JP2002283189A - Thermal displacement measurement method for machine tools - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a multi-degree-of-freedom simultaneous measurement method for a thermal displacement caused by the movement of a linear motion element of a machine tool, including a component other than the positioning deviation. SOLUTION: A movable part (SH, 54) of a machine tool is repeatedly linearly moved in a predetermined axial direction, and a movable part generated along with the linear movement and a stationary part (TB, 52) paired with the movable part. Is measured at a predetermined timing associated with the linear motion. Simultaneously with the measurement, the thermal displacement occurring between the movable part and the stationary part in at least one direction different from the direction of the linear motion is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring a thermal displacement accompanying a linear movement of a machine tool such as a machining center or a lathe.

[0002]

2. Description of the Related Art With the growing interest in thermal deformation of machine tools in recent years, the International Organization for Standardization is establishing standards for thermal displacement of machine tools in order to eliminate problems in international trade of machine tools. Yes (international standard ISO
230 "General rules for machine tool testing Part 3: Determination of thermal effects" (Te
st code for machine tools Part 3: Determin
ation of thermal effects: Hereinafter referred to as ISO230-3. )reference).

[0003] ISO 230-3 specifies a method for evaluating the effects of three states, that is, a change in environmental temperature, a rotation of a spindle, and a movement of a linear axis, on a thermal displacement of a machine tool. Among these measurement methods, the method of evaluating the effect of the motion of the linear axis includes a process of measuring the thermal effect of positioning accuracy at both ends of the linear motion using a laser interferometer. That is, the laser interferometer is attached to the spindle end of the machine tool,
The laser reflecting mirror is fixed to the end of the table of the machine tool, and in this state, the table or the spindle head is moved linearly along the guide surface, and the change due to the thermal displacement of the positioning deviation at both ends of the feed movement is detected. As a device for detecting thermal displacement, a dial gauge may be used in addition to the laser interferometer.

[0004]

As the heat source relating to the feed movement of the machine tool, friction between the feed screw and the nut, the bearing of the feed screw, the drive motor, the friction on the sliding surface, and the like can be considered.
In a machine whose positioning is controlled by the semi-closed loop method, heat generated from these causes the feed screw to expand, thereby lowering the positioning accuracy. Therefore, it is very necessary to measure and evaluate the influence of the thermal displacement on the positioning deviation. However, considering the recent increase in the speed of the feed drive system and the increase in the adoption of a so-called closed loop system for directly detecting the position of a linear motion element, thermal displacement components in directions other than the positioning deviation cannot be ignored, and the It is considered necessary to evaluate the measurement in the same way as the thermal displacement measurement by rotation.

Accordingly, an object of the present invention is to propose a multi-degree-of-freedom simultaneous measuring method including a component other than the positioning deviation for a thermal displacement caused by the movement of a linear motion element of a machine tool, and an apparatus used therefor. .

[0006]

Hereinafter, the present invention will be described. In addition, in order to facilitate understanding of the present invention, reference numerals in the accompanying drawings are added in parentheses, but the present invention is not limited to the illustrated embodiment.

According to the measuring method of the present invention, the movable part (SH, 54) of the machine tool is repeatedly linearly moved in a predetermined axial direction, and the movable part generated along with the linear movement is paired with the movable part. The thermal displacement in the direction of the translational motion between the stationary portion (TB, 52) and the linear motion is measured at a predetermined timing associated with the translational motion, and at the same time as the measurement, at least one of the directions different from the direction of the translational motion is measured. The thermal displacement generated between the movable part and the stationary part in one direction is measured.

According to this method, since the thermal displacement in the direction different from the linear motion is measured simultaneously with the measurement of the positioning deviation in the direction of the linear motion, the behavior accompanying the thermal displacement of the machine tool can be analyzed from various aspects. It is possible to more accurately grasp the state of thermal displacement. For example, when measuring the thermal displacement accompanying the linear motion of the machine tool in the X-axis direction, only the positioning deviation in the X-axis direction has been previously measured as the object to be measured. Thermal displacement in a direction different from the direction, such as translational deviation in the Y-axis direction or Z-axis direction, angular deviation around the X-axis, Y-axis or Z-axis, can be detected. It becomes possible to specifically grasp whether thermal displacement has occurred.

In the measuring method according to the present invention, the movable part is, for example, a spindle head, a table, a saddle, a reciprocating tool rest of a lathe, or the like, of a machine tool driven to generate a predetermined machining motion between a tool and a work. A stationary part means a part that is kept stationary relative to its movable part. The stationary part does not necessarily have to be a part that should always be fixed. It is desirable to set the timing of the measurement at intervals such that the transition of thermal deformation accompanying the linear motion can be sufficiently grasped. Various sensors of a non-contact type or a contact type may be used for measuring the thermal displacement.

In the measuring method according to the present invention, one of the movable part and the stationary part is correlated with a distance to a detection object (21) arranged in the direction of the linear movement or a change in the distance. A first sensor (S
, S2) and a second sensor (2) that outputs a signal correlated with a distance to a detection target (22, 23) or a change in the distance, which is arranged in at least one axial direction orthogonal to the direction of the linear motion. S3 to S7), and the other of the movable portion and the stationary portion is provided with a surface to be measured (21, 22, 2) as a detection target of the first and second sensors.
3) is provided so that each sensor can detect a corresponding measured surface when the movable section reaches a predetermined measurement position with respect to the stationary section, and when the movable section reaches the measurement position, Based on an output signal from each sensor, the direction of the linear motion and the thermal displacement in at least one direction different from the direction of the linear motion may be measured.

[0011] With respect to the direction of the rectilinear movement, the measurement positions may be set at both ends of a movement range of the movable portion.
For example, when the movable part performs a linear motion in the X-axis direction, the measurement position may coincide with both ends of the movement range of the movable part in the X-axis direction. By measuring the thermal displacement at both ends of the linear motion in this way, it is possible to know in more detail the state of occurrence of the thermal displacement in another direction different from the direction of the linear motion. Note that the measurement position may or may not coincide with both ends of the movement range of the movable unit in directions other than the linear movement. If they do not match, that is, if the measurement position is out of the motion range other than in the direction of the linear motion, move the movable part to move the sensor to the measurement position in the direction in which the deviation is provided. become.

