DE102022105135A1 - Process 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 as a result of jet lag when cutting a workpiece with a liquid or gas jet. In the method, the ray lag is fully compensated in convex sections of the cutting path, but only partially compensated in concave sections. This allows a high processing speed to be achieved with high contour accuracy.

Description

Technical field of application

The present invention relates to a method for reducing contour errors that can occur as a result of jet lag when cutting a workpiece with a liquid or gas jet, in particular in water jet cutting.

Waterjet cutting is a very versatile cutting process. Soft materials are gently cut with the pure high-pressure water jet. Applications in automotive engineering are, for example, cutting headliners, panels or rubber seals. Outside the automotive industry, food or diapers are separated with the pure water jet. The addition of abrasives makes it possible to cut hard materials. Typical materials are steel, non-ferrous metals, ceramics, composites or stone. The cut by the abrasive water jet is very gentle. There are no high temperatures and therefore no temperature-related structural changes. The mechanical stress is very low. Therefore, even very filigree shapes can be produced without bending. With brittle materials, the tendency to develop cracks is reduced compared to other methods such as milling. The most common application for waterjet cutting is the production of components from sheet metal or plates. The 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 against the direction of movement 4, as is shown in 1 is shown schematically. In the left part of 1 the still unprocessed material 6 of the workpiece 1 can be seen, the cut surface 7 produced is indicated in the right-hand part. The beam always runs on the underside of the workpiece 1 and causes contour errors there. The beam lag 5 and thus also the resulting error increases with greater depth and higher cutting speed. Therefore, the following applies: The thicker the starting material and the higher the accuracy requirements, the lower the possible cutting speed.

In addition to the beam lag, there is another effect. The radius of the water jet changes between the jet entry point and 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 jet in the material (similar to that in air). A smaller radius is created via beam reflections within the kerf, leading 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. In principle, the influence of the error caused by the beam lag is significantly greater than that caused by the effect of the radius change.

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 and thus create the desired kerf. The Z-axis is usually used before machining to set the nozzle spacing, the actual cutting movement then takes place via the X and Y axes. With these machines, the nozzle is oriented in the Z direction, i.e. it points normally to the workpiece during processing. It is therefore not possible to influence the beam wake.

More complex water jet machines have additional machine axes for orienting the nozzle. With serial kinematics, two additional axes are installed for this purpose, with parallel kinematics there are three axes. This allows the orientation of the nozzle and thus the jet axis to 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, thus achieving 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, the change in radius by an orientation normal to the direction of movement, as is shown by way of example in 2 is shown.

In this figure, a nozzle 3 inclined in the feed direction 4 is shown as an example in the left part. The inclination or orientation of the nozzle and thus the jet axis of the water jet 2 is set in such a way that the jet lag is compensated for when cutting the workpiece 1, so the jet entry point 8 is above the jet exit point 9, as is shown in FIG 2 can be seen. In the right-hand part of this figure, compensation for the jet conicity is illustrated by a nozzle 3 inclined at an angle to the movement or feed direction 4 . When the nozzle 3 is in a vertical position, a slanted cut surface 10 is produced due to the jet conicity, and a vertical cut is produced with a suitable inclination generated surface 11, as shown in the right part of the figure 2 can be seen.

The goal is to minimize the jet lag as much as possible, since this is responsible for the majority of the form error at the lower edge of the component. The beam lag is proportional to the processing or feed rate. If there is no possibility of nozzle orientation, a low processing speed corresponding to the accuracy specifications must be selected. With the possibility 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 exactly as possible below the entry point of the jet in the workpiece by means of a suitable nozzle orientation in the direction of movement. In the schematic representation of 3 This means moving the beam exit point M ₂ , as obtained with a perpendicular beam orientation, to the point M _2n by a suitable correction angle α, by which the beam orientation is changed relative to the perpendicular.

The angle α can be determined, for example, via a series of tests with different feed rates, possibly in combination with a beam model. The material and the blasting parameters are not changed. When using a suitable jet model, the jet trail 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 for possible deviations when cutting workpieces with a cutting beam. Mathematical prediction models are used that examine the cutting front for several layers of the workpiece to determine which correction angles of the beam axis are required.

