JP2011045985A - Method and device for correcting thermal displacement of numerical control machine tool - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and device for correcting the thermal displacement of a numerical control machine tool, accurately correcting a thermal displacement while suppressing a load for calculation processing in a numerical control unit. <P>SOLUTION: The front shaft part and the nut part moving range of a ball screw shaft are divided into five calculation zones at intervals of 80 mm, and the rear shaft part of the ball screw shaft is used as one calculation zone. Based on the feed rate and feed data of a table, the heating amounts generated in the six calculation zones are obtained every 50 ms. Based on the total heating amounts Q<SB>1</SB>-Q<SB>6</SB>and Q<SB>T</SB>obtained by accumulating the heating amounts obtained in 6,400 ms and a non-steady heat conduction equation, the temperature distributions in the six calculation zones are calculated every 6,400 ms. From the temperature distributions, the amounts of thermal displacement in the six calculation zones are calculated every 6,400 ms. Based on the calculated amounts of thermal displacement, the correction amounts for correcting the pitch error correction amounts of the divided positions of sixteen correction zones, respectively, are calculated for every 6,400 ms. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a thermal displacement correction method for a numerically controlled machine tool and a thermal displacement correction apparatus thereof, and more particularly to a configuration configured to correct an error due to thermal displacement of a ball screw mechanism that occurs during operation of a machine tool.

  The ball screw mechanism is widely adopted in machine tools as a positioning mechanism. Since the ball screw mechanism generates a pitch error between the rotation amount of the ball screw shaft and the amount of movement of the nut due to manufacturing tolerances or the like, the pitch error is corrected based on a preset table of pitch error correction amounts.

  In addition, the ball screw mechanism causes thermal expansion due to a temperature rise due to frictional resistance between the ball screw shaft, the nut, and each bearing portion, thereby causing thermal displacement. A semi-closed loop type is generally used in current NC machine tools. However, in this type of NC machine tool, the thermal displacement of the ball screw shaft appears as a positioning error as it is. Therefore, a method of applying pretension to the ball screw shaft and absorbing thermal expansion has been used as a countermeasure.

  Recently, a thick ball screw shaft has been used and the feed rate has become very fast, so the amount of heat generated has increased, and a very large tensile force has to be applied when trying to cope with the pre-tension method. For this reason, there has been a problem that the structure of the ball screw mechanism is deformed, or an excessive force is applied to the thrust bearing and seizes.

  Therefore, Patent Documents 1 to 3 disclose thermal displacement correction methods for a ball screw shaft that do not apply excessive pretension to the ball screw shaft and do not require a special measuring device. In Patent Documents 1 to 3, the amount of heat generated in each section of the ball screw shaft is calculated from the rotation speed of the servo motor, and the temperature distribution is obtained using a model in which the nut moving part of the ball screw shaft is divided into a plurality of sections. Next, the thermal displacement amount of the ball screw shaft is predicted from moment to moment, and this thermal displacement amount is corrected in-process by giving the NC device (control means) as a correction amount for correcting the pitch error correction amount. . According to this method, the calculated correction amount can be approximated to the actual elongation of the ball screw shaft.

For example, as shown in FIG. 8, in the ball screw mechanism, pitch error correction is performed for each correction section divided by a set length of 20 mm for the nut portion movement range (between X0 and X300 in machine coordinates) 81b of the ball screw shaft 81. . In this case, the thermal displacement correction amount is obtained for each calculation section divided by the same set length as the correction section for pitch error correction.
When the calculation interval is short, the number of calculation intervals increases, and there is a problem that the calculation cannot be performed within a predetermined time (calculation cycle). The set length of 20 mm described above is short as the calculation section length, and thus the calculation cycle has to be shortened. For this reason, the calculation processing load is high in order to obtain the thermal displacement correction amount by the numerical control device.

  On the other hand, when the ball screw shaft 81 is divided so that the length of the calculation section becomes long, the number of calculation sections decreases, and the calculation processing load in the numerical control device decreases. However, since the calculation accuracy of the thermal displacement amount is lowered, there is a problem that the machining accuracy is lowered. Therefore, it is necessary to set the length of the calculation section to an appropriate length in order to achieve the target machining accuracy while suppressing the calculation processing load in the numerical control device.

