DE102022105135B4 - Method for reducing contour errors when cutting with a liquid or gas jet - Google Patents

Abstract

The present invention relates to a method for reducing contour errors that occur due to jet lag when cutting a workpiece with a liquid or gas jet. In the method, the beam wake is completely compensated for in convex sections of the cutting path, but only partially compensated for in concave sections. This makes it possible to achieve high processing speeds with high contour accuracy.

Description

Technical application area

The present invention relates to a method for reducing contour errors that can occur due to jet overtravel when cutting a workpiece with a liquid or gas jet, in particular during water jet cutting.

Water jet cutting is a very versatile cutting process. Soft materials are gently cut with the pure high-pressure water jet. Applications include the cutting of roof linings, panels or rubber seals in automobile construction. Outside the automotive industry, food or diapers are separated using the pure water jet. By adding abrasives, cutting hard materials becomes possible. Typical materials are steel, non-ferrous metals, ceramics, composites or stone. The abrasive water jet cuts very gently. There are no high temperatures and therefore no temperature-related structural changes. The mechanical load is very low. This means that even very delicate shapes can be created without bending. With brittle materials, the tendency for cracks to form is reduced compared to other processes such as milling. The most common application for water jet cutting is the production of components from sheets or plates. The economic efficiency of the process is determined by the cutting speed. Therefore, the ultimate goal is to achieve the shortest possible processing time for a given quality level.

When cutting a workpiece 1 with a water jet, the water jet 2 emerging from the nozzle 3 is deflected counter to the direction of movement 4, as shown in 1 is shown schematically. In the left part of the 1 The still unprocessed material 6 of the workpiece 1 can be seen, and the cut surface 7 created is indicated in the right part. The jet always runs slightly behind on the underside of the workpiece 1 and causes contour errors there. The jet wake 5 and thus also the resulting error increases with greater depth and higher cutting speed. Therefore, the thicker the starting material and the higher the accuracy requirements, the lower the possible cutting speed.

In addition to the jet wake, there is another effect. The radius of the water jet changes between the jet entry point and the jet exit point on the workpiece; there is a jet conicity. The radius of the beam exit point can be larger or smaller than the radius of the beam entry point. A larger radius is explained by a fanning out of the beam in the material (similar to air). A smaller radius is created by beam reflections within the kerf, which lead to beam focusing. Which of these two cases occurs and how strong the effect is depends on the material to be cut and the process parameters of the water jet. Basically, the influence of the error caused by the beam wake is significantly greater than the effect of the change in radius.

Simple water jet machines only have three machine axes for positioning the water jet. These axes move the nozzle in the X, Y and Z directions, creating the desired kerf. The Z axis is usually used to adjust the nozzle distance before machining; the actual cutting movement then takes place via the X and Y axes. On these machines, the nozzle is oriented in the Z direction, so it points normally towards the workpiece during machining. It is therefore not possible to influence the jet wake.

More complex water jet machines have additional machine axes for orienting the nozzle. In serial kinematics, two additional axes are installed for this purpose; in parallel kinematics there are three axes. This means that the orientation of the nozzle and thus the jet axis can be freely adjusted within the limits of the machine kinematics. This also makes oblique cuts (e.g. cones) possible.

The adjustability of the nozzle orientation can be used to compensate for the effects of jet lag and jet taper to achieve higher accuracy. The jet lag is compensated for by a suitable orientation of the nozzle or the jet axis in the direction of movement or feed, and the change in radius is compensated for by an orientation normal to the direction of movement, as exemplified in 2 is shown.

In this figure, a nozzle 3 inclined in the feed direction 4 is shown in the left part as an example. The inclination or orientation of the nozzle and thus the jet axis of the water jet 2 is adjusted in such a way that the jet lag when cutting the workpiece 1 is compensated for, i.e. the jet entry point 8 lies above the jet exit point 9, as shown in FIG 2 can be recognized. In the right part of this figure, compensation for the jet conicity is illustrated by a nozzle 3 inclined obliquely to the direction of movement or advance 4. When the nozzle 3 is in a vertical position, an oblique cut surface 10 is created due to the jet conicity, and a vertical cut is created when the nozzle 3 is in a suitable inclination Surface 11 is generated, as shown in the right partial figure 2 can be recognized.

