METHOD FOR MEASURING A WORKPIECE IN A MACHINE TOOL

Description

FIELD OF TECHNOLOGY

[0001] The present invention relates to a method for calibrating a workpiece and / or its clamping devices in a machine tool. The position of the workpiece's coordinate system relative to the machine tool's coordinate system is determined so that program-controlled machining of the workpiece can begin. STATE OF THE ART

[0002] Aligning a workpiece's coordinate system with a machine tool's coordinate system is becoming increasingly important in CNC-controlled machining. Recording the alignment is particularly important for near-net-shape blanks, such as those produced by investment casting or additive processes, or those that have been machined in previous machining steps using different setups and / or on different machines. It is usually sufficient to know the actual position of the workpiece (in the sense of a deviation from the ideal position as expected by an NC program) relative to the machine tool. This allows the deviation from an ideal position to be taken into account during subsequent machining operations using coordinate transformations during machining using an NC program.

[0003] If the clamping devices used to secure the workpiece in the machining space of the machine tool are to be taken into account, for example for collision monitoring during the machining process, the clamping devices must be considered in the same way as the workpiece. Geometric models of both the workpiece and the clamping device must be available, and their coordinate systems must be aligned with the coordinate system of the machine tool. Since a distinction between clamping device and workpiece is not necessary for the following considerations, the term workpiece will also be used to include the clamping devices of the workpiece, and it is also possible to use the method according to the invention to calibrate a clamping device (without a clamped workpiece). For the sequence of the method, it is completely irrelevant whether a clamping device, a workpiece to be machined, or both together are to be calibrated.

[0004] Modern numerical controls today feature a graphical user interface (GUI) that allows a view into the virtual workspace of a machine tool. For example, a simulation of the workpiece on a table is displayed on a screen, with the machine axes (e.g., linear and rotary axes of a 5-axis machine) represented according to their current state. Even when clamping a workpiece, an operator can compare the virtual representation with the actual setup to ensure that the position of the workpiece (and thus, if applicable, its clamping devices) in the workspace roughly corresponds to the assumptions of the NC program. Small deviations in the individual degrees of freedom can then be measured and taken into account through coordinate transformations when executing an NC program.Often, a degree of freedom is already precisely known from the clamping setup, for example, by a flat surface of the workpiece with which it is placed on the machine table. The position of the workpiece is thus precisely defined perpendicular to the table and with respect to rotations around the directions lying in the table plane, and does not need to be determined by probing.

[0005] The measurement or determination of the position of a workpiece is usually performed by probing the workpiece with a probe, which can, for example, be inserted into the tool holder on the machine tool spindle. To do this, the probe is moved to predefined points on the workpiece using the machine tool's movable axes, and the contact point is recorded. The coordinates of the contact point in a Cartesian coordinate system can be calculated from the machine tool's kinematics, the axis positions at the moment of contact, and the probe geometry.

[0006] For example, the position of a surface of a workpiece can be determined by probing three points. The displacement of the workpiece along this surface is not yet known. By probing differently oriented, even curved, surfaces or general areas of a workpiece, the position of the workpiece can be determined in all six degrees of freedom. These can be the three linear and mutually perpendicular spatial directions, often denoted by X, Y, and Z, and the respective rotations A, B, and C around these spatial directions. Various mathematical methods for determining the position of a body based on measured points are described, for example, in the paper "A Method for Registration of 3D Shapes" by Paul J. Besl and Neil D. McKay (IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol. 14, No. 2, February 1992).

[0007] DE 102008045678 A1 discloses a numerical control system for a machine tool that is configured for the predictive monitoring of mutually movable parts of the machine tool for impending collisions. The monitoring is based on a geometric and kinematic description of the machine tool. Clamping devices for securing a workpiece to a workpiece table are also included in the monitoring system. Clamping device descriptions are stored in the numerical control system. These clamping devices actually present in the machining space of the machine tool are such that they are an integral part of the geometric and kinematic description of the machine tool. Furthermore, probing cycles are stored for each clamping device, which define the probing points for determining the position of the clamping device.However, maintaining such clamping device descriptions, including a probing cycle, represents a certain amount of effort that is not always justified. When calibrating a single workpiece, it would be very time-consuming to generate such data and store it on a numerical control system. Furthermore, if a probing point cannot be used in the specified probing cycle due to local damage, the calibration process fails.

