Accuracy stabilization device and accuracy stabilization method for machine tool

Abstract

To maintain processing accuracy, and suppress reduction of processing efficiency caused by measurement whose necessity is low.SOLUTION: An accuracy stabilization device 20 comprises: a temperature measurement part 21 for measuring by temperature sensors 11-17, a temperature of a prescribed part; an estimated thermal displacement amount calculation part 22 for calculating an estimated thermal displacement amount, on the basis of the temperature measured by the temperature measurement part 21; an accuracy affection degree calculation part 23 for calculating an accuracy affection degree which is a degree of an affection to accuracy of a machining center due to thermal displacement, on the basis of temperatures on a plurality of portions measured by the temperature measurement part 21; a measurement determination part 25 for determining necessity of measurement of an actual thermal displacement amount of the machining center, according to the accuracy affection degree and a preset measurement determination reference; an actual measurement thermal displacement amount measurement part 26 for measuring an actual measurement thermal displacement amount when it is determined that measurement of the actual measurement thermal displacement amount by the measurement determination part 25 is necessary; and a determination reference change part 27 for changing the measurement determination reference on the measurement determination part 25, according to an error amount between the estimated thermal displacement amount and the actual measurement thermal displacement amount.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to an apparatus and method for stabilizing accuracy of a machine tool, which predicts situations in which thermal displacement compensation errors in the machine tool will increase and stabilizes accuracy by performing measurements under such situations. [Background technology]

[0002] When machining using a machine tool, the thermal displacement of the machine due to room temperature changes, thermal displacement due to heat generated by the spindle, and changes over time cause changes in the shape and positional relationship of the tool and workpiece, resulting in a deterioration in the machining accuracy of the workpiece. Therefore, as a method for suppressing the thermal displacement of the machine tool, a thermal displacement compensation method is widely used in which temperature sensors are attached to various parts of the machine tool structure, the amount of displacement is calculated based on a preset thermal displacement model from the measured temperature, and the amount of axis movement is changed accordingly. However, there is a limit to the accuracy of the thermal displacement compensation, and errors occur when the temperature change is large. In particular, when the room temperature changes suddenly, such as when starting up an air conditioner, the error in the thermal displacement compensation may become large. Therefore, it is desirable to detect the timing when the error in the thermal displacement compensation becomes large. In addition, in order to prevent the deterioration of machining accuracy, for example, it is possible to calibrate the machine tool by measuring the actual amount of thermal displacement and setting the origin according to the amount of displacement. Patent document 1 discloses an invention in which an adjustment value for adjusting the thermal displacement correction amount is calculated using the thermal displacement correction amount and the actual thermal displacement amount, and the need for measurement is determined based on the changing state of the adjustment value. Patent Document 2 discloses an invention that predicts in real time the effect of changes in environmental temperature on the accuracy of a machine tool and appropriately diagnoses conditions that will cause large thermal displacement. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6285396 [Patent Document 2] JP 2019-136846 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, the invention of Patent Document 1 is a feedback method that performs processing and measurement in a fixed pattern, obtains the change pattern of the adjustment value for each processing, and determines the measurement timing from the change pattern. Therefore, in high-mix low-volume production, the processing and processing environment of the fixed pattern are not necessarily repeated, and there is a problem that it is difficult to determine the necessity of measurement. Furthermore, the invention of Patent Document 2 predicts the risk of poor machining accuracy due to thermal displacement in response to temperature differences or temperature changes at specific locations of the machine tool. However, in reality, when thermal displacement can be accurately predicted based on a preset thermal displacement model, there may be cases where there is no risk of poor machining accuracy. In such cases, if the user determines that there is a risk of poor machining accuracy and suspends machining or calibrates the machine, there is a problem of reduced productivity.

