Field of the invention

The invention relates to a machining center having test pieces for an axis compensation of the machining center. The invention further relates to a method for carrying out an axis compensation of a machining center.

Description of the related art

There are various embodiments of machining centers known from prior art. Machining centers are widely used for automatically machining workpieces like slugs or semi-finished products. Due to the required high machining precision machining of very small drilling bits, milling bits and the like is a particularly challenging task for any machining center. Generally, machining centers are configured to provide a relative movement between the workpiece to be machined and a machining tool. For example, grinding a workpiece requires a rotating grinding disc to be moved relative to the workpiece. A machining center, for instance a grinding center usually comprises a rack, a machining table for supporting the workpiece, a spindle for clamping the machining tool and a plurality of slides each being translationally displaceable along a translational axis and/or heads each being rotational about a rotational axis. The machining table, the spindle, the slides and the heads are directly or indirectly supported by the rack which, hence, absorbs static and dynamic forces and dampens both vibrations and temperature variations during machining operations.

The relative movement between the workpiece and the machining tool is generated by individual movements of the components of the machining center relative to each other, namely relative movements of the rack, the machining table, the slides, the heads and the spindle. The components of a machining center define a so called kinematic chain between the workpiece and the machining tool which is characteristic for a specific machining center.

The complete set of relative orientations and/or positions of the translational and rotational axes of a kinematic chain of a machining center is usually referred to as the geometry of the machining center. The geometry of a machining center determines the degrees of freedom of the relative movement between the workpiece and the machining tool and, thus, determines the class of workpieces which can be machined by the machining center.

The translational and rotational axes are defined by corresponding linear and radial/axial bearings, respectively, which are integral with or attached to the slides, the heads and/or the rack of the machining center. In order to move the machining table, the slides and the heads relative to each other, a machining center usually has a plurality of drives each being associated with an axis.

Moreover, the machining center may have a plurality of sensors to detect the actual positions of the components, i.e. the machining table, the slides and the heads of the machining center relative to the corresponding axes.

A position of a component of the machining center relative to the corresponding axis may be indicated by a positive or negative linear or angular distance from a defined zero point of the corresponding axis, where a positive distance indicates a distance in a positive direction and a negative distance indicates a distance in the negative direction which is opposite to the positive direction. In particular, a positive angular direction is defined in the usual way according to the well- known right-hand rule when applied to the respective rotational axis.

The machining center additionally may have a programmable control unit being configured for processing, transferring and storing digital information. In most cases the control unit provides a logical representation of the machining center, i.e. a model of the geometry of the machining center in order to allow for easily programming any necessary movement of a component.

The control unit is logically connected to the drives and sensors of the machining center. Thus, the control unit is able to control the movements of the

components of the machining center by detecting the actual positions of the components and operating the drives for adequately changing these positions.

The control unit may be CNC (Computerized Numerical Control) capable, i.e. may be programmed by so called CNC programs and be able to execute the CNC programs. A CNC program usually allows for coding and, hence, for accurately determining a sequence of movements of the components of the machining center with their respective spatial extension, speed and/or duration in order to provide a required over all movement of the kinematic chain.

Machining centers already have and in future will have to meet ever higher requirements concerning manufacturing precision, availability and economic efficiency in order to guarantee a constantly high rate of workpieces having a high quality. The quality of a workpiece, however, is limited by the precision of the relative movement between the workpiece and the machining tool provided by the machining center. In other words, any geometric deviation of a real machining center from its ideal geometry unavoidably results in corresponding imperfections of the machined workpiece.

Such a geometric deviation of a machining center may be caused by any geometric component variation within allowed manufacturing tolerances thereof and/or by geometric assembly variations allowed by the play of each component connection. Additionally, static and dynamic loads may on the long run result in a further geometric deviation of a machining center from the ideal geometry. With this respect to such geometric deviations any indication, above and below, concerning a relative orientation or position of two axes of a machining center has to be understood in a nominal sense only, i.e. a parallelism of two axes may include small deviations from parallelism and an orthogonality of two axes may include small deviations from orthogonality. In other word parallelism and orthogonality describe the ideal case while the real case may slightly deviate from the ideal case.

Generally, the overall geometric deviation of a multi-axis machining center comprises so called component deviations, axis deviations and multi-axis movement deviations. There are 6 component deviations per axis, 5 axis deviations per rotational axis, 3 axis deviations per translational axis and 3 multi- axis movement deviations. The total number of geometric deviations N of a multi-axis machining center can be calculated by

N = 6 (N _rot + Ntrans) + 5 N _rot + 3 Ntrans + 3, where N _rot is the number of rotational axes and N _tra ns is the number of translational axes of the multi-axis machining center. In order to reduce or exclude a negative impact of the geometric deviations onto the quality of the workpiece the geometric deviations of a machining center may be compensated. Any compensation of geometric deviations of a machining center is commonly referred to as axis compensation.

