds of improvement on gas supplying nozzle have been attained.

FIG. 31 is a longitudinal sectional view of a laser beam nozzle which is installed in the machining head of a conventional laser machining apparatus. The laser beam nozzle is disclosed, for example, in Japanese patent publication No. 61-60757. In the figure, the numeral 1 denotes a main assist gas nozzle, the numeral 2 denotes a sub assist gas nozzle, the numeral 3 denotes a surface of the material being cut, and the numeral 10 denotes an assist gas source.

The operation of laser beam machining apparatus is explained hereafter. The laser beam nozzle installed in a conventional laser beam machining apparatus is constructed as mentioned above. The nozzle has a multiple structure which comprises a gas flow path on the center axis and concentric multiple gas flow paths arranged around the center gas flow path. The oxygen supplied to the material being cut from the gas flow path on center axis, has comparatively high pressure and is discharged at a high speed. The oxygen is supplied mainly into the cutting groove and used partially for the oxidized burning reaction. The remaining oxygen is used to blow off and eliminate the melted and oxidized material. The oxygen supplied from the outside flow path flows at a comparatively low speed. The air, flowing from the outside, stabilizes the gas flow near the center axis and keeps the oxygen concentration at a high level. Moreover, the direction of jet flow from the outlet of the sub assist gas is arranged parallel to the outlet of the main assist gas and keeps the assist gas in a laminar flow. Since there are thick walls at the border of outlets of the main assist gas and the outlet of each sub assist gas, and also since the interference between the main assist gas and the sub assist gas is suppressed, main assist gas is kept in the laminar flow condition accordingly.

In the laser machining, it is necessary to supply continuously a required amount of assist gas (oxygen gas) into the narrow cutting groove whose width is less than 1.0 mm during cutting the work, although the oxidized burning reaction occurs due to the assist gas (oxygen gas) supplied to the cutting surface of the metal which is heated to a high temperature by the energy of the laser beam. Combustion products (gas or melted metal) are produced near the cutting surface due to the oxidized reaction. If the jet flow of the assist gas (oxygen gas) is in a laminar flow condition, the assist gas must reach the metal surface by dispersing through the ambient combustion product (gas composition). It is effective accordingly to make a jet flow of assist gas turbulent, to disturb the boundary layer in the combustion reaction area near the cutting surface and to replace the combustion product (gas composition) with fresh assist gas in order to increase the speed of combustion.

The laser beam nozzle installed in machining head of a conventional laser machining apparatus is arranged so that the jet direction of the sub assist gas outlet is parallel to that of the main assist gas outlet, and discharges the sub assist gas in a laminar flow, so that the interference with the main assist gas is greatly suppressed. Therefore, the main assist gas is kept in a laminar flow and stabilized. Although the purity of the assist gas (oxygen purity) is kept at a high level, since the gas flow at the center portion is stabilized, it is necessary to increase supply pressure of the main assist gas so as to effectively supply the assist gas (oxygen gas) into the narrow cutting groove. Since a boundary layer of law assist concentration (oxygen concentration) is formed on the oxidized reaction surface of the metal due to small velocity fluctuation (strength of turbulence) of the center portion of the nozzle flow, it is difficult for fresh assist gas (oxygen gas) to reach reaction surface directly. Therefore, there is a problem that the supplied assist gas (oxygen gas) does not contribute effectively to the oxidized reaction. Also, there is another problem that the self burning may occur when the oxygen which is not used effectively may remain at the narrow cutting groove due to some condition of the flow quantity. Since a thick wall exists at the boundary of the jet outlets of the main assist gas and each sub assist gas, the assist gas jet pressure or the jet velocity distribution from the front edge of the laser beam nozzle does not vary continuously. Accordingly, the jet flows of the main assist gas, each sub assist gas and the surrounding air are separated at the thick wall of their respective boundary. Therefore, there is a further another problem that the main assist gas purity (oxygen purity) decreases, since diffusion mixing improves between the main assist gas and the sub assist gas, and the isolation effect is decreased between the outermost sub assist gas and the surrounding air.

It is a primary object of the present invention to provide a machining head in a laser machining apparatus, which may decrease roughness of the cutting surface, may attain to suppression of the self burning, may supply assist gas (oxygen gas) having small flowing quantity of ambient air, high purity and large turbulence to the work surface, may promote an oxidized reaction, and may machine the work stably, at high speed and with high quality, and also may machine the work stably and precisely using a machining head.