[0005] Therefore, an object of the present disclosure is to provide an accuracy stabilization device and an accuracy stabilization method for a machine tool that can measure actual thermal displacement at an appropriate time, while maintaining machining accuracy and suppressing a decrease in machining efficiency due to unnecessary measurements. [Means for solving the problem]

[0006] In order to achieve the above object, a first configuration of the present disclosure includes a temperature measuring means for measuring a temperature of a predetermined portion of a machine tool; and an estimated thermal displacement amount calculation means for calculating an estimated thermal displacement amount based on the temperature measured by the temperature measurement means, and the accuracy stabilization device corrects thermal displacement by issuing a position command for the feed axis of the machine tool so as to cancel out the estimated thermal displacement amount calculated by the estimated thermal displacement amount calculation means, thereby stabilizing the machining accuracy of the machine tool. The accuracy stabilizing device further comprises: an accuracy influence calculation means for calculating an accuracy influence level, which is a degree of influence of thermal displacement on the accuracy of the machine tool, based on the temperatures of the plurality of portions measured by the temperature measurement means; a measurement determination means for determining the necessity of measuring the actual thermal displacement amount of the machine tool based on the accuracy influence degree and a preset measurement determination criterion; an actual thermal displacement measuring means for measuring the actual thermal displacement when the measurement determining means determines that it is necessary to measure the actual thermal displacement; and a determination criterion changing means for changing the measurement determination criterion in the measurement determination means in accordance with an amount of error between the estimated thermal displacement amount and the actually measured thermal displacement amount. Another aspect of the first configuration is characterized in that, in the above configuration, the accuracy influence calculation means calculates the accuracy influence based on a temperature change rate of each of the portions. Another aspect of the first configuration is that, in the above configuration, the accuracy influence degree indicates the degree of influence of expansion / contraction of each of the parts due to thermal displacement, change in straightness, and geometric error of the feed axis, which affect the positional accuracy and / or inclination of the cutting edge in the feed axis direction of a tool attached to the machine tool, and the accuracy influence degree calculation means calculates at least one of the accuracy influence degrees. Another aspect of the first configuration is characterized in that, in the above configuration, the measurement judgment criterion is an accuracy impact standard value, which is a standard value set for each of the accuracy impact degrees, and the measurement judgment means judges that measurement is necessary when at least one of the multiple accuracy impact degrees exceeds the accuracy impact standard value. Another aspect of the first configuration is characterized in that, in the above configuration, the judgment criterion change means changes the measurement judgment criterion depending on the amount of error in at least one measurement by the actual thermal displacement measurement means. Another aspect of the first configuration is characterized in that, in the above configuration, the machining origin correction means further comprises a machining origin correction means for correcting a machining origin in accordance with an amount of error between the estimated thermal displacement amount and the actually measured thermal displacement amount. In order to achieve the above object, a second configuration of the present disclosure is an accuracy stabilization method that calculates an estimated thermal displacement amount estimated from the temperature of a specified part of a machine tool, and corrects the thermal displacement by issuing a position command for the feed axis of the machine tool so as to cancel out the estimated thermal displacement amount, thereby stabilizing the machining accuracy of the machine tool. The accuracy stabilization method includes a temperature measurement step of measuring temperatures of the plurality of portions; an estimated thermal change amount calculation step of calculating the estimated thermal change amount based on the temperatures of the respective portions measured in the temperature measurement step; an accuracy influence calculation step of calculating an accuracy influence degree, which is a degree of influence of thermal displacement on accuracy of the machine tool, based on the temperature measured in the temperature measurement step; a measurement judgment step of judging the necessity of measuring the actual thermal displacement amount of the machine tool based on the accuracy influence degree and a preset measurement judgment criterion; an actual thermal displacement measurement step of measuring the actual thermal displacement when it is determined in the measurement determination step that it is necessary to measure the actual thermal displacement; and changing the measurement judgment criterion in the measurement judgment step according to an amount of error between the estimated thermal displacement amount and the actually measured thermal displacement amount. Effect of the Invention

