Nozzle assembly for laser beam cutting

A nozzle assembly for laser beam cutting has a nozzle body shaped like a ncated cone fitting the focussed laser beam and surrounding the same. The assembly has a passage bore for the laser beam, with a nozzle sleeve concentrically surrounding the nozzle body and forming an annular gap therewith. An outlet bore is coaxial with the passage bore for a cutting gas stream from an annular gap connected to a gas source. The outlet bore is located on the workpiece side in front of the passage bore and has a diameter (D) exceeding the diameter (d) of the passage bore. In order to insure that the deflection of the gas stream from the nozzle takes place parallel to the axis of the laser beam, with a minimum of flow loss, the nozzle assembly is built so that the outlet cross section of the outlet bore is approximately the same as the transition cross section of the annular gap to the outlet bore.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is a national stage of PCT/DE95/100050 filed 18 Jan. 1995 
and based upon German national application P44 02 000.7 of 25 Jan. 1994 
under the International Convention. 
FIELD OF THE INVENTION 
The invention relates to a nozzle assembly for laser beam cutting, which 
has a frustoconical nozzle body surrounding the focussed laser beam, 
having a nozzle sleeve concentrically surrounding the nozzle body and 
forming an annular gap therewith, the sleeve having an outlet opening 
coaxial with a passage bore for a stream of cutting gas flowing from the 
annular gap connected to a gas source, whereby the outlet opening is 
located on the side of the workpiece in front of the passage bore and has 
a diameter which exceeds the diameter of the passage bore. 
BACKGROUND OF THE INVENTION 
A nozzle assembly is known from JP-84589 A in: "Patent Abstracts of Japan" 
1993, Vol. 17/No. 410, Sec. M-1455. The wall of the nozzle sleeve forming 
the annular gap projects by more than 0.5 to 5 mm beyond the end of the 
nozzle body, in order to achieve a perfect shield against the 
environmental air. 
U.S. Pat. No. 4,121,085 describes a nozzle assembly for laser beam cutting, 
which has a nozzle body in the shape of a truncated cone fitting the 
focussed laser beam and surrounding it, with a passage bore for the laser 
beam, with a nozzle sleeve concentrically surrounding the nozzle body at a 
distance. The sleeve, coaxially with the passage bore has an outlet 
opening for the cutting gas beam on the workpiece side in front of the 
passage bore of the nozzle body. The delivery cross section of the outlet 
opening is smaller than the cross section of the inlet into the space 
between the nozzle body and the nozzle sleeve. In this way a reverse gas 
flow is supposed to be generated, with the purpose of cutting the air flow 
to the workpiece. 
DE 36 37 568 A1 describes a nozzle assembly for laser beam cutting, which 
has nozzle body shaped like a truncated cone, fitted to and surrounding 
the focussed laser beam, with a passage bore for the laser beam, with a 
nozzle sleeve concentrically surrounding the nozzle body forming an 
annular gap therewith, the sleeve having an outlet opening, which is 
coaxial with the passage, for a cutting gas supplied by a gas source. In 
this known nozzle arrangement the nozzle sleeve is screwed onto the nozzle 
body and is provided on the inside surface of the sleeve with individual 
longitudinal grooves, separated from each other. When the sleeve is 
slightly unscrewed, a conical annular gap is created between the sleeve 
and the nozzle body. The annular gap surrounds the passage of the nozzle 
body at a distance which is determined by the wall thickness of the nozzle 
body in the area of the passage bore. The exiting cutting gas beam is like 
a hollow cylinder. This known nozzle arrangement can also be used for 
cutting, but is not specialized for this operation and creates a 
relatively large cutting kerf due to the hollow cylindrical shape of the 
burning gas beam. 
DE 38 24 047 A1 describes a nozzle assembly with a nozzle sleeve, which 
concentrically surrounds a nozzle body shaped like a truncated cone and 
has a bore arranged on the workpiece side in front of the passage bore. By 
means of this bore of the known nozzle assembly a negative pressure is 
generated on the workpiece surface, in that the annular gap between the 
nozzle sleeve and the nozzle body is subjected to suction. A considerable 
beam disturbance takes place in the area of the outlet bore, so that this 
particular nozzle arrangement is not suited for laser beam cutting, 
especially in the case of high cutting efficiency and/or high cutting gas 
pressures, for instance with laser power exceeding 3 kW and gas pressure 
of more than 10 bar. 
