Method and device for correcting straightness error of machine tool

Abstract

This invention provides a method and apparatus for correcting straightness errors in machine tools, capable of appropriately correcting straightness errors caused by the straightness of the upper surface of the bed in a machine tool with a table feed drive axis. The method involves calculating the straightness curve of the upper surface of the bed based on the straightness curve of the upper surface of the bed and the commanded position of the table drive axis; calculating the workpiece tilt estimation value based on the fixed position information of the workpiece relative to the reference position of the table and the calculated straightness curve of the upper surface of the table; calculating the straightness error estimation value of the table drive axis at the spindle head position based on the commanded position of the table drive axis, the calculated straightness curve of the upper surface of the table, and the calculated tilt estimation value of the workpiece; and using the calculated straightness error estimation value as a correction amount to perform correction control on at least one of the spindle head drive axes.

Description

Technical Field

[0001] This invention relates to a method and apparatus for correcting the straightness error of machine tools. Background Technology

[0002] Figure 1 This is a schematic diagram of a typical gantry machining center. Additionally, in Figure 1 In this case, the X-axis direction is set as the front-back direction of the gantry machining center M, the Y-axis direction is set as the left-right direction, and the Z-axis direction is set as the up-down direction.

[0003] The gantry machining center M, which is a machine tool, is a 3-axis machining center, consisting of a bed 1, a worktable 2, a column 4, and a spindle head 3.

[0004] The bed frame 1 is, for example, installed or fixed to the floor. Additionally, as... Figure 1 As shown, an X-axis guide 2a is formed on the upper surface of the bed 1. A worktable 2, capable of fixing a workpiece (not shown), is provided on the X-axis guide 2a in a manner that allows it to move in the X-axis direction. That is, by means of the X-axis guide 2a, which serves as a linear drive axis, the worktable 2 can move relative to the bed 1 by one degree of translation.

[0005] Furthermore, the column 4 is erected on the rear side of the bed 1. A Y-axis guide 4a is formed on the front surface of the column 4. A slide saddle 5 is provided on the Y-axis guide 4a so as to be movable in the Y-axis direction. A Z-axis guide 5a is provided on the front surface of the slide saddle 5. A spindle head 6 is provided on the Z-axis guide 5a so as to be movable in the Z-axis direction. A specified tool T is mounted on the lower end of the spindle head 6. Therefore, the spindle head 3 can move in both the Y-axis and Z-axis directions. That is, the spindle head 3 can perform a two-degree-of-freedom translational movement relative to the bed 1 via mutually perpendicular linear drive axes, namely the Y-axis guide and the Z-axis guide, which are also perpendicular to the X-axis. Therefore, in combination with the movement of the worktable 2, the spindle head 3 can perform a three-degree-of-freedom translational movement relative to the bed 1. Each axis guide 2a, 4a, and 5a is driven by a servo motor (not shown) controlled by a numerical control device (not shown), which can process the workpiece into any shape.

[0006] One of the main causes of motion error in the aforementioned portal machining center M is the straightness error of the linear drive axis. For example, the straightness error of the X-axis manifests as an error in the Y-axis or Z-axis direction, which varies depending on the position on the X-axis. When the straightness error of the X-axis is large, machining defects may occur when the worktable 2 is moved during workpiece machining due to this straightness error.

[0007] As a countermeasure against straightness error in linear drive shafts, for example, Patent Document 1 discloses a method for correcting straightness error. This method uses a position correction amount in a direction perpendicular to the position of one linear drive shaft to perform correction control on other linear drive shafts based on the position of the linear drive shaft. According to the method in Patent Document 1, for example, the straightness of the Z-axis in the Y direction is corrected by adjusting the Y-axis movement correction amount based on the Z-axis movement.

[0008] Patent Document 1: Japanese Patent Application Publication No. 58-114845

[0009] Here, taking the straightness error in the Z-axis direction of the X-axis guide 2a as an example, the straightness error of the X-axis of the portal machining center M is explained in detail. Figure 2 It is shown Figure 1 A schematic diagram and a graph of an example of the measurement results are shown for the determination of the straightness error in the Z-axis direction of the X-axis guide 2a of a portal machining center.

