Wire electrical discharge machine having upper/lower guide thermal displacement correction function

There is provided a wire electrical discharge machine that includes a temperature detection unit, an actual position correction amount calculation unit, a correction amount calculation unit, a correction amount adjustment unit, a position correction amount adjustment unit, and a corrective movement amount calculation unit. The actual position correction amount calculation unit calculates an actual position correction amount for an upper/lower guide section in accordance with reference position coordinates at a reference temperature and with actual position coordinates at a temperature different from the reference temperature. The correction amount calculation unit uses a prepared correction amount arithmetic expression. The correction amount adjustment unit calculates a correction amount adjustment value from the actual position correction amount and the position correction amount.

RELATED APPLICATIONS

The present application claims priority to Japanese Application Number 2014-077264, filed Apr. 3, 2014, the disclosure of which is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wire electrical discharge machine and, more particularly, to a wire electrical discharge machine capable of correcting positional displacement (thermal displacement) of a wire electrode that is caused by thermal displacement of the machine.

2. Description of the Related Art

FIG. 5is a schematic diagram illustrating a prior art wire electrical discharge machine. The wire electrical discharge machine machines a workpiece29by causing an electrical discharge between a wire electrode1and the workpiece29.

The workpiece29placed on a workpiece mount is machined while it is immersed in a machining fluid in a work tank6. The machining fluid including machined cuttings generated during machining is discharged from the work tank6to a wastewater tank26through a pipeline (not shown). The machining fluid collected in the wastewater tank26is filtered to remove the machined cuttings and then transferred to a freshwater tank27. The freshwater tank27is provided with a machining fluid temperature regulator28. The machining fluid temperature regulator28adjusts the temperature of the machining fluid stored in the freshwater tank27by pumping up the machining fluid from the freshwater tank27through a piping37and passing it through the machining fluid temperature regulator28. The temperature-adjusted machining fluid is pumped up from the freshwater tank27by a pump (not shown) and fed back to the work tank6through pipelines35,36.

The positional relationship between the wire electrode1and the workpiece29is adjusted by a control unit24that drives motors for individual shafts. An X-axis drive motor17adjusts the relative positional relationship by moving an X-axis saddle11, and a Y-axis drive motor18adjusts the relative positional relationship by moving a Y-axis saddle10, to adjust the relative positional relationship between the wire electrode1and the workpiece29. A U-axis drive motor20and a V-axis drive motor21determine the tilt of the wire electrode1. The U-axis drive motor20determines the tilt of the wire electrode1by moving a U-axis saddle13to reposition an upper head section7. The V-axis drive motor21determines the tilt of the wire electrode1by moving a V-axis saddle14to reposition the upper head section7. A Z-axis drive motor19determines the vertical position of the upper head section7.

As described above, the positional relationship between the wire electrode1and the workpiece29is adjusted by combining a plurality of mechanical sections. Position coordinates of each axis that are required for positional control of each axis are detected by a position detector built-in a motor for each axis.

The wire electrical discharge machine is thermally deformed when a change occurs in a temperature environment in which it is installed, in the temperature of a mechanical section, or in the temperature of the machining fluid. Such thermal deformation causes thermal displacement in which the position and tilt of the wire electrode are displaced from the position and tilt commanded for machining purposes. If the position and tilt of the wire electrode are displaced, the positional relationship between the wire electrode1and the workpiece29changes to decrease the accuracy of machining. As such being the case, when high machining accuracy is required, it is necessary to perform machining not to cause the thermal displacement while the wire electrical discharge machine is installed in a temperature-controlled environment such as a thermostatic chamber.

However, controlling the temperature of a machine installation environment requires the use of a temperature control facility, that is, involves a large amount of investment and operating cost. In reality, therefore, many users cannot afford to use a temperature control facility.

