WO2006067387A2

WO2006067387A2 - A laser machining apparatus

Info

Publication number: WO2006067387A2
Application number: PCT/GB2005/004847
Authority: WO
Inventors: Julian Edward William Preston
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2004-12-21
Filing date: 2005-12-15
Publication date: 2006-06-29
Anticipated expiration: 2007-06-21
Also published as: WO2006067387A3; GB0428018D0; GB2421458A

Abstract

An improved laser machining apparatus is described in which the laser machining head (4) containing the laser focussing lens (24) is provided with protective covers (34, 40) on either side of the lens (24) to exclude foreign particles from the surfaces of the lens. In addition re-orientation of the laser column (2), which carries the laser beam (12) from a laser generator (3) to the machining head (4) is suggested as further protection for the lens (24).

Description

A LASER MACHINING APPARATUS

The invention relates to laser machining apparatus.

In particular it concerns the provision of additional protection from atmospheric borne contamination for the machining lens of the laser apparatus

In a typical laser machining apparatus the laser generator is mounted in a convenient stationary position and the laser beam is conducted through a laser column to an objective machining lens positioned close to the workpiece. During operation at least the objective machining lens is surrounded by a contaminated atmosphere in which particles of vaporised workpiece material as well as general atmospheric borne contamination are present. The laser lens has two stages of protection provided by a jet of gas purge emitted through the final laser exit nozzle and a protective cover slide of mineral crystal in front of the exposed side of the laser lens. The gas purge carries light and less energetic particles away from the lens while the cover slide provides a final defence against more energetic particles reaching the lens surface.

However, operational experience shows that the unprotected rear surface of the lens is also prone to damage and is often found to be a limiting factor in the useful working life of the laser lens. The present invention seeks to provide a cure for this drawback by providing additional protection for the laser lens.

According to the present invention an improved laser machining apparatus comprising a laser beam generator, a laser column carrying a laser beam from the laser beam generator to an objective laser machining lens wherein a protective cover transparent to the laser beam is provided between the laser column and the machining lens.

The invention will now be described in more detail with reference to an example illustrated in the accompanying drawings, in which: Figure 1 illustrates a known laser machining apparatus arrangement;

Figure 2 shows the laser column of the apparatus of Figure 1 in its normal orientation;

Figure 3 illustrates a laser machining apparatus arrangement according to the present invention;

Figure 4 shows the laser column of the apparatus of Figure 3 in its modified orientation; and

Figure 5 shows a part cutaway view of the laser machining nozzle of the apparatus of Figure 3.

A known apparatus illustrated in Figures 1 and 2 comprises a laser column, indicated generally at 2, including a laser machining lens assembly 4 mounted on a fixed base plate 6. The laser beam generator, represented by box 3 in the drawing, is mounted on a movable part of the machine and the laser beam is directed to the laser column carried on the machining head 4 by means of an enclosed optical pathway containing mirrors to reflect the beam. The laser column 2 and machining head 4 are mounted on a carriage 5 mounted on parallel, horizontal slides 7a, 7b allowing the machining head movement in the "X" direction. A workpiece 8 to be machined is secured to a workpiece mounting member 10 which is capable of movement relative to the machining head 4 in the "Y" and "Z" directions this providing freedom of movement about the three conventional orthogonal x, y, z axes. The machining head is also capable of mounted about orthogonal axes "A" and "B" see Figure 2, thus providing the overall arrangement with five-axis movement.

Figure 2 shows a part-cutaway view in the X-direction, of the laser column 2 and machining head 4 mounted on movable carriage 5. A laser beam 12 produced by the generator 3 enters the column from the left in the drawing in a horizontal direction and strikes a first mirror 14 angled to reflect the beam 12 in a vertically downward direction 16. The beam then falls onto a second angled mirror 18, which reflects it horizontally towards a third angled mirror 20 immediately below which is mounted the machining lens assembly 4. The mirror 20 reflects the beam downwards into the machining lens assembly 4. The manner in which the movement of the various machine parts is achieved while maintaining laser beam continuity is well known in the art and is not critical to understanding of the present invention; therefore it will not be further described here.

Figures 3, 4 and 5 illustrate the construction of a machining lens assembly 4 according to the present invention and, in comparison with the known apparatus illustrated in Figures 1 and 2 like parts carry like references.

Referring firstly to Figure 5, the lens assembly 4 is shown in more detail. It comprises a housing 22 containing a laser focussing lens 24, a chamber 26, and a nozzle 28 through which the laser beam emerges to impinge on the workpiece 8. The function of the lens is to focus the laser beam onto the workpiece 8 to achieve a desired power density at the workpiece surface.

