Focusing tube and use of same

Abstract

The aim of the invention is to provide a focusing tube (1, 1'), which is designed to focus a high-pressure liquid jet containing abrasive particles and comprises: a focusing channel portion (9, 9'); a discharge opening (8, 8') for the free discharge of the liquid jet from the focusing channel portion (9, 9'); and a longitudinal axis (6, 6') of the focusing channel portion (9, 9'), which longitudinal axis contains the center point (7, 8a') of the discharge opening (8, 8'), wherein the focusing channel portion (9, 9') is delimited by a liquid-impermeable channel wall (11, 11'), extends from the discharge opening (8, 8') at a focusing taper angle (2, 2') and is tapered toward the discharge opening (8, 8'), the service life of the focusing tube being increased in a way that is simple in terms of design. This aim is achieved, according to the invention, in that the focusing taper angle (2, 2') lies in the range of 0.05° to 1°.

Description

[0001] The present invention relates to a focusing tube designed for focusing a high-pressure liquid jet containing abrasive particles, comprising a focusing channel section, an outlet opening for the free exit of the liquid jet from the focusing channel section, and a longitudinal axis of the focusing channel section containing the center point of the outlet opening, wherein the focusing channel section is bounded by a liquid-impermeable channel wall and tapers towards the outlet opening at a focusing taper angle, wherein the legs of the focusing taper angle are two tangents lying in a longitudinal section plane containing the longitudinal axis and bearing against the channel wall at two opposing inner surface points in the longitudinal section plane, wherein the focusing taper angle is in the range of 0.05° to 1°.

[0002] Furthermore, the present invention relates to a use of such a focusing tube.

[0003] The present invention relates to the field of jet cutting, for example waterjet cutting, of workpieces. The cutting process is carried out with a high-pressure liquid jet, which exits the outlet and impacts a workpiece. The focusing channel section provides the necessary acceleration of the liquid jet and thus of the abrasive particles, because it narrows the high-pressure liquid jet. The liquid jet is typically accelerated to at least 400 m / s. Upon entering the focusing channel section, the liquid jet usually has a pressure of at least approximately 1000 bar. The abrasive particles, for example garnet particles, corundum particles, or quartz sand particles, significantly enhance the cutting performance of the liquid jet, enabling the cutting of even relatively hard materials such as rocks and metals.

[0004] However, the abrasive particles lead to increased wear of the focusing tube in the area of ​​the focusing channel section because they impact the channel wall with considerable energy at the high pressures present. As a result, the focusing channel section widens and thus increasingly loses its focusing effect. Consequently, the service life of the focusing tube decreases.

[0005] To reduce such wear, WO 03 / 053634 A1 teaches that the channel wall of the focusing tube should be provided with a lubricating film.

[0006] However, such a wear-reduction measure is structurally complex because the lubricating film is formed by infiltration of the channel wall from the outside with a suitable lubricant. This requires a pressure chamber in which the focusing tube is located. Furthermore, there is a risk that if the pressure chamber fails, the focusing tube will wear out quickly because its porous structure, necessary for infiltration, is not sufficiently stable.

[0007] US 5,018,317 A discloses a waterjet cutting device with a focusing tube according to the preamble of claim 1. US 8,491,355 B2 discloses a high-pressure cutting system. US 2008 / 032610 A1 discloses a waterjet cutting head.

[0008] US 2005 / 156064 A1 shows a descaling nozzle. US 2015 / 321316 A1 shows a nozzle for a high-pressure cutting system.

[0009] The object of the present invention is therefore to provide a focusing tube of the type mentioned above and a use thereof which achieves an increase in service life in a structurally simple manner.

[0010] The problem is solved by a focusing tube according to claim 1. Advantageous embodiments thereof can be found in the dependent claims.

