Method for identifying a geometric error of a multi-axis machine tool and multi-axis machine tool

Abstract

Method for identifying a geometric error of a multi-axis machine tool including a main spindle (11) on which a tool can be attached and a table (6) on which a workpiece is held, which are configured to move relative to each other via two or more translational axes and at least one rotational axis by means of a control device such that the workpiece is machined with the tool, wherein the method is a method for identifying a geometric error with respect to the translational axes and the rotational axis and for correcting a scaling error of the translational axes by means of the control device of the multi-axis machine tool, wherein the method is characterized by the following steps: a measuring step for indexing a position of a measuring gauge (16) which is attached either to the main spindle (11) or the table (6) at positions at several angles around the axis of rotation, and for measuring the position of the measuring gauge (16) at indexed positions using a position measuring sensor which is attached to the other component of the main spindle (11) and the table (6); a step to calculate a geometric error, in order to calculate a geometric error from the measured position of the measuring gauge (16); a step (S1) to calculate an ellipse expression to calculate an ellipse approximation expression by an arc path measured by actuating the axis of rotation, and a step (S3 to S6) to correct an error, to calculate and correct the scaling error of the translation axes from the calculated ellipse approximation expression.

Description

[0001] The present invention relates to a method for identifying a geometric error of a multi-axis machine tool having two or more translation axes and at least one rotation axis, and to a multi-axis machine tool capable of implementing the method.

[0002] In machine tools, for multi-axis machine tools, such as a five-axis machining center that adds two rotary axes to a conventional three-axis machining center, it is desirable to improve accuracy for high-efficiency machining or machining of complex shaped workpieces. Generally, if the number of axes is increased, assembly becomes difficult, and accuracy tends to deteriorate. However, since there is a limit to achieving assembly accuracy, a correction system has been developed to improve accuracy by correcting inclination between adjacent axes or positioning error, i.e., geometric error.

[0003] As a conventional method for identifying geometric errors, multiple measuring instruments, such as a displacement gauge or a rectangular speed square, are used to obtain the geometric error from a measurement result. However, not only does the requirement for multiple measuring instruments pose problems, but uncertainty in the measurement result, such as variations in the reading depending on the measuring skill, also significantly impacts the identification accuracy.

[0004] As a method for identifying a geometric error in a five-axis machining center, a measurement of a simultaneous three-axis arc interpolation motion using a displacement sensor called a ball head is widely known. This method is a method of synchronizing two linear axes and one rotary axis to effect circular motion, thereby maintaining a relative offset between a point on a table and a main spindle, and identifying a geometric error from the amount of center deviation of a obtained circular path. However, since this method requires a specific measuring instrument such as a ball head, and the influence of the measuring instrument adjustment method on the identification accuracy is significant, the measurement cannot be easily performed.

[0005] In addition, the geometric error varies depending on reasons such as the tool attachment state and thermal expansion due to changes in ambient temperature. Therefore, it is desirable to identify a geometric error to perform geometric error correction before performing high-precision machining. However, since the above-described method requires a significant amount of time for measurement, requires a specific measuring instrument, and the identification accuracy depends on the measurement skills of the person performing the measurement, the implementation of the method is difficult.

[0006] Therefore, using a contact probe attached to the machine tool and a sphere as a measurement target, a measurement system is developed based on a principle for identifying a geometric error, which is similar to measuring the accuracy of a simultaneous three-axis arc interpolation motion using a sphere head (Japanese Patent Application Laid-Open No. JP 2011-38902 A (JP-A-2011-38902)). For example, in the case of a five-axis machining center with a rotary table, a process described below is repeated.The process involves installing a target ball on a table, indexing the rotary axes and the axes of a swing motion to positions at multiple angles, repeatedly contacting the contact probe attached to the main spindle with the target ball on the table under various indexing conditions, for example, repeatedly indexing only the rotary axes while the axes of a swing motion are fixed at a predetermined angle, or repeatedly indexing only the axes of a swing motion while the rotary axes are fixed at a predetermined angle, and calculating a center coordinate and a diameter of the ball. The geometric error is identified from the amount of center deviation of the arc trajectory extracted from the measured position of the ball under the multiple indexing conditions.

[0007] A geometric error measurement system is a function for identifying six components of translational error and rotational error in a relationship between adjacent axes, that is, a positional relationship between a specific axis and an adjacent axis among all axes of a machine configuration. However, an individual error, such as a position error, exists in each of the axes, which affects machining accuracy in the case of the linear axis. The position error is measured in advance using a measuring instrument such as a laser length gauge or the like, and is applied to the position error correction.