An intermediate measurement position is set at a predetermined position on a straight line connecting the measurement positions, and when the movable portion reaches the intermediate measurement position, the other of the movable portion and the stationary portion is set at the other position. A surface to be measured (22, 23 on the target 2C) functioning as a detection target by the sensor may be provided facing the second sensor. As described above, if the measurement is performed at both ends and the intermediate position of the linear motion, the influence of the thermal displacement on the straightness of the motion can be detected.

5. The apparatus according to claim 2, wherein a plurality of said second sensors are provided at a distance from each other in one axial direction perpendicular to the direction of said rectilinear movement. Thermal displacement measurement method. In this case, it is possible to detect an angular deviation around an axis orthogonal to the axis direction by calculating a difference between detection values of a plurality of sensors arranged in one axis direction orthogonal to the direction of the rectilinear motion. it can. In addition, it is desirable that the plurality of sensors arranged in one axial direction are arranged symmetrically with respect to the direction in which they are arranged, with the main shaft center position interposed therebetween.

The second sensor may be provided in each of two axial directions orthogonal to the direction of the linear movement.
In this way, thermal displacement can be detected in more directions, and the state of thermal displacement can be known more accurately.

[0015] The first sensor and the second sensor may be provided in the movable part. When provided on the movable part, for example, measurement can be performed using the same sensor at a plurality of locations separated in the feed direction, and the number of sensors used is reduced.

According to another measuring method of the present invention, a spindle head (SH) and a table (TB) of a machine tool are repeatedly and relatively linearly moved in a predetermined axial direction, and the spindle head (SH) and the table (TB) are moved along with the linear movement. The thermal displacement in the direction of the linear motion occurring between the linear motion and the table is measured at a predetermined timing associated with the linear motion, and at the same time as the measurement, the thermal displacement is measured in at least one direction different from the direction of the linear motion. The thermal displacement generated between the main shaft and the table is measured.

According to this measuring method, the thermal displacement generated between the main shaft of the machine tool and the table is measured not only in the direction of the linear motion but also in other directions. The influence of the generated thermal displacement can be analyzed from various aspects, and the state of the thermal displacement can be grasped more accurately.

A first sensor (S1, S1) which outputs a signal correlated to a distance to a detection object (21) arranged in the direction of the linear motion or a change in the distance is provided at the spindle head.
2) and a second sensor (S3 to S3) for outputting a distance to a detection target (22, 23) arranged in at least one axial direction orthogonal to the direction of the linear motion or a signal correlated to a change in the distance. S7), and a target (2L, 2R, 2C) having a surface to be measured (21) as a detection target of the first and second sensors on the table.
Is provided so that each sensor can detect a corresponding measured surface when the spindle head relatively moves to a predetermined measurement position with respect to the table, and the spindle head performs predetermined measurement with respect to the table. Based on an output signal from each sensor when relatively moved to the position, the direction of the linear motion and the thermal displacement in at least one direction different from the direction of the linear motion may be measured.

According to a further aspect of the present invention, there is provided a measuring method in which a measuring direction is arranged in one of an X-axis direction and a Y-axis direction of a machine tool, and a detection target (2
A first sensor (S1, S2) for outputting a signal correlated with the distance of 1) or a change in the distance, and at least one of the other in the X-axis direction or the Y-axis direction and the Z-axis direction measurement direction And a second sensor (S3 to S3) that outputs a signal correlated with the distance of the detection target (22, 23) or a change in the distance in the measurement direction.
S7) and the sensor support jig (1) provided with the spindle head (S7).
H), and the surfaces to be measured (21, 22, 23) functioning as detection targets of the first sensor and the second sensor, respectively, on the table (TB) of the machine tool.
The target (2L, 2R, 2C) provided with is arranged such that each sensor can detect a corresponding measured surface when the spindle head relatively moves to a predetermined measurement position with respect to the table, The spindle head and the table are connected to the X
The linear motion is repeatedly performed in either the axial direction or the Y-axis direction, and the spindle head is relatively moved to the measurement position with respect to the table at a predetermined timing associated with the linear motion. The direction of the rectilinear motion generated between the main shaft and the table based on the output signals of the first sensor and the second sensor at the time, and the thermal displacement in at least one direction different from the direction of the rectilinear motion. Is measured.

According to this measuring method, the thermal displacement generated between the main shaft of the machine tool and the table is measured not only in the direction of the linear motion but also in other directions. The influence of the generated thermal displacement can be analyzed from various aspects, and the state of the thermal displacement can be grasped more accurately. It is only necessary to attach a sensor support jig to the spindle head and a target to the table, which makes setup easy.
The sensor support jig is desirably mounted on the main shaft. Thereby, the relationship between the sensor supporting jig and the target can be approximated to the relationship between the tool and the workpiece at the time of machining, and the thermal displacement can be measured, which is convenient.

In the sensor support jig, a pair of first sensors (S1 and S2) are arranged so that their respective measurement directions sandwich the central axis of the main shaft in either the X-axis direction or the Y-axis direction. And a pair of targets (2L, 2R) are provided on the table.
The surface to be measured (2) corresponding to each of the first sensors
1, 21) are arranged side by side in the direction of the linear motion so as to face the direction of the linear motion, and face the positive direction of the linear motion provided on one of the pair of targets (2L). A first sensor (S1) in which the surface to be measured has its measurement direction oriented in the negative direction of the linear motion.
And a first sensor in which a measurement surface provided in the other target (2R) of the pair of targets in the negative direction of the linear motion is directed in the positive direction of the linear motion ( The measurement position may be set so as to be detected in S2). In this case, the positioning deviation at both ends of the linear motion is detected by the pair of first sensors.

When the spindle head moves relative to the table to an intermediate measurement position set between the pair of targets, the second sensor (S3, S4) is placed on the table. An intermediate position target (2C) including a surface to be measured (22) that functions as an object to be detected by the sensor may be provided.

A plurality of second sensors (S3, S4) direct their respective measurement directions to the same side in the other of the X-axis direction and the Y-axis direction, and the X-axis direction or the Y-axis direction. It may be provided at a distance from each other in any one of the axial directions. In this case, along with the positioning deviation in one of the X-axis and Y-axis directions, the thermal displacement in the other axis direction and the angular deviation around the Z-axis are detected. The thermal displacement can be obtained more accurately by averaging the detection values of a plurality of sensors provided at a distance from each other in either the X-axis direction or the Y-axis direction. In this case, it is desirable that the plurality of second sensors arranged in one of the X-axis direction and the Y-axis direction be symmetrically arranged from the center of the main axis with respect to the arrangement direction.