[0011] The focusing tube, which is designed for focusing a high-pressure liquid jet containing abrasive particles, has a focusing channel section, an outlet opening for the free exit of the liquid jet from the focusing channel section, and a longitudinal axis of the focusing channel section containing the center point of the outlet opening. The focusing channel section is bounded by a liquid-impermeable channel wall and tapers towards the outlet opening at a focusing taper angle. The legs of the focusing taper angle are two tangents lying in a longitudinal plane containing the longitudinal axis and bearing against the channel wall at two opposing inner surface points in the longitudinal plane. The focusing taper angle is in the range of 0.05° to 1°.It has been shown that the focusing taper angle chosen in this way surprisingly and significantly reduces wear. The service life is correspondingly increased. Surprisingly, the noise emission during operation of the focusing tube is also reduced. Outside the range of 0.05° to 1°, these two positive effects no longer occur.

[0012] In the context of this disclosure, "high pressure" refers to a pressure of the liquid jet at the inlet to the focusing channel section of at least approximately 1000 bar up to approximately 6000 bar or more. Accordingly, the channel wall must be robustly designed, for example, by being sufficiently thick and made of a hard metal ( cemented carbide ) or cermet is formed.

[0013] hard metal (cemented carbide) Cermet and tungsten carbide are, within the meaning of the present disclosure, composite materials in which hard particles, constituting the predominant component of the composite, form a skeletal structure whose spaces are filled by a more ductile metallic binder. The hard particles can be composed, in particular, at least predominantly of tungsten carbide, titanium carbide, and / or titanium carbonitride, with smaller quantities also present, for example, of other hard particles, especially carbides of elements from groups IV to VI of the periodic table. The ductile metallic binder typically consists at least predominantly of cobalt, nickel, iron, or a base alloy of at least one of these elements. However, other elements may also be dissolved in the metallic binder in smaller quantities.A base alloy is defined as one in which this element forms the predominant component of the alloy. The most common is cemented carbide (…). cemented carbide ) for use, in which the hard material particles are formed at least predominantly by tungsten carbide and the metallic binder is a cobalt or cobalt-nickel base alloy; the weight fraction of the corresponding tungsten carbide particles is in particular at least 70 weight percent, preferably at least 80 weight percent, more preferably at least 90 weight percent.

[0014] In the context of this disclosure, "free outlet" means that the liquid jet can exit the outlet opening unimpeded. The outlet opening can be an external or internal outlet opening of the focusing tube. An external outlet opening is formed when the channel wall, viewed in the direction of liquid jet flow, terminates immediately behind the outlet opening. The outlet opening is then located, for example, in a flat end face of the focusing tube. An internal outlet opening is formed when an overhang created by the channel wall extends from the outlet opening in the direction of liquid jet flow. This overhang can, for example, be a chamfer or rounding of the channel wall. The chamfer can, for example, be conical.

[0015] It is expressly stated here that the focusing tube may have a channel end section between the focusing channel section and the outlet opening. This channel end section may be configured differently from the focusing channel section and may open directly into the outlet opening. The channel end section is preferably bounded by the liquid-impermeable channel wall and extends from the focusing channel section to the outlet opening. It may, in particular, have a constant cross-section, preferably a circular cross-section. In the latter case, the channel end section is therefore cylindrical and has its longitudinal axis as the central axis of the cylinder. This is advantageous because the liquid jet can be accelerated even further through the channel end section without increasing the number of particle-wall collisions.This is facilitated by the fact that the liquid jet is calmed by the focusing channel section and can thus enter the final channel section.

[0016] The liquid jet can be a water jet, but other, more viscous liquid jets are also conceivable and possible. Typically, the water jet also contains air, so a mixture of water, air, and the abrasive particles is formed.

[0017] The abrasive particles could be, for example, garnet particles, corundum particles or quartz sand particles.

[0018] For the purposes of this disclosure, the center point is the centroid of a planar surface defined by a boundary curve of the outlet opening. The outlet opening or the boundary curve can have any symmetrical or asymmetrical shape. For a circular or substantially circular outlet opening, the center point is the center of the corresponding circle; for a square, substantially square, rectangular (non-square), or substantially rectangular (non-square) shape, it is the intersection of the diagonals of the corresponding square or rectangle; and for an elliptical or substantially elliptical shape, it is the intersection of the major axis with the minor axis of the corresponding ellipse. "Substantially square" and "rectangular" mean, for example, that one or more corners are rounded.The outlet opening can also be egg-shaped, kidney-shaped, triangular, or essentially triangular. Essentially triangular means, for example, that one or more corners are rounded.