With respect to the high number of geometric deviations a complete axis compensation would take an enormous amount of time and, hence, be very expensive. In practice, the number of geometric deviations to be compensated has to be drastically reduced by neglecting those deviations which have little impact onto the overall geometric deviation. In other words, only the geometric deviations being most relevant for the overall geometric deviation should be compensated for sake of efficiency. In the following, hence, only selected orientation deviations and position deviations of certain axes of a machining center are considered. The reasons and assumptions for other geometric deviations to be negligible or to be ignored are known to skilled persons or can be readily learnt from the respective technical literature. Setting forth this issue here would go beyond the scope of the disclosure. Apart from that, the present invention is not affected thereby.

There are two alternative ways for correcting geometric deviations of a machining center which eventually may also be applicable in combination, the mechanical axis compensation carried out by adjusting the machining center physically and the logical axis compensation carried out by adjusting the theoretical model of the machining center represented in the control unit thereof.

For a mechanical axis compensation the machining center may comprise one or more mechanical adjusting means each associated with an axis of the machining center. The mechanical adjusting means may be screws to be engaged in defined positions in order to mechanically adjust the position and/or orientation of a slide, a head or a bearing relative to the associated axis very accurately, i.e. in steps of few m° per engagement position.

For a logical axis compensation the geometric model of the machining center represented in the control unit may define one or more compensating parameters each associated with a geometric deviation of an axis of the machining center. The compensating parameters may be set to defined values in order to logically adjust the orientations and/or positions of the model axes to the corresponding physical axes. In other words, the perfect theoretical model of the machining center is adapted to the imperfect real geometry of the machining center. The settings of the adjustment means, for instance the optimal engagement position of an adjusting screw and/or the values of the compensating parameters to best compensate the geometric deviations of the machining center are usually determined by means of a plurality of testing methods which have to be carried out in a defined sequence and at least partly can only be carried out manually. These methods accordingly require a plurality of different test pieces and suitable probes to be carried out. The different test pieces and probes correspond to different methods and/or method steps, respectively, and must be manually mounted on the machining center one after the other to perform all the steps of the testing methods. Some test pieces may have special sensors to detect geometric deviations.

A very well-known and reliable testing method of this type is the standardized so called ballbar testing. The ballbar testing requires a special test piece which has an elongate distance sensor and two bars which are connected to opposite free ends of the distance sensor by means of ball joints. The bars are secured to the machining table and the spindle of the machining center extending collinear with and parallel to the first rotational axis, respectively.

During the testing a circular relative movement of the two bars one about the other with a defined nominally constant radius is provided by the kinematic chain of the machining center both in a clockwise and in a counter-clockwise direction. Any deviation from the defined nominally constant radius is detected by the distance sensor of the test piece. The detected deviations are used to calculate corresponding adjustment parameters. Obviously, the ballbar testing method is hard to carry out automatically. After finishing the testing methods the detected geometric deviations are used to accordingly adjust the adjusting screws and manually set the corresponding compensating parameters in the control unit. After all, carrying out these methods usually takes a long time. As an axis compensation has to be repeated rather often due to continuous static and variable dynamic loads onto the kinematic chain these methods, thus, immediately affect the efficiency of a machining center. Apart from that, by carrying out these methods a required accuracy of axis compensation may not yet be reached resulting in an insufficient quality of the machined workpieces.

Five-axis machining centers define a kinematic chain having five axes each being either translational or rotational. DE 103 43 613 Al discloses a five-axis grinding center especially configured for grinding small bits from a cylindric slug. Another more general five-axis machining center is described by Dassanayake et al. in "A strategy for identifying static deviations in universal spindle head type multi-axis machining centers" (published in International Journal of Machine Tools and Manufacture, Elsevier, Amsterdam, NL, vol. 46, no. 10, 1 August 2006, pages 1097-1106, XP024902514, ISSN 0890-6955, DOI

10.1016/J.IJMACHTOOLS.2005.08.010).

For example, a five-axis machining center may comprise a rack, a first and a second slide being supported by the rack and translationally displaceable along a first translational axis and a second translational axis, respectively, a machining table being supported by the first slide and rotatable about a first rotational axis, a third slide being supported by the second slide and translationally displaceable about a third translational axis, a head being supported by the third slide and rotatable about a second rotational axis and a spindle supported by the head. The spindle may define an additional rotational axis which is conventionally not counted for categorization of machining centers as a rotation of the machining tool is common to any machining center. Accordingly, a five-axis machining center having 3 translational axes and 3 rotational axes (including the rotational spindle axis), has 63 geometric deviations. The present invention relates to compensating 13 geometric deviations thereof. Summary of the invention

The problem to be solved by the invention is to provide a machining center having alternative test pieces for improving the precision of and reducing the time required for an axis compensation of the machining center. A further problem to be solved by the invention is to propose a method for an axis compensation of a machining center which improves the precision of and reduces the time required for the axis compensation.

Solutions of the problem are described in the independent claims. The dependent claims relate to further improvements of the respective invention. A machining center according to an embodiment of the invention, particularly a five-axis grinding center may comprise a control unit for automatically operating the machining center. Furthermore, the machining center may comprise a machining table for supporting a workpiece and a grinding head. The grinding head preferably has a spindle for clamping a machining tool and a first touch probe being attached to the grinding head and electrically connected to the control unit to provide a first touch probe signal to the control unit.