[0007] According to the present disclosure, while performing temperature measurement, it is possible to monitor the places that affect the thermal displacement in real time, and to diagnose the environment in which the difference between the estimated thermal displacement amount and the measured thermal displacement amount becomes large. As a result, when it is assumed that the accuracy is stable due to the thermal displacement correction, the judgment criteria for the measurement of the measured thermal displacement amount are relaxed to reduce the measurement frequency, and when the prediction of the thermal displacement is wrong and the accuracy is unstable even after the thermal displacement correction, the judgment criteria for the measurement of the measured thermal displacement amount are strengthened to increase the measurement frequency, so that it is possible to omit the thermal displacement measurement that is less necessary in a situation where the thermal displacement is easily predicted. Therefore, it is possible to reduce the lost time due to interruption of processing. In addition, since the measured thermal displacement amount can be grasped, it is possible to perform the calibration of the machine tool, such as resetting the processing origin, and it is possible to stabilize the processing accuracy. In other words, it is possible to perform the measurement of the measured thermal displacement amount at an appropriate timing, and it is possible to suppress the decrease in processing efficiency due to the measurement that is less necessary while maintaining the processing accuracy. [Brief description of the drawings]

[0008] [Figure 1] 1 is an explanation showing an example of the configuration of a machining center and locations where temperature sensors are installed. [Diagram 2] FIG. 2 is a configuration diagram of an accuracy stabilizing device. [Diagram 3] 1 is a flowchart of an accuracy stabilization method. [Figure 4] 13 is a flowchart showing an example of a process of changing a judgment criterion in a judgment criterion changing unit; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. FIG. 1 is a schematic diagram of a machining center M having three mutually orthogonal translation axes, which is an example of a machine tool. In the machining center M, the spindle head 2 is supported via a saddle 5 on the front of the column 4, and is capable of translational movement with two degrees of freedom relative to the bed 1 by the X-axis and Z-axis, which are translational axes and perpendicular to each other. The table 3 is capable of translational movement with one degree of freedom relative to the bed 1 by the Y-axis, which is a translational axis and perpendicular to the X-axis and Z-axis. Therefore, the spindle head 2 is capable of translational movement with three degrees of freedom relative to the table 3. The operation of the XYZ axes is performed by servo motor drive controlled by a numerical control device (not shown). A workpiece is fixed to the table 3, a tool is attached to the spindle of the spindle head 2 and rotated, and the relative position and relative attitude of the workpiece and the tool are controlled to machine the workpiece. The number of axes of the machine tool according to the present disclosure is not limited to three, but may be four or five. Furthermore, the table 3 or the spindle head 2 may have one or more degrees of freedom of rotation due to a rotating axis. The machine tool is not limited to a machining center, but may be a lathe or a grinding machine.

[0010] Temperature sensors 11-17 are attached to the machining center M. The temperature sensors 11-17 measure the temperature inside the machine or the surrounding environment, making it possible to predict the effect of temperature changes on deformation of the machining center M. Temperature sensor 11 measures the temperature of the spindle head 2, temperature sensor 12 measures the temperature of the saddle 5, temperature sensors 13 and 14 measure the temperatures before and after the column 4, temperature sensor 15 measures the temperature of the table 3, temperature sensor 16 measures the temperature of the bed 1, and temperature sensor 17 measures the environmental temperature around the bed 1. Note that the locations and number of temperature sensors and the combination of temperature measurement locations are not limited to this example.

[0011] FIG. 2 is a configuration diagram showing an example of the accuracy stabilizing device 20 of the machining center M. The accuracy stabilization device 20 is incorporated in a numerical control device and includes a temperature measurement unit 21, an estimated thermal displacement calculation unit 22, an accuracy influence calculation unit 23, a judgment criterion memory unit 24, a measurement judgment unit 25, an actual thermal displacement measurement unit 26, a judgment criterion change unit 27, and a processing origin correction unit 28. The temperature measuring unit 21 is composed of temperature sensors 11 to 17 installed in various parts. The temperature measuring unit 21 is an example of the temperature measuring means of the present disclosure. The estimated thermal displacement amount calculation unit 22 calculates an estimated thermal displacement amount of the relative positional relationship between the cutting edge position of the tool and the workpiece based on a preset thermal displacement model equation from the measurement results of the temperature sensors 11-16 in the temperature measurement unit 21 of the machining center M. This thermal displacement model equation may be changed as appropriate depending on the environment. In addition, the configuration of the temperature sensor used to calculate the estimated thermal displacement amount is not limited to this example. The estimated thermal displacement amount calculation unit 22 is an example of an estimated thermal displacement amount calculation means of the present disclosure.