The aim is, above all, to minimize the jet lag as much as possible, as this is responsible for the majority of the shape errors on the lower edge of the component. The jet tracking is proportional to the processing or feed speed. If there is no possibility of nozzle orientation, a low processing speed must be selected that corresponds to the accuracy specifications. With the option of adjusting the beam orientation, significantly higher speeds can be achieved. This makes the process more economical.

To compensate for the jet lag, the exit point of the jet from the workpiece is placed as precisely as possible below the entry point of the jet into the workpiece by means of a suitable nozzle orientation in the direction of movement. In the schematic representation of the 3 This means moving the beam exit point M ₂ , as obtained with a vertical beam orientation, into the point M _2n through a suitable correction angle α, by which the beam orientation is changed relative to the vertical.

The angle α can be determined, for example, through series of tests with different feed speeds, possibly in combination with a jet model. The material and the beam parameters are not changed. When using a suitable jet model, the jet wake and, if necessary, the jet radius at entry and exit from the workpiece are then obtained for each combination of processing speed and material thickness. The compensation angle α can be derived from this.

The US 2011/0287692 A1 describes a method for automatically determining beam alignment parameters to correct possible deviations when cutting workpieces with a cutting beam. This uses mathematical prediction models that examine the cutting front for multiple layers of the workpiece to determine which beam axis correction angles are required.

The US 9658613 B2 discloses a device for automated modeling of the cutting process with a jet cutting tool based on empirical data.

The US 6996452 B2 describes a method for automating the control of jet alignment parameters of a fluid jet to dynamically control the orientation of the jet relative to the material being cut as a function of velocity and other process parameters. Prediction models are used to determine the orientation parameters.

By compensating for the jet lag, higher cutting speeds are possible because the jet lag accounts for the largest part of the manufacturing error on the underside of the workpiece. In addition, if the jet wake is completely eliminated, the water jet behaves similarly to a rigid tool, such as a milling cutter. Therefore, simple CAM algorithms from milling can continue to be used, only the tool orientation is changed compared to milling.

At high processing speeds, a large jet lag occurs, which must be compensated for by a correspondingly strong compensation. This strong compensation results in significant movements of the machine's rotation axes, which are responsible for the orientation of the nozzle. The dynamic limitations of the rotation axes (speed, acceleration, jerk) can be exceeded. The result is a reduction in processing speed due to the machine control, which was not taken into account in the CAM system. If this reduction does not occur, the rotation axes cannot follow the programmed movement. In both cases, the beam exit point is not at the calculated location because either the compensation is too strong or the machine axes cannot follow due to their inertia. Contour violations occur.

The DE 10 2006 039 246 A1 describes a method for cutting a workpiece with a cutting jet, in which the feed speed of the cutting jet along the cutting line is dynamically varied during a first time period of the cutting process so that the formation of a first groove is prevented, and during the remaining until the end of the cutting process period is essentially maintained.

The DE 10 2012 211 933 A1 discloses a method for tail compensation during beam cutting using a beam cutting device, in which the beam orientation is dynamically adjusted to the cutting direction. The compensation is carried out by changing the beam orientation in a plane spanned by the beam axis around the cutting direction in such a way that the beam entry and exit points lie one above the other.

The object of the present invention is to provide a method for reducing Contour errors when cutting a workpiece with a liquid or gas jet, in particular a water jet, which causes a lower load on the axes of the jet machine used and enables high contour accuracy.

Presentation of the invention

The task is solved using the method according to claim 1. Advantageous embodiments of the method are the subject of the dependent patent claims or can be found in the following description and the exemplary embodiment.