In order to be able to cut workpieces at high cutting speeds it is known to 
use cutting gas supplied at high velocities for blowing away the material 
from the cutting clearance. The velocity of the cutting gas lies within 
the ultrasonic range. From DE 36 30 127 A1 such a nozzle assembly for 
laser beam cutting is known, which has a nozzle body shaped like a 
truncated cone, surrounding and fitting a focussed laser beam, with a 
passage bore for the laser beam and which has a nozzle sleeve 
concentrically surrounding the nozzle body, which together with the nozzle 
body forms several gas stream channels connected to a gas source. The 
channels end on the workpiece side of the passage bore for the laser beam 
in a common chamber, from which, at correspondingly measured pressures, a 
single gas beam emerges through an outlet bore at supersonic velocities. 
Thereby a considerably slower gas shield is provided against the 
entrainment of gas from the surrounding atmosphere. The outlet bore is 
formed by a protection cap screwed to the nozzle sleeve, which is meant as 
a protection against contaminations resulting from the cutting. In the 
above-mentioned nozzle arrangement the diameter of the outlet bore is 
smaller than the diameter of the passage bore for the laser beam. A gas 
stream results which could still be improved upon from the point of view 
of flow losses and of the course of the cutting gas stream in the cutting 
clearance. 
OBJECT OF THE INVENTION 
It is therefore the object of the invention to improve a nozzle assembly so 
the features described in the introduction, so that the gas beam leaving 
the nozzle is guided through the nozzle mouth towards the workpiece with 
minimal stream losses, namely impulse or flow rate losses. 
SUMMARY OF THE INVENTION 
This object is achieved by making the exit cross section of the outlet bore 
approximately equal to the transition cross section of the annular gap 
towards the outlet bore. 
It is important for the invention that a flow optimization takes place in 
the area of the nozzle outlet. In the outlet of the converging gap, the 
gas beam present in annular shape due to the annular gap is deflected 
parallel to the nozzle axis and exits fully as a solid free stream from 
the nozzle mouth, or from the outlet bore of the nozzle. The deflection of 
the annular stream in the area of the nozzle mouth takes place with a 
minimum of impulse loss, since it does not form compression waves. The 
free stream expands only outside the nozzle mouth in the direction of the 
laser beam axis to reach ultrasonic velocities. 
As a result of the low flow loss during the deflection of the annular beam 
into the outlet bore, the leakage of gas beam in the laser passage bore is 
also reduced. The reduced leak gas stream lowers also the admixture of 
environmental air in the gas stream. 
In order to achieve an optimal cutting gas stream inside the nozzle 
assembly, whose flow losses are as low as possible, the nozzle assembly is 
built so that the annular gap and the gas pressure are measured so that 
the gas is accelerated only up to sonic velocity as a result of the 
surface reduction of the gap cross section towards the outlet bore. 
Compression waves can be safely avoided, so that flow losses are 
minimized. 
A further reduction of flow losses can be achieved by decreasing as 
continuously as possible the transition cross section of the annular gap 
towards the outlet bore due the configuration of the wall of the nozzle 
sleeve. 
Preferably, the gap widths of the annular gap are dimensioned approximately 
as follows: 
EQU s=D/(2 cos (.alpha..sub.s))*(1-(1-cos (.alpha..sub.s)).sup.0.5) 
s=gap width 
D=diameter of the outlet bore 
.alpha..sub.s =inclination angle of the annular gap. 
Particularly at higher cutting speeds it is desirable that the nozzle be at 
a slightly greater distance from the workpiece. In this case it has to be 
insured that the gas beam does not expand and maintains its mechanical 
action within the cutting clearance. In order to achieve this, the nozzle 
assembly is built so that the outlet bore communicates with a nozzle 
recess, whose configuration allows for the expansion of the beam up to 
supersonic speed in a guided manner. 
The diameter of the passage bore of the nozzle body is to be kept as small 
as possible, so that an influence on the gas stream is avoided and thereby 
the leakage losses through the inner space of the nozzle can be kept low. 
This is achieved when a suitable design of the nozzle assembly is 
characterized in that the diameter of the passage bore is equal to or 
greater than 1.5 times the diameter of the laser beam in the area of the 
passage bore. 