The housing 22 has a cylindrical upper portion 30 formed integrally with a wider, circular part of the housing defining the chamber 26. The foot of the laser column 2 is adapted to received into a recess formed therein this part of the housing 22, which is retained in place by an internally threaded ring 32 screwed onto a corresponding part on the column 2. By unscrewing the ring 32 the whole laser lens assembly 4 is demountable from the laser column 2.

As is well known in apparatus of this type a gas supply (not shown) is connected to the chamber 26. This gas is ejected through the nozzle 28 towards the workpiece 8 as a plume, which sweeps vaporised material away from the orifice of nozzle 28. However, a certain amount of vaporised material and spatter enters chamber 26 through the orifice and some of it is deposited on the interior surfaces of the machining lens 4. In order to protect the laser focussing lens 24 its lower surface is protected by a sacrificial cover slide 34 made of a mineral crystal transparent to the laser beam. The cover slide - A -

34 is located against an internal shoulder 36 at the bottom of the cylindrical housing potion 30 with the lens 24 positioned above the slide. This assembly is retained in place by means of a collet 38 screwed into the internal bore of the housing portion 30. The cover slide is not fixed in the housing nor is it secured to the lens 24. Thus the whole assembly may be demounted and dismantled and the cover slide 34 replaced as a separate item.

As mentioned above operational experience is that the unprotected upper surface of the lens 24 is also prone to damage from dust and other material present in the internal air space of the laser column. This is often found to be a limiting factor in the useful working life of the laser lens 24. In order to provide a cure for this drawback additional protection is provided for the exposed surface of laser lens.24 in the form of a second, sacrificial cover slide 40. In the exemplary embodiment the collet 38 is split into upper and lower portions with the slide 40 located between the two portions.38a, 38b. Thus the lower collet portion 38b acts to retain the lens 24 in position with the slide 40 above it. The upper collet portion 38a may then be removed to allow replacement of slide 40. Use of sacrificial cover slides 34, 40 to protect both exposed surfaces of lens 24 is found to increase substantially the useful life of the lens. Since the cost of the lens is significantly greater then the cost of the cover slides this increase represents a substantial operating cost saving.

In the embodiment described above the laser beam issues from the machining head in the Y-direction, according to a further aspect of the present invention, illustrated in Figures 3 and 4, the laser column and machining head is rotated through ninety degrees (90°) so that the laser beam issues in the X-direction. Clearly, the workpiece 8 and workpiece mounting member 10 are transposed and turned to occupy a vertical position in the path of the beam. The advantage of this re-oriented arrangement lies in the re-positioning of the mirrors 14, 18, 20 in the laser column 2.

In the known arrangement of Figures 1 and 2 the mirror surfaces are all oriented so that they reflect the laser beam either upwards or downwards. Although the laser column may be assembled initially in a clean atmosphere, so that virtually no dust is trapped in the contained atmosphere, inevitably atmospherically borne dust particles are introduced during workplace maintenance. For example when the laser machining head 4 is removed at least the one end of the laser column is open to the ingress of atmospherically borne particles. Thus the present arrangement suffers the disadvantage that dust particles may settle onto the surface of the upward reflecting mirror 18 gradually degrading its reflectivity, and consequently causing a loss of beam power. According to this aspect of the invention the surfaces of all of the mirrors 14, 18, 20 lie in a vertically extending plane so that dust particles tend not to fall onto any of the mirror surfaces. These particles can e difficult to remove without causing some degradation of the reflecting surface. At present the only available solution to this problem is to replace the laser column including the mirrors. Therefore it is further proposed that the mirrors shall also be protected by cover slide of the same laser transparent material. These additional slides are positioned either at the ends of the laser column to preserve the cleanliness of the atmosphere now sealed within the column or in front of at least the mirror most at risk or all of the mirrors.

Claims

An improved laser machining apparatus comprising a laser beam generator, a laser column carrying a laser beam from the laser beam generator to an objective laser machining lens characterised in that a protective cover transparent to the laser beam is provided between the laser column and the machining lens.

An improved laser machining apparatus as claimed in claim 1 wherein the objective laser machining lens comprise a lens unit demountable from the laser column and the protective cover is positioned in the demountable unit.

An improved laser machining apparatus as claimed in claim 2 further characterised in that the demountable lens unit comprises a lens assembly having a protective cover on either side of the lens.

An improved laser machining apparatus as claimed in any preceding claim wherein the laser machining lens is mounted at the distal end of a laser column capable of movement about three axis relative to the laser beam generator.

An improved laser machining apparatus as claimed in claim 4 wherein the laser column directs the laser beam to the machining lens by means of reflecting mirrors

An improved laser machining apparatus comprising a laser beam generator, a laser column carrying a laser beam from the laser beam generator to an objective laser machining lens characterised in that the laser column and the machining head are arranged to deliver a laser beam in a horizontal direction and all reflecting surfaces in the laser column are disposed in a vertical plane.