The second sensor (S5 to S7) may be provided with its measurement direction in the Z-axis direction and toward the side approaching the table. In this case, the thermal displacement in the Z-axis direction is also detected.

The second sensors (S5 to S7) oriented in the Z-axis direction may be provided at a plurality of different positions in a plane orthogonal to the central axis of the main shaft. In this case, the inclination with respect to the plane orthogonal to the Z axis can be detected.

The second sensors (S5, S6) oriented in the Z-axis direction are provided at a plurality of positions in a plane orthogonal to the central axis of the main shaft and symmetrically with respect to the central axis of the main shaft. Is also good. In this case, the thermal displacement in the Z-axis direction on the axis of the main shaft can be accurately detected by calculating the average value of the detection values of the sensors.

The second sensor (S5, S7) oriented in the Z-axis direction is in a plane orthogonal to the center axis of the main shaft and orthogonal to the direction of the linear motion in the X-axis direction or the Y-axis direction. May be arranged side by side at a distance from each other. In this case, by taking the difference between the detection values of the second sensor, it is possible to detect the angular deviation around the axis of linear motion.

According to still another aspect of the present invention, there is provided a measuring method in which a measuring direction is arranged in a direction of an axis of a main shaft of a lathe (50), and a signal correlated with a distance of a detection target in the measuring direction or a change in the distance. The first sensor (S1,
And S2) outputting a signal correlated with the distance of the detection target in the measurement direction or a change in the distance with the measurement direction oriented in a direction orthogonal to the axial direction of the main shaft.
A sensor support jig (1) provided with the above-mentioned sensors (S3 to S7) is mounted on a carriage (54) of a lathe, and the first tool is provided between a spindle of the lathe and a tailstock (53). Target (2) provided with surfaces to be measured (21, 22, 23) functioning as detection targets of the second sensor and the second sensor, respectively.
L, 2R, 2C) are arranged so that each sensor can detect the corresponding surface to be measured when the carriage moves to a predetermined measurement position, and the carriage is moved linearly repeatedly in the axial direction of the spindle. The carriage is moved to the measurement position at a predetermined timing associated with the rectilinear movement, and the main shaft and the carriage are moved based on output signals of the first sensor and the second sensor at that time. The thermal displacement in the direction of the rectilinear motion and the other direction is measured.

According to this method, not only the positioning deviation in the axial direction of the main shaft of the lathe but also the thermal displacement in another axial direction (for example, the cutting direction of the tool) can be detected.

In the present invention, the Z-axis direction of the machine tool refers to the axial direction of the main shaft, and the X-axis and Y-axis directions refer to two-axis directions orthogonal to the Z-axis direction. In the present invention, the thermal displacement in a direction different from the direction of the linear motion includes translational deviation in at least one of two axial directions orthogonal to the linear motion, and three orthogonal directions including an axis in which the linear motion is performed. Angular deviation about at least one of the axes.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an outline of a 6-DOF thermal displacement measuring apparatus according to an embodiment of the present invention. The figure shows that the movable part of the machine tool that moves linearly is the spindle head SH or table TB.
Shows the setup in the case of. The measuring apparatus includes a sensor support jig 1 to which seven non-contact displacement sensors S1 to S7 (see FIG. 2) are attached, and three targets 2L, 2R, and 2C. In the present specification, when it is not necessary to distinguish the sensors S1 to S7, they are represented by reference numeral S, and when it is not necessary to distinguish the targets 2L, 2R, 2C, they are represented by reference numeral 2. Also, the targets 2L, 2R, and 2C may be simply referred to as targets L, R, and C, respectively (for example, FIG. 8).
reference).

The sensor support jig 1 is attached to a spindle end via a tool holder (not shown) or the like.
Is mounted on the table TB. Each target 2 is arranged on one straight line parallel to the feed direction. In the test, the spindle head SH and the table TB are relatively linearly reciprocated between the targets 2L, 2R, and 2C. The measurement of the thermal displacement is performed by stopping the sensor support jig 1 on the target 2 and detecting a change in a gap between the target 2 and the sensor S.

ISO 230-3 specifies a method for measuring the change in the positioning deviation only at the positions at both ends of the reciprocating feed motion. This is because, in general, the main cause of the change in the positioning deviation is thermal expansion of the lead screw, and the position where the positioning deviation changes most is located at one of the positions at both ends of the feed movement. However, in the method of the present embodiment, a translational deviation component in a direction orthogonal to the feed direction and an angular deviation component around each axis are also measured. From the translational deviation measurement results at a plurality of points, it is possible to evaluate the so-called straightness, which is the deviation from the straight line of the motion. Straightness is a basic evaluation item of linear motion accuracy, and is very important in predicting machining accuracy. Therefore, in the present embodiment, the target 2C is provided near the center of the feed motion, and the influence on the straightness due to thermal deformation is evaluated together with the translational deviation components obtained at both ends of the reciprocating motion.

FIG. 2 shows the sensor support jig 1. 2A is a plan view, FIG. 2B is a front view, and FIG. 2C is a right side view. The sensor support jig 1 includes a plate member 10 made of carbon steel for fixing the sensors S1 to S7 and a bar member 11 for coupling to a spindle end. In order to minimize the influence of the thermal deformation of the jig 1 itself, the shape of the support jig 1 is symmetrical with respect to the center axis of the bar 11. Further, the thickness of the plate member 10 is designed to be relatively large in order to make the plate member 10 less susceptible to changes in the temperature of the environment. The bar 11 has a simple round bar shape in order to be compatible with various machines and types of spindle ends, and is connected to the spindle via a tool holder for an end mill.

On the sensor supporting jig 1, seven non-contact displacement sensors S1 to S7 are radially arranged. These sensors S output signals corresponding to the amount of gap between the sensor S and the surface of the target 2 shown in FIG. The sensor S only needs to be capable of outputting a signal correlated with the distance to the measurement target or a change in the distance, and uses a non-contact type sensor such as a capacitance type or high frequency induction type proximity switch, an ultrasonic sensor, or the like. Alternatively, a contact-type sensor that detects a displacement by bringing a probe such as a plunger into contact with a measurement target may be used. However, using a non-contact type sensor is advantageous in terms of accuracy in that the sensor is not affected by the state of contact with the measurement target.