According to one aspect of the present invention, there is provided a machining head of a laser machining apparatus in which the laser beam passes through, the machining head comprises a main assist gas nozzle at the center portion of the machining head for supplying a main assist gas; one or more annular sub assist gas nozzles which surround the main assist gas nozzle, whose jet outlet of the most inside diameter is larger or equal to the jet outlet of main assist gas nozzle.

According to another aspect of the present invention, there is provided a machining head wherein the main assist gas nozzle comprises at least one circulation flow forming means which allow a part of the main assist gas to circulate.

According to another aspect of the present invention, there is provided a machining head wherein the circulation flow forming means comprises twisted wings or grooves mounted on an inside wall of the main assist gas nozzle.

According to another aspect of the present invention, there is provided a machining head wherein the circulation flow forming means comprises a plurality of sub flow nozzles each of which has a sub nozzle outlet in the main assist gas nozzle and discharges the main assist gas in a circumferential direction from the sub nozzle outlet.

According to another aspect of the present invention, there is provided a machining head wherein the jet outlet of main assist gas nozzle is positioned at an upper gas stream side of said jet outlets of a respective sub assist gas nozzle.

According to another aspect of the present invention, there is provided a machining head, wherein the machining head further comprises a static pressure conversion means which converts the dynamic pressure of the assist gas which flows into the sub assist gas nozzle through sub assist gas supply inlets into static pressure.

According to another aspect of the present invention, there is provided a machining head wherein the static pressure conversion means comprises a static pressure conversion surface which faces against the sub assist gas inlet; and a bank which is located at an inside edge of the static pressure conversion surface and forms a space where the sub assist gas stays.

According to another aspect of the present invention, there is provided a machining head, wherein the machining head further comprises sub assist gas supply inlets which are located around the main assist gas supply inlet and adjust the sub assist gas flow, and the main assist gas and the sub assist gas are supplied from a common gas source to the main assist gas supply inlet and the sub assist gas supply inlets.

According to another aspect of the present invention, there is provided a machining head, wherein the machining head further comprises aperture area adjusting means located at the sub assist gas supply inlets for adjusting sub assist gas flow.

According to another aspect of the present invention, there is provided a machining head, wherein the machining head further comprises at least one fluid resistance body located at the sub assist gas supply inlets for adjusting sub assist gas flow.

According to another aspect of the present invention, there is provided a laser machining apparatus which comprises a main assist gas nozzle and one or more sub assist gas nozzles, wherein the laser machining apparatus comprises at least one assist gas sources for supplying assist gas to at least one of a main assist gas inlet and a sub assist gas inlet; sensors for detecting the temperature of the machining surface, the width of the cutting groove, or the amount of spark; and a flow quantity control apparatus for adjusting the flow quantity and pressure of the assist gas during machining in response to the signals from the sensors.

According to another aspect of the present invention, there is provided a laser machining apparatus which comprises a main assist gas nozzle and one or more sub assist gas nozzles, wherein the laser machining comprises an operation control means for controlling assist gas flow quantity and assist gas pressure discharged from said machining head by programming control.

FIG. 1 is a longitudinal cross sectional view which illustrates a machining head of a laser machining apparatus which corresponds to a first embodiment.

FIG. 2 illustrates a pressure fluctuation distribution of assist gas flow on the cutting work surface of the embodiment 1.

FIG. 3 illustrates the principle for improving the cutting characteristic of the first embodiment.

FIG. 4 illustrates a flow velocity distribution on the cutting work surface in an arrangement according to the first embodiment.

FIG. 5 illustrates the relationship between the pressure fluctuation value of assist gas flow and the roughness of the cutting surface in the present invention.

FIGS. 6A and 6B illustrate a comparison between the machining velocity and tolerance for laser output power of the first embodiment and the conventional example.

FIG. 7 is a longitudinal cross sectional view which shows the positional relationship between the jet outlets of machining head of the embodiment 1.

FIG. 8 illustrates relationship of the pressure fluctuation value of assist gas flow as a function of the distance L between main and sub jet outlets of the first embodiment.

FIG. 9 illustrates a machining head according to a second embodiment.

FIGS. 10A, 10B, 10C are a cross-sectional view and perspective views which illustrate the inside of a machining head according to a third embodiment.

FIGS. 11A, 11B illustrate main assist gas nozzles of a machining head of a fourth embodiment.

FIGS. 12A, 12B illustrate main assist gas nozzles of a machining head of a fifth embodiment.

FIG. 13 is a longitudinal cross sectional view which illustrates a machining head of a sixth embodiment.

FIG. 14 is a longitudinal cross sectional view which illustrates a machining head of a seventh embodiment.