[0012] The accuracy influence calculation unit 23 calculates the accuracy influence caused by the deformation of each element of the machining center M that affects the machining accuracy, using the measurement results of the temperature sensors 13-17 in the temperature measurement unit 21. The accuracy influence indicates the degree of influence on the expansion / contraction of each part due to thermal displacement, the change in straightness, and the geometric error of the XYZ axes, which affect the positional accuracy and / or inclination of each axial direction of the cutting edge of the tool attached to the spindle of the machining center M. 2, for example, accuracy influence degree A due to the inclination of column 4 is calculated from temperature sensors 13 and 14. Accuracy influence degree B based on the temperature rate change of the portion of table 3 where the temperature is not directly measured due to the temperature difference between the ambient environment temperature and table 3 is calculated from temperature sensors 15 and 17. Accuracy influence degree C based on the temperature rate change of the portion of bed 1 where the temperature is not directly measured due to the temperature difference between the ambient environment temperature and bed 1 is calculated from temperature sensors 16 and 17. The number and configuration of these accuracy influence degrees are not limited to this example. Accuracy influence degree calculation unit 23 is an example of an accuracy influence degree calculation means of the present disclosure.

[0013] In the judgment criteria storage unit 24, accuracy influence criteria values ​​A to C (examples of measurement judgment criteria) which are reference values ​​for the accuracy influence A, the accuracy influence B, and the accuracy influence C, respectively, are set. The measurement determination unit 25 determines the necessity of measuring the actual thermal displacement when at least one accuracy influence degree exceeds an accuracy influence degree reference value. The measurement determination unit 25 is an example of the measurement determination means of the present disclosure. The actual thermal displacement measuring unit 26 measures the actual thermal displacement by a method such as measuring a reference position with a touch probe attached to the spindle, measuring the spindle position coordinates when measuring a reference tool with a touch sensor, etc. The actual thermal displacement measuring unit 26 is an example of the actual thermal displacement measuring means of the present disclosure. The judgment criterion changing unit 27 calculates the difference between the estimated thermal displacement calculated by the estimated thermal displacement calculation unit 22 and the actual thermal displacement measured by the actual thermal displacement measurement unit 26, and changes the reference value of the accuracy influence degree according to the value of this error amount. The judgment criterion changing unit 27 is an example of the judgment criterion changing means of the present disclosure. The machining origin correction unit 28 changes the work origin, which is the reference position for workpiece machining, by the amount of error between the estimated thermal displacement amount and the actually measured thermal displacement amount. By using the changed work origin as a new reference, it is possible to cancel the error in the relative positional relationship between the tool tip position and the workpiece after correction due to the estimated thermal displacement amount. The machining origin correction unit 28 is an example of the machining origin correction means of the present disclosure.

[0014] 3 is a flowchart of the accuracy stabilizing method executed by the accuracy stabilizing device 20. This process is executed by a pre-stored program. First, in step (hereinafter referred to as "S") A0, when it is confirmed that a predetermined time has elapsed, the temperature of each part of the machine tool is measured in SA1 (temperature measurement step). That is, this is performed using at least one or more of the temperature sensors 11 to 17 in the temperature measurement unit 21. In SA2, the accuracy influence calculation unit 23 calculates the accuracy influence based on each temperature, the temperature change rate, and the temperature difference (accuracy influence calculation step). In SA3, estimated thermal change amount calculation section 22 uses at least one of temperature sensors 11 to 17 to calculate an estimated thermal change amount based on the set thermal change model equation (estimated thermal change amount calculation step). In SA4, the measurement decision unit 25 decides whether or not it is necessary to measure the actual thermal displacement based on the calculated accuracy influence degree and the accuracy influence degree reference value stored in the judgment criterion storage unit 24 (measurement decision step). If all accuracy influence degrees are smaller than the accuracy influence degree reference value, it is decided that the influence on the machining accuracy is small and that it is not necessary to measure the actual thermal displacement amount, and the previous machine operation is continued until a predetermined time has passed. On the other hand, if at least one accuracy influence degree is larger than the accuracy influence degree reference value, it is decided that the influence on the machining accuracy is large and that it is necessary to measure the actual thermal displacement amount, and the process proceeds to SA5.