[0010] To determine the straightness of the X-axis guide 2a of the portal machining center M in the Z-axis direction, such as... Figure 2 As shown, a displacement gauge 7 is mounted on the spindle head 3. A ruler 8 is set at measurement point a on the worktable 2, and the position of, for example, the upper surface of the ruler 8 is measured while the worktable 2 is driven. Next, a ruler 8 is set at at least measurement point b at different positions along the X-axis, and the position of the upper surface of the ruler 8 is measured in the same manner as at measurement point a. Based on these measurement results, the straightness error in the Z direction along the X-axis is calculated for each of the measurement points a and b. For example, the measurement result at measurement point a is a straightness error curve 9a, and the measurement result at measurement point b is a straightness error curve 9b.

[0011] In the gantry machining center M, the straightness error of the X-axis in the Z-axis direction is caused by the straightness of the X-axis guide 2a located on the upper surface of the bed 1. In other words, the straightness error of the X-axis in the Z-axis direction in the gantry machining center M does not depend on the position of the measuring point on the worktable 2, but on the position of the measuring point relative to the bed 1. As mentioned above, the measuring points a and b are located at different positions on the worktable 2, and as... Figure 2 As shown, the straightness error curves 9a and 9b obtained by measurement have approximately the same shape. Thus, even if the X-axis position on the worktable 2 is changed, as with measurement points a and b, the Z-axis straightness error of the X-axis guide in the portal machining center M will show the same measurement result. This is because, even if measurement points a and b are set at different positions on the worktable 2, since the measurement is performed directly below the spindle head 3, the X-axis positions of measurement points a and b relative to the bed 1 are the same during measurement.

[0012] As described above, in the straightness correction method disclosed in Patent Document 1, straightness correction is performed using a correction amount corresponding to the position of the linear drive axis. Therefore, when the position of the workpiece on the worktable changes, the position of the linear drive axis needs to be changed accordingly. Moreover, since the correction amount corresponds to the position of the linear drive axis, the correction amount changes before and after the workpiece position change. When the method disclosed in Patent Document 1 is applied to the aforementioned portal machining center M, the change of the linear drive axis is equivalent to a change in the position of the X-axis. Furthermore, the change in the position of the X-axis is equivalent to a change in the position of the worktable 2 on the bed 1. That is, the correction amount is different before and after the change in the position of the worktable 2. However, the shape of the X-axis guide 2a, which manifests as a straightness error of the bed, not only produces a straightness error on the surface of the worktable, but also, as described above, the effect of the bed on straightness is the same regardless of the position of the workpiece on the worktable. Therefore, depending on the situation, it may be impossible to properly correct the straightness that depends on the position on the bed. Summary of the Invention

[0013] Therefore, the object of the present invention is to provide a method and apparatus for correcting straightness error of a machine tool, which can appropriately correct the straightness error caused by the straightness of the upper surface of the machine bed in a machine tool having a table linear drive axis.

[0014] To address the aforementioned issues, the first structure of the present invention is a method for correcting the straightness error of a machine tool. The machine tool includes: a bed; a worktable for fixing a workpiece; a spindle head for rotating a mounted tool; a worktable drive shaft for enabling the worktable to perform linear motion relative to the bed; and two spindle head drive shafts for enabling the spindle head to perform linear motion relative to the bed in a direction perpendicular to the worktable drive shafts. The method is characterized by performing the following steps within the machine tool: calculating the straightness curve of the upper surface of the worktable based on the straightness curve of the upper surface of the bed and the commanded position of the worktable drive shafts; calculating the inclination estimation value of the workpiece based on the fixed position information of the workpiece relative to the reference position of the worktable and the calculated straightness curve of the upper surface of the worktable; calculating the straightness error estimation value of the worktable drive shaft at the spindle head position based on the commanded position of the worktable drive shaft, the calculated straightness curve of the upper surface of the worktable, and the calculated inclination estimation value of the workpiece; and using the calculated straightness error estimation value as a correction amount to perform correction control on at least one of the spindle head drive shafts.

[0015] Another aspect of the first structure of the present invention is characterized in that, in the above structure, the workpiece is fixed to the worktable at multiple points, and in the calculation of the inclination inference value, the difference value of the upper surface curve of multiple worktables corresponding to the fixed position information of multiple fixed positions is used.