[0008] As a position detector of a machine tool, a rotary encoder for controlling the rotation of a motor or a linear encoder, which is a direct detector, is used as a device for detecting the position with a high degree of accuracy. These position detectors are made of a material other than cast iron, which is a common component material of the machine tool. Therefore, a correction table is prepared by adjusting the linear expansion coefficients of the essential materials constituting the position detector, obtaining the temperatures of an object at positions close to the positions of their sensors, and measuring the position error with a laser length measuring instrument.However, at the time of measurement for preparing the correction table, an error, generally called a scaling error, is generated between the stored correction table and the actual behavior of the machine if correction coefficients of linear expansion are not set or the positions of the applied object temperature sensors are not appropriate, and if an ambient temperature of the installed machine changes.As a technique for correcting the scale error, Japanese Patent Laid-Open No. 2006-281335 discloses an invention in which an amount of displacement of a scale used for obtaining an amount of displacement of a fixed position of a workpiece on the table is obtained from a product of a temperature for estimating the thermal displacement obtained based on a difference between the scale temperature and the reference temperature, and a difference between a coordinate of a position of a blade tip and a coordinate of a position of a scale detector, and a coefficient of linear expansion of the scale.

[0009] However, since the scaling error not only changes depending on the temperature but also depends on other causes, it is difficult to correct the scaling error only by the temperature information.

[0010] It is accordingly the object of the invention to provide a method for identifying a geometric error for a multi-axis machine tool, which can identify geometric errors of adjacent axes and scaling errors of corresponding translation axes and can correct the errors with a high degree of accuracy, and to provide the multi-axis machine tool.

[0011] The object is achieved by a method having the features of claim 1 and by a multi-axis machine tool having the features of patent claim 8.

[0012] To achieve the above-described object, a first aspect of the invention provides a method for identifying a geometric error related to two or more translation axes and at least one rotation axis and for correcting a scaling error of the translation axes by a control device for a multi-axis machine tool. The multi-axis machine tool has a main spindle to which a tool can be attached and a table on which a workpiece is held. The main spindle and the table are configured to move relative to each other via the translation axes and the rotation axis by the control device, so that the workpiece is machined with the tool.The method has a measuring step of indexing a position of a gauge mounted on either the main spindle or the table to positions at multiple angles around the rotary axis and measuring the position of the gauge at the indexed positions using a position measuring sensor mounted on the other of the main spindle and the table, a geometric error calculating step of calculating a geometric error from the measured position of the gauge, an ellipse expression calculating step of calculating an ellipse approximate expression by an arc trajectory measured by the operation of the rotary axis, and an error correcting step of calculating and correcting the scaling error of the translation axes from the calculated ellipse approximate expression.

[0013] A second aspect of the present invention provides a method according to the first aspect, wherein the step of correcting the error is performed when a difference between a long diameter and a short diameter of the ellipse approximation expression exceeds a predetermined threshold.

[0014] A third aspect of the present invention provides a method according to the second aspect, wherein the threshold is set based on an amount of change per unit length.

[0015] A fourth aspect of the present invention provides the method according to any one of the first to third aspects, wherein correction of the scaling error is expressed by a primary term, and correction of the scaling error is achieved by setting a new position error correction table with the primary term taken into account in a gradient of the position error correction table of the translation axes in the step of correcting the error.

[0016] A fifth aspect of the present invention provides the method according to any one of the first to fourth aspects, wherein the step of correcting an error includes calculating a difference between detected temperatures of temperature sensors provided near the detectors of the respective translation axes and a detected temperature of a temperature sensor provided at an arbitrary reference position, and distributing the correction amount of the scaling error with a ratio of the difference of the detected temperature for the correction.

[0017] A sixth aspect of the present invention provides the method according to the fifth aspect, in which the correction amount of the scaling error is corrected using a linear expansion coefficient of a structure on which the gauge is installed.

[0018] A seventh aspect of the present invention provides the method according to any one of the first to third aspects, wherein the step of correcting an error includes setting the translation axis serving as a reference in advance and considering a difference in scaling error in the remaining translation axes for correction.

[0019] To achieve the above-described object, according to an eighth aspect of the invention, there is provided a multi-axis machine tool including a main spindle to which a tool can be attached and a table on which a workpiece is held, which are configured to move relative to each other via two or more translation axes and at least one rotational axis by a control device so that the workpiece is machined with the tool, wherein the control device can carry out a method for identifying a geometric error according to any one of the first to seventh aspects of the invention.

[0020] According to the invention, not only the identification of the geometric error of the adjacent axes but also the scaling error of the translation axes can be corrected simultaneously, so that machining with even higher accuracy is achieved.