  Therefore, when the ball screw shaft 81 is divided into calculation sections for thermal displacement correction, the section of the calculation section is made to coincide with the origin (X0) of the machine coordinates, and the length of the calculation section in the nut portion movement range 81b of the ball screw shaft 81 is set. Is divided so as to be an integral multiple of the set length of the pitch error correction interval. For example, when the set length of the calculation section is set to 20 mm × 4 = 80 mm and the division position of the calculation section is matched with the machine coordinate X0, the result is as shown in FIG.

  Here, assuming that the length of the front shaft portion 81a is 100 mm, the length of the nut portion moving range 81b is 300 mm, and the length of the rear shaft portion 81c is 100 mm, the length of the calculation section 1 is 80 mm, The length is 20 mm, the length of the calculation sections 3 to 5 is 80 mm, the length of the calculation section 6 is 60 mm, the length of the calculation section 7 is 80 mm, and the length of the calculation section 8 is 20 mm. In addition, in the calculation section 2, the calculation section 6, and the calculation section 8, a remainder that cannot be divided by the section length (80 mm) of the calculation section is arranged.

  When the pitch error correction amount is corrected using the thermal displacement amount obtained in this calculation interval, the following method is used. First, the sum of the thermal displacement amounts of the calculation interval 1 and the calculation interval 2 is set as the thermal displacement amount at the position X0, and the pitch error correction amount of X0 is corrected. Next, since the length of the calculation section 3 is four times the set length of the correction section, ¼ of the thermal displacement amount of the calculation section 3 is equally distributed to the positions X20, X40, X60, and X80, so that the respective positions are calculated. The amount of thermal displacement is obtained. The pitch error correction amount at each position is corrected using the obtained thermal displacement amount. The pitch error correction amount is corrected in the same manner for the other calculation sections.

JP-A 63-256336 JP-A-4-240045 JP 7-299701 A

However, in the above dividing method, the lengths of the calculation interval 2, the calculation interval 6, and the calculation interval 8 are shortened.
In such a division method, the calculation cycle for calculating the thermal displacement amount is set to be small by the length (20 mm) of the calculation section 2 and the calculation section 8 having the minimum length among the remainders that cannot be divided by the section length of the calculation section. As a result, the processing load on the numerical controller increases.

  On the other hand, as shown in FIG. 13, there is a method of adding the calculation section 2, the calculation section 6, and the calculation section 8 in FIG. 12 to a section adjacent to the fixed side (left side in FIG. 12) of the ball screw shaft 81. In this case, since the length (80 mm) of the calculation section 2 and the calculation section 3 is a length that determines the calculation cycle for calculating the thermal displacement amount, it is possible to suppress the load of calculation processing of the numerical control device. However, in the movement range 81b from the front shaft portion 81a of the ball screw shaft 81 that affects the calculation accuracy of the thermal displacement amount, the calculation section 1 and the calculation section 4 are longer than the set length of the calculation section. Therefore, since the calculation accuracy of the thermal displacement amount is lowered, there is a possibility that the target machining accuracy cannot be achieved.

  An object of the present invention is to provide a thermal displacement correction method for a numerically controlled machine tool capable of performing highly accurate thermal displacement correction while suppressing the processing load in the numerical control device, and a thermal displacement correction device thereof. It is.

  According to a first aspect of the present invention, there is provided a numerically controlled machine tool thermal displacement correction method comprising: a feed screw ball screw mechanism; a servo motor that rotationally drives a shaft screwed with a nut of the ball screw mechanism; and the servo motor is controlled based on control data. In a method for correcting thermal displacement of a numerically controlled machine tool having a control means, a fixed side end connected to the shaft and the servo motor, and a movable side end opposite to the fixed side end are preliminarily provided. When the thermal displacement amount of the shaft is calculated during operation of the machine tool, the entire length of the shaft is divided by a fixed length calculation section from the fixed side end and includes the movable side end The length of the calculation section is set to be equal to or greater than the predetermined length, and the heat generation amount and the temperature distribution of the plurality of calculation sections are calculated.

  In this thermal displacement correction method for a numerically controlled machine tool, a heat generation amount and a temperature distribution in a calculation section are calculated. First, the fixed side end connected to the servo motor on the shaft and the movable side end opposite to the fixed side end are provided in advance, and the total length of the shaft is calculated from the fixed side end by a certain length. While dividing | segmenting in an area, the length of the calculation area containing a movable side edge part is set more than fixed length. Finally, the calorific value and temperature distribution of the plurality of divided calculation sections are calculated.