The electric connection between the first touch probe and the control unit may be provided by an interface unit, both the first touch probe and the control unit are electrically connected to. The interface unit is configured for receiving sample signals from the first touch probe corresponding to the tilting angle of the probe tip thereof, calculating output signals according to the requirements of the control unit therefrom and transmitting the calculated output signals to the control unit.

The machining center may further comprise a first test piece which is configured to be mounted on the machining table. According to the invention the first test piece may have a second touch probe being electrically connectable to the control unit to provide a second touch probe signal to the control unit. The second touch probe, thus, is attached to the machining table when the first test piece is mounted on the machining table. In contrast, the grinding machine disclosed by DE 103 43 613 Al has a first test piece being fixed to a workpiece support and a second touch probe being fixed to a workpiece mount of the workpiece support. There the first test piece and the second touch probe areseparate components and are arranged relative to each other at a distance.

The machining center preferably comprises a second test piece being configured to be connectable to the spindle. Providing two test pieces one of which is mountable on the machine table while the other one is connectable to the spindle and two touch probes one of which is secured to the machining table in the mounted state of the first test piece while the other one is attached to the grinding head allows for avoiding the elaborate ballbar testing.

The machining center may define a first rotational axis and a second rotational axis extending perpendicular to the first rotational axis. The machining table may be rotatable about the first rotational axis. The grinding head may be rotatable about the second rotational axis. The spindle defines a rotational spindle axis, the machining tool extending perpendicular to the second rotational axis. This is different from the teaching of DE 103 43 613 Al, where the grinding head of the grinding machine is not rotatable and, hence, there is no second rotational axis both the first rotational axis of the machining table and the rotational spindle axis may extend perpendicular to.

The first touch probe preferably has a probe tip extending parallel to the second rotational axis. The second touch probe has a probe tip extending parallel to the first rotational axis in the mounted state of the first test piece. In other words, each probe tip extends in a direction parallel to the rotation axis of the component the corresponding touch probe is attached to. The probe tip of the first touch probe and/or the probe tip of the second touch probe may be configured as a tilting head and may comprise an elongate pin and a ball wherein the ball is attached to one free end of the pin and the other free end of the pin is configured to be securely fixed in a mounting of the

corresponding touch probe. The ball protects the object to be sampled by the probe tip while the elongate pin generates a torque sufficient to operate a tilting sensor of the touch probe when the probe tip is tilted by an object to be sampled.

The elongate pin preferably comprises a ceramic material and/or the ball is preferably configured as a ruby sphere, e.g. with a diameter in the range from 4 mm to 8 mm, preferably in the range from 5 mm to 7 mm and most preferred of 6 mm. Ceramic materials have both a high bending strength and a high thermal stability. Ruby spheres are very hard and, hence, do not suffer from attrition. Diameters within the indicated regions can reliably eliminate a surface roughness Ra = 0.4 μιη of the faces of the test mandrels as deviations of an evenness of a surface are averaged by the so called Hertzian stress.

The first test piece preferably comprises a first base plate, particularly having first and second rectangular main faces, the first base plate having fastening means for mounting the base plate on the machining table. The first test piece may further comprise a first test mandrel being attached to the first base plate. Preferably, the first test mandrel and the second touch probe protrude perpendicular from the second main face of the first base plate and particularly are disposed equally spaced-apart from and opposite with respect to the first rotational axis in the mounted state of the first test piece. Of course, the machining table according to the invention may also be represented in the form of a dividing head. The first base plate, the first test mandrel and the second touch probe provide a very simple structure of the first test piece which allows for an easy and low-cost manufacture of the first test piece. Due to the eccentrical arrangement of the first test mandrel form and orientation imperfections of the first test mandrel can be averaged by rotating the first piece about the first rotational axis by 180°.

The second test piece may comprise a second base plate, particularly having rectangular main faces and a collet chuck for connecting the second test piece to the spindle. The collet chuck is preferably attached to a first main face of the second base plate. The second test piece may further comprise a second test mandrel being attached to the second base plate and protruding perpendicular from the second main face of the second base plate, particularly being disposed eccentrical with respect to the rotational spindle axis in the mounted state of the second test piece. A collet chuck corresponding to the spindle allows for quickly and easily connecting the second test piece to the spindle. Again, due to the eccentrical arrangement of the second test mandrel form and orientation imperfections of the second test mandrel can be averaged by rotating the second piece about the second rotational axis by 180°.

The second test mandrel and/or the first test mandrel may comprise a case- hardened steel and may be configured as an elongate bar with a rectangular, particularly quadratic cross-section defining a side touch face and a front touch face, preferably configured identical. The first and second test mandrels preferably have a side touch face and/or a front touch face comprising the case- hardened steel. Case-hardened steel can reduce or avoid an attrition of the touch faces of the first and second test mandrels.