From the point of view of the laser beam focussing means, nozzle assemblies 
have various designs. When focussing lenses are used, it is necessary to 
build the nozzle arrangement so that the focussing lens and/or a shutter 
window transmitting the laser beam are not destroyed by the maximum 
possible cutting gas pressure. When the nozzle assembly is provided with 
such a shutter window traversed by the laser beam, it makes sense to build 
it so that pressure-relief bores are provided in the inner space of the 
nozzle body closed off by a transmitting element in the access area of the 
laser beam. Particularly the pressure-relief bores can reduce possible 
pressure peaks, so that the transmitting shutter window, or the focussing 
lens will not be damaged. 
The nozzle assembly can be built so that the inner space of the nozzle body 
closed off by a transmitting element in the access area of the laser beam 
will be connected to a gas source. Thereby the inner space of the nozzle 
body is supplied in a conventional way by gas, which escapes through the 
passage bore for the laser beam and flows centrally into the gas of the 
annular gap. 
The nozzle assembly can be further developed so that the nozzle body shaped 
like a truncated cone is frustoconical nozzle body surrounded by an outer 
nozzle sleeve forming a further coaxial annular gap therewith, and can be 
the two annular gaps can be connected to the same or to different fluid 
sources. This embodiment is particularly suitable for nozzle assemblies 
which are used in connection with an optical mirror system. When both 
annular gaps are connected to the same gas source, the resulting cutting 
gas beam can be created with pressure and/or pressure flow ratios varying 
over its cross section. When the two annular gaps are connected to 
different gas sources, the corresponding pressure and/or pressure flow 
ratios of the combined separate beams result. In special cases, instead of 
the gas, a liquid can be used as fluid. 
The further annular gap can end in the outlet bore or in a separate gap 
mouth surrounding the outlet bore at a distance. In the first instance the 
outer onflowing fluid becomes a component of the central gas beam, whereby 
the formation of the gas beam can be influenced through the design of the 
mouth of the further annular gap. In the second instance a separate beam 
results, which for example is used for influencing workpiece areas around 
the cutting beam, e.g. to shield them. 
An additional gas beam has often to meet different requirements. For 
instance it can be that the gas consumption should be low. For this case 
the nozzle arrangement so that an annular gap for the formation of an 
additional gas beam connected to a bore which ends next to the outlet 
bore. Preferably this bore is parallel to the nozzle axis. 
For similar special purposes the nozzle assembly can also be built so that 
the nozzle sleeve has a vertical slot emerging from the annular gap in 
which the outlet bore is integrated.

The nozzle assembly 34 of the invention consists basically of a nozzle body 
10, with an inner space 16, which is correspondingly fitted to the 
focussed laser beam not shown in the drawing, which means that it narrows 
down corresponding to the focussed laser beam interior of the nozzle body 
10 and vice versa. At the end of the nozzle body 10 on the workpiece side, 
shown as the lower end in the drawing, a passage 11 for the laser beam is 
provided in the form of a cylindrical bore. The diameter d of this passage 
bore 11 should be as small as possible and corresponds to the diameter of 
the focussed laser beam d.sub.L at this point. As a rule of the thumb the 
following formula approximately applies: d&gt;1.5.times.d.sub.L. 
The nozzle body 10 is surrounded by a nozzle sleeve 13, which has a complex 
construction. In the region of the lower end of the nozzle body 10 the 
nozzle sleeve 13 is shaped similarly to the nozzle body 10 like a 
truncated cone. However it is slightly spaced away from the outer 
circumference 10' of the nozzle body 10, so that an annular gap 12 is 
formed. This annular gap 12 extends from a plenum 23 connected with a gas 
inlet 25, to a nozzle mouth 26, where an outlet bore 14 is provided in the 
nozzle sleeve 13. A continuous, uniform flow connection results between 
the gas inlet 25 and the nozzle mouth 26. The plenum 23 is an annular 
space surrounding the nozzle body 10, to which, apart from the gas inlet, 
a connection bore 27 for pressure measurements is also connected. As a 
result a cutting gas supply through the gas inlet 25 can be delivered 
depending on a pressure measurement so that buildup of working gas 
pressure inside the nozzle reaches a certain predetermined value, or does 
not exceed or fall below this value. The buildup of the working gas 
pressure is caused by the narrowing of the flow cross section of the 
annular gap 12, from the plenum 23 to the nozzle mouth 26. Here the 
cutting gas is accelerated to a maximum up to the speed of sound. 