The US9658613B2 discloses a device for automated modeling of the cutting process with a jet cutting tool based on empirical data.

The US6996452B2 describes a method for automating the control of jet pointing parameters of a fluid jet to dynamically control the pointing of the jet relative to the material to be cut as a function of velocity and other process parameters. Predictive models are used to determine the orientation parameters.

Compensating for beam lag enables higher cutting speeds, since the beam lag accounts for most of the manufacturing error on the underside of the workpiece. In addition, when jet lag is completely eliminated, the water jet behaves in a similar way to a rigid tool, e.g. 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 beam lag occurs, which must be compensated for by a correspondingly strong compensation. This strong compensation leads to considerable movements of the rotation axes of the machine, 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 take place, the rotary 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 injuries occur.

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

Presentation of the invention

The object is achieved with the method according to patent claim 1. Advantageous configurations of the method are the subject matter of the dependent patent claims or can be found in the following description and the exemplary embodiment.

In the proposed method, first a cutting path, which is traversed in a feed direction when cutting the workpiece with the cutting beam, and parameters required for determining the beam lag, in particular the orientation of the beam axis or nozzle and the feed rate of the cutting beam along the cutting path are 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 way. Convex and concave sections of the cutting path as well as the ray trail are determined. The terms convex and concave refer to the cut workpiece. The convex sections thus represent convex cutting surfaces of the cut workpiece, the concave sections concave cutting surfaces of the cut workpiece. The beam lag is determined on the basis of the feed rate, the cutting depth and the material, as is known from the prior art. This can be done on the basis of a model or also on the basis of experimental preliminary investigations. The orientation of the jet axis (or nozzle) contained in the data provided is then adjusted in the convex sections in such a way that the jet entry point and jet exit point on the workpiece lie one above the other when the workpiece is being cut. In contrast, 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 correspondingly adapted data. The workpiece is then cut with the liquid or gas jet as a cutting jet using this adjusted data, possibly after further processing of this data. 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, ie a plasma jet.

In the proposed method, the rigid coupling between the beam entrance and beam exit is eliminated in concave sections of the cutting path, which leads to high dynamic demands on the blasting machine at high processing speeds. In this context, it was recognized that complete compensation of the beam lag in the concave sections is not necessary at all in order to achieve a high cutting quality. In the proposed method, complete compensation of the jet lag is therefore only carried out in convex sections; in concave sections, the orientation of the jet axis or nozzle is only adjusted such that the jet exit point lies in the kerf. This corresponds to only a partial compensation of the beam lag. In this case, the beam exit location is preferably placed at a point of the kerf that can be reached by 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 be present between the convex and/or concave sections, there is no compensation for the overrun. Since the beam trailing in these straight sections is directed in the opposite direction to the processing direction, when cutting along the straight section, the beam exit point is also automatically on this section and does not have to be corrected. This selection of the orientation of the beam axis results in a lower load on the axes of the blasting machine with a high cut quality, since complete compensation of the beam lag with the associated high axis loads is only required in the concave areas.

With certain geometries, the change between the different compensations (complete compensation of the beam lag versus partial compensation) between the convex and concave sections can result in the beam exit point jumping at these points. In order to avoid this, the orientations of the beam axis obtained according to the above description are preferably additionally checked when changing between the concave and convex sections during data processing. A resulting speed of the beam exit location is calculated for each point of the cutting path in the area of this change, and the orientation of the beam axis is corrected if necessary in order to ensure a speed within a predetermined speed interval. This speed interval is selected on the basis of the speed and acceleration options specified by the blasting machine. A positive minimum speed and a positive maximum speed are thus preferably specified. The points of the cutting path at which the speed is calculated in each case are understood to mean points that are selected according to the desired spatial resolution along the cutting path. These can have a constant spacing or vary in spacing. In the same way, the acceleration can optionally also be correspondingly limited and/or a jerk of the beam exit point can be avoided by appropriate specifications.