  In this way, the total length of the shaft is divided into fixed length calculation sections from the fixed side end, and the length of the calculation section including the movable side end is set to be equal to or greater than a certain length, so that the calculation of thermal displacement correction There is no need to match the section position of the section and the section position of the correction section for pitch error correction. Therefore, by setting the section length of the calculation section to be long, the calculation cycle for calculating the thermal displacement amount can be lengthened, and the calculation processing load in the numerical controller can be reduced. In addition, if the calculation section including the movable side end is longer than the other calculation sections, an error occurs when calculating the temperature distribution, but the movable side end is away from the servo motor that is the heat source. There is no problem because the degree of influence is less than that of other calculation intervals.

  The thermal displacement correction method for a numerically controlled machine tool according to claim 2 is the invention according to claim 1, wherein the nut movement range of the shaft is divided into a plurality of correction sections shorter than the calculation section, and each of the plurality of correction sections is divided. The pitch error correction amount for correcting the pitch error is corrected using the thermal displacement amount.

  A thermal displacement correction method for a numerically controlled machine tool according to claim 3 includes: a ball screw mechanism for driving driving; a servo motor that rotationally drives a shaft into which a nut of the ball screw mechanism is screwed; and the servo motor is controlled based on control data. And a control means that divides the nut movement range of the shaft into a plurality of correction sections, and corrects a pitch error for each of the plurality of correction sections. A fixed side end connected to the servo motor and a movable side end opposite to the fixed side end are provided in advance, and the entire length of the shaft is longer than the correction interval from the fixed side end. The servo motor is configured to divide the heat generation amount generated in the plurality of calculation sections in which the length of the calculation section including the movable side end portion is set to the predetermined length or more. A plurality of computations based on a first step obtained every predetermined time based on the rotation speed and control data, a total calorific value obtained by accumulating the calorific values of the plurality of computation sections for a predetermined period, and an unsteady heat conduction equation A second step of calculating a temperature distribution of the section for each predetermined period; a third step of calculating a thermal displacement amount of the plurality of calculation sections of the shaft for each predetermined period from the temperature distribution; and the plurality of calculation sections And a fourth step of calculating a correction amount for correcting the pitch error correction amount of each of the plurality of correction sections for each of the predetermined periods.

  In this thermal displacement correction method for a numerically controlled machine tool, a correction amount for correcting the pitch error correction amount is obtained by the following procedure. A fixed-side end connected to the shaft to the servo motor and a movable-side end opposite to the fixed-side end are provided in advance. First, the total length of the shaft is divided into fixed length calculation sections longer than the correction section from the fixed side end, and the length of the calculation section including the movable side end is set to a certain length or more. The amount of heat generated in the calculation interval is obtained every predetermined time based on the rotation speed of the servo motor and control data. Based on the total calorific value obtained by accumulating the obtained calorific value for a predetermined period and the unsteady heat conduction equation, the temperature distribution of the plurality of calculation sections is calculated for each predetermined period. The amount of thermal displacement in a plurality of calculation sections of the shaft is calculated for each predetermined period from the temperature distribution. Finally, based on the calculated thermal displacement amount, a correction amount for correcting the pitch error correction amount in each of the plurality of correction sections is calculated for each predetermined period. Thus, the same effect as that of claim 1 is obtained.

  According to another aspect of the present invention, there is provided a ball screw mechanism for feed drive, a servo motor for rotationally driving a shaft into which a nut of the ball screw mechanism is screwed, and the servo motor based on control data. And a control means that divides the nut movement range of the shaft into a plurality of correction sections, and corrects a pitch error for each of the plurality of correction sections. A fixed side end connected to the servo motor, a movable side end opposite to the fixed side end, a speed detecting means for detecting the rotational speed of the servo motor, and the total length of the shaft, The calculation side is divided into a calculation section having a fixed length longer than the correction section from the fixed side end, and a length of the calculation section including the movable side end is set to be equal to or more than the predetermined length. A calorific value calculation means for obtaining a calorific value generated in the calculation interval every predetermined time based on a rotation speed of the servo motor and control data, and a total heat generation in which the calorific values in the calculation intervals are accumulated for a predetermined period. A temperature distribution calculating means for calculating a temperature distribution of a plurality of calculation sections for each of the predetermined periods based on the amount and the unsteady heat conduction equation; and a thermal displacement amount of the plurality of calculation sections of the shaft from the temperature distribution. Based on the thermal displacement amount calculation means that calculates every predetermined period and the thermal displacement amount of the plurality of calculation sections, a correction amount that respectively corrects the pitch error correction amount of the plurality of correction sections is calculated for each predetermined period. And a correction amount calculating means.