Japanese Patent Application Laid-Open No. 61-297057 or No. 7-75937 discloses a thermal displacement correction function that provides against thermal displacement of a machine tool by acquiring temperature information with temperature detectors disposed in various sections of a machine, computing a correction value for thermal displacement, and subjecting each axis to drive control based on the computed correction value in order to suppress relative positional displacement between the wire electrode1and the workpiece29. When the above-described thermal displacement correction function is used to give an appropriate correction value, the relative positional relationship between the wire electrode1and the workpiece29remains unchanged even in an environment where the temperature changes. This makes it possible to suppress a decrease in the machining accuracy.

However, mechanical components of the wire electrical discharge machine are assembled from a plurality of mechanical elements and formed of different types of members. Further, as the environment in which the wire electrical discharge machine is installed varies from one user to another, it is difficult to set a thermal displacement correction value suitable for all temperature environments. A discrepancy occurs between a correction value preset for thermal displacement and an actual thermal displacement of the machine depending on the temperature environment and on the difference between one wire electrical discharge machine and another. In some cases, therefore, appropriate corrections cannot be made.

It has been difficult for the users to adjust the difference between a preset correction value and actual thermal displacement and compute a correction value appropriate for a user-specific temperature environment because it has been necessary, for example, to make measurements with an accurate measuring instrument, install a measurement sensor for making measurements, and rewrite a complex correction value computation program.

SUMMARY OF THE INVENTION

In view of the above-described problems in the prior art techniques, an object of the present invention is accordingly to provide a wire electrical discharge machine that makes it easy to set a thermal displacement correction value suitable for a machine-specific temperature environment.

A wire electrical discharge machine having an upper/lower guide thermal displacement correction function according to the present invention corrects upper/lower guide thermal displacement caused by changes in an installation environment and in a machine temperature by relatively moving a wire electrode and a workpiece. The wire electrical discharge machine includes a temperature detection unit, a storage unit, a first storage execution unit, a second storage execution unit, an actual position correction amount calculation unit, a correction amount calculation unit, a correction amount adjustment unit, a position correction amount adjustment unit, and a corrective movement amount calculation unit. The temperature detection unit is configured to detect at least one of the mechanical section temperature, machining fluid temperature, and machine ambient temperature of the wire electrical discharge machine. The storage unit is configured to store temperature information acquired by the temperature detection unit. The first storage execution unit is configured to command individual drive shafts at a reference temperature to relatively move the wire electrode with respect to a reference object, brings the wire electrode into contact with the reference object, and stores the position of the wire electrode in the form of reference position coordinates in the storage unit together with the reference temperature. The second storage execution unit is configured to command the individual drive shafts at a temperature different from the reference temperature to relatively move the wire electrode with respect to a reference object, brings the wire electrode into contact with the reference object, and stores the position of the wire electrode in the form of actual position coordinates in the storage unit together with the temperature different from the reference temperature. The actual position correction amount calculation unit is configured to calculate an actual position correction amount for an upper/lower guide section in accordance with the reference position coordinates and with the actual position coordinates. The correction amount calculation unit is configured to store a prepared correction amount arithmetic expression and calculates a position correction amount for the upper/lower guide section by substituting a temperature difference between the reference temperature and the temperature different from the reference temperature into the correction amount arithmetic expression. The correction amount adjustment unit is configured to calculate a correction amount adjustment value from the actual position correction amount for the upper/lower guide section and from the position correction amount therefor. The position correction amount adjustment unit is configured to correct the position correction amount based on the calculated correction amount adjustment value. The corrective movement amount calculation unit is configured to calculate a corrective movement amount for each of the individual drive shafts of the wire electrical discharge machine in accordance with the position correction amount for the upper/lower guide section, which is corrected by the position correction amount adjustment unit. The movement amount for each of the individual drive shafts is corrected by using the corrective movement amount and used to control each of the individual drive shafts.

As the present invention has the above-described configuration, it provides a wire electrical discharge machine that easily sets a thermal displacement correction value suitable for a machine-specific temperature environment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Elements identical with or similar to those used with prior art techniques will be described by using the same reference numerals as those of the corresponding elements.

FIG. 1Ais a schematic diagram illustrating the adjustment of a thermal displacement correction value for a wire electrical discharge machine according to an embodiment of the present invention.