[0021] In particular, since not only the temperature information but also the direct scaling error is identified from the arc trajectory obtained when identifying the geometric error, correction of the scaling error can be realized with a high degree of accuracy. Fig. 1 shows a perspective view of a five-axis controlled machining center. Fig. Figure 2 shows a descriptive view of a long ellipse component due to a scaling error. Fig. 3A and 3B show a top view and a side view of a target ball in a state where it is attached to a table. Fig. 4 shows a flowchart of a scaling error correction process.

[0022] Embodiments of the invention are described with reference to the drawings.

[0023] Fig. Figure 1 shows an example of a multi-axis machine tool, and it shows a perspective view of a five-axis controlled machining center 1, in which an axis configuration sequence from a workpiece to a tool is a workpiece, a C-axis, an A-axis, a Y-axis, an X-axis, a Z-axis, and a tool. The X-axis, the Y-axis, and the Z-axis in Fig. 1 are three orthogonal axes (translation axes), and the Y-axis direction is defined as a forward / backward direction, the X-axis direction is defined as a lateral direction, and the Z-axis direction is defined as a vertical direction.

[0024] Y-axis guides 3 and 3 are formed on an upper surface of a bed 2, and an AC-axis unit 4 having a pin structure is installed on the Y-axis guides 3 and 3 so as to be movable in the Y-axis direction. The AC-axis unit 4 has a frame 5 formed in a wide U-shape in the lateral direction in a front view, and the frame 5 can rotate and tilt about the A-axis parallel to the X-axis direction by an A-axis drive mechanism (not shown) integrated on the left and right sides. The AC-axis unit 4 has a table 6 for placing a workpiece corresponding to a machining object on the upper surface of the stage 5, and the table 6 is rotatable by 360° about the C-axis parallel to the Z-axis by a C-axis drive mechanism (not shown) integrated in the stage 5.

[0025] A crossbar 7 with an inverted U-shape is fixed to the bed 2 to bridge the Y-axis guides 3 and 3, and an X-axis guide section 8 is formed on the front surface of the crossbar 7. A RAM saddle 9 is provided on the X-axis guide section 8 so that it is movable in the X-axis direction. The RAM saddle 9 is provided with a Z-axis guide section 10, and the Z-axis guide section 10 is provided with a main spindle head 12 having a main spindle 11 at its lower end so that it is movable in the Z-axis direction. The RAM saddle 9, the AC axis unit 4 and the main spindle head 12 are configured to be movable by a ball screw installed parallel to guide surfaces of the respective guide sections and a servo motor coupled to the ball screw.An NC device (control device), not shown, is provided on the five-axis controlled machining center 1, and the drive of the respective elements such as the AC axis unit 4 and the main spindle head 12 in the directions of the respective axes is controlled by the NC device.

[0026] The five-axis controlled machining center 1 rotates and rotates the workpiece mounted on the table 6 around the A-axis and the C-axis and moves it in the Y-axis direction. Meanwhile, multi-surface machining is performed on the workpiece by moving the main spindle 11, to which the tool is attached, along the X-axis and the Z-axis.

[0027] Reference numeral 13 denotes a temperature sensor configured to measure an ambient temperature. A temperature sensor 14 is installed near a position where a Y-axis linear scale (not shown) is installed on the bed 2. A temperature sensor 15 is installed on the crossbar 7 near an installation position of the X-axis linear scale (not shown).

[0028] In the five-axis controlled machining center 1 configured as described above, scaling errors ΔX and ΔY exist in the respective axes in an XY plane. It is generally known that an ellipse is obtained according to an expression (1) given below when a circular track with a radius R is drawn as shown in the Fig. 2 is shown. X2(R−ΔX)2+Y2(R−ΔY)2=1

[0029] If a right angle error γ is actually identified using a contact probe as a position measuring sensor and a target ball as a gauge, a target ball 16 is fixed at a position spaced from a rotation center of the table 6 by R, as shown in the Fig. 3. Assuming that the rotation center of the A-axis is located at a center line of the C-axis, in this example, the center coordinates [X0, Y0, Z0, A0, C0] of the target ball 16 are set by Expression (2) given below if an intersection between the center line of the C-axis and the rotation center of the A-axis is specified as a coordinate origin. In Expression (2), H is a distance from the coordinate origin to the center of the target ball 16. [X0Y0Z0A0C0]=[R0H00]

[0030] In the example in the Fig. 2, there are eight measuring points in a measuring range from 0° to 360°, with the indexed pitch being 45°. Therefore, the C-axis is indexed in 45° pitches, and the coordinate of the target ball 16 is measured by a contact probe (not shown) attached to the main spindle each time indexing is performed (measuring step).