In the proposed method, a cutting path is first taken into account, which is traversed in a feed direction when cutting the workpiece with the cutting jet, as well as parameters required for determining the jet tracking, which include at least the orientation of the jet axis or nozzle and the feed speed of the cutting jet along the cutting path. provided as data. This data is preferably generated and provided in a conventional manner by a CAM system (CAM: computer-aided manufacturing). The data can be in a machine-independent format, such as CLDATA or NCI. This data is then further processed in the proposed method in the following manner. Convex and concave sections of the cutting path as well as the beam wake are determined. The terms convex and concave refer to the cut workpiece. The convex sections therefore represent convex cut surfaces of the cut workpiece, the concave sections represent concave cut surfaces of the cut workpiece. The jet tracking is determined on the basis of the feed speed, the cutting depth and the material, as is known from the prior art. This can be done on the basis of a model or on the basis of preliminary experimental studies. The orientation of the jet axis (or nozzle) contained in the data provided is then adjusted in the convex sections such that the jet entry and jet exit locations on the workpiece lie one above the other when the workpiece is cut. In contrast to this, the orientation of the beam axis in the concave sections is adjusted in such a way that the beam exit point when cutting the workpiece is on the cutting path, but not below the beam entry point, but behind the beam entry point with respect to the feed direction. This data processing results in appropriately adapted data. The workpiece is then cut using this adapted data, if necessary after further processing of this data, with the liquid or gas jet as a cutting jet. The liquid or gas jet is preferably a water jet or an abrasive water jet that contains additional abrasive elements. However, other liquid jets or gas jets can also be used for cutting, for example an ionized gas jet, i.e. a plasma jet.

In the proposed method, the rigid coupling between beam entry and beam exit is eliminated in concave sections of the cutting path, which leads to high dynamic demands on the blast machine at high processing speeds. It was recognized that complete compensation of the jet wake in the concave sections is not necessary to achieve high cutting quality. In the proposed method, complete compensation of the jet wake is therefore only carried out in convex sections; in concave sections, the orientation of the jet axis or nozzle is only adjusted so that the jet exit location lies in the kerf. This only corresponds to partial compensation of the beam wake. The beam exit location is preferably placed at a point on the kerf that can be reached via the shortest path starting from a beam exit location that is present when the beam axis is oriented vertically. In the straight-line areas that may exist between the convex and/or concave sections, there is no compensation for the caster. Since the jet trail in these rectilinear sections is directed in the opposite direction to the processing direction, when a cut is made along the rectilinear section, the jet exit point is automatically on this path and does not need to be corrected. This choice of orientation of the jet axis results in a lower load on the axes of the jet machine with high cutting quality, since complete compensation of the jet wake with the associated high axis loads is only required in the concave areas.

With certain geometries, the change between the different compensations (full compensation of the beam wake versus partial compensation) between the convex and concave sections can cause the beam exit point to jump at these points. In order to avoid this, the orientations of the beam axis obtained according to the previous description are preferably additionally checked when changing between the concave and convex sections during data processing. A resulting velocity of the beam exit location is calculated for each point of the cutting path in the area of these changes and the orientation of the beam axis is corrected as necessary to ensure a speed within a predetermined speed interval. This speed interval is selected based on the speed and acceleration options specified by the jet machine. A positive minimum speed and a positive maximum speed are preferably specified. The points of the cutting path at which the speed is calculated are understood to be points that are selected according to the desired spatial resolution along the cutting path. These can have a constant distance or can vary in distance. In the same way, the acceleration can optionally be limited accordingly and/or a jerk of the jet exit point can be avoided by appropriate specifications.

In a further advantageous embodiment, during data processing to obtain the adapted data for each point of the cutting path, it is also checked whether the selected orientation of the beam axis causes a contour violation on a beam entry side of the workpiece. Such a contour violation can occur especially if the beam axis is strongly inclined compared to the perpendicular orientation to the workpiece surface. The detection of a contour violation requires knowledge of the conicity of the beam, which can be determined based on a geometric beam model. If a contour violation is detected, the beam entry point is corrected while maintaining the beam exit point in the kerf in order to minimize the contour violation. Alternatively or additionally, the orientation of the beam axis can also be corrected in the concave sections while maintaining the beam exit location in the kerf.