A machining fluid is discharged from a work tank6to a wastewater tank26through a piping (not shown). As the machining fluid discharged from the work tank6to the wastewater tank26includes machined cuttings generated during electrical discharge machining, it is transferred to a freshwater tank27through a filter (not shown). A machining fluid temperature regulator28is disposed in the freshwater tank27. When the machining fluid in the freshwater tank27circulates in the machining fluid temperature regulator28through a pipeline37, the machining fluid temperature regulator28adjusts the temperature of the machining fluid. The machining fluid in the freshwater tank27is then fed back to the work tank6through pipelines35,36. Some pumps, used to pump the machining fluid from the freshwater tank27to the work tank6, are not shown in the figures.

The relative positional relationship between a wire electrode1and a reference object4varies when motors of individual shafts are driven by a control unit24of a control device40. An X-axis saddle11is driven by an X-axis drive motor17, which is controlled by the control unit24through a Y-axis drive motor signal line31. A Y-axis saddle10is driven by a Y-axis drive motor18, which is controlled by the control unit24through an X-axis drive motor signal line30. Driving the X-axis saddle11and the Y-axis saddle10in the above manner determines the relative XY-direction positional relationship between the wire electrode1and the reference object4.

A U-axis saddle13is driven by a U-axis drive motor20, which is controlled by the control unit24through a U-axis drive motor signal line33. A V-axis saddle14is driven by a V-axis drive motor21, which is controlled by the control unit24through a V-axis drive motor signal line34. Driving the U-axis saddle13and the V-axis saddle14in the above manner changes the UV-direction position of an upper head section7to determine the tilt of the wire electrode1.

A Z-axis drive motor19, which is driven by the control unit24through a Z-axis drive motor signal line32, determines the height of the upper head section7, which is mounted on a Z-axis saddle12. The wire electrode1is positioned while its position is detected by a position detector built in each of the drive motors17,18,19,20,21for the individual shafts.

The wire electrode1is tightly stretched between an upper guide2mounted on the upper head section7and a lower guide3mounted on a lower head section8. The control device40of the wire electrical discharge machine applies a contact detection voltage between the wire electrode1and the reference object4. As shown inFIG. 1A, while the reference object4is placed on a workpiece mount5and immersed in the machining fluid in the work tank6, the shafts of the wire electrical discharge machine are driven to measure the position of each axis by detecting the contact between the wire electrode1and the reference object4. The measured data about the position of each axis is stored in a storage unit25of the control device40. The reference numeral9denotes a lower arm. The reference numeral15denotes a column.

A mechanical section temperature detector22is disposed in a bed16. A machining fluid temperature detector23is disposed in the machining fluid temperature regulator28. These temperature detectors22,23acquire temperature information about a mechanical section and machining fluid of the wire electrical discharge machine. The number of temperature detectors for acquiring the temperature information is not limited. The present embodiment uses the temperature detectors disposed at two locations. However, an alternative is to use more temperature detectors and use only one temperature detector. The locations at which the temperature detectors are disposed are determined by a machine designer and not limited to the bed16and the machining fluid temperature regulator28. Data about the temperatures of the bed16and machining fluid, which are detected respectively by the mechanical section temperature detector22and the machining fluid temperature detector23, is stored in the storage unit25of the control device40.

The temperatures to be detected are not limited to the mechanical section temperature and the machining fluid temperature. For example, the temperatures of mechanical parts, the ambient temperature of the machine, and the machining fluid temperature may be detected. The acquired temperature information is transmitted to the storage unit25. The reference object4is placed on the workpiece mount5. The reference object4to be used should be formed of an electrifiable material.

FIG. 1Bis a schematic diagram illustrating an exemplary configuration of the control device40. The control device40includes the control unit24and the storage unit25.