The different embodiments of the detail Z in FIG. 1 illustrated in the 
FIGS. 2a to 2d refer mainly to the configuration of the transition between 
the flow cross section of the annular gap 12 and the flow cross section of 
the outlet bore 14. The transition cross section is determined in FIG. 2a 
by the fact that the nozzle body 10 has the shape of a truncated cone, 
while the nozzle sleeve 13 is also shaped like a truncated cone on the 
outside, but on the inside it has a configuration of the wall 36, which 
basically has the shape of a curved funnel and is continuously bent in the 
transition area between the annular gap 12 and the nozzle mouth 26, so 
that a circumferentially running edge does not exist. Correspondingly the 
transition cross section is continuously reduced, as can be seen from the 
diagram located next to FIG. 2a, wherein the flow cross section A is 
plotted as a function of the course coordinate or distance u. To the 
course coordinate u.sub.1 corresponds a flow cross section A.sub.1 which 
is greater than the flow cross section surface A.sub.2 corresponding to 
the course coordinate u.sub.2 at the nozzle mouth 26. The diagram 
represents the desired continuous reduction of the transition cross 
section. It is assumed in this plot that the peripheral surface of the 
nozzle body 10 was fully present up to the cone point 38 and that 
therefore an effective disturbance through the passage bore 11 can not 
occur. It is apparent that the flow cross section surface narrows without 
interruption towards the nozzle mouth 26. 
In the FIGS. 2c and 2d the passage bore 11 and the outlet bore 14 are each 
cylindrical. These configurations can be manufactured in the simplest 
manner. The width of the annular gap 12 is constant up to the cylindrical 
outlet bore 14. By comparing FIGS. 2c and 2d it can be seen that the outer 
peripheral surface of the nozzle body 10 is the same and has the same 
position in each of them. The transition cross section from the annular 
gap 12 to the outlet bore 14 is located here as defined at the peripheral 
edge 39 of the nozzle sleeve 13 and lies perpendicularly to the outer 
peripheral surface of the nozzle body 10. The transition cross section is 
indicated at 40 in FIGS. 2c and 2d. In a comparison of FIGS. 2c and 2d it 
will be apparent that the lower edge 41 of the nozzle body 10 is closer to 
the peripheral edge 39, the smaller is the diameter d of the passage bore 
11. The smaller the diameter d is, the deeper the nozzle body 10 plunges 
in the direction of the outlet bore 14, without changing the position of 
its peripheral surface. 
Furthermore it is of primary importance that the diameter D of the outlet 
bore 14 be greater than the diameter d of the passage bore 11 for the 
laser beam. The gap width s according to FIG. 2 is also important. All 
mentioned values have to be interrelated in such a manner that the gas 
beam formed through the annular gap 12 is deflected in the direction of 
the laser beam axis 28 with possible losses. Thereby one starts from the 
following relation: 
EQU A.sub.D =.pi.D.sup.2 /4 
Further the following applies: 
EQU A.sub.s =.pi.s(D-s cos (.alpha..sub.s)) 
where 
D=diameter of the outlet bore 14 
d=diameter of the passage bore 11 for the laser beam 
s=the gap width 
.alpha..sub.s =inclination angle of the gap 
A.sub.s =outlet area of the gap 
A.sub.D =outlet area of the outlet bore 14. 
For a flow of the gas beam which is as much as possible free from losses, 
it is important that As be approximately equal to AD, it follows from the 
two previously mentioned equations that: 
EQU s=D/(2(cos (.alpha..sub.s))*(1-(1-cos (.alpha..sub.s)).sup.O.5) 
The diameter of the outlet bore 14 is usually around 2 mm. The angle 
.alpha..sub.s, should be as sharp as possible. But since the shape of the 
focussed laser beam limits the angle .alpha..sub.s, normally an angle of 
the magnitude .alpha..sub.s =30.degree. is assigned. The required gap 
width is then s=0.73 mm. 
The nozzle mouth 26 in FIG. 2b corresponds to the one in FIG. 2a, but has 
in addition a nozzle crown 42 which is provided at the nozzle point 43. 
The fastening of the nozzle crown 42 on the nozzle point 43 takes place 
for instance by jointing. It is also possible to make them in one piece. 
The nozzle recess 37, allows the cutting gas beam exiting the outlet bore 
14 to expand in a controlled manner to ultrasonic speed. This is 
particularly advantageous in applications where the cutting gas beam has 
to preserve its constant characteristics over a longer distance between 
the work head and the workpiece. It is especially important that it does 
not widen and that it develops in the cutting clearance the desired 
mechanical features related to blowing away of the burned material. The 
diagram arranged next to FIG. 2b represents the dependence of the flow 
cross section surface A on the course coordinate u within the nozzle crown 
42, whereby the increase of the cross sectional area outside the region of 
the nozzle mouth 26 will be apparent. 