[0016] To address the aforementioned issues, the second structure of the present invention is a straightness error correction device for a machine tool. This machine tool includes: a bed; a worktable for fixing a workpiece; a spindle head for rotating a mounted tool; a worktable drive shaft for enabling the worktable to perform linear motion relative to the bed; and two spindle head drive shafts for enabling the spindle head to perform linear motion relative to the bed in a direction perpendicular to the worktable drive shafts. The straightness error correction device is characterized by including: a bed curve storage unit for storing the straightness curve of the upper surface of the bed; a fixed position information storage unit for storing fixed position information of the workpiece relative to the reference position of the worktable; and a worktable curve calculation unit for calculating the straightness error based on the stored information. The system includes a bed curve storage unit, which calculates the straightness curve of the upper surface of the bed based on the straightness curve of the upper surface of the bed and the command position of the worktable drive shaft; a workpiece tilt calculation unit, which calculates the workpiece tilt estimation value based on the fixed position information stored in the fixed position information storage unit and the calculated straightness curve of the upper surface of the worktable; a straightness error calculation unit, which calculates the straightness error estimation value of the worktable drive shaft at the spindle head position based on the command position of the worktable drive shaft, the calculated straightness curve of the upper surface of the worktable, and the calculated tilt estimation value of the workpiece; and a correction control unit, which uses the calculated straightness error estimation value as a correction amount to perform correction control on at least one of the spindle head drive shafts.

[0017] Another aspect of the second structure of the present invention is characterized in that, in the above structure, the workpiece is fixed to the worktable at multiple points, and in the workpiece tilt calculation unit, the tilt estimation value of the workpiece is calculated using the difference values ​​of multiple values ​​of the worktable surface curve corresponding to the fixed position information of multiple fixed positions respectively.

[0018] According to the present invention, in the machine tool straightness error correction method and straightness error correction device, a straightness error estimation value is calculated based on the worktable surface straightness curve considering the straightness curve of the machine bed surface and the workpiece tilt estimation value, and correction is performed based on the estimation value. Therefore, it is possible to appropriately correct straightness errors caused by the straightness of the machine bed surface in the worktable's linear drive shaft, which cannot be addressed in conventional straightness corrections that use correction amounts corresponding to the position of the linear drive shaft. In other words, straightness correction can be performed considering situations where the workpiece tilts according to a fixed position and the worktable does not move straight.

[0019] For example, by applying the straightness error correction method and straightness error correction device of the present invention, even when the ground on which the machine tool is installed deforms over time, and the machine bed also deforms due to this deformation, thus easily causing straightness errors in the table's linear drive shaft, high machining accuracy can be maintained. Attached Figure Description

[0020] Figure 1 This is an explanatory diagram showing a typical gantry machining center.

[0021] Figure 2 This is an explanatory diagram showing the straightness measurement of a portal machining center.

[0022] Figure 3 This is an explanatory diagram showing the structure of the straightness error correction device of the present invention.

[0023] Figure 4 It is a schematic diagram of the positional relationship between the coordinate systems.

[0024] Figure 5 This is a flowchart relating to the straightness error correction method of the present invention.

[0025] Figure 6 It is a schematic diagram of each coordinate system and each straightness curve.

[0026] Figure 7 It is a schematic diagram of the coordinate systems, the straightness curve of the worktable surface, and the workpiece tilt error.

[0027] Label Explanation

[0028] 1: Bed; 2: Worktable; 3: Spindle head; 30: Command position generation unit; 31: Bed curve storage unit; 32: Fixed position information storage unit; 33: Worktable curve calculation unit; 34: Workpiece tilt calculation unit; 35: Straightness error calculation unit; 36: Correction control unit; M: Portal machining center (machine tool); W: Workpiece. Detailed Implementation

[0029] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0030] Figure 3 This is an explanatory diagram showing the structure of the straightness error correction device S.

[0031] The straightness error correction device S has an instruction position generation unit 30, a bed curve storage unit 31, a fixed position information storage unit 32, a worktable curve calculation unit 33, a workpiece tilt calculation unit 34, a straightness error calculation unit 35, and a correction control unit 36.