FIG. 15 illustrates the pressure fluctuation value of assist gas flow on the work surfaces as a function of the distance in a radial direction regarding the sixth and seventh embodiments and a conventional example.

FIG. 16 is a longitudinal cross sectional view which illustrates a machining head of the embodiment 8 in the invention.

FIG. 17 is a longitudinal cross sectional view which illustrates a machining head of a ninth embodiment.

FIG. 18 is partial perspective view which illustrates a machining head of a tenth embodiment.

FIGS. 19A and 19B are plan views which illustrate other examples of small hole shapes for assist gas supply inlet of the embodiment 10.

FIG. 20 is a partial perspective view which illustrates a machining head of a eleventh embodiment.

FIG. 21 is a longitudinal cross sectional view which illustrates a machining head of a twelfth embodiment.

FIG. 22 is a plan view which illustrates an aperture area adjusting means of a thirteenth embodiment.

FIG. 23 is plan view which illustrates an aperture area adjusting means of a fourteenth embodiment.

FIG. 24 is a longitudinal cross sectional view which illustrates a machining head of a laser machining apparatus of a fifteenth embodiment.

FIG. 25 illustrates the construction of a laser machining apparatus of a sixteenth embodiment.

FIG. 26 illustrates a construction of a laser machining apparatus of a seventeenth embodiment.

FIG. 27 illustrates a construction of a laser machining apparatus of an eighteenth embodiment.

FIG. 28 is a longitudinal cross sectional view which illustrates a machining head of laser machining apparatus of a nineteenth embodiment.

FIGS. 29A and 29B are longitudinal cross sectional views which illustrate another example of a machining head of laser machining apparatus of the nineteenth embodiment.

FIG. 30 is a longitudinal cross sectional view which illustrates a machining head of laser machining apparatus of a twentieth embodiment.

FIG. 31 is a longitudinal cross sectional view which illustrates a conventional machining head of a laser machining apparatus.

Embodiment 1

FIG. 1 is a longitudinal sectional view which illustrates a machining head of a laser machining apparatus corresponding to the first embodiment of the present invention. The machining head comprises a main assist gas jet nozzle outlet 1a having a diameter of D.sub.1 which is located at an innermost position and a main assist gas nozzle duct 1b which is adjacent to the main assist gas jet nozzle outlet 1a. The main assist gas nozzle 1 is where the laser beam passes through, inside a sub assist gas nozzle jet outlet 2. The sub assist gas nozzle jet outlet 2 comprises a sub assist gas nozzle 2a having a diameter of D.sub.2 (D.sub.2 .gtoreq.D.sub.1, D.sub.2 >D.sub.1 in this case) which is located outside of the main assist gas nozzle 1 and a duct 2b adjacent to the sub assist gas nozzle 2a form a multiple part construction (in this case, two part -construction). The jet outlet 1a of the main assist gas nozzle 1 is located upstream of the jet outlet 2a of sub assist gas nozzle 2. This construction causes jet gas pressure and the jet gas flow velocity to vary continuously at the jet outlet, and allows both the pressure fluctuation and the flow velocity fluctuation of the main assist gas flow to be large. The assist gas (oxygen gas) is supplied on the cutting work surface 3 through the path as indicated by the arrows in the figure.

FIG. 2 illustrates a pressure fluctuation distribution of assist gas flow on the cutting work surface 3. The vertical scale shows pressure fluctuation p/P (%) (p: pressure fluctuation, P: absolute pressure). The curve corresponds to the location of the jet outlet of assist gas. Since the flow velocity fluctuation of assist gas which flows through the main assist gas nozzle is large, the pressure fluctuation of main assist gas just below the jet outlet 1a is large, alternately the pressure fluctuation of sub assist gas just below jet outlet 2a is controlled to be low. The desired amount of gas flow is supplied to the cutting groove without increasing original pressure by the pumping effect of the pressure fluctuation.

The large fluctuation of flow velocity means that the turbulence of oxygen gas flow supplied into the cutting groove is large and the gas flow is in the state of turbulent flow. Near the cutting surface of the work, an oxide produced by the reaction and oxygen form a mixed layer, and the oxidized reaction speed depends upon the velocity at which oxygen reaches the reaction surface by dispersing through inside said mixed layer. It is accordingly effective to cause the gas flow to be in the state of turbulent flow and to cause the mixed layer to be unstable for increasing the rate of the oxidized reaction.