Preferably, the first test piece is mounted on the machining table and/or the second test piece is connected to the spindle and/or the second touch probe is electrically connected to the control unit. In this state the five-axis machining center is prepared for an automatic axis compensation according to the invention. The electric connection between the second touch probe and the control unit may be provided by an interface unit both the second touch probe and the control unit are electrically connected to. The interface unit is configured for receiving sample signals from the second touch probe the sample signals corresponding to the tilting angle of the probe tip, calculating output signals according to the requirements of the control unit, wherein the calculated output signals bijectively represent the received input signals, and transmitting the calculated output signals to the control unit.

According to another aspect of the invention a method is provided for carrying out an axis compensation of a machining center. The method may be particularly applied to a machining center according to the invention, the control unit of which defines a plurality of compensating parameters corresponding to geometric deviations of rotational axes and translational axes of the machining center, and which may have a machining table for supporting a workpiece and a grinding head. The grinding head preferably supports a spindle for clamping a machining tool and may have a first touch probe being attached to the grinding head. A first test piece having a first test mandrel configured to be sampled by the first touch probe may be mounted on the machining table.

According to the invention, a second test piece having a second test mandrel may be connected to the spindle. The second test mandrel preferably is configured to be sampled by a second touch probe being comprised by and attached to the first test piece. As a result, both the machining table and the grinding head may have both a test mandrel and a touch probe attached thereto in order to have the test mandrel of the machining table be sampled by the touch probe of the grinding head and have the test mandrel of the grinding head be sampled by the touch probe of the machining table. Due to the reciprocity of the respective test mandrels and touch probes an elaborate ball bar testing can be avoided which after all is hard to automate. The method is preferably applied to a five-axis machining center having a CNC programmable control unit, defining a first translational axis, a second translational axis, a third translational axis, a first rotational axis, a second rotational axis and a rotational spindle axis, wherein each translational axis extends trans- verse, particularly perpendicular to each other translational axis, the first rotational axis and the second rotational axis extend parallel to the first translational axis and the third translational axis, respectively, and the rotational spindle axis extends perpendicular to the second rotational axis, and having mechanical adjusting means associated with the second rotational axis and the rotational spin- die axis, respectively. Five-axis machining centers of this type are approved and widely used.

The method may comprise the steps:

- Preparing the machining center;

- Compensating a plurality of orientation deviations, particularly exactly nine orientation deviations of the rotational axes and the translational axes;

- Compensating a plurality of position deviations, particularly exactly four position deviations of the rotational spindle axis and the translational axes, respectively.

These method steps are at a very high abstraction level and will be described in more detail in the following. At any detail level the steps have to be carried out in the sequence according to the literal sequence, respectively. Dassanayake et al. also describe a calibration method comprising a plurality of orientation deviations and position deviations. These deviations refer to the five-axis machining center disclosed therein, these five-axis machining center, however, have a different relative arrangement of rotational axes and translational axes. Preparing the machining center may comprise the steps:

- Transferring compensation programs to the control unit;

- Setting the compensating parameters to zero;

- Mounting the first test piece on the machining table and electrically connecting the second touch probe to the control unit;

- Determining an orientation deviation and/or position deviation of the first test piece by a first measurement series;

- Connecting the second test piece to the spindle;

- Determining an orientation deviation and/or position deviation of the second test piece by a second measurement series.

For the sake of automation each step of compensation may be coded in a CNC program defining the movement of the kinematic chain required for the respective step of compensation. Preferably, each single step of compensation is coded in a separate program. Before starting the axis compensation any compensating parameter is preferably reset, i.e. set to zero in order to not distort the axis compensation to carry out. After correctly installing the first test piece and the second test piece both test pieces have to be adequately sampled in order to ensure a specified precision of the test pieces. The precision of the test pieces is crucial for the precision of the axis compensation. Compensating a plurality of orientation deviations of the rotational axes and the translational axes may comprise the steps:

- Compensating a deviation from orthogonality of the orientation of the first rotational axis relative to the second translational axis and the third translational axis;

- Compensating a deviation from parallelism of the second rotational axis and the third rotational axis;

- Compensating orientation deviations of the rotational spindle axis; - Compensating a deviation from orthogonality of the second translational axis and the third translational axis;

- Compensation a deviation from orthogonality of the first

translational axis and the second translational axis;

- Compensating a deviation from orthogonality of the first

translational axis and the third translational axis,

wherein the orientation deviations except the orientation deviations of the rotational spindle axis are compensated logically by the control unit, the control unit calculating and setting the corresponding compensating parameters.

Each of the compensation steps is carried out in form of a measurement series. Each measurement series comprises a plurality of measuring points which correspond to defined states of the kinematic chain which are provided by the control unit by means of adequate movements of the components. Each measuring point corresponds to a sampling contact of a probe tip of a touch probe with a touch face of the reciprocal test mandrel. The measured tilting angle detected by the touch probe is transformed by the interface unit and transmitted to the control unit. From the transformed tilting angle the control unit calculates and sets the corresponding compensating parameter which is referred to as a logical compensation.