[0019] The longitudinal axis is arranged parallel to the extent of the focusing channel section. Since it contains the center point of the exit aperture, it penetrates the interior of the focusing channel section. If the focusing channel section is rotationally symmetrical with respect to its longitudinal axis, the longitudinal axis can also be referred to as the central axis.

[0020] The focusing channel section can extend from the exit aperture at the focusing narrowing angle.

[0021] A fluid-impermeable channel wall means that the channel wall is impermeable to both fluid ingress from the outside and fluid egress from the inside through the channel wall, for example by being made of a completely or almost completely sintered material, such as a hard metal ( cemented carbide ) or cermet.

[0022] As the focusing channel section narrows towards the exit opening, it and thus the liquid jet become narrower in that direction.

[0023] The longitudinal section plane contains the longitudinal axis and intersects an inner surface of the channel wall, so that the longitudinal section plane contains two sections that correspond to the inner surface and thus to the course of the focusing channel section in the longitudinal section plane. The points opposite each other in the longitudinal section plane are consequently contained in the sections. One or both of the sections can be straight or curved, for example, as segments of a hyperbola or parabola. The tangents include the focusing taper angle as an internal angle. At the exit orifice or at an inlet opening for the cutting fluid jet to enter the focusing channel section, the channel wall may have a discontinuity, for example, in the form of an edge. In such a case, the points where the tangents can be drawn are only those that are axially spaced from the exit and inlet openings.

[0024] By placing the points opposite each other in the longitudinal section plane, they are contained in a straight line that is perpendicular to the longitudinal axis of the focusing channel section and lies in the longitudinal section plane.

[0025] The focusing reduction angle can be constant. This is advantageous because such an angle can be produced particularly easily, for example, using a spark erosion process such as wire EDM. However, it is also conceivable and possible for the focusing reduction angle to vary.

[0026] According to a further development of the focusing tube, the focusing reduction angle is in the range of 0.1° to 0.8°. By keeping the focusing reduction angle within this range, even better wear reduction and noise emission reduction are achieved.

[0027] According to a further development of the focusing tube, the focusing channel section has a maximum diameter of 0.5 mm to 5 mm at every axial position with respect to its longitudinal axis in a cross-section along this longitudinal axis. Surprisingly, when the maximum diameter is within this range, a further reduction in wear and noise emission is achieved. When the maximum diameter is in the range of 0.65 mm to 3.5 mm, wear and noise emission are reduced even further. The maximum diameter is the inner diameter of the focusing channel section when it has a circular cross-section. In the case of other cross-sectional shapes of the focusing channel section, the maximum diameter is determined by the longest chord that can be drawn between two opposing points on the inner surface of the channel wall.The points are contained within a straight line perpendicular to the longitudinal axis of the focusing channel section. If the focusing channel section has an elliptical cross-section, the longest chord corresponds to the major axis of the ellipse. The focusing channel section can exhibit the shapes described for the outlet aperture in cross-section relative to its longitudinal axis; in particular, the shape of the outlet aperture is continued in the focusing channel section in cross-section. Thus, if the outlet aperture is circular, the focusing channel section is also circular in cross-section; if the outlet aperture is elliptical, it is elliptical, and so on.

[0028] According to a further development of the focusing tube, the focusing channel section is rotationally symmetrical about its longitudinal axis. This is advantageous because such a shape of the focusing channel section can be manufactured particularly easily, for example, by a spark erosion process such as wire EDM or die-sinking EDM.

[0029] According to a further development of the focusing tube, the focusing channel section is frustoconical in shape. This is advantageous because such a shape for the focusing channel section can be manufactured particularly easily, for example, by a spark erosion process such as wire EDM. Manufacturing becomes even simpler if the focusing channel section thus formed is frustoconical and a circular cone axis defined by this shape is aligned with the longitudinal axis of the focusing channel section.

[0030] According to a further development of the focusing tube, the focusing channel section extends over at least 50% of the length of the focusing tube measured parallel to its longitudinal axis. The focusing channel section thus essentially constitutes the focusing tube in its axial direction, which is advantageous for the wear-reduced focusing of the liquid jet.