Therefore, a machining head having a nozzle construction such as described above causes the oxygen gas flow to be in the state of turbulent flow. The machining head of the present embodiment causes the mixed layer to be unstable as explained in FIG. 3. Therefore, it is possible to supply the oxygen gas directly near the reaction surface and to remove simultaneously the mixed gas having high oxide concentration out of the reaction area. Accordingly, the oxidized burning rate is increased remarkably due to an increased amount of oxygen supplied, and high speed cutting becomes possible. Since the supplied oxygen gas is effectively consumed continuously, the self-burning caused by the temperature rise due to laser and the imbalance of the oxidized reaction rate can be prevented and stable machining can be carried out.

Since the flow of ambient air is prevented by the assist gas (oxygen gas) which flows from the assist gas nozzle jet, the purity of oxygen at center flow (namely, the main assist gas) which flows into the cutting groove is maintained and stable cutting can be achieved.

FIG. 4 illustrates a flow velocity distribution on the cutting work surface of the first embodiment. The vertical scale indicates jet gas flow velocity U (m/s) corresponding to the assist gas jet outlet. According to the present embodiment, since the jet gas pressure and jet gas velocity distribution at the assist gas jet outlet are higher near the center portion than the outer portion and continuously varied as indicated in FIG. 3 and FIG. 4, no separation occurs between the main assist gas flow and the sub assist gas flow and at the border with the surrounding air. Accordingly, the problem can be overcome that the purity of the oxygen gas (oxygen purity) decreases due to the decrease of the air cut off effect of sub assist gas.

FIG. 5 illustrates relationship between the roughness of the cutting surface versus the pressure fluctuation value of the assist gas flow of the present invention. The horizontal scale shows the pressure fluctuation value (%) which is the pressure ratio of the pressure fluctuation component p against the pressure P of the main assist gas (p/P) on the work surface just under the center axis. The vertical scale indicates the cutting surface roughness Rmax (.mu.m). The cutting material is mild steel SS400 having a thickness of 12 mm. The machining condition is defined such that the laser power is 1400 W, and the machining velocity is 0.7 m/min. It is clearly understood from FIG. 5 that the larger the pressure fluctuation value is, the less the cutting surface roughness is. Namely, FIG. 5 indicates that high speed cutting in good quality can be achieved by machining using a machining head having a nozzle construction of the embodiment of the present invention.

FIGS. 6A and 6B illustrate tolerances of the machining velocity against the laser output power during a laser cutting test using the machining head of the present embodiment and the conventional machining head. The vertical scale shows machining velocity (m/min), the horizontal scale shows laser output power (kw). The material used is SS400, having a thickness of 12 mm. FIG. 6A indicates the tolerance of the conventional type machining head (pressure fluctuation value is 0.1 (%) and FIG. 6B indicates that of the machining head of the present embodiment. The areas covered with slanted lines in FIGS. 6A, 6B are areas for normal machining for cutting. As seen from the figures, it is understood easily that high-speed cutting may be achieved using the machining head of the present embodiment.

FIG. 7 is a longitudinal cross sectional view which shows the positional relationship between the jet outlets of machining head of the embodiment 1. More precisely, FIG. 7 illustrates the relation between the locations of main assist gas nozzle jet outlets 1a and main assist gas nozzle duct 1b and also the relation of distance between the cutting work surface 3 and the lower edge of the nozzle (jet outlet). The letter L in the figure indicates the distance between the sub assist gas jet outlet 2a and the main assist gas nozzle jet outlet 1a, and the letter H in the figure indicates a distance between a cutting work surface 3 and the lower edge of the sub assist gas jet outlet 2a (jet outlet).

FIG. 8 illustrates relationship of the pressure fluctuation value p/P (%) just below the center axis of nozzle against the distance L between the main jet outlet and the sub jet outlet of the embodiment 1, while H is changed, where H is a distance in millimeter from the cutting work surface 3 to the lower edge of the nozzle 2a. The test is carried out under the condition of diameters of D.sub.1 =1.5 mm, D.sub.2 =5 mm. The vertical scale indicates a pressure fluctuation value p/P (%), the horizontal scale indicates a distance L between the main jet outlet 1a and the sub jet outlet 2a. The curve labelled O indicates a characteristic for H=4 mm, the curve labelled .DELTA. indicates a characteristic for H=2 mm, and the curve labeled .quadrature. indicates a characteristic for H=1 mm.

From FIG. 8, the condition in which the pressure fluctuation value is more than 0.8% is obtained as;

If L.gtoreq.5.times.D.sub.1, then the machining tolerance against the nozzle height H improves. Therefore, a large pressure fluctuation is obtained by simple construction.