[0032] In this invention, the straightness error correction device S is envisioned as a function of a numerical control device (not shown) responsible for controlling the portal machining center M. The numerical control device includes a CPU and a memory connected to the CPU, which are used to perform various actions such as motion control, information storage, and computation. The straightness error correction device S can be configured as a device independent of the numerical control device, or it can be integrated as a function of another computer.

[0033] In the command position generation unit 30, the command position of each axis is generated.

[0034] The bed curve storage unit 31 stores the bed surface straightness curve fb(x) representing the shape of the bed guide surface.

[0035] The fixed position information storage unit 32 stores information about the fixed position of the workpiece W relative to the worktable 2, i.e., fixed position information. Here, for example... Figure 4 As shown, when the workpiece W is fixed in two places, the position information of fixed position 15 and fixed position 16 is stored respectively.

[0036] In the worktable curve calculation unit 33, the worktable surface straightness curve ft(x) is calculated based on the bed surface straightness curve fb(x) stored in the bed curve storage unit 31 and the command position of the X-axis.

[0037] In the workpiece tilt calculation unit 34, the tilt estimation value θt of the workpiece W is calculated based on the fixed position information stored in the fixed position information storage unit 32 and the straightness curve ft(x) of the worktable surface.

[0038] In the straightness error calculation unit 35, the straightness error estimation value dZ at the machining point of the workpiece W, which is located directly below the spindle head 3, is calculated based on the straightness curve ft(x) of the worktable surface, the command position of the X-axis, and the inclination estimation value θt of the workpiece W.

[0039] In the correction control unit 36, the straightness error estimation value dZ is set as the correction value, and the Z-axis is corrected and controlled according to the command position of the X-axis.

[0040] Figure 4 It shows in Figure 1 The positional relationships of each coordinate system when the workpiece W is fixed on the worktable 2 of the gantry machining center M for machining. Additionally, regarding the above... Figure 1 , Figure 2 The same structures involved are omitted from the description.

[0041] like Figure 4As shown, the machine coordinate system 10 is located on the bed 1. Furthermore, the X-direction position of the origin of the machine coordinate system 10 is the same as that of the machining point 14. The machining point 14 is the control target point and the location of the object to be corrected for straightness error.

[0042] The bed coordinate system 11 is located on the upper surface of the bed 1. In this invention, the origin of the bed coordinate system 11 is located at -Obx in the X direction of the machine coordinate system 10.

[0043] The table coordinate system 12 is located on the upper surface of the table 2. In this invention, the table coordinate system 12 moves -Xm from the machine coordinate system 10 as the table 2 moves to the commanded Xm position Xm along the X-axis in the machine coordinate system 10. Therefore, as... Figure 4 As shown, the origin of the worktable coordinate system 12 is located at position Xb in the bed coordinate system 11, which has been moved (Obx-Xm) along the X direction.

[0044] The workpiece coordinate system 13 is located on the upper surface of the workpiece W. In this invention, the workpiece coordinate system 13 is set at a coordinate position of (Owx, Owy, Owz) in the table coordinate system 12. Here, the X-axis command position in the workpiece coordinate system 13 is Xw. That is, as shown... Figure 4 As shown, the X-coordinate Xt of machining point 14 in the worktable coordinate system 12 can be expressed as (Xw + Owx). Furthermore, as... Figure 4 As shown, the X-coordinate Xt of machining point 14 in worktable coordinate system 12 and the X-axis command position Xm in machine coordinate system 10 represent the same position on the X-axis.

[0045] The workpiece W is fixed at two locations, 15 and 16, relative to the worktable 2 along the X-axis. The X-coordinate of the fixed location 15 in the workpiece coordinate system 13 is Xs1. The X-coordinate of the fixed location 16 in the workpiece coordinate system 13 is Xs2.

[0046] The following is based on Figure 5 The flowchart and the diagram illustrating the calculation method. Figure 6 and Figure 7 The straightness error correction method of the present invention will be explained. Specifically, a method for correcting the straightness error in the X-axis and Z-axis directions of a portal machining center M will be described.

[0047] In step S1, calculate Figure 6 , Figure 7 The straightness curve 22 of the upper surface of the worktable in the worktable coordinate system 12 shown.