The sensors S1 and S2 are mounted outward on a straight line passing through the central axis of the bar 11, respectively. The sensors S3 and S4 are mounted outward on the same side of the plate 10 and each sensor S
3. Distances in the X-axis direction up to S4 are equal to each other. The sensors S5, S6, and S7 are mounted so as to protrude from the bottom surface of the plate material 10, respectively. The sensors S5 and S6 are arranged on one diagonal of the plate 10 and the sensor S7 is the plate 1
0 are arranged on the other diagonal line. The distances from the central axis of the bar 11 to the sensors S5 to S7 are equal to each other.

A measurement sequence can be easily designed by installing a sensor 12 for detecting a trigger signal, for example, an optical fiber sensor and detecting a trigger mark attached to the target 2.

FIGS. 3A to 3D show the target 2L.
The target 2C is shown in FIG. 3A is a plan view of the target 2L, FIG. 3B is a front view of the target 2L, FIG. 3C is a right side view of the target 2L, and FIG. 3D is a IIId portion of FIG. (E) is a plan view of the target 2C. Target 2
R has a symmetrical shape with the target 2L and is not shown in detail.

As shown in these figures, each target 2
Is detachably mounted on the block 20.
By changing the block 20, the height of the target 2 can be changed. As shown in FIGS. 3A to 3C, the targets 2L are two orthogonal to each other when viewed from above.
It has two side surfaces 21 and 22 and a bottom surface 23 orthogonal to each side surface 21 and 22. By detecting a change in the gap between the sensor S and the side surfaces 21 and 22 or the bottom surface 23 (a gap g4 between the sensor S4 and the side surface 22 is shown in FIG. 3D as an example), the thermal displacement is measured. You.

On the other hand, as shown in FIG. 3 (e), the target 2C arranged at the center of the stroke of the feeding motion is provided with a side surface 21 extending in one direction and a bottom surface 23 orthogonal to the side surface. However, in the target 2C, the side surface 22 for detecting the positioning deviation is omitted.

The method for detecting thermal displacement using the target 2L is as follows. The change in the positioning deviation from the change in the gap between the side surface 21 of the target 2L and the sensor S1 is determined by the
The thermal displacement in the Y-axis direction is detected from the average value of the gap change between the sensors S3 and S4, and the angle deviation C about the Z-axis is detected from the difference between the detection values of the sensors S3 and S4. Further, a change in the gap with the bottom surface 23 of the target 2 is detected by three sensors S5 to S7. Sensor S5,
The thermal displacement in the Z-axis direction is detected from the average of the detected values in S6.
The remaining angular deviation A about the X axis is obtained from the difference between the detection values of the sensors S5 and S7, and the angular deviation B about the Y axis is the sensor S
6. Determined from the difference between the detected values in S7. The sensor support jig 1 approaches the target 2L from one direction of the X axis. Therefore, no target is set for the sensor S2, and the sensor S2 is not used in relation to the target 2L. Target 2R is target 2L
The method of detecting thermal displacement using the target 2R is the same as that of the target 2 except that S2 is used instead of the sensor S1 to detect a change in positioning deviation.
Same as L.

The above thermal displacement detection method is summarized below.
The output of the sensor S is defined as a positive direction in which the direction in which the gap between the sensor 2 and the surface of the target 2 becomes large is the positive direction, and the sensor Si (where i = 1
The gap change corresponding to (7) is denoted by △ gi. The distance between the sensors Si and Sj (j = 1 to 7) is defined as Lij.

[0043]

(Equation 1)

(Equation 2)

(Equation 3)

(Equation 4)

(Equation 5)

(Equation 6) The target 2C illustrated in FIG.
Unlike L, there is no side surface (wall surface orthogonal to the X-axis direction) for detecting the positioning deviation. That is, on the target 2C, only the thermal displacement components in five directions excluding the positioning deviation are detected. As described above, the influence of the thermal deformation on the positioning deviation is remarkable at both end points of the motion, and if it is evaluated at this position, it is considered that there is not much need to evaluate at the center.

However, if necessary, the target 2C can also be constructed in the same structure as the targets 2L and 2R (a structure having a side surface orthogonal to the X-axis direction), and the positioning deviation can be detected on the target 2C. It is. In that case, for the measurement, it is necessary to perform a feed motion in a direction different from the linear motion to be evaluated. For example, when the X axis is to be evaluated, in order to avoid interference with the target 2C,
The sensor support jig 1 reciprocates in the X-axis direction at a position shifted in advance in the positive Z-axis direction, and moves in the Z-axis direction on the target 2C only when thermal displacement is detected. May be approached to the bottom surface 23 of the target 2C. Involving such movement of the axis other than the object to be evaluated may affect the measurement. However, the distance of the exercise is smaller than that of the straight-running exercise to be evaluated, and if the frequency is sufficiently small, it can be almost ignored.

The material of the target 2 (including the block 20) may be determined according to the purpose of the test or the like.
In this embodiment, carbon steel was used in accordance with the same concept as the measurement method of -3. Carbon steel is recommended for the jig and target for thermal displacement measurement specified in ISO230-3. ISO230-3
In general, before the measurement was made, in the measurement of thermal displacement, the amount of low thermal expansion material was used so that the thermal expansion component of the device was not included in the measurement value, or the thermal deformation was corrected based on the results of temperature measurement performed simultaneously. And so on. However, in consideration of the processing accuracy of the machine tool, the temperature of the workpiece itself changes according to the table and the environmental temperature, and the workpiece expands thermally, thereby causing a relative displacement between the tool workpieces which directly affects the processing accuracy. Therefore, the idea that evaluation including this effect is more appropriate was introduced in ISO230-3. In consideration of the fact that there are many iron-based materials in ISO 230-3, carbon steel should be used as the material for the target of the measuring device corresponding to the workpiece, the sensor fixture, the test bar equivalent to the tool, etc. Is recommended.

FIG. 4 shows the movement path of the sensor support jig 1 with respect to the target 2 in the thermal displacement measurement test. The test in this embodiment is based on the principle that the axis to be tested reciprocates linearly and continuously, and the sensor support jig 1
Are sequentially stopped on three targets 2L, 2R, and 2C to measure the thermal displacement. However, ISO230
As compared with the method of -3, the number of data is large because the number of targets 2 is one more and the displacement component to be detected is six times as large. In addition, when the number of suspensions performed at the time of measurement increases, the cooling effect during the stop increases, which affects the thermal displacement measurement value originally desired to be evaluated. Therefore, instead of performing stop measurement on all targets in the reciprocating motion path, reciprocation is performed without performing measurement for a certain period of time, and then the sensor support jig 1 is temporarily stopped on each target 2 for measurement. Repeat to do. The target 2 may be moved by driving the table TB in place of the sensor support jig 1, but in the following description, the test is performed by moving the sensor support jig 1 for convenience.