Embodiment 2

FIG. 9 illustrates a machining head according to a second the embodiment. Although the machining head in the above embodiment comprises one main assist gas nozzle 1 and one sub assist gas nozzle 2, it is needless to say that the sub assist gas nozzle 2 may be constituted as a multiplex type construction as indicated in the longitudinal cross sectional view of FIG. 9. In the second embodiment, if the jet outlet 1a of the main assist gas is located upstream of any one of the plurality of jet outlets 2a, the second embodiment has effects similar to these of the first embodiment.

Embodiment 3

FIGS. 10A, 10B and 10C are a cross sectional view and two perspective views which illustrate the inside of the machining head according to a third embodiment. In the third embodiment, a plurality of static wings twisted in a screw shape are mounted on the inner surface of the main assist nozzle 1 for causing twisting flows which increases the pressure and flow velocity fluctuation of the main assist gas flow. FIG. 10A illustrates a longitudinal sectional view of the machining head of this third embodiment.

FIG. 10B illustrates a perspective view showing the inside of the main assist gas nozzle 1. A plurality of the twisted static wings 4 are installed on the straight portion inside the duct wall of the main assist gas nozzle 1. These static wings 4 generate a circulating gas flow which flows inside the main assist gas nozzle. The jet flow has a large turbulent flow strength at the center axis when it includes a circulating component at the jet outlet portion. The turbulent ratio may reach more than 10% in case where the distance from the nozzle to the cutting work surface is substantially equal to the nozzle diameter. Therefore, the oxidized burning reaction is effectively caused, oxidation velocity is accelerated and the machining quality and the stability are improved accordingly. The high-speed cutting may be attained simultaneously.

In the embodiment for circulating at least a part of main assist gas flow, it is possible to generate a flow velocity fluctuation in the main assist gas even if the relative distance L from the lower edge of the main assist gas nozzle (namely, jet outlet 1a) to the lower edge of the sub assist gas nozzle (namely, jet outlet 2a) is set to zero.

In the above embodiment, the diameter of laser beam at the lower edge of the main assist gas nozzle must be large so as to locate the main assist gas more closely to the machining optical lens side. Therefore, it is comparatively difficult to align the laser beam axis with the nozzle center axis and to adjust the distance between the laser beam focal point and the work surface. However, in this embodiment, the above adjustment can be easily achieved since the flow velocity of the main assist gas varies even if the relative distance L between the jet outlets of both nozzles is set to zero.

FIG. 10C is a perspective view which illustrates the inside of the machining head of the third embodiment. The static wings may be installed at a contraction flow portion of the lower stream of the main assist gas nozzles shown in FIG. 10C.

Embodiment 4

FIGS. 11A, 11B are perspective view which shows the main assist gas nozzle of the machining head according to a fourth embodiment. In this fourth embodiment illustrated in FIG. 11A, a plurality of twisted grooves 5 are provided at a straight portion inside the duct wall of the main assist gas nozzle 1 as circulation flow forming means. The turbulent flow strength at the central part of the main assist gas jet flow increases since a part of the main assist gas (oxygen gas) flows through the main assist gas nozzle 1 and generates a circulating flow along the twisted grooves 5. Therefore, in this case, oxidization velocity is promoted, and the cutting velocity and the cutting quality may be improved.

The twisted groove 5 may be installed in a contraction flow portion located at the lower stream of the main assist gas nozzle 1 as illustrated in FIG. 11B.

Embodiment 5

FIGS. 12A, 12B illustrate main assist gas nozzles of the machining head of the embodiment 5. FIG. 12A is its top view, FIG. 12B is its longitudinal cross sectional view. In this embodiment 5, main assist gas

nozzles 6a are installed at the inside wall of the main assist gas nozzle 1 as the circulation flow forming means, and sub flow nozzles 6 of the main assist gas are installed symmetrically with respect to one another, which discharge gas in a circumferential direction in parallel to the main assist gas nozzle 1 (namely, the sub flow of main assist gas is discharged in the tangential direction of the outside circle of the main flow of the main assist gas). In the above construction, part of the main assist gas is discharged through the sub flow nozzles 6 of main assist gas and the main assist gas is circulated accordingly. Therefore, the same effects are achieved in this fifth embodiment as in that of in third and fourth the embodiments.

Embodiment 6

FIG. 13 is a longitudinal cross sectional view which illustrates the machining head according to the sixth embodiment. In this sixth embodiment, a static pressure conversion means (a static pressure conversion surface 7, in this case) which converts the dynamic pressure of the sub assist gas flow supplied through the gas supply inlet 2c to static pressure are installed at the location where the gas flow pass through the gas duct 2b. Broken line arrows show the assist gas flows. Since the gas directed toward the inside of the assist gas nozzle 2b is recovered to the static pressure after colliding against static pressure conversion surface 7, the assist gas flows homogeneously in the lower stream of static pressure conversion surface 7 and is discharged through the jet outlet 2a of the sub assist gas. The gas flow outside the nozzle, therefore, becomes homogeneous with less turbulence and inflow of ambient air is effectively prevented.