The aforedescribed basic constructions of the nozzles can be advantageously 
used in various configurations. FIG. 3 shows a nozzle assembly which is 
used in association with a nozzle mounting 29 together with an optical 
mirror system 35, which guides the laser beam along the broken and 
dash-dot line. The dash-dot line represents the laser beam axis 28 of the 
focussed laser beam, which is directed through the nozzle assembly 34, and 
though the nozzle body 10 onto the workpiece to be cut. 
In FIG. 4 the laser beam is focussed with a transmitting element 15, namely 
a lens, which upwardly closes off the inner space 16 of the nozzle body 
10. A cutting gas stream with a static pressure is formed, which does not 
depend on the mechanical destruction threshold of the transmitting element 
15. Pressure-relief bores 32 can be provided for the pressure relief of 
the transmitting element, namely the lens 15. 
For some cutting applications it could be necessary to introduce in the 
center of the gas stream a type of gas which is different from the 
peripheral gas. This for instance is advantageous in flame cutting, when 
close to the middle of the gas stream which coincides with the laser beam 
axis, pure oxygen is introduced in order to improve the energy coupling, 
while at the periphery of the gas stream a gas which inhibits the 
oxidation process, such as nitrogen or a liquid such as water, is used. A 
radial mixing or layering can be achieved by introducing centrally one gas 
or gas mixture, namely through the inner space 16 of the nozzle body 10, 
through which the focussed laser beam is also sent. The second component 
of the gas stream can then be supplied through the annular gap 12, so that 
the desired coherent, but radially and axially differently mixed gas 
stream results. The dosage of the blend is performed through the pressure 
ratio and/or flow ratio of the combined individual gas flows. In addition 
the dosage, respectively the flow can be set by a mechanical adjustment of 
the gap, in the sense of a change in the gap width. For the central supply 
of gas a transmitting closing element 15 is required, as shown in FIG. 5. 
Through the bore 30 of the nozzle mounting 29 the gas supply of gas kind 1 
takes place and the bore 31 serves for the pressure measurement 1 of this 
gas. The second kind of gas is introduced through the gas inlet 25 of the 
sleeve 13, at whose bore 27 the second pressure measurement takes place. 
If a transmitting closing element 15 according to FIG. 5 is not feasible, 
according to FIG. 6 a nozzle can be employed which besides the nozzle 
sleeve 13 has a further outer nozzle sleeve 18 surrounding the first, 
which together with the outer circumference of sleeve 13 encloses a 
further coaxial annular gap 17. While the inner annular gap 12 is 
connected to the reservoir 23.sup.1, the outer annular gap 17 is in 
connection with a further reservoir 23.sup.2, each of them having a 
separate gas inlet for the two gas kinds, each also having a connection 
27.sup.1 and 27.sup.2 for pressure measurement. 
The two kinds of gas join at the nozzle mouth. According to FIG. 6 the 
outer annular gap 17 ends in the outlet bore 14. By contrast FIG. 7 shows 
that the further annular gap 17 has a gap mouth 20 which is at a distance 
19 from the outlet bore 14 and also surrounds the latter with the same 
distance 19. A corresponding formation of separate concentric gas beams is 
to be expected. 
An equally concentric arrangement of gas stream separated from each other 
is to be expected with the construction of the nozzle according to FIG. 8. 
This nozzle has however the particular feature that both annular gaps 12, 
17 are connected to the same plenum 23, whereby the annular gap 17 has 
supply bores 33 distributed over its circumference. 
It is self-understood that a nozzle assembly according to FIG. 8 has a 
comparatively high consumption of gas because of the outer, spaced away 
additional gas stream. When this gas consumption has to be reduced, a 
nozzle assembly according to FIG. 9 can be employed, wherein the annular 
gap 12 not only feeds the outlet bore 14, but also a bore 21 which is 
arranged parallel to the beam axis 28 and at a distance therefrom, and 
correspondingly produces a second stream with a comparatively large cross 
section. 
It is possible to shape the additional gas stream so that it is present at 
a distance from the laser beam axis 28, as well as connected with the gas 
stream of the outlet bore 14. For this purposes the nozzle sleeve 13 is 
provided with a slot 22 extending vertically from the annular gap 12 and 
having an oblong mouth, wherein the outlet bore 14 is integrated. This is 
shown in FIG. 10.