[0048] First, based on the X-axis command position Xw in the workpiece coordinate system 13, the origin X position Xb of the worktable coordinate system 12 in the bed coordinate system 11 is calculated using equation (1). Position Xb is the reference position of the worktable 2 relative to the bed 1 in this invention.

[0049] Xb=Obx-Xt=Obx-Xm=Obx-(Xw+Owx) (1)

[0050] Next, the function representing the shape of the X-axis guide surface of the bed 1 in the bed coordinate system 11, namely the straightness curve of the upper surface of the bed, is set as fb(x), and the origin X position of the worktable coordinate system 12 in the bed coordinate system 11, calculated according to the command position of the X-axis, is set as Xb. The function representing the straightness curve of the upper surface of the worktable in the worktable coordinate system 12, namely the straightness curve ft(x), is obtained by equation (2).

[0051] ft(x)=G·fb(x+Xb) (2)

[0052] Here, the straightness curve fb(x) of the upper surface of the bed is given by a function such as an nth-degree polynomial, a line segment based on multiple point groups, and an nth-degree interpolation curve. For example, the straightness curve fb(x) of the upper surface of the bed is obtained by directly measuring the X-axis guide of the upper surface of the bed 1 beforehand.

[0053] Furthermore, G is a transfer function based on the rigidity of the worktable 2 and the X-axis guide mechanism. When the rigidity on the worktable 2 side is high, the shape of the straightness curve fb(x) on the upper surface of the bed is difficult to transfer to the straightness curve ft(x) on the upper surface of the worktable. On the other hand, when the rigidity on the worktable 2 side is low and the straightness curve fb(x) on the upper surface of the bed is directly transferred, the straightness curve ft(x) on the upper surface of the worktable is obtained by equation (3). The rigidity of the bed 1 and the worktable 2 can be obtained, for example, by referring to mechanical information related to the specifications.

[0054] ft(x)=fb(x+Xb) (3)

[0055] In step S2, calculate Figure 7 The inclination inference value θt of the workpiece W is shown. Based on the straightness curve ft(x) of the worktable surface calculated in step S1 and the X positions Xs1 and Xs2 of the fixed positions 15 and 16 of the workpiece W in the workpiece coordinate system 13, the inclination inference value θt of the workpiece W is obtained by equation (4).

[0056] θt={ft(Owx+Xs2)-ft(Owx+Xs1)} / (Xs2-Xs1) (4)

[0057] For example, if the workpiece W is fixed at one location, or if the interval between fixed locations 15 and 16 is narrow, the inclination value θt of the workpiece W is calculated using equation (5) based on the differential value δft / δx(x) of the straightness curve ft(x) on the surface of the worktable. If the fixed location is one location, this fixed location is treated as fixed location 15. Furthermore, if the interval between fixed locations 15 and 16 is narrow, only fixed location 15 is considered.

[0058] Furthermore, without considering the tilt of the workpiece W, the tilt inference estimate θt in step S2 can also be 0.

[0059] θt=δft / δx(Owx) (5)

[0060] In step S3, the straightness error estimate dZ in the Z direction of the X-axis at machining point 14 is calculated. Based on the inclination estimate θt of workpiece W obtained in step S2, the straightness curve ft(x) of the worktable surface calculated in step S1, and the position of machining point 14 in worktable coordinate system 12 obtained according to the X-axis command position Xw in workpiece coordinate system 13, the straightness error estimate dZ in the Z direction of the X-axis at machining point 14 is obtained by equation (6).

[0061] dZ = (Xw - Xs1) θt+ft(Xs1) (6)

[0062] In step S4, the straightness error estimation value dZ obtained in step S3 is used as the correction amount for the Z-axis, and the Z-axis is corrected and controlled according to the X-axis command position. This correction control corrects the straightness error in the Z-direction of the X-axis caused by the straightness error on the upper surface of the bed 1. Therefore, the straightness error in the Z-direction of the X-axis is reduced, and the machining accuracy is improved.

[0063] The straightness error correction method and straightness error correction device S of the portal machining center M, configured as described above, calculate the straightness error inference value based on the workpiece tilt inference inference value and the straightness curve of the worktable surface considering the straightness curve of the upper surface of the machine bed, and perform correction based on the inference value. Therefore, it is possible to perform straightness correction considering situations where the workpiece W tilts according to fixed positions 15 and 16 and the worktable 2 does not move straight.