As described later, the storage unit25includes a temperature information storage unit41, a first storage execution unit42, and a second storage execution unit43. The temperature information storage unit41stores temperature information. The first storage execution unit42stores a reference temperature and reference position coordinates related to the reference temperature. The second storage execution unit43stores actual position coordinates related to a temperature different from the reference temperature and the temperature different from the reference temperature. The temperature information storage unit41, the first storage execution unit42, and the second storage execution unit43are all connected to the mechanical section temperature detector22and machining fluid temperature detector23in the control device40. The first storage execution unit42and the second storage execution unit43are both connected to the drive motors17,18,19,20,21for the individual shafts in the control device40.

Further, as described later, the control unit24includes an actual position correction amount calculation unit44, a correction amount calculation unit45, a correction amount adjustment unit46, a position correction amount adjustment unit47, and a corrective movement amount calculation unit48. The actual position correction amount calculation unit44is connected to the first storage execution unit42and the second storage execution unit43in order to calculate an actual position correction amount for the upper/lower guide section. The correction amount calculation unit45is connected to the temperature information storage unit41in order to calculate a position correction amount for the upper/lower guide section. The correction amount adjustment unit46is connected to the correction amount calculation unit45and the actual position correction amount calculation unit44in order to calculate a correction amount adjustment value. The position correction amount adjustment unit47is connected to the correction amount calculation unit45and the correction amount adjustment unit46. The corrective movement amount calculation unit48is connected to the position correction amount adjustment unit47in order to calculate a corrective movement amount for each drive shaft of the wire electrical discharge machine.

In the control device, the control unit24may be formed of a single CPU, and the storage unit25may be formed of a single memory.

An example of an operation performed by the wire electrical discharge machine having the above-described configuration according to an embodiment of the present invention will now be described with reference to steps of a process depicted inFIG. 2.FIGS. 3A and 3BandFIGS. 4A and 4Bare diagrams illustrating an operation that is performed to detect wire position coordinates by using a reference object. The reference object4is fastened to the workpiece mount5.FIGS. 3A and 3BandFIGS. 4A and 4Bare diagrams illustrating a case where the Y-direction tilt of the wire electrode1is measured.FIGS. 3A and 3Bare diagrams illustrating an operation that is performed to detect wire position coordinates by using a reference object (upper position).FIGS. 4A and 4Bare diagrams illustrating an operation that is performed to detect wire position coordinates by using a reference object (lower position).

The drive motors (18,21,17,20) for the Y- and V-axes or for the X- and U-axes are driven to bring the wire electrode1into contact with the reference object4. When the wire electrode1comes into contact with the reference object4, a current flowing in the wire electrode1flows to the reference object4. This change in the current is detected to detect the wire position coordinates of each axis at which the wire electrode1comes into contact with the reference object4(step S101).

The detection in step S101of the wire position coordinates at which the wire electrode1comes into contact with the reference object4will now be described with reference to a case where the Y- and U-axes are driven.

First of all, the coordinates of the Y- and U-axes at which a measurement starts are stored as (Y00, V00). Subsequently, as shown inFIG. 3A, the U-axis is moved to move the upper guide2in the U direction so that only the wire electrode1near the upper guide2is tilted to come into contact with the reference object4. Then, as shown inFIG. 3B, the Y-axis is driven to move the tilted wire electrode1in the Y direction so that the wire electrode1comes into contact with the reference object4.

The wire electrode1is electrified. Therefore, when the wire electrode1comes into contact with the reference object4, a current flows to the reference object4. Thus, the current flow to the reference object4is detected to detect the contact between the wire electrode1and the reference object4. The Y coordinates at which the wire electrode1is brought into contact with the reference object4are stored as the position coordinates “Y upper” of the upper guide2(the storage unit25of the control device40).

As for the position coordinates “Y lower” of the lower guide3, too, the contact between the wire electrode1and the reference object4is detected while the wire electrode1is tilted as shown inFIGS. 4A and 4B. The detected position coordinates “Y upper” and position coordinates “Y lower” are used so that data about the Y- and V-axis wire position coordinates (Y0, V0) at which the wire electrode1comes into contact with the reference object4is calculated by the equations Y0=Y lower, V0=V00+Y upper−Y lower.