[0008] US 5208763 A discloses, in addition to automated measurement of the position of a workpiece, for which a model must be available, an interactive mode in which a user first selects a surface to be probed in the model, then moves a probe to the corresponding location on the tool and probes it. Once sufficient measurement points have been determined in this way, the actual position is determined for all measurement points by minimizing the squares of the deviations between the assumed position of the workpiece and the measured position. However, this requires considerable experience and knowledge from the operator to initially select suitable points that, after measurement, allow a sufficiently accurate determination of the workpiece's position for all degrees of freedom.

[0009] WO 2016169589 A1 discloses a coordinate measuring machine with a measuring head, which allows a user to perform a computer-assisted measurement sequence on provided model data with acceptable target properties in the form of manufacturing tolerances. The user first manually creates a complete measurement sequence for controlling the measuring head depending on inspection characteristics, which is then optimized with computer support. In this process, the numerical values describing the geometry are modified so that they exhibit a measurement uncertainty and are smaller by a defined amount than the acceptable manufacturing tolerances of the model data.

[0010] DE 10 2019 007348 A1 discloses a measurement program selection support device that supports an operator, for example, in selecting a measurement program during a measurement process with a probe, by providing additional information via a mixed-reality display. For this purpose, the virtual display information is overlaid with image data of the real space. SUMMARY OF THE INVENTION

[0011] It is therefore an object of the invention to provide an improved method for measuring the position of a workpiece, which supports an operator of a machine tool as well as possible in the selection of points to be probed, which requires little or no special prior knowledge from the operator, and which reduces the effort required for measuring compared to the prior art.

[0012] This object is achieved by a method according to claim 1. Advantageous details of this method also emerge from the claims dependent on claim 1.

[0013] A method for measuring a workpiece in a machine tool is disclosed. With this method, an operator of a

[0014] It supports the machine tool in using a probe to calibrate a workpiece and / or a clamping device within the machine tool's workspace so that the model of the workspace displayed on a numerical control screen corresponds sufficiently well to the actual workspace. The operator can move the probe freely around the workpiece and automatically sees selected contact points along with quality information. If the quality is sufficient, the operator can initiate a contact process at the touch of a button and receives immediate feedback on the progress of the calibration process.

[0015] More specifically, a method for measuring a workpiece in the working space of a machine tool with numerical control by means of a probe is disclosed, comprising the steps: 1: Providing 3D models of the workspace, the probe and the workpiece and displaying these models as a virtual workpiece and virtual probe in a virtual workspace on a screen of the numerical control, 2: Positioning the workpiece in the workspace of the machine tool and the virtual workpiece in the virtual workspace so that a position of the workpiece in the workspace and the position of the virtual workpiece in the virtual workspace have an initial match, 3: Manually positioning the probe relative to the workpiece, with automated selection of a planned contact point and a contact direction based on a minimum distance of the virtual probe to the virtual workpiece, and displaying the planned contact point and the contact direction in the virtual workspace, 4: Determining the quality of the planned contact point and releasing the contact if the quality is sufficient, whereby the quality can be, for example,is determined based on the local curvature of the workpiece at the contact point, 5: Initiating the probing with probing enabled and determining the coordinates of the contact point, 6: Recalculating the position of the workpiece using the coordinates of the contact point and updating the position of the virtual workpiece in the virtual workspace, 7: Repeating steps 3 - 6 until a final match between the position of the virtual workpiece in the virtual workspace and the position of the workpiece in the workspace is achieved.

[0016] In step 1, a suitable model must be provided for the clamping situation in the workspace of the machine tool. These models are usually available for the fixed components of the machine tool, such as the table, the tool spindle, and the boundaries of the workspace, and are the same for all machining operations with this machine tool. For the workpiece (and its clamping devices), such models can be obtained from CAD data or via prepared 3D scans and are often available in common formats such as STL or STEP. This describes a workpiece as a polyhedron composed of rectilinear surface elements (usually triangles). The surface elements of the polyhedron have a surface normal, and the corner points have vertex normals, which approximate the normal direction of the real body at this point.Vertex normals are usually part of CAD data, alternatively they can be approximated as the averaged direction of the adjacent surfaces.

[0017] The surface of a polyhedron can be described as a polygonal mesh (also called a mesh). This allows arbitrarily shaped surfaces of a body to be described in such a way that things like surface normals, vertex normals, local curvature of the surface, and distances to other bodies can be calculated efficiently. The mesh—like any real, solid body—should have a closed surface, i.e., be "watertight."