  In this numerically controlled machine tool thermal displacement compensator, first, the speed detecting means detects the rotational speed of the servo motor. Next, the calorific value calculation means divides the entire length of the shaft into a calculation section having a fixed length longer than the correction section from the fixed side end, and sets the length of the calculation section including the movable side end to a fixed length. The amount of heat generated in the plurality of calculation intervals set as described above is obtained every predetermined time based on the rotation speed of the servo motor and control data. The temperature distribution calculation means calculates the temperature distribution of the plurality of calculation sections for each predetermined period based on the total heat generation amount obtained by accumulating the heat generation amount for a predetermined period and the unsteady heat conduction equation. The thermal displacement amount calculation means calculates the thermal displacement amount of the plurality of calculation sections of the shaft for each predetermined period from the temperature distribution. Finally, the correction amount calculation means calculates a correction amount for correcting the pitch error correction amount of each of the plurality of correction sections for each predetermined period based on the thermal displacement amount. Thus, the same effect as that of claim 1 is obtained.

  According to the first aspect of the present invention, the fixed side end connected to the shaft to the servo motor and the movable side end opposite to the fixed side end are provided in advance so that the heat of the shaft can be obtained during operation of the machine tool. When calculating the amount of displacement, the entire length of the shaft is divided from the fixed side end into a fixed length calculation section, and the length of the calculation section including the movable side end is set to a certain length or more, Since the calorific value and temperature distribution in the calculation section are calculated, it is not necessary to match the break position of the heat displacement correction calculation section and the break position of the pitch error correction correction section. Therefore, by setting the section length of the calculation section to be long, the calculation cycle for calculating the thermal displacement amount can be lengthened, and the calculation processing load in the numerical controller can be reduced.

  Furthermore, by setting the section length of the calculation section so as not to become unnecessarily long, highly accurate thermal displacement correction can be performed. By setting the section length of the calculation section to an appropriate size, it is possible to achieve the target machining accuracy while suppressing the load of calculation processing in the numerical controller.

  According to the invention of claim 2, the nut movement range of the shaft is divided into a plurality of correction sections shorter than the calculation section, and the pitch error correction amount for correcting the pitch error for each of the plurality of correction sections is determined using the thermal displacement amount. Therefore, the calculation cycle for calculating the thermal displacement amount can be lengthened, and the calculation processing load in the numerical controller can be reduced.

  According to invention of Claim 3, while dividing the full length of a shaft into the calculation area of fixed length longer than a correction area from a fixed side edge part, the length of the calculation area containing a movable side edge part is more than fixed length Therefore, the calculation cycle for calculating the thermal displacement amount can be lengthened, and the calculation processing load in the numerical controller can be reduced. Further, by setting the section length of the calculation section to an appropriate size, it is possible to achieve the target machining accuracy while suppressing the load of calculation processing in the numerical controller.

  According to the invention of claim 4, since the speed detecting means, the calorific value calculating means, the temperature distribution calculating means, the thermal displacement calculating means, and the correction amount calculating means are provided, the same effect as in claim 3 is provided. Play.

1 is an overall perspective view of a numerically controlled machine tool according to an embodiment of the present invention. It is a side view of this machine tool. It is a block diagram of an X-axis ball screw mechanism. It is a block diagram of the control system of a machine tool. It is explanatory drawing explaining the multiple calculation area which divided | segmented the ball screw shaft into multiple. It is explanatory drawing explaining memory | storage data, such as the total emitted-heat amount of a some calculation area. It is explanatory drawing explaining the emitted-heat amount and temperature distributed to the several calculation area. It is explanatory drawing of the correction area for a pitch error correction amount. It is explanatory drawing which shows the thermal displacement amount in each section division position from a fixed bearing. It is a flowchart of a thermal displacement correction control program. It is a flowchart of a correction amount calculation processing program. FIG. 6 is a diagram corresponding to FIG. 5 of the prior art. FIG. 6 is a view corresponding to FIG. 5 of another prior art.

  Hereinafter, modes for carrying out the present invention will be described.