[0031] Subsequently, a geometric error is identified from the measured coordinate. Similar to the disclosure of JP 2011-38902 A, the geometric error is identified by approximating multiple measured values ​​of the center positions to an arc and calculating a center position error, a C-axis tilt error, and X-axis and Y-axis tilt errors from a primary or secondary component of the approximated arc (geometric error calculation step).

[0032] Then, the scaling errors of the X-axis and Y-axis are confirmed by using the X-axis coordinates and the Y-axis coordinates [Xi, Yi] from the measured coordinates of the target ball 16. The ellipse expression as Expression (1) can be applied if the error is only the scaling error. However, if the center of the ellipse is assumed to be [Xc, Yc] and the inclination of the ellipse is assumed to be θ, a general expression of the ellipse is actually Expression (3) given below from the geometric error, such as a perpendicularity or position error of the center of the rotation axis. [(Xi−Xc)cosθ+(Yi−Yc)sinθR−ΔX]2+[−(Xi−Xc)sinθ+(Yi−Yc)cosθR−ΔY]2=1

[0033] A coefficient of expression (3) is calculated by a least square method from the measured data [Xi, Yi] to obtain an approximate expression of the ellipse, and a correction of the scaling error is performed. A detailed procedure of the correction is described with reference to a flowchart in the Fig. 4 described.

[0034] In order to determine the scaling errors of the X-axis and the Y-axis, first of all, the expression (3), namely an ellipse approximation expression, is obtained from the measured data of the X-axis coordinate and the Y-axis coordinate (S1, step of calculating an ellipse expression).

[0035] Subsequently, (R - ΔX) and (R - ΔY), which correspond to the obtained long diameter and the obtained short diameter, are compared, and it is determined whether a difference exists and a long ellipse is formed (S2). Based on a threshold value ε [mm / mm] set in advance (the amount of change per unit length), it is determined that the long ellipse component is present when (ε × R) <|ΔX - ΔY| is satisfied.

[0036] If the long ellipse component is present as determined at S2, the respective temperatures T1, T2, and T3 of the corresponding temperature sensors 13 to 15 are obtained (S3). Conversely, if no long ellipse component is present, the process is terminated without performing the scaling error correction.

[0037] Subsequently, the temperature T1 of the temperature sensor 13, which corresponds to an ambient temperature, is set as a reference temperature, and a temperature difference of the X-axis ΔT X = T3 - T1 and a temperature difference of the Y-axis ΔT Y = T2 - T1 are calculated (S4). However, the reference temperature is not limited to the ambient temperature, and temperatures at any position can be used, such as the temperature of the reference temperature sensor of an oil control device, or the temperature of a temperature sensor installed in a position not affected by a hydraulic instrument.

[0038] From the difference of the scaling errors (ΔX - ΔY) the coefficients k x and k yto correct the gradients of the position correction of the respective axes as in expression (4) by weighting the amount of temperature change (S5). [kXkY]=(ΔX−ΔY)R[ΔTXΔTX+ΔTYΔTyΔTX+ΔTY]

[0039] Although the temperature sensor corresponding to the reference temperature is used here, a temperature of the table on which the target ball is installed differs from the reference temperature if cutting water is used, and the temperature difference may affect the measured value R. Therefore, a temperature sensor 17 ( Fig.1) is provided which is configured to measure a temperature of an upper surface of the table 6 on which the target ball is to be installed, and R' in the expression (5) given below is applied for R in the expression (4), where T4 is the table temperature and α is a linear expansion coefficient of the machine, so that correction at a higher level is enabled. R′=R⋅α⋅(T4−T1)

[0040] Finally, correction coefficients are considered in the position error correction tables F(X) and F(Y) as in the following given expression (6), and new position error correction tables F'(X) and F'(Y) are set, thereby correcting the scaling errors (S6, S3 to S6 are steps for correcting an error). [F′(X)F′(Y)]=[kX⋅F(X)kY⋅F(Y)]

[0041] According to a method for identifying a geometric error of the above-described embodiment, not only the identification of the geometric error of the adjacent axes but also the scaling errors of the translation axes can be corrected simultaneously, so that machining with even higher accuracy is achieved.

[0042] In particular, not only the temperature information but also the direct scaling error is identified from the arc trajectory obtained when identifying the geometric error, and correction of the scaling error can be realized with a higher degree of accuracy.

[0043] Although the correction gradient of a position error correction table is distributed according to the temperature difference of the respective sections in the above-described embodiment, the same effect is achieved by a method of setting a linear axis serving as a reference in advance and taking into account the difference in the scaling error on other axes.