The path PT1 in FIG. 4 indicates a path for causing thermal displacement, and the paths PT2 to PT5 indicate paths for measurement. In the figure, the reciprocating path PT1 and the measurement paths PT3 and PT4 are set on two different straight lines parallel to each other. This illustrates a path when the trigger sensor 12 is used. Hereinafter, the measurement method shown in FIG. 4 will be described with reference to the flowchart shown in FIG.

In the present embodiment, first, the operation of the X-axis feed motor 31 is controlled by the operation control unit 30 of the machine tool so that the sensor support jig 1 is repeatedly reciprocated along the reciprocating path PT1 (step SP1). . After the number of reciprocating movements reaches a predetermined number, the operation control unit 30 controls the operation of the Y-axis feed motor 32 to move the sensor support jig 1 through the path PT2.
Along the axis to the measurement path PT3 (step SP2 → SP3). At this time, the trigger sensor 12
It is determined whether or not has detected the trigger mark 24 attached to the bottom surface 23 of the target 2C (step SP4). If detected, the sensor support jig 1 is stopped and the sensors S2 to S4 are detected.
The measurement processing of the gap amounts g2 to g7 using S7 is performed (step SP5). Triggering (trigger sensor 12
It is desirable that the time from the detection) to the stop of the sensor support jig 1 be minimized.

Thereafter, the sensor supporting jig 1 is moved in the X-axis direction along the path PT3 (step SP6), and when the trigger mark 24 of the next target 2C is detected, the gap amount using the sensors S3 to S7 The measurement processing of g3 to g7 is performed (step SP7 → SP8). Further, the sensor support jig 1 is moved in the X-axis direction along the path PT4 (step SP9), and when the trigger mark 24 of the next target 2L is detected, the gap amount g1 using the sensors S1, S3 to S7. , G3 to g7 (Step SP10)
→ SP11). Thereafter, the sensor support jig 1 is returned to the reciprocating path PT1 along the path PT5 (step SP
12) The sensor support jig 1 is repeatedly reciprocated again along the path PT1 (step SP11 → SP1).
2).

In the measurement described above, the sensors S1 to S7
Is output to a predetermined displacement calculator 33, where the sensors S1 to S7 and the side surfaces 21, 22, or the bottom surface 2 of the measurement target are measured.
3 is calculated. As the displacement calculation unit 33, a measurement processing device used in a known proximity sensor or the like can be used. On the other hand, the output signal of the trigger sensor 12 is guided to the operation control unit 30. The operation control unit 30 has X
A program for controlling the operations of the axis feed motor 31 and the Y-axis feed motor 32 to move the sensor support jig 1 as described above is installed. The measurement method of the present invention is realized when the CPU of the operation control unit 30 executes the program.

By using the method and apparatus as described above, a thermal displacement measurement test can be realized with a relatively simple configuration and without increasing the number of data. Strictly, it is necessary to consider the effects of motion error and thermal displacement accompanying the movement in the Y-axis direction. In the former case, the effect can be estimated by measuring the repeatability of the movement deviation due to the movement in the Y-axis direction in advance. In the latter, the distance d of the Y-axis movement is set to a minimum of about several mm required for triggering, and the measurement paths PT3 to P3 are compared with the reciprocating movement of the path PT1.
It is considered that if the number of exercises of T4 is made sufficiently small, it can be almost ignored. If it is determined that these problems cannot be ignored, for example, the reciprocation path PT1
By setting the measurement paths PT3 and PT4 on the same straight line and counting the number of times of triggering, it is possible to solve the problem by adopting a method of matching the timing of the dwell (pause) with the measurement.

The present invention is not limited to the above embodiment, but can be implemented in various forms. For example, the targets 2L, 2R, and 2C may be integrally formed. The rectilinear motion targeted by the present invention is not limited to the motion of the machining center in the X-axis direction, and can be similarly applied to the Y-axis or the Z-axis. When measuring the thermal displacement in the Z-axis direction, only the target 2L or 2R is set on the table so as to be positioned on the extension of the main shaft, and the sensors S1, S3 to S7 are used when the main shaft moves to the lower end. The thermal displacement may be measured repeatedly. A pair of side surfaces 21 may be provided on one target 2L or 2R so that the displacements in the X-axis direction can be simultaneously detected by the sensors S1 and S2.

The machine tool to which the present invention is applied is not limited to a machining center, but may be any machine tool that performs a linear motion.

FIG. 6 shows an embodiment in which the measuring method of the present invention is applied to a lathe. In this embodiment,
In the lathe 50, a headstock 52 and a tailstock 53 are arranged on the upper surface of a bed 51, and a reciprocating carriage (a tool rest) 54 is arranged between them so as to be movable left and right. 3 targets 2L,
2R and 2C are connected to each other or formed integrally, and a shank 25 is provided on the left target 2L. While the shank portion 25 is gripped by the chuck 55, the right target 2R is pressed by the center 56 to arrange the targets 2L, 2R, and 2C in a line in the direction of the linear reciprocating movement of the carriage 54, while the sensor support jig is used. 1
Is mounted on a portion of the carriage 54 where the blade is mounted. The rotation of the main shaft is restricted by a suitable fixing means so that the main shaft does not rotate during the measurement test.

After mounting the test apparatus in this way, the carriage 54 is reciprocated in the axial direction of the main shaft, and the thermal displacements generated in each direction at that time are measured by the sensors S1 to S7 attached to the sensor support jig 1. May be detected by

In the above embodiment, the sensor S is provided on the spindle head or the carriage as a movable portion. However, the present invention is not limited to this, and the sensor is provided on a stationary portion such as a table.
A jig having a surface to be measured may be mounted on the movable part. Further, the sensors S1 to S7 do not always need to be mounted in the three-axis directions.
Only sensors oriented in either the axial or Z-axis directions may be used. One sensor can be used for one axial direction.