The configuration of the machine tool M will be described with reference to FIGS.
The machine tool M performs a desired machining (for example, “milling”, “drilling”, “cutting”) on the workpiece by moving the workpiece and the tool 6 relative to each other in the XYZ orthogonal coordinate system independently. Etc.). As shown in FIG. 1, a machine tool M includes a base 1, a machine main body 2 that is provided on an upper part of the base 1 and performs workpiece cutting, and a machine main body 2 and a base 1 that are fixed to the upper part of the base 1. A box-shaped splash cover (not shown) covering the upper part is mainly configured. The base 1 is a substantially rectangular parallelepiped cast product that is long in the Y-axis direction (the lower right in FIG. 1 is the front of the machine tool M, and the Y-axis direction is the front-rear direction of the machine tool M).

Next, the machine body 2 will be described.
The machine body 2 is fixed to a column seat 3 on the rear part of the base 1 and extends vertically upward, a spindle head 5 that can be moved up and down along the column 4, and can be rotated inside the spindle head 5. 5A, a tool changer (ATC) 7 which is provided on the right side of the spindle head 5 and attaches and replaces the tool holder of the tool 6 at the tip of the spindle 5A, and is provided on the upper part of the base 1 and can be attached and detached. The table 8 is fixed to the main body. A box-shaped control box 9 is provided on the back side of the column 4, and a numerical control device 50 (see FIG. 4) for controlling the operation of the machine tool M is provided inside the control box 9.

Next, a moving mechanism that moves the table 8 in the X-axis direction and the Y-axis direction will be described.
As shown in FIGS. 1 and 4, an X-axis motor 71 formed of a servo motor moves and drives the table 8 in the X-axis direction (the left-right direction of the machine body 2 in FIG. 1). A Y-axis motor 72 formed of a servo motor moves and drives the table 8 in the Y-axis direction. This moving mechanism has the following configuration. A rectangular parallelepiped support 10 is provided below the table 8. The support base 10 is provided with a pair of X-axis feed guides extending along the X-axis direction, and the table 8 is movably supported on the pair of X-axis feed guides.

  As shown in FIG. 3, a nut portion 8 a is disposed on the lower surface of the table 8. The nut portion 8 a constitutes a ball screw mechanism by screwing with an X-axis ball screw shaft 81 connected to the X-axis motor 71 via the coupling 17. A fixed bearing 18 fixed to the support base 10 supports an end portion 81 e of the X-axis ball screw shaft 81 on the X-axis motor 71 side (fixed side). The movable bearing 19 supports an end 81 f on the opposite side (movable side) of the X-axis ball screw shaft 81. The movable bearing 19 is movable in the axial direction of the X-axis ball screw shaft 81.

  A pair of Y-axis feed guides extending along the longitudinal direction of the base 1 are provided above the base 1. The Y-axis feed guide supports the support 10 so as to be movable. An X-axis motor 71 provided on the support base 10 moves and drives the table 8 in the X-axis direction along the X-axis feed guide. A Y-axis motor 72 provided on the base 1 moves and drives the support base 10 in the Y-axis direction along the Y-axis feed guide. The Y-axis moving mechanism is a ball screw mechanism as in the X-axis.

  The X-axis feed guide is provided with telescopic covers 11 and 12 that contract telescopically on both the left and right sides of the table 8. In the Y-axis feed guide, a telescopic cover 13 and a Y-axis rear cover are provided before and after the support base 10, respectively. Even if the table 8 is moved in any of the X-axis direction and the Y-axis direction by these plural covers, the telescopic covers 11, 12, 13 and the Y-axis rear cover are always the X-axis feed guide and the Y-axis feed guide. Covering. The telescopic covers 11, 12, 13 and the Y-axis rear cover prevent chips and coolant liquid scattered from the processing area from falling on each rail.

Next, the elevating mechanism of the spindle head 5 will be described.
As shown in FIGS. 1 and 2, the column 4 supports a Z-axis ball screw shaft extending in the vertical direction. The spindle head 5 is supported by a nut portion screwed onto the Z-axis ball screw shaft. The Z-axis motor 73 (see FIG. 4) rotationally drives the Z-axis ball screw shaft in the forward and reverse directions, so that the spindle head 5 is driven up and down in the Z-axis direction (the vertical direction of the machine body 2 in FIG. 1). Therefore, based on a control signal from the CPU 51 (see FIG. 4) of the numerical controller 50, the Z-axis motor 73 drives the spindle head 5 up and down by the shaft control unit 63a (see FIG. 4).