[0031] The wear-reduced focusing is further improved if the focusing channel section extends over at least 70%, or even more preferably over at least 90%, of the length of the focusing tube.

[0032] The focusing tube has an inlet channel section, the inlet channel section extending from an inlet opening for the liquid jet entering the focusing tube to a transfer opening formed jointly with the focusing channel section. The inlet channel section has a longitudinal axis containing the center of the inlet opening and, outside of the transfer opening, has a maximum diameter at every axial position with respect to its longitudinal axis in a cross-section that is larger than the maximum diameter of the focusing channel section. This is advantageous because the inlet channel section, due to its larger maximum diameter, ensures that the liquid jet can enter the focusing channel section with a smoother flow. The longitudinal axis of the inlet channel section extends analogously to the longitudinal axis of the focusing channel section.

[0033] Focusing channel section. The inlet opening can have one of the shapes described for the outlet opening, in particular, it can be circular. The maximum diameter of the inlet channel section is defined analogously to the diameter of the focusing channel section as an inner diameter or as the longest chord between two opposite points on an inner surface of the channel wall. The transfer opening is an outlet opening of the inlet channel section and simultaneously an inlet opening of the focusing channel section. The transfer opening is thus associated with both the focusing channel section and the inlet channel section. A discontinuity in the channel wall can be formed at the transfer opening and the inlet opening, for example, in the form of an edge. In such a case, the points where the tangents can be drawn are only those that are axially spaced from the transfer opening and the inlet opening.The inlet channel section can be shaped like a frustocone, particularly a frustoconical cone, analogous to the focusing channel section. However, it is also conceivable and possible for the inlet channel section to be cylindrical, particularly a circular cylindrical shape.

[0034] According to a further development of the focusing tube, the longitudinal axis of the focusing channel section and the longitudinal axis of the inlet channel section are arranged coaxially. Due to this coaxial arrangement, the liquid jet can enter the focusing channel section via the transfer opening without deflection. The wear otherwise associated with deflection is therefore avoided.

[0035] According to a further development of the focusing tube, the inlet channel section is bounded by the fluid-impermeable channel wall, tapers towards the transfer opening, and extends at an inlet taper angle. The legs of the inlet taper angle are two tangents lying in a longitudinal plane containing the longitudinal axis of the inlet channel section and abutting two opposing inner surface points of the channel wall in this longitudinal plane. Outside the transfer opening, the inlet taper angle is larger than the focusing taper angle. This is advantageous because the inlet channel section, due to this taper, pre-focuses the fluid jet, resulting in even better flow stabilization. The inlet taper angle is defined analogously to the focusing taper angle.

[0036] According to a further development of the focusing tube, the inlet constriction angle is in the range of 10° to 90°. This leads to even better flow stabilization of the liquid jet.

[0037] According to a further development of the focusing tube, the inlet narrowing angle is in the range of 27° to 37°, which further improves the flow stabilization.

[0038] According to a further development of the focusing tube, the inlet channel section transitions seamlessly into the transfer opening. This reduces wear in the area of ​​the transfer opening because the impact energy of the abrasive particles is reduced compared to a stepped transition from the inlet channel section to the focusing channel section.

[0039] According to a further development of the focusing tube, the length of the focusing channel section, measured parallel to the longitudinal axis of the focusing channel section, is at least five times, preferably at least ten times, and even more preferably at least twenty times, greater than the length of the inlet channel section measured parallel to the longitudinal axis. This provides a length ratio that is particularly well suited for flow stabilization and focusing the cutting jet.

[0040] The problem is also solved by the use according to claim 14.

[0041] The focusing tube according to any one of claims 1 to 13 is used for cutting a workpiece by passing a liquid jet containing abrasive particles through the focusing channel section. This is advantageous because the cutting power of the liquid jet required for cutting can be maintained for a longer period due to the reduced wear in the focusing channel section. The liquid jet can be a water jet. The abrasive particles can be, for example, garnet particles, corundum particles, or quartz sand particles. The pressure of the liquid jet at the inlet to the focusing channel section can be in the range of 1000 bar to 6000 bar or more. The liquid jet can be a water jet. Typically, the water jet also contains air, so that a mixture of water, air, and the abrasive particles is formed. The focusing tube can be made of a hard metal. (cemented carbide) or be formed from cermet. The workpiece can be made of a metal.