Compensating the orientation deviations of the rotational spindle axis may comprise the steps:

- Compensating a deviation from orthogonality of the rotational spindle axis and the second rotational axis;

- Compensating a deviation from orthogonality of the rotational spindle axis and the third translational axis;

- Compensating a deviation from parallelism of the rotational spindle axis and the first rotational axis, wherein the orientation deviations related to orthogonality may be compensated mechanically by adjusting the corresponding adjusting means and the

orientation deviation related to parallelism may be compensated logically by the control unit, the control unit calculating and setting the corresponding compensating parameters.

As the mechanical adjustment being carried out when compensating the deviation from orthogonality of the rotational spindle axis and the second rotational axis changes the orientation of the already compensated second rotational axis, the previous compensation step has to be repeated. Compensating a plurality of position deviations of the rotational spindle axis and the translational axes, respectively, may comprise the steps:

- Compensating an offset deviation between the rotational spindle axis and the second rotational axis;

- Compensating the zero point deviations of the translational axes, wherein the offset deviation is compensated logically by the control unit, the control unit calculating and setting the corresponding compensating parameters.

Compensating the zero point deviations of the translational axes may comprise the steps:

- Compensating deviations of the zero points of the third

translational axis and the second translational axis, respectively;

- Compensating a deviation of the zero point of the first translational axis,

wherein the zero point deviations may be compensated logically by the control unit, the control unit calculating and setting the corresponding compensating parameters. Alternatively, the compensation parameters corresponding to the zero point deviations may be manually calculated from the detected tilting angle and entered into the control unit. Preferably a measurement series is carried out automatically by means of a corresponding CNC program. CNC is a well-established and widely used way of coding movements of a kinematic chain of a machining center.

Description of Drawings In the following the invention will be described by way of example, without limitation of the general concept of the invention, on examples of embodiment with reference to the drawings.

Figure 1 shows a perspective view of a five-axis machining center according to an embodiment of the invention; Figure 2 shows a perspective view of a grinding head of the five-axis machining center shown in figure 1;

Figure 3 shows a control unit of the five-axis machining center shown in figure 1 and a second touch probe connected to the control unit via an interface unit; Figure 4 shows a schematic perspective view of the axes of the five-axis

machining center shown in figure 1;

Figure 5 shows a perspective view of a first test piece according to an

embodiment of the invention;

Figure 6 shows a perspective view of a second test piece according to an

embodiment of the invention;

Figure 7 shows a perspective view of orientation deviations of the five-axis machining center shown in figure 1 to be compensated;

Figure 8 shows a perspective view of position deviations of the five-axis

machining center shown in figure 1 to be compensated; and Figure 9 shows a flow diagram of the axis compensation method.

In figures 1 to 4 a preferred embodiment of a machining center 10 according to the invention is shown. The machining center 10 is a five-axis machining center having a rack 11 being part of and supporting further components of the kinematic chain of the machining center 10. However, the invention may be mutatis mutandis applied to any other multi-axis machining center.

In the following, the machining center 10 is described referring to a left-handed Cartesian coordinate system which is space-fixed relative to the rack 11 of the machining center 10. This choice, however, does not apply any restriction to the present invention. Different coordinate systems like a right-handed Cartesian coordinate system, a cylindrical or a spherical coordinate system may be used as well. Essentially, an appropriate coordinate system must only provide three linearly independent directions uniquely determining any position within the three-dimensional space. The Cartesian coordinate system is orientated such that the y axis and the z axis of the coordinate system define a horizontal yz plane while the x axis defines a vertical direction extending perpendicular to the yz plane. Again, this special orientation does not at all restrict the present invention, and different orientations of the Cartesian coordinate system may be used as well. Referring to this coordinate the machining center 10 defines a first rotational axis A extending parallel with the x axis and a second rotational axis C extending parallel with the z axis. Further, the machining center 10 defines three translational axes X, Y, Z extending parallel with the x, y, z axes of the coordinate system, respectively. In other words, each translational axis X, Y, Z extends perpendicular to each other translational axis X, Y, Z, the first rotational axis A and the second rotational axis C extend parallel to the first translational axis X and the third translational axis Z, respectively. Thus, there are five axes altogether. That is why the machining center 10 is referred to as a five-axis machining center.

The machining center 10 comprises a machining table 12 for supporting and fixing a workpiece (not shown) to be machined. The machining table 12 is rotatably supported by a radial/axial bearing defining the rotational axis A of the machining center 10. The radial/axial bearing supporting the machining table 12 is hidden in the figures. The machining center 10 further comprises a Z-slide 13. The Z-slide 13 is supported by and displaceable connected to a linear bearing 14 which defines the translational axis Z and is attached to the rack 11. The Z-slide 13 supports the radial/axial bearing supporting the machining table 12.

The machining center 10 comprises a Y-slide 15. The Y-slide 15 has a frame 16 extending mainly parallel with the x axis, i.e. in a vertical direction and is supported by and displaceable connected to a linear bearing 17 which defines the translational axis Y and is attached to the rack 11. The machining center 10 further comprises an X-slide 18. The X-slide 18 is supported by and displaceable connected to a linear bearing 19 defining the translational axis X and being attached to the frame 16 of the Y-slide 15.