[0057]

EXAMPLE A 6-degree-of-freedom thermal displacement measuring device proposed above was prototyped, and a thermal displacement measuring experiment was conducted on an actual machine tool. The object of the experiment was the X-axis motion of the portal machining center. FIG. 7 shows an experimental apparatus. In the machining center to be tested, two upper and lower linear motion guides LM are attached along the cross rail CR, and a saddle SD (identical to the spindle head SH in FIG. 1) is supported on a movable portion of the linear motion guide LM. The saddle SD is driven in the X-axis direction (left-right direction in the figure) by a ball screw BS driven by a motor M mounted on the upper part of the left column CL. The thermal displacement due to the movement of the X axis is mainly caused by the expansion of the screw shaft due to friction between the screw shaft of the ball screw BS and the nut, and the spindle head SH due to the friction between the rail and the movable part (carriage) in the linear motion guide LM. And the thermal deformation of the cross rail CR are expected. Therefore, as shown in FIG.
Near the center of the upper surface of the lens, near the mounting position of the movable part of the linear motion guide LM (LT, LB, RT, RB),
The interior space, the sensor support jig 1, the target 2, the table TB, and the bed BD were each set, and the temperature was measured at the same time as the thermal displacement was measured. The experimental conditions are as follows.

X-axis movement distance: 500 mm Feed speed: 1700 mm / min Number of reciprocations: 10.5 dowels: 2 seconds Experimental time: 56 minutes FIG. 8 shows the results of temperature measurement. The graph shows the temperature change from the start of the experiment. FIG. 3A shows a temperature change at a position distant from the X-axis drive system as a heat source, and FIG. 3B shows a temperature change at a position near the heat source. In the figure, targets 2L, 2R, and 2C are denoted by L, R,
Represented by C. “Ambient” is the temperature measurement point in the machine in FIG. 7, and “Sensor Fixture” is the temperature at the temperature measurement point of the sensor support jig 1 in FIG.

From FIG. 8A, it can be seen that the temperature change of the table and the bed which are almost stopped is minute, and the temperature of the target fixed to the table does not change. On the other hand, the ambient temperature (Ambient) tends to decrease slightly, and the sensor support jig attached to the spindle end drops by about 0.5 ° C. in accordance with the environmental temperature. By the way, the components most affected by the thermal expansion of the sensor support jig are an X-axis component and a Y-axis component. However, the thermal expansion amount in the X and Y directions estimated from the temperature measurement result is about 0.3 μm, and the influence is small. Also, since the sensor support jig has a symmetrical shape, it is considered that it has no effect on the measurement of the angle deviation.

FIG. 8B shows that the temperature of the spindle head near the cross rail and the carriage tends to increase.
Comparing the temperatures near the carriage, the lower cross (B) of the two cross rails is closer to the upper (T).
The temperature is rising more. In addition, the right side (R) of the spindle head tends to have a higher temperature than the left side (L).

FIG. 9 shows a change in positioning deviation due to thermal displacement. From the figure, the influence of thermal expansion of a typical ball screw can be seen at the positions of the targets R and L. In particular, it can be seen that the increasing tendency of R is remarkable, and about one hour later, a thermal displacement of 200 μm has occurred. On the other hand, the thermal displacement of L is small. As shown in FIG. 7, the drive motor M is arranged on the left side, that is, on the target L side, of the X axis of the machining center to be tested. The thrust restraining force acting on the bearings at both ends supporting the ball screw BS is larger on the motor side bearing than on the other side, and therefore, a larger thermal displacement occurs on the target R side. It is considered something. This is the tendency of a ball screw drive shaft of a general semi-closed control system. It is considered that the results of this example are consistent with the results obtained by a test in accordance with the method specified in ISO 230-3, which is currently being standardized.

FIG. 10 shows the translation deviation Y in the horizontal direction. As can be seen from the figure, the results at the three points of the targets L, C, and R all show a monotonous decrease. This indicates that the spindle end has moved to the worker side, that is, the front side in FIG. 7. This may be caused by the thermal expansion of the spindle head in the Y-axis direction, the angular displacement of the spindle head around the X-axis independent of the position in the X-axis direction, or the This is considered to be caused by any of the following deformations. However, it is impossible to identify the cause only by the result of the thermal displacement in the Y-axis direction. As will be described later, the state of thermal deformation can be estimated by comparing the result of the angle deviation.

The maximum thermal displacement at the three points is about 5 μm, which is considerably smaller than the result of the positioning deviation, but cannot be ignored when performing high-precision machining. Comparing the values at the three points, the measured values of the targets L and R at both ends are almost the same, but the target C has a larger thermal displacement than these. This result indicates that the front surface of the cross rail CR may be deformed in the front direction. The cause may be deformation due to frictional heat between the rail of the linear motion guide and the carriage. FIG.
The rise in the temperature of the cross rail shown in (b) supports such deformation of the cross rail.

FIG. 11 shows the translational deviation Z in the vertical direction. The deviation Z was almost the same at all three points, indicating a monotonically increasing tendency. The maximum is about 8 μm, which is small compared to the change in the positioning deviation like the thermal displacement in the Y-axis direction.
It is a value that cannot be ignored due to processing accuracy. By the way, in actual machining, since the spindle is rotated, heat generation in the spindle system causes thermal displacement in the Z-axis direction. This value is known to be on the order of several tens of μm in a general-purpose machining center having no thermal displacement correction function, and in such a machine, there is no problem as long as the result of this experiment is on the order. Maybe. However, it is a fatal problem for machine tools that claim ultra-precision. Furthermore, progress in measures for thermal displacement and thermal displacement correction for the spindle system in recent years has been remarkable. In the near future, even a general-purpose machine will have a thermal displacement of 10 μm due to spindle rotation even in a general-purpose machine.
It is expected that: Therefore, the evaluation of the Z-direction thermal displacement accompanying the linear motion by the measuring method of the present invention will be a very important evaluation item in the future.

FIG. 12 shows the experimental results of the angle deviation A. 3
It can be seen that the same behavior was observed at all of the three measurement points, with a slight increase from immediately after the start to 4 minutes, and a monotonous decrease thereafter. This tendency indicates that the spindle head does not depend on the measurement point,
This indicates that the operator has fallen with time with respect to the back side as viewed from the operator. In the temperature measurement results shown in FIG. 8B, the lower part of the spindle head tends to have a higher temperature than the upper part. Therefore, it is considered that the temperature gradient inside the structure as seen in this temperature difference causes such thermal deformation. According to the definition of the direction of the thermal displacement, A, which is an angular deviation about the X axis, is one of the causes of the displacement in the Y axis direction at the spindle end. Therefore, Y shown in FIG.
It is considered that the tendency of the axial thermal displacement to decrease monotonically includes a component due to the decrease in the angle deviation A.

FIG. 13 shows the experimental results of the angle deviation B about the Y axis. The change in the deviation B was almost constant in the target C, but slightly decreased in the targets R and L.