In the proposed method, the data with the cutting path is preferably provided together with the parameters required for determining the beam wake by a CAM system. The data provided by this CAM system is then processed in a data processing module according to the proposed method and preferably in turn fed to a postprocessor, which then uses the adapted data to generate a control program for a jet machine used for cutting, with which the machine is then controlled for cutting the workpiece becomes. The customized data provided by this data processing module preferably has the same format as the data provided by the CAM system.

The proposed method can be used very advantageously for water jet cutting, but is also suitable for other jet cutting methods in which a liquid or gas jet is used. Examples of areas of application have already been listed in the introduction to the description.

Brief description of the drawings

• 1 eine schematische Darstellung des Strahlnachlaufs beim Wasserstrahlschneiden;
• 2 eine schematische Darstellung der Kompensation des Strahlnachlaufs und der Konizität gemäß dem Stand der Technik;
• 3 eine schematische Darstellung einer beispielhaften Strahlgeometrie und eines Korrekturwinkels für eine vollständige Korrektur des Strahlnachlaufs;
• 4 eine schematische Darstellung des Strahlnachlaufs bei konkaver Geometrie und der beim vorgeschlagenen Verfahren durchgeführten Teilkompensation in konkaven Abschnitten;
• 5 eine schematische Darstellung zur Veranschaulichung einer Verletzung der Oberkante des Werkstücks bei stark geneigter Orientierung der Strahlachse; und
• 6 ein Beispiel für die Einbindung der Datenverarbeitung gemäß dem vorgeschlagenen Verfahren in den Datenfluss bestehender Systeme.

The proposed method is explained in more detail below using an exemplary embodiment in conjunction with the drawings. Show here:

• 1 a schematic representation of the jet wake during waterjet cutting;
• 2 a schematic representation of the compensation of the beam wake and the taper according to the prior art;
• 3 a schematic representation of an exemplary beam geometry and a correction angle for a complete correction of the beam wake;
• 4 a schematic representation of the beam wake with concave geometry and the partial compensation in concave sections carried out in the proposed method;
• 5 a schematic representation to illustrate an injury to the upper edge of the workpiece when the beam axis is strongly inclined; and
• 6 an example of integrating data processing according to the proposed method into the data flow of existing systems.

Ways of carrying out the invention

The 1 until 3 have already been explained in more detail in the introduction to the description. In the proposed method, the compensation of the beam wake is only fully compensated in the convex sections of the cutting path. Deviations only occur to avoid contour violations or due to the limitations of the blasting machine used for cutting. In the remaining sections, there is no compensation of the beam wake in straight sections and only partial compensation in concave sections. The inventor recognized that full compensation does not make sense not only in straight cuts, but also in more complex geometries.

This is done using the 4 explained in more detail. This figure shows a concave geometry or a concave section of the left and right Cutting path that is cut in the indicated feed direction 4. With a curved contour, the beam exit point 9 lies on a tangent on the cutting path at the beam entry point 8, at a distance predetermined by the beam model (beam wake 5). Compensation according to the current state of the art now means pulling the beam exit point 9 below the beam entry point 8. However, it can be seen from the figure that the distance of the jet exit point 9 to the kerf or cutting path is much shorter than to the jet entry point 8. Therefore, according to the present invention, the nozzle or jet axis is oriented when cutting along this concave section in such a way that the jet exit point 9 does not move along the tangent but on a shorter path to the kerf.

In the present method, the orientation of the beam axis is therefore only adjusted in concave sections of the cutting path so that the beam exit point lies in the kerf or on the cutting path. The rigid coupling between the beam exit point and the beam entry point is eliminated and a certain beam tracking is expressly permitted. So there is only partial compensation.

Preferably, the beam exit point is drawn to the kerf along the shortest path. This is advantageous because it requires the least amount of compensation. However, in practice, due to external boundary conditions, it is often necessary to be able to place the beam exit point in the concave sections at any point on the kerf between the point with the least necessary compensation and the beam entry point. The right part of the 4 shows several options as an example (beam exit points 12). The reasons for this are explained in more detail below.