As for the X- and U-axes, too, the wire electrode1is tilted and brought into contact with the reference object4, as is the case with the Y- and V-axes, in order to measure the measurement position coordinates (X0, U0) of the X- and U-axes at which the wire electrode1is brought into contact with the reference object4.

The wire position coordinate data about each axis, which is detected in step S102by using the reference object4, is stored as the reference position (step S105). Temperature information prevailing at the time of reference position detection is detected by each temperature detector (mechanical section temperature detector22and machining fluid temperature detector23) (steps S103and S104), and stored in a storage device as the reference temperature (step S106). As a result, the reference position and the reference temperature, which represent the position and tilt of the wire electrode1and temperature information that prevail before thermal displacement, are detected and stored.

Subsequently, the detected temperature is monitored to determine whether it has varied from the reference temperature (step S107). Then, in a state where the detected temperature has varied from the reference temperature to cause thermal displacement, each axis is driven again (seeFIGS. 3A and 3BandFIGS. 4A and 4B), and the data about the wire position coordinates (Y1, V1) of the Y- and V-shafts at which the wire electrode1comes into contact with the reference object4and the data about the wire position coordinates (X1, U1) of the X- and U-shafts at which the wire electrode1comes into contact with the reference object4are detected (step S108) in the same manner as in step S101.

The wire position coordinate data about each axis, which is detected in step S109by using the reference object in a state where the temperature has varied from the reference temperature, is stored as the measurement position (step S112). The temperature information detected at the time of measurement position detection by each temperature detector (mechanical section temperature detector22and machining fluid temperature detector23) (steps S110and S111) is stored in the storage unit25as the measured temperature (step S113).

The reference position (step S105) and the measurement position (step S112) are read from the storage unit25of the control device40(step S114). The amount of positional displacement of the upper guide2and of the lower guide3, which is caused by thermal displacement, is calculated from the difference between the reference position and the measurement position (step S115), and an optimal correction amount is calculated in accordance with the result of measurement (step S116).

Further, the reference temperature and the measured temperature are read from the storage unit25of the control device40(step S117), and the difference between the reference temperature (step S106) and the measured temperature (step S113) is calculated as a temperature change amount (step S118).

A correction amount for the calculated temperature change amount is computed by using a correction amount arithmetic expression stored in the storage device by a manufacturer (step S119). The optimal correction amount based on the measurement result, which is calculated in step S116, is compared against the correction amount computed from the correction amount arithmetic expression that is stored and used for computation in step S119in order to compute a correction amount adjustment value (step S120). The correction amount adjustment value computed in step S120is used to adjust the correction amount arithmetic expression for the measurement result (step S121).

The adjustment of the correction amount arithmetic expression in steps S114to S121will now be described with reference to the Y- and V-shafts.

When the reference position is (Y0, V0) and the measurement position is (Y1, V1), the measured positional displacement amount A1of the upper guide2and the measured positional displacement amount A2of the lower guide3are calculated respectively from the following equations (step S115).
A1=Y0−Y1+V1−V0  (1)
A2=Y0−Y1  (2)

Thus, the optimal position correction amount B1for the upper guide2and the optimal position correction amount B2for the lower guide3, which are based on the above positional displacement amounts, are calculated from the following equations (step S116).
B1=−A1  (3)
B2=−A2  (4)

The reference temperatures (T01, T02) and measured temperatures (T11, T12), which are detected by the mechanical section temperature detector22and the machining fluid temperature detector23and stored in the storage unit25, are read (step S117).

The temperature change amounts (T21, T22) are calculated from the following equations (step S118).
T21=T11−T01  (5)
T22=T12−T02  (6)

Let us assume, for example, that the correction amount arithmetic expressions for the upper and lower guide positions, which are prepared by the manufacturer of the wire electrical discharge machine and stored in the storage unit25, are as follows.
D1=C1T1+C2T2  (7)
D2=C3T1+C4T2  (8)

D1is a correction amount for the upper guide position. D2is a correction amount for the lower guide position. T1is a temperature change amount of the mechanical section temperature detector22. T2is a temperature change amount of the machining fluid temperature detector23. C1, C2, C3, and C4are arbitrary coefficients. The correction amount arithmetic expressions are determined in accordance, for instance, with the results of experiments conducted by the manufacturer. Equations (7) and (8) are merely examples.