[0018] A model of the probe used must also be available. The probe will usually be a calibrated, touch-trigger probe that emits a signal after contacting the workpiece and a small (known) deflection of the probe ball, after which the positions of all machine axes are read. From this, based on the known machine kinematics and the probe geometry stored, for example, in the tool table, the coordinates of the contact point can be calculated. Using a measuring probe that can output the amount of its deflection is also possible with slightly more computational effort. Instead of a probe ball, other contact bodies are also possible, such as disks or cylinders.

[0019] The provided models, along with the relevant machine tool components, are displayed as simulation graphics on the screen of a numerical control system, thus enabling a virtual view into the machine tool's workspace. Such a display is now common on modern control systems and allows a view of the machining process, for example, when the view in the actual machining area is obstructed by cooling lubricants, or when the control system is positioned in such a way that no view into the workspace is possible.

[0020] In step 2, the position (e.g. the position of the center and the alignment of the workpiece axes) of the workpiece in the workspace must be brought into initial agreement with the position of the virtual workpiece in the virtual workspace to such an extent that it can be meaningfully probed in the subsequent process. To do this, the operator can try to clamp the workpiece in the workspace as it is specified in the virtual workspace, displayed on the screen of the numerical control. Conversely, it is also possible to bring the workpiece into the workspace and then align the virtual workpiece with the real clamping. Using directional keys or other input devices, the virtual workpiece can be moved and rotated on the screen so that it matches reality as closely as possible. A two- or multi-stage process using both methods can also achieve the desired result.For spatial orientation, for example, T-slots on the table of the workpiece machine are used, which allow for rough positioning of the workpiece - real or virtual. Tests have shown that positioning the probe in the work area opposite a prominent point on the workpiece is a useful orientation aid, and then aligning the virtual workpiece accordingly using the controller's directional keys.

[0021] An initial match or accuracy should be achieved, which can be defined, for example, such that no point on the workpiece is more than a few centimeters away from its virtual counterpart. As a rule of thumb, half the dimension of the smallest element of the workpiece to be measured should also be the target for initial match, for example, half the diameter of a hole that is to be probed to calibrate the workpiece.

[0022] If the workpiece is probed further in the process, a certain tolerance range or expected range results within which a probe event should be registered. If a probe event is detected too early or not within the tolerance range later in the process, this indicates an excessive deviation between the actual setup and the virtual representation. An appropriate error message can then be generated, and the initial match can be further improved, meaning a return to step 2.

[0023] In step 3, the operator positions the probe in the workspace. To do this, the probe can be moved within the workspace using directional keys, a handwheel, or other input devices. This movement is also replicated in the virtual workspace and displayed on the control system screen. To reach areas of the workpiece that are not directly accessible, angular axes of the machine can be moved, for example, to tilt the workpiece or the probe. Functions that keep the distance between the tool (in this case, the probe) and the workpiece constant are very useful. Collision monitoring can also be helpful, even if the position of the workpiece is not yet known with optimal accuracy. Collisions between the probe and other machine elements can still be reliably detected because their positions are known at all times.

[0024] Finally, the operator moves the probe close to areas of the workpiece that appear to be suitable for probing. The control system continuously calculates a probing vector from the probe ball to the workpiece. This specifies a probing direction and a (planned) probing point on the workpiece. This calculation is carried out by determining the shortest distance between the mesh of the probe ball and the mesh of the workpiece (i.e. between the virtual images of the probe ball and the workpiece). Very fast numerical methods exist for this calculation (e.g. Bounding Volume Hierarchy) so that the probing vector can be calculated and displayed at short time intervals of, for example, 20 milliseconds. The operator can therefore see on the screen at any time which probing point is currently being targeted and can adjust the positioning of the probe.

[0025] In step 4, the quality of the probe point is continuously calculated, and probing is only enabled if this quality is sufficient. This quality takes into account, for example, measurement accuracy, incorrect probe directions, and the chord error of the modeling. The enclosing surface, its neighboring surfaces, and their vertex and surface normals are known for the probe point.

[0026] For high measurement accuracy, the workpiece must be probed as perpendicularly as possible: edges and excessively curved surfaces must be avoided. To achieve this, the angles between the surface normals near the probe point and the probe vector can be considered; these should always be less than 30 degrees, preferably less than 20 degrees, or even better, less than 10 degrees. The term "near" in this context can be understood as follows: All surface elements located in an area around the probe point whose diameter corresponds to the probe body plus the remaining uncertainty of the workpiece's orientation are considered. Alternatively, the surface elements immediately adjacent to the probed surface element can be considered. Large deviations between the surface normals and the probe vector indicate a highly curved surface or even an edge in the workpiece.The local radius of curvature can be approximated using the surfaces surrounding the probe point and their vertex normals; this radius should be significantly larger than the radius of the probe sphere. Probing is not possible in areas with large curvatures (and thus small radii of curvature) or even edges.