  As shown in FIGS. 1 and 2, the tool changer 7 includes a tool magazine 14 and a tool change arm 15. The tool magazine 14 stores a plurality of tool holders that support the tool 6. The tool exchange arm 15 grips the tool holder attached to the main shaft 5A and another tool holder, and conveys and exchanges them. The tool magazine 14 includes a plurality of tool pots (not shown) and a transport mechanism (not shown) inside. The tool pot supports the tool holder. The transport mechanism transports the tool pot in the tool magazine 14.

Next, the electrical configuration of the numerical controller 50 will be described.
As shown in FIG. 4, the numerical control device 50 includes a microcomputer, and includes an input / output interface 54, a CPU 51, a ROM 52, a RAM 53, axis control units 61 a to 64 a and 75 a, and a servo amplifier. 61-64, differentiators 71b-74b, etc. are provided. The axis controllers 61a to 64a are connected to the servo amplifiers 61 to 64, respectively. The servo amplifiers 61 to 64 are connected to the X-axis motor 71, the Y-axis motor 72, the Z-axis motor 73, and the main shaft motor 74, respectively. The shaft control unit 75 a is connected to the magazine motor 75.

  The X axis motor 71 and the Y axis motor 72 are for moving the table 8 in the X axis direction and the Y axis direction, respectively. The Z-axis motor 73 drives the spindle head 5 to move up and down in the Z-axis direction. The magazine motor 75 is for rotating the tool magazine 14. The main shaft motor 74 is for rotating the main shaft 5A. The X-axis motor 71, the Y-axis motor 72, the Z-axis motor 73, and the main shaft motor 74 are provided with encoders 71a to 74a, respectively.

  The axis controllers 61a to 64a receive a movement command amount from the CPU 51 and output a current command (motor torque command value) to the servo amplifiers 61 to 64. The servo amplifiers 61 to 64 receive a current command and output a drive current to the motors 71 to 74. The axis controllers 61a to 64a receive position feedback signals from the encoders 71a to 74a and perform position feedback control. The differentiators 71b to 74b differentiate the position feedback signals input from the encoders 71a to 74a, convert them into speed feedback signals, and output the speed feedback signals to the axis controllers 61a to 64a.

  The axis controllers 61a to 64a receive the speed feedback signal from the differentiators 71b to 74b and control the speed feedback. Current detectors 61b to 64b detect drive currents output from servo amplifiers 61 to 64 to motors 71 to 74. The drive current detected by the current detectors 61b to 64b is fed back to the axis controllers 61a to 64a. The shaft controllers 61a to 64a perform current (torque) control based on the fed back drive current. The axis control unit 75 a receives the movement command amount from the CPU 51 and drives the magazine motor 75.

The ROM 52 is a main control program for causing the machining program of the machine tool M to function, a control program for thermal displacement correction control shown in FIG. 10, and a control program for correction amount calculation processing for calculating the correction amount of the pitch error correction amount shown in FIG. Etc. are remembered. The RAM 53 is a table of parameters relating to the mechanical structure such as the length and diameter of the ball screw shaft 81, parameters relating to physical properties such as density and specific heat, and heat distribution coefficients (ratio) η _N , η _F , η _{B and} pitch error correction amount. Etc. are stored. The RAM 53 appropriately stores a plurality of machining programs for machining various workpieces.

The pitch error correction amount table is a table for correcting pitch errors of the X-axis, Y-axis, and Z-axis ball screw mechanisms. That is, in the ball screw mechanism, a pitch error between the rotation amount of the ball screw shaft 81 and the movement amount of the nut portion 8a is unavoidable due to manufacturing tolerances and the like, so that the pitch error is corrected based on a preset pitch error correction amount table. It is like that.
In this embodiment, an example in which the thermal displacement of the X-axis ball screw shaft 81 is corrected will be described.

  As shown in FIG. 8, the nut portion movement range 81b of the X-axis ball screw shaft 81 is divided into a plurality of correction sections shorter than a calculation section described later, specifically, 15 correction sections with a set length of 20 mm, Pitch error correction is performed for each correction section. The pitch error correction amount for correcting the pitch error is a correction interval at an interval of 20 mm in the X-axis direction from the position X0 to the position X300 in the adjustment stage before shipment after the manufacturer manufactures the machine tool M. Move every time. An error with respect to the command value, that is, an error which is (target value−actual movement amount) is precisely measured, a pitch error correction amount table is created, the table is stored in the RAM 53 in advance and shipped. Similarly, a table of pitch error correction amounts is created for the Y-axis and Z-axis directions.