[0044] In addition, although the case where the scaling errors of the X-axis and the Y-axis are corrected is described in the above-described embodiment, the C-axis is fixed to a predetermined angle, and then the ellipse approximation expression can be identified in the same manner from the trace of the Y-axis coordinate and the Z-axis coordinate from the measurement result obtained by positioning the A-axis at a plurality of indexing conditions when the scaling errors of the Y-axis and the Z-axis are corrected.

[0045] Furthermore, the invention is also applicable when the arrangements of the target ball and the contact probe are changed between the main spindle and the table.

[0046] The multi-axis machine tool is not limited to the five-axis controlled machining center. If the multi-axis machine tool has two or more translation axes and at least one rotary axis, a configuration in which two or more rotary axes are provided on the main spindle side, or a configuration in which one of the rotary axes is provided on the main spindle side and one on the table side, is also applicable. Therefore, the invention is not limited to the machining center-based machine tool, but a compound machine tool based on a lathe is also applicable.

[0047] As long as the position can be measured in the direction of the translation axes of the table, the invention can be applied not only to the linear scale but also to other detectors such as a rotary encoder.

[0048] It is expressly stated that all features disclosed in the description and / or the claims are intended to be disclosed separately and independently of one another for the purpose of original disclosure and also for the purpose of limiting the claimed invention, regardless of the combination of features in the embodiments and / or the claims. It is expressly stated that all value ranges or indications of subassemblies are intended to disclose all possible intermediate values ​​or intermediate subassemblies for the purpose of original disclosure and also for the purpose of limiting the claimed invention, and in particular, the limits of the value ranges.

Claims

[1] A method for identifying a geometric error of a multi-axis machine tool including a main spindle (11) to which a tool can be attached and a table (6) on which a workpiece is held, which are configured to move relative to each other via two or more translation axes and at least one rotational axis by a control device such that the workpiece is machined with the tool, the method being a method for identifying a geometric error with respect to the translational axes and the rotational axis and for correcting a scaling error of the translational axes by the control device of the multi-axis machine tool, the method being characterized by the following steps: a measuring step for indexing a position of a measuring gauge (16) attached to either the main spindle (11) or the table (6) at positions at multiple angles around the rotation axis, and for measuring the position of the measuring gauge (16) at indexed positions using a position measuring sensor attached to the other of the main spindle (11) and the table (6); a geometric error calculating step for calculating a geometric error from the measured position of the gauge (16); an ellipse expression calculating step (S1) for calculating an ellipse approximation expression by an arc trajectory measured by operating the rotation axis, and an error correcting step (S3 to S6) to calculate and correct the scaling error of the translation axes from the calculated ellipse approximation expression. [2] A method for identifying a geometric error of a multi-axis machine tool according to claim 1, wherein the step (S3 to S6) of correcting an error is carried out when a difference between a long diameter and a short diameter of the ellipse approximation expression exceeds a predetermined threshold. [3] A method for identifying a geometric error of a multi-axis machine tool according to claim 2, wherein the threshold value is set based on an amount of change per unit length. [4] A method for identifying a geometric error of a multi-axis machine tool according to any one of claims 1 to 3, wherein a correction of the scaling error is expressed by a primary term, and a correction of the scaling error is achieved by setting a new position error correction table with the primary term taken into account in a gradient of the position error correction table of the translation axes in the step (S3 to S6) of correcting an error. [5] A method for identifying a geometric error of a multi-axis machine tool according to any one of claims 1 to 4, wherein the step (S3 to S6) of correcting an error comprises calculating a difference between detected temperatures of temperature sensors provided near detectors of the respective translation axes and a detected temperature of a temperature sensor provided at an arbitrary reference position, and distributing a correction amount of the scaling error with a ratio of the difference of the detected temperature for the correction. [6] A method for identifying a geometric error of a multi-axis machine tool according to claim 5, wherein the correction amount of the scaling error is corrected using a linear expansion coefficient of a structure on which the measuring gauge (16) is installed. [7] A method for identifying a geometric error of a multi-axis machine tool according to any one of claims 1 to 3, wherein the step (S3 to S6) of correcting an error comprises setting the translation axis serving as a reference in advance and considering a difference in scaling error in the remaining translation axes for the correction. [8] A multi-axis machine tool including a main spindle (11) to which a tool can be mounted and a table (6) on which a workpiece is held, which are configured to move relative to each other by a control device over two or more translation axes and at least one rotational axis such that the workpiece is machined with the tool, wherein the control device can carry out a method for identifying a geometric error according to any one of claims 1 to 7.

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