In this sixth embodiment, the static pressure conversion surface 7 is installed at the flat portion parallel to the bottom surface of the nozzle outlet 2a. However, a similar effect is obtained if the static pressure conversion surface 7 is not parallel to the bottom surface 2a or it is not installed at the flat surface, if it collides with the gas which is directed to the flow path of the circumference flow nozzle (namely, sub assist gas nozzle) and causes the gas flow to recover to the static pressure.

Embodiment 7

FIG. 14 is a longitudinal cross sectional view which illustrates a machining head according to a seventh embodiment. In this seventh embodiment, an annular return wall surface 8 protrudes at the edge of static pressure conversion surface 7 which is provided inside the sub assist gas duct 2b of the embodiment 6. The return wall surface 8 forms a gas residence space 8a, where the upper edge of the return wall surface and inside circumference nozzle are adjusted. In the case shown, the height is adjusted so that the appropriate clearance exists between the upper edge of the return wall surface 8 and the outside wall of the main assist gas nozzle 1.

The broken line arrows in FIG. 14 illustrate the directions of gas flow. The gas flow supplied through the gas supply inlet 2c temporarily fills the gas residence space 8a, which is comprised of the return wall surface 8, the inside wall of the sub assist gas nozzle 2 and the static pressure conversion surface 7, and then the static pressure of the gas flow is recovered. The gas flows with homogeneous flow velocity from the upper edge of the return wall surface 8 to the duct 2b of the sub assist gas nozzle, then the gas is discharged toward the outside through the jet outlet 2a. Therefore, the outside circumference flow becomes a steady and homogeneous flow with less turbulence and the inflow of the air improved more effectively.

FIG. 15 illustrates a characteristic curve which shows the pressure fluctuation value of the assist gas flow on the work surface versus distance from the center axis toward the radial direction regarding the sixth and seventh embodiments and the conventional example. The vertical scale indicates the pressure fluctuation value p/P (%), and the horizontal scale indicates distance r (mm) from the center axis of the nozzle toward radius direction. In FIG. 15, the solid line shows the characteristic curve for the seventh embodiment, the broken line shows that of sixth embodiment and the short dash line shows that for the conventional example. From the figure, in the case of the seventh embodiment, it is easily understood that the pressure fluctuation value of sub assist gas at the cutting work surface is smallest and the gas flow is most homogeneous with a steady state. Therefore, the amount of inflow of the surrounding air is decreased and the oxygen purity is kept at a high level.

Embodiment 8

FIG. 16 is a longitudinal cross sectional view which illustrates a machining head of the laser machining apparatus according to an eighth embodiment. In the eighth embodiment, an expanded flow path 9 is formed at the front edge of the sub assist gas nozzle 2 where the flow path expands gradually towards the sub assist gas nozzle jet outlet 2a. According to the above construction, the sub assist gas (oxygen gas) discharged from the sub assist gas nozzle 2 does not accelerated, and flows parallel to the nozzle center axis without being separated from the inside nozzle wall. Therefore, the inflow of surrounding air into the surrounding flow can be minimized immediately down stream of the nozzle jet outlet, and high oxygen purity can be maintained, and steady machining can be attained. In this embodiment, the flow path area gradually expands, but a similar effect is obtained if the flow path area remains constant.

Embodiment 9

FIG. 17 is a longitudinal cross sectional view which illustrates a machining head according to a ninth embodiment. In the ninth embodiment, the outermost wall of the multi-nozzles, namely, the sub assist gas nozzle 2, is formed as a cone-shaped trapezoid which has virtual top angle .theta. less than 45 degrees. According to the above construction, it is possible to minimize the amount of assist gas which stays between the laser beam nozzle and the work surface and to prevent self burning. Moreover, it is possible to minimize the amount of inflow of surrounding air down stream of the surrounding nozzle, namely, from the sub assist nozzle outlet. Further, high oxygen purity is maintained and stable machining can be attained.

Embodiment 10

FIG. 18 is partial perspective view which illustrates a machining head according to a tenth embodiment. In this tenth embodiment, a common assist gas (oxygen gas) source 10 is installed upstream of the nozzle. The gas supplied from the assist gas source 10 branches to the main assist gas supply inlet 1c and to the plurality of small holes 11 on the surrounding annular portion of the main assist gas nozzle 1 at the center nozzle. The assist gas (oxygen gas) flows into the sub assist gas nozzle duct 2b through the small holes 11.