FIG. 14 shows the experimental results of the angular deviation C about the Z axis. The reason why the deviation C tends to increase in all three targets L, C, and R is considered to be due to thermal deformation due to a temperature gradient inside the spindle head. The maximum thermal displacement increases in the order of the targets L, C, and R. This is considered to be due to the bending deformation of the cross rail. As already shown, it can be expected from the result of the displacement in the Y-axis direction in FIG.
The result confirms this consideration.

The machine tool according to the present embodiment is provided with a Z-axis thermal displacement function. From the results of the thermal displacement test by the spindle rotation performed separately, the maximum thermal displacement in the Z-axis direction at the spindle rotational speed of 20000 / min is approximately −20 μm when the thermal displacement correction function is enabled, and approximately −20 μm when the thermal displacement correction function is disabled. The result is 60 μm. By the way, such processing that requires high-speed rotation necessarily requires high-speed feeding. Therefore, it is expected that the thermal displacement caused by the rotation of the main shaft and the thermal displacement caused by the movement of the linear shaft will be superimposed during actual machining. In other words, when the machine being tested is actually used for machining, even if the thermal displacement correction function for the spindle is enabled due to the movement of the feed shaft, the thermal displacement will be significantly different from the expected one. . In this embodiment, the thermal displacement in the Z-axis direction due to the linear motion has a positive value, and as a result, the thermal displacement is smaller when the main shaft is rotated. This is only a side benefit. If this effect is not predicted, for example, even if the NC program is modified to cancel out this effect by predicting the thermal displacement during processing only from the results of the spindle thermal displacement test, if the feed motion is fast, I think It is not possible to improve the processing accuracy.
Further, as a machine tool maker, it is not possible to improve the processing accuracy only by correcting the thermal displacement caused by the rotation of the spindle.

As described above, the effectiveness of the thermal displacement measurement and evaluation method proposed in the present invention is apparent even when attention is paid only to the thermal displacement in the Z-axis direction. Further, in order to predict the machining accuracy, it is important to measure the component of the angle deviation. For example, even when the component of the translational deviation at the end of the spindle for detecting the deviation is small, if the angle deviation is large, the translational deviation at the distal end will increase if a tool longer in the Z-axis direction is used. In addition, as discussed in the above experimental results, translational displacement and angular displacement are correlated, and the test results of these individual directional components alone will clarify the behavior of thermal deformation of machine tools and the causes of their generation. It is impossible.

As described above, in order to clarify the location, the cause, the causal relationship, and the like of the thermal displacement due to the linear motion, not only the positioning deviation currently specified in ISO but also the present measuring method Evaluation of thermal displacement is required.

[0071]

As described above, according to the present invention, since the positioning deviation in the direction of the linear motion of the machine tool is measured and the thermal displacement in the direction different from the linear motion is measured simultaneously, The behavior associated with the thermal displacement of the machine can be analyzed from multiple angles, and the state of the thermal displacement can be grasped more accurately.

[Brief description of the drawings]

FIG. 1 is a perspective view schematically showing a measuring device according to an embodiment of the present invention.

FIG. 2 is a view showing a sensor support jig used in the measurement method of the present invention.

FIG. 3 is a view showing a target used in the measurement method of the present invention.

FIG. 4 is a diagram showing a relative movement path between a sensor support jig and a target in the measurement method of the present invention.

FIG. 5 is a flowchart showing a procedure for performing measurement along the route shown in FIG. 4;

FIG. 6 is a diagram showing an embodiment when the present invention is used for a lathe.

FIG. 7 is a view showing a relationship between a gate-type machining center used in the embodiment of the present invention and temperature measurement points.

FIG. 8 is a view showing a result of measuring a temperature by the apparatus of FIG. 7;

FIG. 9 is a view showing thermal displacement in the X-axis direction measured by the device of FIG. 7;

FIG. 10 is a view showing thermal displacement in the Y-axis direction measured by the device of FIG. 7;

FIG. 11 is a view showing thermal displacement in the Z-axis direction measured by the apparatus of FIG. 7;

FIG. 12 is a view showing an angle deviation A about the X axis measured by the apparatus of FIG. 7;

FIG. 13 is a view showing an angle deviation B about the Y axis measured by the apparatus of FIG. 7;

FIG. 14 is a diagram showing an angular deviation C about the Z axis measured by the device of FIG. 7;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sensor support jig 2L, 2R, 2C Target 10 Plate material 11 Bar material 12 Trigger sensor 20 Block 21, 22 Side surface 22 Side surface 23 Bottom surface 24 Trigger mark 25 Shank unit 30 Operation control unit 33 Displacement calculation unit 50 Lathe 51 Bed 52 Headstock 53 tailstock 54 carriage 55 chuck 56 center BD bed BS ball screw CL column CR cross rail LM linear motion guide PT1 reciprocating movement path PT3, PT4 measurement path S1-S7 sensor SD saddle SH spindle head TB table

Claims (18)