The advantages of the partial compensation according to the invention only in the concave sections could be validated experimentally. For this purpose, a direct comparison of the partial compensation described and the complete compensation of the lag according to the state of the art was carried out. A gear-like component was manufactured at a very high processing speed in order to test the limits of the compensation methods. With partial compensation, the beam exit point for the entire cutting path was drawn into the kerf along the shortest path. The two compensation methods produced completely different damage patterns in this test. The prior art compensation method produced significant damage on concave component sections, but performed very well on convex sections. With partial compensation, the concave sections were manufactured particularly well, while the convex sections were heavily rounded. The method according to the invention, in which complete compensation of the beam wake occurs in the convex sections of the contour (according to the prior art) and the partial compensation described above in the concave sections, combines the strengths of both methods and eliminates the weaknesses. This meant that the entire component could be manufactured without any significant damage. In particular, there is no need to reduce the processing speed in order to reduce damage in the respective areas. In practice, a significantly higher processing speed can also be selected.

Different, partly optional steps of data processing according to the proposed method are explained below using an exemplary embodiment with a water jet, which can be carried out with a software program.

Step 1: Division into convex and concave areas

The software receives the cutting path along with the programmed orientation and the process parameters (feed speed, pressure, etc.) from a CAM system. The software reads the cutting path from the NC program and divides it into convex and concave areas. This is done through a geometry analysis.

Step 2: Calculating the beam orientation

Now the beam orientation is adjusted for the concave areas using the new compensation method (partial compensation), and in the convex areas by complete compensation according to the state of the art. With the new compensation method, the beam exit point in this example is drawn into the kerf along the shortest path.

Step 3: Adjust the position of the beam exit point

If you were to execute an NC program generated by the first two steps on the water jet machine, this could lead to an unsatisfactory result. By switching between the two compensation methods, the beam exit point may “jump” at these points. When changing from concave to convex, the jet exit point would accelerate very sharply, and when changing the other way around, it would slow down very sharply or even recede. Even within the concave area, due to the calculation method, the beam exit point could suddenly recede men. Therefore, the entire cutting path is preferably traversed and the resulting speed of the beam exit point is calculated for each point. By correcting the jet exit points, it is now ensured that the speed is within a predetermined interval. After this correction step, the beam exit point always moves forward, never falls below the predetermined minimum speed and never exceeds the maximum speed. It is also possible to control not only the speed but also the acceleration and jerk of the jet exit point in an analogous manner.

Step 4: Correction of component injuries on the top of the workpiece

As explained in the introduction to the description, the beam has a beam conicity. The jet always expands in the air, but it can also narrow in the component. If the water jet 2 is now tilted strongly to the side during the cut in order to pull the jet exit point into the kerf, contour injuries 13 can occur on the top of the workpiece 1. These occur when the tilt angle exceeds the expansion angle of the beam 2, as shown in 5 is indicated schematically.

One solution involves then recalculating the position of the programmed path point (beam entry point). The path point is moved away from the workpiece in order to avoid contour violation.

In a second approach, the beam exit point in the concave areas is not moved into the kerf along the shortest path, but is corrected to another point in the kerf that is closer to the beam entry point. This reduces the contour violations on the top and also the necessary adjustments to the dynamics of the jet exit point.

The software can preferably offer both options for this. This allows the user to find the most favorable parameters for their process.

In the manufacturing systems used today, the tool path or cutting path is calculated within a so-called CAM system (CAM - computer aided manufacturing). This outputs the production program in a general, technology- and machine-independent format such as CLDATA or NCI. The programs are only translated into the NC language of the respective machine in the postprocessor. In addition, individual information tailored to the respective machine is added in this step. This can be code for controlling special devices, for example in water jet machines these are pumps, valves, abrasive dosing. With this software architecture, the information for beam compensation is created in the CAM system and output using the machine-independent program.

Another method currently used is to pass a tool path to the controller without compensation information. The user then enters the information needed for the compensation calculation on the water jet machine. These are the process parameters used (pressure, abrasive load, etc.) and the information about which material should be cut and in what thickness. With this method, the controller adds the beam compensation information shortly before or during production. A previously created material database is used for this purpose.