[0027] Even small discrepancies between reality and the model can lead to significant uncertainties in the calculated coordinates. Therefore, the probe points should be located in regions of the workpiece that are modeled as accurately as possible. Since a mesh linearizes freeform surfaces, strong curvatures are subject to a modeling error (chord error), and flat surfaces are preferable. Therefore, probe points in such areas should not be enabled.

[0028] Another criterion for the quality of a contact point is that the virtual probe ball must be located outside the virtual workpiece. If the virtual probe ball intersects the virtual workpiece, or is even located entirely within the virtual workpiece, then the contact direction cannot be determined, and a suitable contact point is definitely not available.

[0029] If a probing vector with sufficient quality is determined for the current position of the probe, consisting of the probing point and direction, this is signaled to the operator. A good method for this is to display the probing vector in red if the quality is insufficient, and in green if the quality is sufficient and probing is therefore permitted. Another method could be to only display a probing vector when the quality is sufficient, or to cross out the probing vector or display a warning next to the probing vector if the quality is insufficient. A check mark next to the probing vector as a sign that quality is sufficient and probing is permitted is also intuitive for the operator.

[0030] Steps 3 and 4 run continuously in a loop, one after the other, or simultaneously or in parallel, whereby the displayed probing vector can change several times between sufficient quality and insufficient quality while the probe is being moved in step 3 - depending on the result in step 4. A probing process does not yet take place.

[0031] This process ends when the operator initiates a probing operation in step 5. This requires that the probing operation is enabled at that moment. The operator can then initiate the probing process along the current probing vector by pressing a button or using another input device. A button with a start function can be found on every numerical control of a machine tool to initiate certain processes depending on the context. This start button is also suitable in this context.

[0032] With probing, the control system can determine the coordinates of the contact point in the conventional way and make them available for further processing. If the contact point is not within the expected range, or if no contact point is registered after a certain distance, the process is aborted with an error message. The match between the real setup and the virtual setup is then not precise enough, and thus the calculation of the quality of a (planned) contact point is no longer reliable. The process should then be restarted from the beginning or from step 2.

[0033] If a probe was triggered in step 5 and the coordinates of the probe point were successfully determined, the additional information obtained is processed in step 6. For this purpose, the position of the virtual workpiece is recalculated to take into account the additional position information obtained from the obtained probe point coordinates. A simple example would be a probe point on a workpiece surface spanned by the Y and Z directions, which is probed in the X direction: Using the X coordinate of the probe point, the position of the virtual workpiece can now be determined much more accurately than by the initial placement of the workpiece "by eye" in step 2. Even with this single probe point, the match between the virtual setup and the real setup is better than the initial match.By probing another point on this surface, offset in the Y direction from the first contact point, the rotation of the workpiece around the Z axis can also be determined. Additional contact points thus further increase the accuracy. It is generally recommended to distribute contact points across the entire workpiece as much as possible to achieve the best possible leverage for rotations.

[0034] In step 6, additional supporting information can also be provided to the operator, which clearly conveys the progress and quality of the entire process.

[0035] Each contact point is permanently displayed in the virtual workspace. The workpiece should then conform to all of the visualized contact points. This allows the operator to identify, for example, deviations between the model and reality. The distance of all contact points from the model can be statistically evaluated; the mean square deviation provides information about the quality. A guideline here is a deviation of approximately 0.5 millimeters, as this uncertainty is ideal for collision monitoring, for example.

[0036] In the style of a traffic light, a color-coded display can also be used to indicate whether sufficient information is available to determine the respective spatial axis directions. For example, if an object is only probed in the X direction, it is not possible to make any statements about the Z component. This can be visualized, for example, by color-coded axis labels with a green X and a red Z. This makes it very easy to identify which probe directions could further improve the calibration process.

[0037] Directions that are already determined by the clamping situation should be automated or locked by the operator and displayed in the progress bar with green color or grayed out, the latter as an indication that this axis direction is not involved in the calibration process.

[0038] In addition, the progression of the specific workpiece position can be displayed in a diagram, either overall or per coordinate. By plotting the position change achieved by the last touch point, you can intuitively determine whether the calibration process is converging. While the first touch points still result in relatively large changes or corrections, subsequent touch points should only result in small changes.