[0064] For example, by applying the straightness error correction method and straightness error correction device S of the present invention, even when the ground where the portal machining center M is installed deforms over time and the bed 1 also deforms due to its influence, thus easily causing straightness error of the table linear drive axis, high machining accuracy can be maintained.

[0065] The structure of the machine tool straightness error correction method and straightness error correction device in this invention is not limited in the manner described in the above embodiments, and can be appropriately modified as needed without departing from the spirit of the invention.

[0066] For example, although the calculation and error correction of the straightness error in the Z direction of the X-axis have been described in the above embodiments, the calculation and error correction can also be performed on the straightness error in the Y direction, and both the straightness error in the Z direction and the straightness error in the Y direction can be taken as objects.

[0067] Furthermore, as long as there is a table drive shaft that enables the table to move linearly relative to the bed and two spindle head drive shafts that enable the spindle head to move linearly relative to the bed in a direction perpendicular to the table drive shaft, the straightness error correction method and straightness error correction device of the present invention can also be applied to machine tools other than gantry machining centers.

[0068] The fixed position of the workpiece is not limited to two places; it can be one place or more than three places.

Claims

1. A method for correcting the straightness error of a machine tool, the machine tool having: Bed frame; A worktable, which can hold the workpiece in place; Spindle head, which enables the mounted tool to rotate; A table drive shaft that enables the table to perform linear motion relative to the bed; and Two spindle head drive axes enable the spindle head to move linearly relative to the bed in a direction perpendicular to the table drive axis. Its features are, The straightness error correction method for this machine tool involves the following steps: The straightness curve of the upper surface of the bed is calculated based on the straightness curve of the upper surface of the bed and the command position of the worktable drive shaft. The inclination value of the workpiece is calculated based on the fixed position information of the workpiece relative to the reference position of the worktable and the calculated straightness curve of the upper surface of the worktable. The straightness error estimate of the worktable drive shaft at the spindle head position is calculated based on the commanded position of the worktable drive shaft, the calculated straightness curve of the upper surface of the worktable, and the calculated inclination estimate of the workpiece. as well as The calculated straightness error estimate is used as a correction value to correct at least one of the spindle head drive shafts.

2. The method for correcting the straightness error of a machine tool according to claim 1, characterized in that, The workpiece is fixed to the worktable at multiple points. In the calculation of the inclination inference value, the difference value of the straightness curve of the upper surface of the worktable corresponding to the fixed position information of the multiple fixed positions is used.

3. A straightness error correction device for a machine tool, the machine tool having: Bed frame; A worktable, which can hold the workpiece in place; Spindle head, which enables the mounted tool to rotate; A table drive shaft that enables the table to perform linear motion relative to the bed; and Two spindle head drive axes enable the spindle head to move linearly relative to the bed in a direction perpendicular to the table drive axis. Its features are, The straightness error correction device of this machine tool has the following features: A bed curve storage unit stores the straightness curve of the upper surface of the bed. A fixed position information storage unit stores fixed position information of the workpiece relative to the reference position of the worktable; The worktable curve calculation unit calculates the straightness curve of the upper surface of the worktable based on the straightness curve of the upper surface of the bed stored in the bed curve storage unit and the command position of the worktable drive shaft. The workpiece tilt calculation unit calculates the tilt estimate of the workpiece based on the fixed position information stored in the fixed position information storage unit and the calculated straightness curve of the worktable surface. The straightness error calculation unit calculates the straightness error estimate of the worktable drive shaft at the spindle head position based on the commanded position of the worktable drive shaft, the calculated straightness curve of the upper surface of the worktable, and the calculated tilt estimate of the workpiece; and The correction control unit uses the calculated straightness error estimation value as a correction amount to perform correction control on at least one of the spindle head drive shafts.

4. The machine tool straightness error correction device according to claim 3, characterized in that, The workpiece is fixed to the worktable at multiple points. In the workpiece tilt calculation unit, the tilt estimation value of the workpiece is calculated using the difference values ​​of multiple values ​​of the straightness curve of the worktable surface corresponding to the fixed position information of multiple fixed positions respectively.

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