In a further advantageous embodiment, the data processing to obtain the specified matched data for each point of the cutting path is also checked whether the selected orientation of the beam axis causes a contour violation on a beam entry side of the workpiece. This type of contour damage can occur above all if the beam axis is sharply inclined in relation 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 on the basis of 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 lag 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 blasting machine used for cutting, with which the machine is then controlled for cutting the workpiece becomes. The adapted data supplied by this data processing module preferably have the same format as the data provided by the CAM system.

The proposed method can be used very advantageously in water jet cutting, but is also suitable for other jet cutting methods in which a liquid or gas jet is used. Exemplary areas of use have already been mentioned 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. Here show:

• 1 a schematic representation of the jet trail during water jet cutting;
• 2 a schematic representation of the compensation of the beam lag 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 lag;
• 4 a schematic representation of the beam lag with a concave geometry and the partial compensation carried out in the proposed method in concave sections;
• 5 a schematic representation to illustrate an injury to the upper edge of the workpiece with a strongly inclined orientation of the beam axis; and
• 6 an example of the integration of data processing according to the proposed method in the data flow of existing systems.

Ways to carry 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 lag is fully compensated only in the convex sections of the cutting path. Deviations only occur to avoid contour damage 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 only make sense in straight cuts, but also in the case of more complex geometries.

This is based on the 4 explained in more detail. This figure shows a concave geometry or a concave section of the cutting path on the left and right, which is cut in the indicated feed direction 4 . In the case of a curved contour, the beam exit point 9 lies on a tangent adjacent to the cutting path at the beam entry point 8, at a distance specified by the beam model (beam trail 5). Compensation according to the prior 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 from 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 during cutting along this concave section in such a way that the jet exit point 9 does not travel along the tangent but on a shorter path to the kerf.

In the present method, the orientation of the beam axis is therefore only set in concave sections of the cutting path in such a way that the beam exit point lies in the cutting kerf or on the cutting path. The rigid coupling between Beam exit point and beam entry point are eliminated, a certain beam lag is expressly allowed. 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 it is often necessary due to external boundary conditions to be able to place the beam exit point in the concave sections at any point of the kerf between the point with the lowest necessary compensation and the beam entry point. The right part of 4 shows an example of several possibilities (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 prior art was carried out. A gear-like component was manufactured with 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. In this test, the two compensation methods produced completely different damage patterns. The prior art compensation method produced significant damage on concave component sections but performed very well on convex sections. In the case of partial compensation, the concave sections were particularly well made, while the convex sections were greatly rounded. The method according to the invention, in which the beam lag is completely compensated (according to the prior art) in the convex sections of the contour and the partial compensation described above takes place in the concave sections, combines the strengths of both methods and eliminates the weaknesses. This meant that the entire component could be manufactured without significant damage. In particular, there is no need to reduce the processing speed in order to reduce damage in the respective areas. A significantly higher processing speed can therefore also be selected in practice.

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 together with the programmed orientation and the process parameters (feed rate, 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: Calculation of the beam orientation

The beam orientation is now adapted for the concave areas according to the new compensation method (partial compensation), in the convex areas by full compensation according to the prior art. With the new compensation method, the beam exit point in this example is drawn into the kerf along the shortest possible route.

Step 3: Adjusting the position of the beam exit point

If you were to run an NC program generated by the first two steps on the water jet machine, this can 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 beam exit point would then accelerate very strongly, and when changing in the opposite direction, decelerate very strongly or even reverse. Due to the calculation method, the beam exit point could also suddenly recede within the concave area. Therefore, the entire cutting path is preferably run through and the resulting speed of the beam exit point is calculated for each point. Correcting the jet exit points ensures that the speed is within a previously defined interval. After this correction step, the beam exit point always moves forward, never falls below the previously defined minimum speed and never exceeds the maximum speed. In addition to the speed, it is also possible to control the acceleration and the jerk of the beam exit point in an analogous manner.