[0042] Further advantages and expediencies of the invention will become apparent from the following description of exemplary embodiments with reference to the accompanying figures.

[0043] The figures show: Fig. 1: a schematic longitudinal sectional view of a focusing tube according to a first embodiment; Fig. 2: an end view of the focusing tube made of Fig. 1 Fig. 3: a perspective schematic representation of a focusing tube according to a second embodiment; Fig. 4: a schematic interrupted longitudinal section view of the focusing tube made of Fig. 3 Fig. 5: a detailed enlargement of the longitudinal section view from Fig. 4 ; Fig. 6: a diagram in which the wear of a focusing tube as defined in the present disclosure and the wear of a focusing tube used as a reference are each plotted as a function of the operating time.

[0044] Fig. 1 und Fig. 2 Figure 1 schematically shows a focusing tube 1 according to a first embodiment. Based on the longitudinal section view from Fig. 1 It becomes clear how the focusing tapering angle is to be determined in accordance with the present revelation.

[0045] The focusing reduction angle 2 has two legs, which are in Fig. 1 are marked with reference symbols 3 and 4. The focusing reduction angle 2 is in the range of 0.05° to 1° and has only been included for clarity. Fig. 1 The legs 3 and 4 lie in a longitudinal section plane 5, which is aligned with the drawing plane of Fig. 1 coincides. The longitudinal section plane 5 contains a longitudinal axis 6. The longitudinal axis 6 contains a center point 7 of an exit opening 8, as can be seen from a combination of Fig. 1 und Fig. 2 as is evident.

[0046] Since the outlet opening 8 is circular, the center point 7 is the center of a corresponding circle. The longitudinal axis 6 extends towards a focusing channel section 9, which is bounded by a channel wall 11 and extends from the outlet opening 8 into the interior of the focusing tube 1, as shown. Fig. 1 The focusing channel section 9 tapers towards the outlet opening 8, so that a water jet containing abrasive particles and operating at a high pressure of at least 1000 bar is focused to the diameter of the outlet opening 8 when flowing through the focusing channel section 9 in the direction of the outlet opening 8, and thus exits freely in a focused state from the outlet opening 8.

[0047] The longitudinal section plane 5 also contains two points 3a and 4a, which correspond to an inner surface 10 of the channel wall 11 and are connected in the longitudinal section plane 5 by a straight line 12 that is perpendicular to the longitudinal axis 6. The legs 3 and 4 are tangents that lie at points 3a and 4a.

[0048] From the combination of Fig. 1 und 2 It becomes clear that the focusing channel section 9 is shaped like a frustoconical circular cone. The lines of intersection belonging to the inner surface 10 are therefore straight and coincide with the legs or tangents 3 and 4. However, it is conceivable and also possible that the focusing channel section 9 has a different shape, so that the lines of intersection would, for example, be convexly curved inwards.

[0049] Fig. 3 bis 5 Figure 1 shows a focusing tube 1' according to a second embodiment. The focusing tube 1' is constructed analogously to the focusing tube 1. Thus, the focusing tube 1' has a focusing channel section 9' which extends from an outlet opening 8' into the interior of the focusing tube 1' parallel to a longitudinal axis 6', tapers towards the outlet opening 8', and is bounded by a channel wall 11'. The channel wall 11' consists of a sintered hard metal ( cemented carbide ). The channel wall 11' is therefore impermeable to liquids.

[0050] The longitudinal axis 6' contains the center point 8a' of the outlet opening 8'. The longitudinal axis 6' and thus the center point 8' are contained in a longitudinal section plane 5', which with respect to the Fig. 1 und 2 The longitudinal section plane 5 described is positioned analogously.