[0039] These indicators (adjustable virtual probing points, color-coded spatial directions, and the progression of corrections obtained for each probing point) show the operator, individually or in combination, whether the probing process is complete or whether further probing is required at other locations. Automated evaluation of these criteria is also possible, so that the achievement of final agreement can be recognized without operator intervention, and the process can be terminated.

[0040] In the final step, 7, a decision is made as to whether the achieved correspondence between the virtual position of the workpiece and the actual position is sufficiently accurate—in other words, whether the final correspondence has been achieved. If this is the case, the workpiece calibration process ends. If this is not yet the case, the process branches to step 3 and searches for another contact point.

[0041] At this point, it should be emphasized again that the term "workpiece" refers to the actual workpiece to be machined and / or a clamping device for it. The case of calibrating a clamping device without a workpiece to be machined is also expressly covered by this description of the method and the claims.

[0042] It has been shown that the method described here supports an operator in a very intuitive way in calibrating a clamping situation in a machine tool using a probe. Unlike conventional methods, there is no need to invest in programming fixed probing cycles. Anomalies in the workpiece, such as damage or markings, can be easily excluded during probing. Calibrating workpieces of any shape is easy if only a model of the workpiece is available. The iterative feedback on the information obtained with each probing point provides the operator with good indications of additional probing points and thus quickly gains experience.

[0043] To summarize very simply but aptly, the automated steps running in the background of the process enable a machine tool operator to complete the complex process of measuring a workpiece by moving a probe around the workpiece. The probe is then executed when a start button is pressed to enable the probe to be touched. Feedback on the information obtained is provided immediately.

[0044] The process makes it easy to measure even unusually shaped workpieces with freeform surfaces or inclined surfaces (e.g., pyramids). By using the machine's rotary axes, even hard-to-reach measurement points can be included without additional computational effort for the operator.

[0045] Further advantages and details of the present invention will become apparent from the following description of various embodiments with reference to the figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Show Figure 1 a machine tool with a numerical control, Figures 2, 3 the alignment of a virtual workpiece in two directions, Figures 4, 5 various permissible and impermissible contact points, Figure 6 a flow chart of the process for measuring a workpiece. DESCRIPTION OF THE EMBODIMENTS

[0047] The Figure 1 shows a view into the workspace A of a machine tool WM. A workpiece WS is arranged on a workpiece table WT. The position of this workpiece WS is to be determined using a probe T by probing the workpiece WS with the probe ball TK. For this purpose, a numerical control (NC) is available, which allows a view into a virtual workspace vA on a screen BS.

[0048] The elements of workspace A that are important for operator B are also represented in the virtual workspace vA: the virtual workpiece table vWT with the virtual workpiece vWS, as well as the virtual probe vT with the virtual probe ball vTK. For this purpose, the required 3D models are provided in the NC control system according to step 1 described above. A start button S is also shown as a central control element for the process of measuring the workpiece WS, which triggers a probing process in step 5.

[0049] In the Figures 2 and 3 The direction keys RT of the numerical control NC are shown, which enable the positioning of the virtual workpiece vWS required in step 2 of the process. By pressing the direction keys of the desired axis direction X, Y, Z, the virtual workpiece vWS can be moved on the virtual workpiece table vWT so that its position corresponds to the real workpiece WS. Figure 2the virtual workpiece vWS is moved in the positive X direction, in the Figure 3 in the positive Z direction in order to bring the virtual workpiece vWS into initial alignment with the real workpiece WS.

[0050] The Figures 4 and 5 show the display on the screen BS during steps 3 and 4. In a display of the virtual probe vT and the virtual workpiece vWS, the (planned) probing point AP and the probing direction AR determined in step 3 are displayed, as well as an indication of the quality of the probing point or the successful release of the probing process, for example by means of hook and lightning symbols, crossed or not crossed arrows, arrows in red or green color or in any other way. Figure 4A permissible contact point AP on a surface of the virtual workpiece vWS is shown, as well as an impermissible contact point AP, which concerns an edge of the virtual workpiece vWS. The Figure 5 Both probe positions shown are not permitted because the virtual probe ball vTK is located within the virtual workpiece vWS or intersects it.