The method according to the invention works according to a different, novel principle. Here the traditional software chain CAD system -> CAM system -> postprocessor -> machine control between CAM system and postprocessor is broken and another software layer is added, as in 6 is shown. This software layer according to an exemplary embodiment of the present invention is described in 6 referred to as a compensation layer. In this compensation layer, the necessary calculation for beam compensation is carried out and the result is passed on to the postprocessor.

This has several advantages for the user. First of all, the interface between CAM system and post processor is well defined via formats such as CLDATA or NCI. If the software module used for data processing according to the present method reads, for example, a program in CLDATA format, modifies it and finally outputs it again in CLDATA format, integration into existing systems is possible without any problems. Future updates can be carried out particularly easily. The existing software systems can continue to be used. So no training is necessary for a new CAD or CAM system.

Reference symbol list

1

workpiece

2

water jet

3

jet

4

Direction of movement/feed direction

5

Jet tracking

6

unprocessed material

7

cutting surface

8th

Beam entry location

9

Beam exit location

10

oblique cutting surface

11

vertical cutting surface

12

Beam exit location after (partial) compensation

13

Contour violation

Claims (8)

Method for reducing contour errors that occur due to a jet lag (5) when cutting a workpiece (1) with a liquid or gas jet (2), in particular when water jet cutting - a cutting path which is traversed in a feed direction (4) when cutting the workpiece (1), as well as parameters required for determining the jet wake (5), which include at least an orientation of the jet axis and a feed speed of the liquid or gas jet (2). along the cutting path, as data is provided, - in the data -- convex and concave sections of the cutting path are determined with respect to the cut workpiece (1) as well as the beam trail (5), -- the orientation of the beam axis in the convex sections is adjusted in such a way that the beam entry (8) and beam exit locations (9, 12) on the workpiece (1) lie one above the other when the workpiece (1) is cut, and -- the orientation of the beam axis in the concave sections is adjusted in such a way that the beam exit location (9, 12) lies behind the beam entry location (8) on the cutting path when cutting the workpiece (1) with respect to the feed direction (4) in order to provide adapted data received, and - The workpiece (1) is then cut with the liquid or gas jet (1) using the adapted data, if necessary after further data processing. Procedure according to Claim 1 , characterized in that the orientation of the beam axis in the concave sections is adjusted such that the beam exit location (9, 12) lies at a point on the cutting path that is determined by a beam exit location (5) determined without adjusting the orientation of the beam axis due to the beam wake (5). 9) can be reached via the shortest route. Procedure according to Claim 1 or 2 , characterized in that in order to obtain the adapted data after adjusting the orientation of the beam axis in the convex and concave sections when changing between concave and convex sections, a resulting speed of the beam exit location (9, 12) is additionally calculated for several points of the cutting path and the Orientation of the beam axis is corrected as necessary to ensure that the speed of the beam exit location (9, 12) is always within a predeterminable speed interval. Procedure according to Claim 3 , characterized in that the predeterminable speed interval comprises a positive minimum value and a positive maximum value of a speed. Procedure according to one of the Claims 1 until 4 , characterized in that for obtaining the adjusted data after adjusting the orientation of the beam axis in the convex and concave sections or after correcting the orientation of the beam axis according to Patent claim 3 In addition, it is checked for several points of the cutting path whether the orientation of the beam axis creates a contour violation on a beam entry side of the workpiece (1), and if a contour violation is detected, the beam entry location and/or in the concave sections the orientation of the beam axis while maintaining the beam exit location (9, 12) is corrected on the cutting path to minimize contour violation. Procedure according to one of the Claims 1 until 5 , characterized in that the data with the cutting path and the parameters required for determining the beam wake (5) are provided by a CAM system. Procedure according to Claim 6 , characterized in that the adapted data are fed to a postprocessor, which generates a control program from the adapted data for a blasting machine used for cutting, with which the blasting machine is then controlled to cut the workpiece (1). Computer program product with program code stored on a machine-readable carrier for carrying out the data processing of the method according to one or more of the preceding claims in order to create the adapted data from the data provided generate when the program code is executed on a data processing device.

Images