[0051] Compared to focusing tube 1, focusing tube 1' additionally has an inlet channel section 13', which extends from an inlet opening 14' into the interior of focusing tube 1' and tapers towards a transfer opening 15'. The transfer opening 15' is an internal opening of focusing tube 1' formed jointly with focusing channel section 9'. The transfer opening 15' can be described as an outlet opening 15' of inlet channel section 13' and simultaneously as an inlet opening 15' of focusing channel section 9'. When a water jet containing abrasive particles and operating at a high pressure of at least 1000 bar enters the inlet opening 14' from a mixing chamber in which the abrasive particles have been mixed with the water jet, the water jet flows through the inlet channel section 13'.Because the inlet channel section 13' tapers towards the transfer opening 15' and the inlet channel section 13' has a larger inner diameter outside the transfer opening 15' than the focusing channel section 9', the water jet flow is calmed and the water jet is pre-focused. After the water jet enters the focusing channel section 9' through the transfer opening 15', it is focused within the focusing channel section 9' to the diameter of the outlet opening 8'. This focusing causes the water jet, and thus the abrasive particles, to be accelerated to an exit velocity of at least 400 m / s with respect to exiting the outlet opening 8'.

[0052] Out of Fig. 4 It is particularly evident that the focusing channel section 9' has a focusing narrowing angle 2'. The focusing narrowing angle 2' is, for example, 0.18°. However, other focusing narrowing angles 2' in the range of 0.05° to 1° are conceivable and also possible. The focusing narrowing angle 2' has two legs 3' and 4'. Legs 3' and 4' are tangents lying in the longitudinal section plane 5'. The two legs 3' and 4', or rather the tangents 3' and 4', lie at two points 3a' and 4a' opposite each other in the longitudinal section plane 5' on an inner surface 10' of the channel wall 11'. The focusing reduction angle 2' is constant because the focusing channel section 9' is frustoconical and rotationally symmetric about the longitudinal axis 6'.

[0053] The inlet channel section 13' has an inlet tapering angle 16' defined analogously to the focusing tapering angles 2 and 2'. Thus, the inlet tapering angle 16' has two legs 17' and 18', which lie in the longitudinal section plane 5', because the focusing channel section 9' and the inlet channel section 13' are arranged coaxially. The legs 17' and 18', or rather their tangents 17' and 18', lie at two points 17a' and 18a' opposite each other in the longitudinal section plane 5' on an inner surface 19' of the channel wall 11'. The inlet channel section 13' has a longitudinal axis 6' that coincides with the longitudinal axis 6' of the focusing channel section 9'. The longitudinal axis 6' of the inlet channel section 13' or the focusing channel section 9' contains the center point 20' of the circular inlet opening 14'. The inlet tapering angle is 35°. However, other inlet tapering angles in the range of 10° to 90° are conceivable and also possible.

[0054] The diagram from Fig. 6 The figure shows the percentage diameter increase of an outlet opening of a focusing tube Exp. and a reference focusing tube Ref., denoted as r, as a function of operating hours h. Both focusing tubes Exp. and Ref. were subjected to a water jet containing abrasive particles at 6000 bar with constant jet parameters in the area of ​​their focusing channel section. In focusing tube Exp., the focusing channel section was tapered towards the outlet opening at a focusing angle of 0.18°, analogous to focusing channel section 9'. In contrast, the focusing channel section of focusing tube Ref. had a constant inner diameter, i.e., no tapering towards the outlet opening. Apart from this, focusing tubes Exp. and Ref. were identical. Fig. 6It is evident that the focusing reduction angle of 0.18°, chosen as an example for the range of 0.05° to 1°, ensures that the wear of the focusing tube Exp. is significantly less than the wear of the focusing tube Ref. after an operating time of 40 hours. Thus, the diameter of the outlet opening of the focusing tube Exp. has increased by approximately 16% after 100 operating hours, whereas the diameter of the outlet opening of the focusing tube Ref. has increased by approximately 26% after 100 operating hours.