Additionally, the machining center 10 comprises a grinding head 20. The grinding head 20 is rotatably supported by a radial/axial bearing 23. The radial/axial bearing 23 defines the second rotational axis C and is attached to the X slide 18. The grinding head 20 has a spindle 21 clamping a machining tool 22 in the form of a grinding disc. The spindle 21 defines a third rotational axis S, the so called rotational spindle axis S, which extends perpendicular to the rotational axis C.

Furthermore, the grinding head 20 comprises a first touch probe 24 being attached to the grinding head 20. The first touch probe 24 has a probe tip extending 25 parallel to the second rotational axis C and being configured as a tilting head. The first touch probe 24 is electrically connected to the control unit 26 via an interface unit 27 to provide a first touch probe signal to the control unit 26.

The machining center 10 has three linear drives (not shown) being associated with the translational axes X, Y, Z and translationally driving the corresponding X-, Y-, Z-slides along the linear bearings 14, 17, 19, respectively, on the one hand and two rotational drives (not shown) being associated with the rotational axes A, C and rotationally driving the machining table 12 and the grinding head 21 in the radial/axial bearings 23, respectively, on the other hand. One more drive is provided for driving the spindle 21 and the machining tool 22 clamped thereby about the rotational spindle axis S.

The machining center 10 comprises a plurality of sensors (not shown). The sensors are associated to the translational/rotational axes X, Y, Z, A, C of the machining center 10 and configured to detect the linear/angular position of the machining table 12, the Z slide 13, the Y-slide 15, the X-slide 18 and the grinding head 20 along/about the corresponding linear bearing 14, 17, 19 or radial/axial bearing 23, respectively. The machining center 10 additionally has mechanical adjusting means (not shown) associated with the second rotational axis C and the rotational spindle axis S, respectively.

From Figure 4 can be seen that the Z-slide 13 may be moved between translational positions -82 mm and 310 mm of the translational axis Z, the Y- slide 15 may be moved between translational positions -425 mm and 675 mm of the translational axis Y and the X-slide 18 may be moved between translational positions -12 mm and 605 mm of the translational Z axis. The grinding head 21 may be rotated between angular positions -202° and 25° of the rotational axis C while the machining table 12 may be rotated without any angular restriction. Of course, other embodiments according to the invention may show different domains of translational and/or rotational movement. Furthermore, the machining center 10 preferably comprises a CNC- programmable control unit 26 which is connected to the linear and rotational drives and the sensors of the machining center 10. The control unit 26 may define a plurality of compensating parameters corresponding to geometric deviations δφ _ζχ , δφ _γχ , δφ _ΖΥ , 5(p _sz , 5(p _cx , 5(p _SY , δφ _γζ , δφ _χγ , δφ _χζ , ASC _Y , ASA _Y , ASA _Z , ACT _X of rotational axes A, C, S and translational axes X, Y, Z of the machining center 10.

In figure 5 a preferred embodiment of a first test piece 40 according to the invention is shown. The first test piece 40 is configured to be mounted on the machining table 12. The first test piece 40 comprises a first base plate 41 with first and second rectangular main faces 42, 43. The first base plate 41 has fastening means 44 for mounting the first base plate 41 on the machining table 12. The fastening means 44 are formed as six through-holes each corresponding to a female-threaded bore of the machining table 12 and a plurality of pins protruding from the first main face 42 each corresponding to a bore (not shown) of the machining table 12.

The first test piece 40 further comprises a first test mandrel 45 being attached to the base plate 41. The first test piece 40 has a second touch probe 46 which is attached to the first base plate 41. The first test mandrel 45 and the second test probe 46 protrude perpendicular from the second main face 43 of the first base plate 41 and are disposed eccentrical, equally spaced-apart from and opposite to each other with respect to the rotational axis A in the mounted state of the first test piece 40. The second touch probe 46 has a probe tip 47 extending parallel to the first rotational axis A in the mounted state of the first test piece 40 and being configured as a tilting head.

The probe tip 25 of the first touch probe 24 and the probe tip 47 of the second touch probe 46 each comprise an elongate pin 50 and a ball 51. The ball 51 is attached to one free end 52 of the pin 50 and the other free end 53 of the pin is configured to be securely fixed in a mounting of the corresponding touch probe 24, 45.

In figure 6 a preferred embodiment of a second test piece 30 according to the invention is shown. The second test piece 30 is configured to be connectable to the spindle 21 and comprises a second base plate 31 with rectangular main faces 32, 35. The second test piece 30 may further comprise a collet chuck 33 for connecting the second test piece 30 to the spindle 21 of the grinding head 20. The collet chuck 33 may be attached to a first main face 32 of the second base plate 30. The second test piece 30 comprises a second test mandrel 34 being attached to the second base plate 31 and protruding perpendicular from the second main face 35 of the second base plate 31.