Next, a method for calculating the amount of thermal displacement generated accompanying numerical control during operation of the machine tool M will be described.
In this calculation method, as shown in FIG. 5, the amount of heat generated in three regions of the front shaft portion 81 a of the ball screw shaft 81, the nut portion movement range 81 b, and the rear shaft portion 81 c of the ball screw shaft 81 is obtained. Based on this calorific value, the calorific value of six calculation sections obtained by dividing the ball screw shaft 81 in the length direction over the entire length is obtained.

[Calculation of total calorific value]
The ball screw shaft 81 shown in FIG. 5 has a front shaft portion 81a and a rear shaft portion 81c of 100 mm, a nut portion moving range 81b of 300 mm, and a total length of 500 mm.
The length of the calculation section is 80 mm. Therefore, if the fixed side end portion 81e is divided at equal intervals in the calculation interval, the end of the nut portion moving range 81b on the movable bearing 19 side coincides with the division of the calculation interval 5. The rear shaft portion 81c is 80 mm and 20 mm when divided by the calculation section. In the present application, since the calculation section including the movable side end portion 81f is set to be equal to or greater than the calculation section of 80 mm, the length of the calculation section 6 is 100 mm.

  An X-axis motor that determines the calculation section in which the nut portion 8a exists based on the X-axis feed data (control data) of the machining program every predetermined time (for example, 50 ms) and obtains from the detection signal of the encoder 71a Based on the feed speed of the table 8 determined from the actual rotational speed of 71, the heat generation amount is obtained by the following equation and stored in the data area of the RAM 53.

Q = K ₁ × F ^T
Here, Q is the calorific value, F is the feed speed of the table 8, K ₁ and T are predetermined constants.

Using the above calorific value calculation formula, the calorific value due to the movement of the nut portion 8a in each calculation interval is calculated 128 times every 50 ms for a predetermined period (for example, 6400 ms). The calorific values of these predetermined periods are summed for each calculation section, and the total calorific values Q _{1 to} Q ₆ are stored in the RAM 53 in correspondence with the calculation sections 1 to 6 as shown in FIG. The total calorific value Q _T of the calorific values Q _{1 to} Q ₆ of the _six computation sections 1 to 6 generated during the predetermined period is also calculated and stored in the RAM 53.

[Distribution of total calorific value]
In the distribution method of the total calorific value Q _T shown below, heat conduction to other parts does not occur in the nut portion moving range 81b, the front shaft portion 81a and the rear shaft portion 81c of the ball screw shaft 81, and the This is based on the fact that it can be regarded as being approximately independent, and the ratio of each heat generating portion to the total heat generation amount Q _T is substantially constant regardless of the feed rate.

When the total heat generation amount Q _T , the front bearing portion heat generation amount Q _F generated by rotation of the fixed bearing 18, the nut portion moving range heat generation amount Q _N , and the rear bearing portion heat generation amount Q _B generated by rotation of the movable bearing 19, The distribution calorific value of each heat generating part is calculated from the following equation.

Q _F = η _F × Q _T
Q _N = η _N × Q _T
Q _B = η _B × Q _T
Here, the ratios η _F , η _N , and η _B are constant according to the above knowledge, and Q _F , Q _N , and Q _B are measured by an actual machine, and the ratios η _F , η _N , and η _B are obtained in advance. Use.

[Nut movement range calorific value distribution]
Next, the distributed calorific value Q _N of the nut portion movement range is distributed to six calculation sections. Based on the total calorific values Q _{1 to} Q ₆ and the total calorific value Q _T of the _six calculation sections stored in the data area, the heat generation in the six calculation intervals for the nut portion moving range calorific value Q _{N is} obtained from the following equation. The quantity distribution ratios X _{1 to} X ₆ are obtained.

X ₁ = Total calorific value Q ₁ / Q _{T of} calculation section 1
:
X ₆ = total calorific value Q ₆ / Q _{T of} calculation section 6
In this way, after the distribution ratios X _{1 to} X ₆ of the six calculation sections are obtained, the distribution calorific values Q _N1 to Q _N of the six calculation sections are calculated from the distribution ratio and the nut movement range heat generation quantity Q _N by the following formula. _{Find N6} .