[0015] In SA5, the actual thermal displacement amount is measured in the actual thermal displacement amount measuring unit 26 (actual thermal displacement amount measuring step). In SA6, the difference between the estimated thermal change amount calculated in SA3 and the actual thermal change amount measured in SA5 is calculated as an error amount. In SA7, it is determined whether the amount of error calculated in SA6 falls within a preset reference range. If the absolute value of the amount of error does not fall within the reference range, in SA8, the judgment criterion changing unit 27 changes the accuracy influence reference value, which is the measurement judgment criterion stored in the judgment criterion storage unit 24, so as to lower it. This increases the frequency with which SA4 judges that measurement is necessary until the amount of error falls within the preset reference range. An example of changing the measurement judgment criterion will be described later. In SA7, if the absolute value of the amount of error falls within the reference range, in SA9, the judgment criterion changing unit 27 changes the measurement judgment criterion stored in the judgment criterion storage unit 24 so as to increase the accuracy influence reference value. This reduces the frequency with which it is determined that measurement is necessary in SA4. Examples of changing the measurement judgment criterion will be described later (SA6 to SA9: judgment criterion changing steps).

[0016] On the other hand, after the processing of SA6, in SA10, the workpiece origin is changed in the machining origin correction unit 28 based on the amount of error calculated in SA6. This makes it possible to cancel the error corresponding to the amount of deformation of the machining center M. Subsequent calculations of the estimated amount of thermal displacement by the estimated amount of thermal displacement calculation unit 22 are performed based on the temperature and position at the time of actual measurement. In this way, when machining is performed by the machining center M, the accuracy stabilizing device 20 issues a feed axis position command to the numerical control device so as to cancel the estimated thermal displacement calculated by the estimated thermal displacement calculation unit 22. Therefore, in the machining center M, it is possible to stabilize the machining accuracy by correcting the thermal displacement. In this case, if it is assumed that accuracy has been stabilized by thermal displacement compensation, the measurement criteria for the actual thermal displacement are relaxed, and the measurement frequency is reduced. On the other hand, if the thermal displacement prediction is incorrect and accuracy is unstable even after thermal displacement compensation, the measurement criteria for the actual thermal displacement are strengthened, and the measurement frequency is increased. This makes it possible to eliminate thermal displacement measurements that are not necessary, and reduce lost time due to interruptions to processing.

[0017] Fig. 4 shows an example of a process for changing the accuracy influence reference value, which is the measurement judgment criterion in SA7 to SA9 in Fig. 3. Each step will be described below. In SB1, the error amount, which is the difference between the estimated thermal displacement amount calculated by the estimated thermal displacement amount calculation unit 22 and the actual thermal displacement amount measured by the actual thermal displacement amount measurement unit 26, is defined as |E| (absolute value), and the reference value of the error amount (maximum allowable error amount) is defined as |E ref |(absolute value), and the error amount |E| is the error standard value |E ref Determine whether it exceeds |. Here, the error amount |E| is the error standard value |E ref If it exceeds |, the value of accuracy impact A in SB2 is I A is the value I of the accuracy impact standard value A Aref It is to be noted that the accuracy influence value is a positive value, and the greater the amount of change in temperature from the reference temperature, the temperature change rate, or the temperature difference, the greater the value. Here I A I Aref If the accuracy impact standard value (I Aref ) new is set based on the following formula: α is an approximate coefficient of the influence of the accuracy influence degree A on the thermal displacement amount, and is set in advance. Details regarding the setting of the change amount when updating the accuracy influence degree reference value will be described later.