The second test mandrel 34 and the first test mandrel 45 are preferably configured identical and made from a case-hardened steel and may be configured as an elongate bar with a quadratic cross-section defining a side touch face 55 and a front touch face 54, respectively.

For carrying out an automatic axis compensation of the machining center 10 according to the invention the machining center may be prepared first. Preparing 100 the machining center 10 may comprise six steps as follows. To start with, compensation CNC programs may be transferred 110 to the control unit 26 and the compensating parameters defined by the control unit are preferably set 120 to zero. In a next step the first test piece 40 may be mounted 130 on the machining table 12, the second touch probe 46 of which is preferably electrically connected to the control unit 26 via the interface unit 27 to provide a second touch probe signal to the control unit 26. Then an orientation and/or position deviation of the first test piece 40 may be determined 140 by a first

measurement series. Hereafter the second test piece 30 may be connected 150 to the spindle 21. Following this, an orientation deviation and/or position deviation of the second test piece 30 may be determined 160 by a second measurement series.

Above and below, a measurement series is to be conceived as comprising a plurality of measuring points corresponding to defined states of the kinematic chain which are provided by the control unit 26 by means of adequate movements of the components 12, 13, 15, 18, 20 relative to each other and relative to the rack 11. Each measuring point corresponds to a sampling contact of a probe tip 25, 47 of a touch probe 24, 46 with a touch face 54, 55 of the reciprocal test mandrel 34, 45.

The measured tilting angle detected by the touch probe 24, 46 is transformed by the interface unit 27. Therefore, the interface unit 27 preferably receives sample signals from a touch probe 24, 46, the sample signals corresponding to the tilting angle of the respective probe tip 25, 47. Then the interface unit 27 may calculate output signals according to the requirements of the control unit 26. Therein, the calculated output signals bijectively represent the received input signals. After that the interface unit 27 may transmit the calculated output signals to the control unit 26.

After the preparation 100 of the machining center 10 has been finished nine orientation deviations δφ _Ζ χ, δφ _γχ , δφ _Ζ γ, 5cp _S z, 5cp _C x, 5cp _S Y, δφ _γζ , δφ _χγ , δφ _χζ of the rotational axes A, C, S and the translational axes X, Y, Z which are shown in figure 7 may be compensated 200.

To start with, deviations δφ _ζχ , δφ _γχ from orthogonality of the orientation of the first rotational axis A relative to the second translational axis Y and the third translational axis Z may be compensated 210 logically by a third measurement series, respectively. Above and below, a logical compensation is to be conceived as being carried out automatically by the control unit 26. Thereby, the control unit 26 preferably calculates and sets a compensating parameter corresponding to a deviation determined by a measurement series. Next a deviation δφ _Ζ γ from parallelism of the second rotational axis C and the third rotational axis Z may be compensated 220 logically by a fourth

measurement series.

Orientation deviations 5cp _S z, 5cp _C x, 5c _S Y of the rotational spindle axis S may be compensated 230 subsequently. In the first instance, a deviation 5cp _S z from orthogonality of the rotational spindle axis S and the second rotational axis C may be compensated 231 manually according to a fifth measurement series.

As the mechanical adjustment being carried out when compensating 231 the deviation from orthogonality of the rotational spindle axis S and the second rotational axis C changes the orientation of the already compensated second rotational axis C, the previous compensation 220 step has to be repeated.

Thereafter a deviation 5cp _C x from orthogonality of the rotational spindle axis S and the third rotational axis Z may be compensated 232 manually according to a sixth measurement series. Above a manual compensation has to be conceived as being carried out manually by adjusting a corresponding mechanical adjusting means of the machining center according to a measurement series.

Compensation 230 of the rotational spindle axis S may be finished by

compensating 233 logically a deviation δφ _5Υ from parallelism of the rotational spindle axis S and the first rotational axis A by a seventh measurement series. After the compensation of the rotational spindle axis S has been finished a deviation δφ _γζ from orthogonality of the second translational axis Y and the third translational axis Z may be compensated 240 logically by an eighth measurement series. Afterwards, a deviation δφ _χγ from orthogonality of the first translational axis X and the second translational axis Y may be compensated 250 logically by a ninth measurement series.

To end up with, a deviation δφ _χζ from orthogonality of the first translational axis X and the third translational axis Z may be compensated 260 logically by an tenth measurement series.

After the compensation 200 of the nine orientation deviations δφ _ζχ , δφ _γχ , δφ _ζγ , δφ _5Ζ , δφ _αχ , δφ _5Υ , δφ _γζ , δφ _χγ , δφ _χζ of the rotational axes A, C, S and the

translational axes X, Y, Z has been finished four position deviations ASC _Y , ASA _Y , ASA _Z , ACTx of the rotational spindle axis S and the translational axes X, Y, Z, respectively, which are shown in figure 8 may be compensated 300.

First, an offset deviation ASC _Y between the rotational spindle axis S and the second rotational axis C may be compensated 310 logically by an eleventh measurement series.