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

As explained in the introduction to the description, the beam has a beam conicity. The jet always widens in the air, while it can also narrow in the component. If the water jet 2 now tilted sharply to the side during the cut in order to draw the beam exit point into the kerf, contour injuries 13 can occur on the upper side of the workpiece 1. These occur when the angle of inclination exceeds the angle of divergence of beam 2, as shown in 5 is indicated schematically.

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

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

The software can preferably offer both options for this. In this way, the user can find the most favorable parameters for his 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 this step, individual information tailored to the respective machine is also added. This can be code for controlling special devices, for water jet machines these are, for example, pumps, valves, abrasive dosing. With this software architecture, the information for the beam compensation is created in the CAM system and output using the machine-independent program.

Another method currently used is to pass a toolpath to the controller without any compensation information. The user then enters the information required for the compensation calculation on the waterjet machine. These are the process parameters used (pressure, abrasive loading, etc.) and the specification of which material is to be cut and in what thickness. With this method, the controller adds the beam compensation information just before or during manufacture. A previously created material database is used for this purpose.

The method according to the invention works according to a different, new 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 shown in 6 is shown. This software layer according to an exemplary embodiment of the present invention is 6 called the compensation layer. The necessary calculation for beam compensation is carried out in this compensation layer and the result is transferred to the post-processor.

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

Reference List

1

workpiece

2

water jet

3

jet

4

Direction of movement/feed direction

5

beam lag

6

unprocessed material

7

cut surface

8th

beam entry point

9

beam exit location

10

oblique cut surface

11

vertical cutting surface

12

Beam exit location after (partial) compensation

13

contour violation

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US 2011/0287692 A1 [0012]
• US 9658613 B2 [0013]
• US 6996452 B2 [0014]

Claims (8)

A method for reducing contour errors that occur as a result of a beam lag (5) when cutting a workpiece (1) with a liquid or gas jet (2), in particular when water jet cutting, in which - a cutting path, which is traversed when cutting the workpiece (1) in a feed direction (4), as well as parameters required for determining the jet lag (5), in particular the orientation of the jet axis and the 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 in relation to the cut workpiece (1) and the beam lag (5) are determined, -- the orientation of the beam axis in the convex sections is adjusted in such a way that the beam entry (8) and beam exit (9, 12) locations on the workpiece (1) lie one above the other when the workpiece (1) is being cut, and -- the orientation of the beam axis in the concave sections is adjusted in such a way that the beam exit point (9, 12) when cutting the workpiece (1) is behind the beam entry point (8) on the cutting path with respect to the feed direction (4) in order to obtain adjusted data received, and - The workpiece (1) is then cut using the adjusted data, optionally after further data processing, with the liquid or gas jet (1). procedure after claim 1 , characterized in that the orientation of the beam axis is adjusted in the concave sections in such a way that the beam exit point (9, 12) is at a point on the cutting path which is determined by a beam exit point ( 9) can be reached via the shortest route. procedure after claim 1 or 2 , characterized in that to obtain the adjusted data after the adjustment of 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 needed to ensure that the speed of the beam exit location (9, 12) is always within a predetermined speed interval. procedure after claim 3 , characterized in that the predefinable speed interval includes a positive minimum value and a positive maximum value of a speed. Procedure according to one of 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 portions or after correcting the orientation of the beam axis according to patent claim 3 it is also checked for several points of the cutting path whether the orientation of the beam axis causes a contour violation on a beam entry side of the workpiece (1), and if a contour violation is detected, the beam entry point and/or the orientation of the beam axis in the concave sections while maintaining the beam exit point (9, 12) is corrected on the cutting path to minimize contour damage. Procedure according to one of Claims 1 until 5 , characterized in that the data with the cutting path and the parameters required for determining the beam lag (5) are provided by a CAM system. procedure after claim 6 , characterized in that the adjusted data are supplied to a post-processor, which generates a control program for a blasting machine used for cutting from the adjusted data, with which the blasting machine is then controlled for cutting the workpiece (1). Computer program product with program code stored on a machine-readable medium for carrying out the data processing of the method according to one or more of the preceding claims in order to generate the adapted data from the data provided when the program code is executed on a data processing device.

Images