[0018]

number

[0019] SB3 accuracy impact standard value (I Aref ) new After setting, or in SB2, the value of accuracy impact A I A is the value I of the accuracy impact standard value A Aref If it does not exceed the value of I B is the value of the accuracy impact standard value B, I Bref It is determined whether or not the value exceeds the threshold value. Here I B I Bref If the accuracy impact standard value for the new accuracy impact B (I Bref ) new is set based on the following formula: β is an approximate coefficient of the influence of the accuracy influence degree B on the thermal displacement amount, and is set in advance.

[0020]

number

[0021] Accuracy impact standard value (I Bref ) new After setting, or in SB4, the value of accuracy impact B I B is the value of the accuracy impact standard value B, I Bref If it does not exceed the value of I C is the value of the accuracy impact standard value C, I Cref It is determined whether or not the value exceeds the threshold value. Here I C I Cref If the accuracy impact C exceeds the threshold, SB7 sets the accuracy impact standard value (I Cref ) new is set based on the following formula: γ is an approximate coefficient of the influence of the accuracy influence degree C on the amount of error, and is set in advance.

[0022]

number

[0023] On the other hand, in SB1, the error amount |E| is the error standard value |E ref If the accuracy impact A value I A is the value I of the accuracy impact standard value A Aref It is determined whether or not the value exceeds the threshold value. Here I A I Aref If the accuracy impact standard value (I Aref ) new I Aref From I A Change to. Accuracy impact standard value I for SB9 A After changing the value of accuracy impact A in SB8, A is the value I of the accuracy impact standard value A Aref If it does not exceed the value of I B is the value of the accuracy impact standard value B, I Bref It is determined whether or not the value exceeds the threshold value. Here I B I Bref If the accuracy impact standard value for accuracy impact B (I Bref ) new I Bref From I B Change to. Accuracy impact standard value I for SB11 B After changing the value of accuracy impact B in SB10, B is the value of the accuracy impact standard value B, I Bref If it does not exceed the value of I C is the value of the accuracy impact standard value C, I Cref It is determined whether or not the value exceeds the threshold value. Here I C I Cref If the accuracy impact standard value (I Cref ) new I Cref From I C Change to.

[0024] An example of a process for changing the accuracy influence reference value will be described below. If it is assumed that the error amount exceeds the reference value of the error amount when the accuracy impact degree exceeds the reference value of the accuracy impact degree, the model can be expressed by the following equation (1).

[0025]

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[0026] For example, when creating a thermal displacement model for each machine, in the following formula (2) based on past thermal displacement measurement data, α, β, and γ may be determined in advance by performing multiple regression analysis based on measurement data of the influence degree and the amount of error.

[0027]

number

[0028] In addition, the initial value of the reference value of the accuracy influence degree may be determined so as to satisfy the following formula (3).

[0029]

number

[0030] When only the accuracy impact degree A exceeds the accuracy impact degree reference value A, equation (1) becomes the following equation (4).

[0031]

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[0032] In equation (4), the deviation between the error amount and the error standard value cannot be accurately estimated by the accuracy influence degree, so an adjustment term Δ i1 is placed. The accuracy impact standard value A is adjusted by changing it, and the change in the accuracy impact standard value is expressed as ΔI Aref As the adjustment term Δ i1is replaced by the following equation (5).

[0033]

number

[0034] In equation (5), ΔI Aref When this is rearranged, we obtain the following equation (6).

[0035]

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[0036] It can be seen that thermal displacement that cannot be estimated by the thermal displacement model formula in the estimated thermal displacement amount calculation unit 22 in FIG. 2 occurs due to the influence of deformation of the element for which the accuracy impact degree A is calculated. Therefore, it is desirable to increase the frequency of thermal displacement measurement when deformation of the element for which the accuracy impact degree A is calculated occurs by lowering the accuracy impact degree reference value A. ΔI A >0, and the coefficient α that the accuracy influence degree A exerts on the error amount is not accurate. Therefore, if the accuracy influence degree standard value is set on the safe side so that the accuracy influence degree standard value A is always lowered, the change amount of the accuracy influence degree standard value A from the formula (6) is as shown in the following formula (7). Bref , I Cref The change amount for is the same as that in equation (7).