Then the zero point deviations ASA _Y , ASA _Z , ACT _X of the translational axes X, Y, Z, are compensated 320.

Deviations ASA _Y , ASA _Z of the zero points of the third translational axis Z and the second translational axis Y, respectively, may be compensated 321 logically by a twelfth measurement series.

A deviation ACT _X of the zero point of the first translational axis X may be compensated 322 logically by a thirteenth measurement series. Thereby, the automatic axis compensation according to the invention is finished. The proposed method may comprise thirteen measurement series both for preparing the machining center 10 and carrying out the thirteen axis

compensations. Nine measurement series associated with the compensations 210, 220, 233, 240, 250, 260, 310, 321, 322 may be carried out by the control unit 26 automatically and only two compensations being associated with the compensations 231, 232 may be carried out manually. There is no need to convert the machining center 10 during the automatic axis compensation, i.e. the first and second test pieces 30, 40 may be each mounted and unmounted only once, initially and finally, respectively.

Skilled persons understand that both each single step and the sequence of steps to choose for any automatic axis compensation according to the invention have to be adapted to the specific multi-axis machining center on which the method is to be carried out. Particularly, the sequence of steps to choose must obviously satisfy the following conditions:

First, the orientation deviations of the axes have to be compensated in order to ensure orthogonality and parallelism of the axes as defined by the kinematic chain of the multi-axis machining center. The compensation of position deviation of the axes can only carried out when any orientation deviation is compensated. Second, in case a certain compensating step affects an orientation and/or position of an axis already compensated, steps corresponding to the orientation and/or position of the affected axis - and possibly any subsequent step - have to be repeated.

A significant benefit of the machining center 10 having two touch probes 24, 46 and two test mandrels 34, 45 to be reciprocally sampled by the touch probes 24, 46 and the corresponding method for an axis compensation of the machining center 10 is obviously in avoiding the elaborate ballbar testing. Accordingly, the axis compensation can be carried out automatically to a great extent. Only the compensating steps related to the mechanical adjustment means require a manual operation. Apart from that, the control unit (26) can manage the axis compensation on its own. As a result, an enormous amount of time can be saved, particularly as the axis compensation has to be repeated several times in order to reach a required precision and/or at a relative high frequency related to operation cycles of the machining center 10. For instance, the axis compensation according to the invention may require up to 8 times less time than known alternative methods. Apart from that, the automatic axis compensation according to the invention requires two repetitions at maximum while known alternative methods usually require more than two repetitions which further increases the efficiency benefit.

Another benefit results from the high precision of the axis compensation according to the invention. The achieved precision can be simply checked by carrying out a known alternative method after finishing the automatic axis compensation according to the invention. Such a check has shown that the known alternative method cannot at all improve the adjustment parameter settings obtained by the axis compensation according to the invention, i.e. with the already compensated machining center 10 there are substantially no orientation or position deviations left. For example, with a machining center being axis compensated by the method according to the invention an approval workpiece, i.e. a drilling bit or a milling bit can be machined within a

manufacturing tolerance of ± 0,5 μιη related to a relevant complex surface. List of reference numerals

10 machining center

A first rotational axis

C second rotational axis

S rotational spindle axis

X first translational axis

Y second translational axis

Z third translational axis

11 rack

12 machining table

13 Z slide

14 linear bearing

15 Y slide

16 frame

17 linear bearing

18 X slide

19 linear bearing

20 grinding head

21 spindle

22 machining tool

23 radial/axial bearing

24 first touch probe

25 probe tip

26 control unit

27 interface unit

30 second test piece

31 base plate

32 main face

33 collet chuck test mandrel

main face

first test piece

base plate

main face

main face

fastening means

test mandrel

second touch probe

probe tip

pin

ball

free end

free end

front touch face

side touch face

preparing the machining center

transferring compensation programs to the control unit

setting the compensating parameters to zero

mounting the first test piece on the machining table and electrically connecting the second touch probe to the control unit

determining an orientation deviation and/or position deviation of the first test piece by a first measurement series

connecting the second test piece to the spindle

determining an orientation deviation and/or position deviation of the second test piece by a second measurement series

compensating orientation deviations compensating a deviation from orthogonality of the orientation of the first rotational axis relative to the second translational axis and the third translational axis

compensating a deviation from parallelism of the second rotational axis and the third rotational axis

compensating orientation deviations of the rotational spindle axis compensating a deviation from orthogonality of the rotational spindle axis and the second rotational axis

compensating a deviation from orthogonality of the rotational spindle axis and the second rotational axis

compensating a deviation from parallelism of the rotational spindle axis and the first rotational axis

compensating a deviation from orthogonality of the second translational axis and the third translational axis

compensation a deviation from orthogonality of the first translational axis and the second translational axis

compensating a deviation from orthogonality of the first translational axis and the third translational axis

compensating position deviations

compensating an offset deviation between the rotational spindle axis and the second rotational axis

compensating the zero point deviations of the translational axes compensating a deviation of the zero points of the third translational axis and the second translational axis, respectively

compensating a deviation of the zero point of the first translational axis