In this embodiment, the amount of main assist gas (oxygen gas) flow discharged from the center flow nozzle, namely, main assist gas nozzle 1 and the amount of sub assist gas (oxygen gas) flow discharged from the surrounding flow nozzle, namely, the sub assist gas nozzle 2 are variable according to the preset value of area of the small holes 11 and the number of the small holes 11. Therefore, a desired amount of the main assist gas and the sub assist gas can be easily obtained by selecting the appropriate nozzle, namely, the nozzle with the appropriate area and appropriate number of small holes 11, according to the quality and thickness of the material to be machined.

FIGS. 19A, 19B are plan views which illustrate other examples of small hole shapes for the assist gas supply inlet according to a tenth embodiment. In the above embodiment, the small holes 11 have circular shapes. However, it is needless to say that a similar effect is obtained if any shape of small holes 11 is used, for example, a rectangular or oval shape as illustrated in the plan views of FIGS. 19A, 19B, respectively.

Embodiment 11

FIG. 20 is a partial perspective view which illustrates a machining head according to an eleventh embodiment. In the above tenth embodiment, the flow path is adjusted by installing small holes 11 which are located at the annular portion surrounding the main assist gas supply inlets 1c and supply the sub assist gas to the sub assist gas nozzle 2.

However, in this the eleventh embodiment, the flow quantity of the assist gas is adjusted by a mesh type fluid resistance body 12 installed at the above annular portion as illustrated in FIG. 20. The fluid resistance body 12 may be a porous material. In this eleventh embodiment, the gas flow supplied to the sub assist gas nozzle 2 is homogeneous and a stable gas flow having less turbulence can be realized by selecting an appropriate mesh size of the fluid resistance body or appropriate porous material.

Embodiment 12

FIG. 21 is a longitudinal cross sectional view which illustrates a machining head of the laser machining apparatus according to a twelfth embodiment. In this twelfth embodiment, an aperture area adjusting means 13 is installed at the sub assist gas supply inlet of the sub assist gas nozzle 2. The aperture area of the aperture area adjusting means 13 can be adjusted manually or automatically using a driving apparatus. By using such a construction, the flow quantity of assist gas and the flow ratio of main assist gas versus the sub assist gas can be adjusted without replacing the nozzle, so that work can be carried out more effectively.

Embodiment 13

FIG. 22 is a plan view which illustrates an aperture area adjusting means according to a thirteenth embodiment, which separates the main assist gas flow from the sub assist gas flow. In the thirteenth embodiment, a concrete example of construction of the aperture area adjusting means 13 of the above embodiment 12 is illustrated. In the embodiment, an annular disc 14 contacts the annular portion which has a plurality of small holes 11 for directing the assist gas to the sub assist gas nozzle. The annular disc 14 has a plurality of similar small holes 15 thereon and also has a knob 16 at the outside thereof. The annular disc 14 is moved manually to adjust the aperture area, where the small holes 11 overlap with the small holes 15, which supplies the sub assist gas to the sub assist gas nozzle. Therefore, the flow quantity of the assist gas to be supplied to sub assist gas nozzle can be easily adjusted by moving the annular disc 14 which varies aperture area of gas flow inlet.

Although the annular disc 14 is operated manually by the knob 16 connected to the outside nozzle in the embodiment, it is needless to say that automatic driving means may also be useful.

Embodiment 14

FIG. 23 is plan view which illustrates an aperture area adjusting means according to a fourteenth embodiment. In the fourteenth embodiment, another concrete example of an aperture area adjusting means is illustrated. The aperture area adjusting means of the fourteenth embodiment comprises a flow restriction means 17 which restricts the plurality of small round holes 11 which supplies assist gas to the sub assist gas nozzle. The aperture area adjusting means 17 also has a knob 16 which permits manual adjustment of the degree of restriction of the aperture area from the outside of the nozzle. The aperture area of the gas supply inlet for supplying the sub assist gas is varied by adjusting the restriction means 17, whereby the supply gas flow quantity can be adjusted.

Although aperture area adjusting means 17 is operated manually by the knob 16 connected to the outside nozzle in the embodiment, it is needless to say that automatic driving means may be useful.