[0037]

number

[0038] When only the accuracy impact degree A exceeds the accuracy impact degree reference value A, equation (1) becomes the following equation (8).

[0039]

number

[0040] In equation (8), although the accuracy impact A exceeds the accuracy impact standard value A, the amount of error is below the error standard value, so Δ i2 is placed. The accuracy impact standard value A is adjusted by changing it, and the adjustment term Δ i2 The accuracy impact standard value change amount ΔI Aref Replace with.

[0041]

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[0042] In equation (9), ΔI Aref When this is rearranged, we obtain the following equation (10).

[0043]

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[0044] When the accuracy influence degree A exceeds the accuracy influence degree reference value A but the amount of error is below the error reference value, the accuracy influence degree A at this time can be set as a new accuracy influence degree reference value A. The accuracy influence reference value B and the accuracy influence reference value C can also be changed in the same manner as in formula (10).

[0045] In this way, the accuracy stabilization device 20 of the machining center M of the above-mentioned form has a temperature measurement unit 21 that measures the temperature of a specified portion using the temperature sensors 11 to 17, and an estimated thermal displacement amount calculation unit 22 that calculates an estimated thermal displacement amount based on the temperature measured by the temperature measurement unit 21, and corrects the thermal displacement by issuing a position command for the feed axis of the machining center M so as to cancel out the estimated thermal displacement amount calculated by the estimated thermal displacement amount calculation unit 22, thereby stabilizing the machining accuracy. The system further includes an accuracy impact calculation unit 23 which calculates an accuracy impact degree, which is the degree of impact of thermal displacement on the accuracy of the machining center M, based on the temperatures of multiple locations measured by the temperature measurement unit 21; a measurement judgment unit 25 which judges the need to measure an actual thermal displacement amount of the machining center M based on the accuracy impact degree and a preset measurement judgment criterion; an actual thermal displacement amount measuring unit 26 which measures the actual thermal displacement amount when the measurement judgment unit 25 judges that it is necessary to measure the actual thermal displacement amount; and a judgment criterion change unit 27 which changes the measurement judgment criterion in the measurement judgment unit 25 based on the amount of error between the estimated thermal displacement amount and the actual measured thermal displacement amount.

[0046] According to this configuration, while performing temperature measurement, it is possible to monitor the locations that affect the thermal displacement in real time, and to diagnose the environment in which the difference between the estimated thermal displacement amount and the actually measured thermal displacement amount becomes large. As a result, when it is assumed that the accuracy is stable due to the thermal displacement correction, the judgment criteria for the measurement of the actually measured thermal displacement amount are relaxed and the measurement frequency is reduced, and when the prediction of the thermal displacement is wrong and the accuracy is unstable even with the thermal displacement correction, the judgment criteria for the measurement of the actually measured thermal displacement amount are strengthened and the measurement frequency is increased, thereby making it possible to omit thermal displacement measurement that is less necessary in a situation where the thermal displacement is easy to predict. Therefore, it is possible to reduce lost time due to interruption of processing. In addition, since the actual thermal displacement can be grasped, it is possible to calibrate the machine tool, such as by resetting the machining origin, thereby stabilizing machining accuracy. This allows the actual thermal displacement to be measured at an appropriate timing, and the machining precision can be maintained while suppressing a decrease in machining efficiency due to unnecessary measurements.

[0047] In the above embodiment, the processing origin is corrected based on the error between the estimated thermal displacement amount and the actually measured thermal displacement amount, but instead of correcting the processing origin, when the actually measured thermal displacement amount is measured frequently (for example, when the measured number of times reaches a set number within a predetermined time), the thermal displacement model formula may be changed in the estimated thermal displacement amount calculation unit. Other calibrations may also be performed. The accuracy stabilization device is not limited to being incorporated in the numerical control device as in the above embodiment, but may be installed separately from the numerical control device. Temperatures may be acquired from a plurality of machine tools, and the timing for measuring the actual thermal displacement may be set individually for each of them. [Explanation of symbols]

[0048] 1··bed, 2··spindle head, 3··table, 4··column, 5··saddle, 11-17··temperature sensor, 20··accuracy stabilization device, 21··temperature measurement unit, 22··estimated thermal displacement amount calculation unit, 23··accuracy influence calculation unit, 24··criteria memory unit, 25··measurement judgment unit, 26··actual thermal displacement amount measurement unit, 27··criteria changing unit, 28··machining origin correction unit.