[0051] The Figure 6 shows the procedure described in the general part with its essential steps 1 - 7: 1: Providing 3D models of the workspace A, the probe T and the workpiece WS and displaying these models as a virtual workpiece vWS and a virtual probe vT in a virtual workspace vA on a screen BS of the numerical control NC. 2: Positioning the workpiece WS in the workspace A of the machine tool WM and the virtual workpiece vWS in the virtual workspace vA such that the position of the workpiece WS in the workspace A and the position of the virtual workpiece vWS in the virtual workspace vA initially match. 3: Manually positioning the probe T relative to the workpiece WS, with automated selection of a planned probing point AP and a probing direction AR based on a minimum distance of the virtual probe vT from the virtual workpiece vWS, and displaying the planned probing point AP and the probing direction AR in the virtual workspace vA.4: Determining the quality of the planned contact point AP and enabling the contact if the quality is sufficient, whereby the quality is determined, for example, based on the local curvature of the workpiece WS at the contact point AP. 5: Initiating the contact (by operator B) when probing is enabled, feedback to the user on the progress of the calibration process and determining the coordinates of the contact point AP. If the contact is not triggered yet and the probe T is moved further instead, the process branches back to step 3. 6: Recalculating the position of the workpiece WS using the coordinates of the contact point AP and updating the position of the virtual workpiece vWS in the virtual workspace vA. 7: Repeating steps 3 - 6 until a final match between the position of the virtual workpiece vWS in the virtual workspace vA and the position of the workpiece WS in the workspace A is achieved.

[0052] The numerical control (NC) is configured to run this process in steps 1 to 7, providing optimal support to operator B in his task of calibrating the workpiece WS. During the actual calibration of the workpiece, very simple and intuitive operator interactions are only necessary in steps 3 and 5 to achieve the desired result.

Claims

1. Method for aligning a workpiece (WS) in the working space (A) of a machine tool (WM) having a numerical control means (NC) using a probe (T), comprising the steps of: (1) providing 3D models of the working space (A), the probe (T) and the workpiece (WS) and displaying these models as a virtual workpiece (vWS) and a virtual probe (vT) in a virtual working space (vA) on a screen (BS) of the numerical control means (NC), (2) positioning the workpiece (WS) in the working space (A) of the machine tool (WM) and the virtual workpiece (vWS) in the virtual working space (vA), so that a position of the workpiece (WS) in the working space (A) and the position of the virtual workpiece (vWS) in the virtual working space (vA) initially match, (3) manually positioning the probe (T) relative to the workpiece (WS), thereby automatically selecting a planned probing point (AP) and a probing direction (AR) based on a distance between the virtual probe (vT) and the virtual workpiece (vWS), and displaying the planned probing point (AP) and the probing direction (AR) in the virtual working space (vA), (4) determining a quality of the planned probing point (AP) and enabling probing at a sufficient quality, the quality being determined at least on the basis of the local curvature of the virtual workpiece (vWS) at the planned probing point (AP), (5) triggering the probing by an operator when probing is enabled, providing feedback to the operator on the progress of the aligning process and determining coordinates of the probing point (AP), (6) recalculating the position of the workpiece (WS) using the coordinates of the probing point (AP) and updating the position of the virtual workpiece (vWS) in the virtual working space (vA), (7) repeating steps (3) - (6) until a final match between the position of the virtual workpiece (vWS) in the virtual working space (vA) and the position of the workpiece (WS) in the working space (A) is achieved.

2. Method according to Claim 1, in accordance with which, in step (4), a deviation in the probing direction (AR) from the surface normals of the workpiece (WS) in the vicinity of the probing point (AP) is used for determining the quality, where these deviations may be at most 30°, preferably at most 20°, particularly preferably at most 10°, in order to assess the probing point with sufficient quality.

3. Method according to Claim 1 or 2, in accordance with which, in step (4), probing points (AP) on edges of the workpiece (WS) are excluded from probing and determined with insufficient quality.

4. Method according to any of the preceding claims, in accordance with which, in step (4), an indication of the enabling or non-enabling is displayed graphically on the screen (BS).

5. Method according to any of the preceding claims, in accordance with which, in step (6), information about the match already achieved between the position of the virtual workpiece (vWS) and the workpiece (WS) of the method is additionally displayed on the screen (BS).

6. Method according to Claim 5, in accordance with which the course of the correction of the position of the virtual workpiece (vWS) caused by the respective probing point (AP) is displayed for several of the probing points (AP) already executed.

7. Method according to Claim 5 or 6, in accordance with which each probing point (AP) is permanently displayed in the virtual working space (vA).

8. Numerical control means (NC) for a machine tool (WS), designed for carrying out a method according to any of the preceding claims.