[Claims] A movable portion of the machine tool is linearly moved repeatedly in a predetermined axial direction, and the linear motion of the movable portion generated along with the linear motion and a stationary portion forming a pair with the movable portion is generated. A thermal displacement in a direction is measured at a predetermined timing associated with the linear motion, and simultaneously with the measurement, a thermal displacement is generated between the movable portion and the stationary portion in at least one direction different from the direction of the linear motion. A method for measuring a thermal displacement of a machine tool, comprising measuring a thermal displacement occurring. 2. A method according to claim 1, wherein one of the movable part and the stationary part is provided with a first sensor that outputs a signal that is correlated to a distance to a detection target or a change in the distance that is arranged in a direction of the linear motion. And a second sensor that outputs a signal correlated with a distance to a detection target or a change in the distance, which is arranged in at least one axis direction orthogonal to the direction of the linear motion, and the movable unit and the stationary unit. In each of the other portions, each sensor corresponds to a surface to be measured as a detection target of the first and second sensors when the movable portion reaches a predetermined measurement position with respect to the stationary portion. Provided to be able to detect the surface to be measured, based on an output signal from each sensor when the movable portion has reached the measurement position, the direction of the linear motion and at least one different from the direction of the linear motion About the direction Claim, characterized in that measuring the thermal displacement respectively 1
4. The method for measuring thermal displacement according to 1. 3. The thermal displacement measuring method according to claim 2, wherein the measurement positions are set at both ends of a movement range of the movable portion with respect to a direction of the linear motion. 4. An intermediate measurement position is set at a predetermined position on a straight line connecting the measurement positions, and when the movable portion reaches the intermediate measurement position in one of the movable portion and the stationary portion. 4. The thermal displacement measuring method according to claim 3, further comprising a measurement surface facing the second sensor and functioning as a detection target by the sensor. 5. The apparatus according to claim 2, wherein a plurality of the second sensors are provided at a distance from each other in one axial direction orthogonal to the direction of the linear motion.
The method for measuring thermal displacement according to any one of the above. 6. The thermal displacement measuring method according to claim 2, wherein the second sensor is provided in each of two axial directions orthogonal to a direction of the linear motion. . 7. The thermal displacement measuring method according to claim 1, wherein the first sensor and the second sensor are provided on the movable part. 8. A spindle head of a machine tool and a table are repeatedly and relatively linearly moved in a predetermined axial direction, and heat generated in the direction of the linear movement generated between the spindle head and the table with the linear movement. The displacement is measured at a predetermined timing associated with the linear motion, and at the same time, the thermal displacement generated between the main shaft and the table in at least one direction different from the direction of the linear motion is measured. A method for measuring thermal displacement of a machine tool. 9. A first sensor which outputs a signal correlated with a distance to a detection target or a change in the distance, which is disposed in a direction of the linear motion, and a direction orthogonal to the direction of the linear motion. And a second sensor that outputs a signal correlated with a distance to a detection target or a change in the distance, which is arranged in at least one axial direction, and the first and second sensors are provided on the table. A target having a surface to be measured as a detection target is provided such that each sensor can detect a corresponding surface to be measured when the spindle head relatively moves to a predetermined measurement position with respect to the table, On the basis of an output signal from each sensor when the spindle head relatively moves to a predetermined measurement position with respect to the table, at least one of the direction of the linear motion and the direction of the linear motion different from each other. Thermal displacement measuring method according to claim 8, characterized in that measuring the thermal displacement related directions. 10. A first device which is arranged so that a measurement direction is directed to one of an X-axis direction and a Y-axis direction of a machine tool, and outputs a signal relating to a distance of a detection target or a change in the distance in the measurement direction. The sensor, the X-axis direction or the other in the Y-axis direction, and the measurement direction is disposed at least one of the Z-axis direction measurement direction, the distance of the detection target with respect to the measurement direction or a change in the distance. A sensor support jig including a second sensor that outputs a correlated signal is mounted on the spindle head, and on the table of the machine tool, the first sensor and the second sensor are respectively detected as detection targets. A target having a functioning surface to be measured is arranged so that each sensor can detect a corresponding surface to be measured when the spindle head moves relative to the table to a predetermined measurement position. The spindle head and the table are repeatedly and relatively linearly moved in either the X-axis direction or the Y-axis direction, and the spindle head is moved relative to the table at a predetermined timing associated with the linear movement. Relatively moving to the measurement position, the direction of the linear motion generated between the main shaft and the table based on the output signals of the first sensor and the second sensor at that time, and the direction of the linear motion A method for measuring thermal displacement of a machine tool, comprising measuring thermal displacement in at least one direction different from the direction. 11. The sensor support jig includes a pair of first
Are provided with their respective measurement directions directed outward with respect to one of the X-axis direction and the Y-axis direction with the center axis of the main shaft interposed therebetween, and a pair of targets on the table. Are arranged side by side in the direction of the rectilinear motion such that the surfaces to be measured corresponding to the respective first sensors face the direction of the rectilinear motion, and are provided on one of the pair of targets. The surface to be measured facing the positive direction of the linear motion is detected by the first sensor whose measurement direction is oriented in the negative direction of the linear motion, and the linear motion provided on the other of the pair of targets is detected. The measurement position is set so that the surface to be measured facing in the negative direction is detected by the first sensor whose measurement direction is oriented in the positive direction of the linear motion. Thermal displacement measuring method according to claim 10. 12. The apparatus according to claim 12, wherein said spindle head is moved to an intermediate measurement position set between said pair of targets when said spindle head moves relative to said table.
12. The thermal displacement measuring method according to claim 11, wherein an intermediate position target having a surface to be measured that functions as a detection target of the sensor is provided opposite to the sensor. 13. A plurality of second sensors directing their respective measurement directions to the same side in the other of the X-axis direction and the Y-axis direction, and the measurement direction of the X-axis direction or the Y-axis direction. The thermal displacement measuring method according to claim 10, wherein the thermal displacement measuring methods are provided at a distance from each other in one of the axial directions. 14. The heat sensor according to claim 9, wherein the second sensor is provided with its measurement direction in the Z-axis direction and toward the side approaching the table. Displacement measurement method. 15. The apparatus according to claim 1, wherein the second sensors oriented in the Z-axis direction are provided at a plurality of different positions in a plane orthogonal to a center axis of the main shaft.
5. The method for measuring thermal displacement according to item 4. 16. The apparatus according to claim 16, wherein the second sensors oriented in the Z-axis direction are provided in a plane orthogonal to a center axis of the main shaft and at a plurality of positions symmetrical with respect to the center axis of the main shaft. The method for measuring thermal displacement according to claim 14, wherein: 17. The second sensor oriented in the Z-axis direction is in a plane orthogonal to a center axis of the main shaft and in a direction orthogonal to the direction of the linear motion in the X-axis direction or the Y-axis direction. The thermal displacement measuring method according to claim 14, wherein a plurality of the thermal displacement measuring methods are provided at a distance from each other. 18. A first sensor which is arranged with the measuring direction oriented in the axis direction of the main shaft of the lathe and outputs a signal related to the distance of the detection target in the measuring direction or a change in the distance, and an axis of the main shaft. Reciprocating the lathe with a second sensor that is disposed with the measurement direction oriented in a direction perpendicular to the direction and that outputs a signal related to the distance of the detection target in the measurement direction or a change in the distance. Attached to the table, between the spindle of the lathe and the tailstock, a target having a surface to be measured respectively functioning as a detection target of the first sensor and the second sensor, the carriage, Each sensor is arranged so as to be able to detect the corresponding measured surface when it has moved to a predetermined measurement position, and the carriage is repeatedly linearly moved in the axial direction of the main shaft, where the linear movement is associated with the linear movement. The carriage is moved to the measurement position at a fixed timing, and the linear movement of the linear movement generated between the main shaft and the carriage based on output signals of the first sensor and the second sensor at that time. A method for measuring thermal displacement of a machine tool, comprising measuring thermal displacements in a direction and other directions.