When the temperature change amounts (T21, T22) calculated in step S118are substituted into the correction amount arithmetic expressions (Equations (1) and (2)) to compute a correction amount E1for the upper guide2and a correction amount E2for the lower guide3, the following equations are obtained (step S119).
E1=C1T21+C2T22  (9)
E2=C3T21+C4T22  (10)

The correction amount adjustment values F1, F2for comparing and adjusting the position correction amounts E1, E2for the upper and lower guides2,3, which are computed from the correction amount arithmetic expressions of equations (3) and (4), and the optimal position correction amount B1for the upper guide2and optimal position correction amount B2for the lower guide3, which are computed in step S116and based on the results of actual measurements, are computed from the following equations (step S120).
F1=B1/E1  (11)
F2=B2/E2  (12)

When the correction amount adjustment values F1, F2for the upper guide2and the lower guide3computed in step S120are used to optimize the position correction amounts computed from Equations (7) and (8) in accordance with the measurement results, the following equations are obtained.
D1F1D1  (13)
D2F2D2  (14)

Arithmetic expressions for correction amounts Y, V for the V- and Y-axes are as follows.
Y=D2  (15)
V=D1−D2  (16)
Thus, the arithmetic expressions for the correction amounts Y, V for the Y- and V-axes are as follows when the measurement results are taken into account.
Y=F2D2  (17)
V=F1D1−F2D2  (18)
Hence, when corrective movements are to be made, adjustments are made so as to correct in accordance with Equations (17) and (18) (step S121).

When the above correction value adjustments are made for setup purposes, the correction value arithmetic expressions are adjusted in accordance with the measurement results. This ensures that corrective movements are made in accordance with the adjusted correction amounts, which are computed for the Y- and V-axes. As for the X- and U-axes, too, rewriting the computed correction amounts in the same manner as for the Y- and V-axes converts the correction values in accordance with detection results.

In a temperature environment for the wire electrical discharge machine in which actual machining is performed, the wire electrode1used for machining is directly used to detect and calculate the amounts of changes in the position and tilt of the wire electrode1due to thermal displacement and the amount of the resulting temperature change. As the correction values for thermal displacement are computed in accordance with the results of detection and calculation, it is possible to set correction values more suitable for respective conditions.

Further, the position coordinates of each axis are detected by bringing the wire electrode1into contact with the reference object4, and the temperature information detected by each temperature detector22,23is stored, so that correction value computations for thermal displacement are automatically performed by the control unit24of the control device40. As a result, correction value setup can be easily performed with a minimum of human error.

Position detection is also achieved by using the wire electrode1that is actually used for machining and by moving the shafts of the machine. Therefore, correction value adjustments can be made without having to prepare a measuring instrument, a detection sensor, a detector, or the like. In addition, measurements can be made under conditions that are closer to those for machining.

The foregoing description has been made on the assumption that the temperatures of the mechanical section and machining fluid are used while the temperature detector22is disposed in the bed16, which is one mechanical section, and the temperature detector23is disposed in the machining fluid temperature regulator28. However, the adjustments can be made in the same manner even if the machining fluid temperature is detected in a different mechanical section or in a different location or the number of temperature detection unit is increased. Further, only one of the temperature detector22and the temperature detector23may be used as the temperature detection unit to correct the thermal displacement of the upper/lower guide in accordance with the detected temperature information. Moreover, a temperature detector may detect the temperature of an environment in which the machine is installed and the thermal displacement of the upper/lower guide may be corrected in accordance with information about the temperature of an environment in which the machine is installed.

Consequently, the above-described embodiment makes it possible to compute a thermal displacement correction value suitable for a machine-specific temperature environment and perform thermal displacement correction value setup with ease without preparing an additional measuring instrument.

The machine-specific temperature environment includes, for example, the temperatures of mechanical parts, the ambient temperature of the machine, and the machining fluid temperature.