Claims

1. A temperature measuring means for measuring a temperature of a predetermined portion of the machine tool; an estimated thermal displacement amount calculation means for calculating an estimated thermal displacement amount based on the temperature measured by the temperature measurement means, and a position command for a feed axis of the machine tool is issued so as to cancel the estimated thermal displacement amount calculated by the estimated thermal displacement amount calculation means, thereby stabilizing the machining accuracy of the machine tool, an accuracy influence calculation means for calculating an accuracy influence degree, which is a degree of influence of thermal displacement on accuracy of the machine tool, based on temperatures of the plurality of portions measured by the temperature measurement means; a measurement determination means for determining the necessity of measuring the actual thermal displacement amount of the machine tool based on the accuracy influence degree and a preset measurement determination criterion; an actual thermal displacement measuring means for measuring the actual thermal displacement when the measurement determining means determines that it is necessary to measure the actual thermal displacement; a criterion changing means for changing the measurement criterion in the measurement judging means in accordance with an amount of error between the estimated thermal displacement amount and the actually measured thermal displacement amount; 1. An accuracy stabilization device for a machine tool, comprising:

2. 2. The accuracy stabilization device for a machine tool according to claim 1, wherein said accuracy influence calculation means calculates said accuracy influence based on a temperature change rate of each of said portions.

3. The accuracy stabilization device for a machine tool as described in claim 2, characterized in that the accuracy influence degree indicates the degree of influence of expansion / contraction of each of the parts due to thermal displacement, change in straightness, and geometric error of the feed axis, which affect the positional accuracy and / or inclination of the cutting edge in the feed axis direction of a tool attached to the machine tool, and the accuracy influence degree calculation means calculates at least one of the accuracy influence degrees.

4. 4. The accuracy stabilization device for a machine tool as described in any one of claims 1 to 3, characterized in that the measurement judgment criterion is an accuracy influence reference value which is a reference value set for each of the accuracy influence degrees, and the measurement judgment means judges that measurement is necessary when at least one of the multiple accuracy influence degrees exceeds the accuracy influence reference value.

5. 5. An accuracy stabilization device for a machine tool as claimed in claim 1, wherein the judgment criterion changing means changes the measurement judgment criterion in accordance with the amount of error in at least one measurement by the actual thermal displacement measuring means.

6. 6. An accuracy stabilizing device for a machine tool according to claim 1, further comprising a machining origin correcting means for correcting a machining origin in accordance with an amount of error between the estimated thermal displacement amount and the actually measured thermal displacement amount.

7. 1. A method for stabilizing machining accuracy of a machine tool, comprising: calculating an estimated thermal displacement amount estimated from a temperature of a predetermined portion of the machine tool; and correcting the thermal displacement by issuing a position command for a feed axis of the machine tool so as to cancel the estimated thermal displacement amount, the method comprising: a temperature measuring step of measuring temperatures of a plurality of the portions; an estimated thermal change amount calculation step of calculating the estimated thermal change amount based on the temperatures of the respective portions measured in the temperature measurement step; an accuracy influence calculation step of calculating an accuracy influence degree, which is a degree of influence of thermal displacement on accuracy of the machine tool, based on the temperature measured in the temperature measurement step; a measurement judgment step of judging the necessity of measuring the actual thermal displacement amount of the machine tool based on the accuracy influence degree and a preset measurement judgment criterion; an actual thermal displacement measurement step of measuring the actual thermal displacement when it is determined in the measurement determination step that it is necessary to measure the actual thermal displacement; a criterion changing step of changing the measurement judgment criterion in the measurement judgment step in accordance with an amount of error between the estimated thermal displacement amount and the actually measured thermal displacement amount; A method for stabilizing accuracy of a machine tool, comprising:

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