Claims

1. Focusing tube (1, 1') which is configured for focusing a highly pressurized liquid jet that contains abrasive particles, having a focusing duct portion (9, 9'), an exit opening (8, 8') for the liquid jet to freely exit the focusing duct portion (9, 9'), and a longitudinal axis (6, 6') of the focusing duct portion (9, 9') that contains the centre (7, 8a') of the exit opening (8, 8'), wherein the focusing duct portion (9, 9') is delimited by a liquid-impermeable duct wall (11, 11') and at a focusing taper angle (2, 2') tapers in the direction of the exit opening (8, 8') , wherein the legs (3, 4) of the focusing taper angle (2, 2') are two tangents (3, 4) which lie in a longitudinal sectional plane (5, 5') that contains the longitudinal axis (6, 6') and bear on two internal surface points (3a, 4a, 3a', 4a') of the duct wall (11, 11') that lie opposite one another in the longitudinal sectional plane (5, 5'), wherein the focusing taper angle (2, 2') is in the range from 0.05° to 1°, characterized in that said focusing tube (1, 1') has an inlet duct portion (13'), wherein the inlet duct portion (13') extends from an entry opening (14') for the liquid jet to enter into the focusing tube (1, 1') to a transfer opening (15'), which is formed conjointly with the focusing duct portion (9, 9'), has a longitudinal axis (6') that contains the centre (20) of the entry opening (14'), and outside the transfer opening (15'), in terms of the longitudinal axis (6') thereof in a cross section relating to this longitudinal axis (6'), at each axial position has a maximum diameter which is larger than the maximum diameter of the focusing duct portion (9, 9').

2. Focusing tube (1, 1') according to claim 1, characterized in that the focusing taper angle (2, 2') is in the range from 0.1° to 0.8°.

3. Focusing tube (1, 1') according to one of the preceding claims, characterized in that the focusing duct portion (9, 9'), in terms of the longitudinal axis (6, 6') thereof in a cross section relating to this longitudinal axis (6, 6'), at each axial position has a maximum diameter of 0.5 mm to 5 mm.

4. Focusing tube (1, 1') according to one of the preceding claims, characterized in that the focusing duct portion (9, 9') is configured so as to be rotationally symmetrical about the longitudinal axis (6, 6') thereof.

5. Focusing tube (1, 1') according to one of the preceding claims, characterized in that the focusing duct portion (9, 9') is configured so as to be frustoconical.

6. Focusing tube (1, 1') according to one of the preceding claims, characterized in that the focusing duct portion (9, 9') extends across at least 50% of a length of the focusing tube measured parallel to the longitudinal axis (6, 6') of said focusing duct portion (9, 9').

7. Focusing tube (1, 1') according to claim 6, characterized in that the focusing duct portion (9, 9') extends across at least 70% of the length of the focusing tube.

8. Focusing tube (1, 1') according to one of the preceding claims, characterized in that the longitudinal axis (6, 6') of the focusing duct portion (9, 9') and the longitudinal axis (6') of the inlet duct portion (13') are disposed so as to be mutually coaxial.

9. Focusing tube (1, 1') according to one of the preceding claims, characterized in that the inlet duct portion (13') is delimited by the liquid-impermeable duct wall (11'), tapers in the direction of the transfer opening (15'), and extends at an inlet taper angle (16'), wherein the legs (17', 18') of the inlet taper angle (16') are two tangents (17', 18') which lie in a longitudinal sectional plane (5') that contains the longitudinal axis (6') of the inlet duct portion (15') and bear on two internal surface points (17a', 18a') of the duct wall (11') that lie opposite one another in this longitudinal sectional plane (5'), wherein the inlet taper angle (16') outside the transfer opening (15) is larger than the focusing taper angle (2, 2')10. Focusing tube (1, 1') according to claim 9, characterized in that the inlet taper angle (16') is in the range from 10° and up to 90°.

11. Focusing tube (1, 1') according to claim 10, characterized in that the inlet taper angle (16') is in the range from 27° and up to 37°.

12. Focusing tube (1, 1') according to one of the preceding claims, characterized in that the inlet duct portion (13) transitions in a stepless manner to the transfer opening (15).

13. Focusing tube (1, 1') according to one of the preceding claims, characterized in that a length of the focusing duct portion (9, 9') measured parallel to the longitudinal axis (6, 6') of the focusing duct portion (9, 9') is larger than a length of the inlet duct portion (13') measured parallel to the longitudinal axis (6') of the inlet duct portion (13') by a factor of at least five.

14. Use of a focusing tube (1, 1') according to one of the preceding claims for cutting a workpiece in that a flow of the liquid jet containing abrasive particles passes through the focusing duct portion (9, 9').

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