Embodiment 15

FIG. 24 is a longitudinal cross sectional view which illustrates a machining head of laser machining apparatus according to a fifteenth embodiment. In the fifteenth embodiment, a gas (oxygen gas) source for main assist gas nozzle 1 and a gas source for sub assist gas nozzle 2 are separately provided. Therefore, it is possible to supply the appropriate flow quantity of assist gas and to supply oxygen with appropriate flow quantity ratio of the main assist gas corresponding to the sub assist gas.

Embodiment 16

FIG. 25 illustrates a construction of a laser machining apparatus according to a sixteenth the embodiment. The machining apparatus of the embodiment comprises respective assist gas (oxygen gas) sources 18, a control means 20 and sensors 19. The assist gas sources 18 supplies the assist gas to a machining head having a nozzle construction as shown in FIG. 24. The sensors 19 detect information regarding the temperature of the material surface, the groove width, and the amount of luminescence due to sparks during machining. The control means 20 adjusts the gas flow quantity and the gas flow pressure of the main assist gas and the sub assist gas which are discharged from the nozzles depending on the information from the sensors 19. According to this construction, it is possible to avoid abnormal machining and to attain stable machining.

The machining head having a nozzle construction has respective assist gas source in the sixteenth embodiment. But, in case that the same assist gas source is used, the same effect is obtained by providing a control apparatus for controlling the apparatus for automatically driving the sub assist gas flow control apparatus and the gas flow from the same gas source as shown in the thirteenth and fourteenth embodiments, instead of the control apparatus shown in FIG. 25.

Embodiment 17

FIG. 26 illustrates a construction of a laser machining apparatus according to a seventeenth embodiment. The machining apparatus of the seventeenth embodiment comprises a sensor 19 which predicts or detects self burning. The machining apparatus operates to decrease at least one of the gas pressure and the gas flow quantity of the main assist gas and the sub assist gas when self burning is predicted or detected according to the information from the sensor 19. According to this construction, it is possible to avoid an outbreak of self burning and to attain stable machining.

Embodiment 18

FIG. 27 illustrates a construction a laser machining apparatus according to an eighteenth embodiment. The machining apparatus of the eighteenth embodiment comprises an operation control means 21 for controlling the laser machining apparatus and a flow control means 20 for controlling the assist gas sources 18. The operation control means 21 adjusts the gas flow quantity and the gas pressure of the main assist gas and the sub assist gas using programming control, for example, during piercing and corner cutting. Thereby, it is possible to adjust the flow quantity at the beginning of the piercing or to control accurately the flow quantity during cutting the corner portion. According to this construction, it is possible to avoid an outbreak of burning during piercing and cutting the corner portion and to attain more stable cutting and machining.

Embodiment 19

FIG. 28 is a longitudinal cross sectional view which illustrates a machining head of laser machining apparatus according to a nineteenth embodiment. The machining head of the embodiment comprises a main assist gas nozzle 1 and a sub assist gas nozzle 2 which are separated. The main assist gas nozzle 1 is mounted and fixed on a portion 22 which protrudes from the inside of the sub assist gas nozzle 2 as shown in FIG. 28. According to this construction, since the main assist gas nozzle 1 can be detachably mounted, many combinations of different nozzles having different small hole area and hole numbers for supplying the assist gas can be freely selected depending on the material and thickness of the work. Therefore, an appropriate flow quantity and appropriate flow ratio of the main assist gas versus the sub assist gas can be selected.

Although double annular nozzle construction is explained in the nineteenth embodiment, a similar effect is obtained in case of multiple annular nozzle construction. In the multiple annular construction, each nozzle is provided separately and fixed inside the wall of nozzle 2. The fixing portion may comprise not only the protruding portion as shown in FIG. 28, but also the construction as shown in FIG. 29A and FIG. 29B. FIG. 29A illustrates a level difference 22a inside of the nozzle 2 and FIG. 29B illustrates a taper portion 22b inside of the nozzle 2.

Embodiment 20

FIG. 30 is a longitudinal cross sectional view which illustrates a machining head of laser machining apparatus according to a twentieth embodiment. In this embodiment, the material of the main assist gas nozzle 1 is different from that of the sub assist gas nozzle 2. The material of the main assist gas nozzle 1 is a metal (brass, for example) which is easy to machine. The material of the sub assist gas nozzle 2 located most outside is a metal which has a high melting point and high hardness. According to this construction, the outermost portion is hardly damaged by spattering due to oxide caused by the high temperature during cutting or splash of melted metal, and protects the inside nozzle 1, thereby the lifetime of the nozzle is extended.

Although the laser cutting apparatus is explained in the above embodiments, it is needless to say that similar effect may be obtained for the laser welding apparatus which comprises the same construction as the laser machining apparatus.