Q _N1 = X ₁ × Q _N
:
Q _N6 = X ₆ × Q _N
Based on FIG. 5, the temperature of each part and the distributed heat generation amount of each calculation section can be expressed as shown in FIG.

[Calculation of temperature distribution]
As described above, the distribution heat generation amount of the six calculation sections is obtained, and the temperature distribution is calculated based on these distribution heat generation amounts. The temperature distribution can be obtained by solving the following unsteady heat conduction equation under the initial condition {θ} _{t = 0} = {θ ₀ }. Θ ₀ is the initial temperature.

[C] d {θ} / dt + [H] {θ} + {Q} = 0
Here, [C]: heat capacity matrix, [H]: heat conduction matrix, {θ}: temperature distribution, {Q}: calorific value, t: time.

[Calculation of thermal displacement]
As shown in FIG. 7, from seeking temperature theta ₁ through? ₆ six computation area of the ball screw shaft 81, on the basis of these temperature theta ₁ through? _6, six operation section break position of the ball screw shaft 81 ( The amount of thermal displacement at positions corresponding to θ ₁ to θ ₆ in FIG. 7 is calculated. The amount of thermal displacement at the six calculation section break positions can be obtained from the following equation.

ΔL = ∫ ^L ₀ β × θ (L) dL (1)
Here, ΔL: thermal displacement amount, β: linear expansion coefficient of the ball screw shaft material.
The integration symbol indicates integration in the range of 0 to L, and L indicates the length to the calculation interval delimitation positions for the six calculation intervals. Specifically, the integration over a range of 0 to 80, 0 to 160, 0 to 240,.

[Calculation of correction amount]
After obtaining the thermal displacement amounts at the six calculation section break positions of the ball screw shaft 81, correction amounts for correcting the pitch error correction amounts at the 16 correction section break positions are calculated. In this embodiment, the moving range of the nut is X0 to X300 (300 mm range), and the length of each correction section is 20 mm. Therefore, the correction amounts are calculated at 16 positions of X0, X20, X40,. The correction amounts of the 16 correction section break positions can be obtained from FIG. 9 and the equation of [correction amount calculation formula] described later.

FIG. 9 is an explanatory diagram for obtaining a correction amount for correcting the pitch error correction amount. In FIG. 9, the vertical axis represents the amount of thermal displacement with respect to the position of the fixed bearing 18, the upper horizontal axis represents the position of each part of the ball screw shaft 81 with respect to the fixed bearing 18, and the lower horizontal axis represents 16 pieces. The correction section delimiter positions (X0, X20..., X300) are indicated.
Here, D _F1 is the amount of thermal displacement in the calculation interval 1,
_DF2 is the total amount of thermal displacement in calculation interval 1 and calculation interval 2,
:
D _F6 is the total amount of thermal displacement in the calculation interval 1 to calculation interval 6.

As shown in FIG. 9, the correction amount at the delimiter positions (X0, X20,..., X300) of the 16 correction sections is obtained from the following equation.
[Correction amount calculation formula]
X0 correction amount = thermal displacement amount in computation section 1 + thermal displacement amount in computation section 2 × {(length between left delimiter position of computation section 2 and X0) / length of computation section 2}
Correction amount of X20 = thermal displacement amount of calculation section 1 + thermal displacement amount of calculation section 2 × {(length between left delimiter position of calculation section 2 and X20) / length of calculation section 2} −correction amount of X0 Correction amount of X40 = thermal displacement amount of calculation section 1 + thermal displacement amount of calculation section 2 × {(length between left delimiter position of calculation section 2 and X40) / length of calculation section 2} −correction amount of X20 Correction amount of X60 = thermal displacement amount of calculation section 1 + heat displacement amount of calculation section 2 × {(length between left delimiter position of calculation section 2 and X60) / length of calculation section 2} −correction amount of X40 Correction amount of X80 = thermal displacement amount of calculation section 1 + thermal displacement amount of calculation section 2 + heat displacement amount of calculation section 3 × {(length between left delimiter position of calculation section 3 and X80) / calculation section 3 Length}-Correction amount of X60
:
X300 correction amount = thermal displacement amount in computation section 1 + thermal displacement amount in computation section 2 + thermal displacement amount in computation section 3 + thermal displacement amount in computation section 4 + thermal displacement amount in computation section 5 + heat in computation section 6 Displacement amount × {(length between the left delimiter position of calculation section 6 and X300) / length of calculation section 6} −correction amount of X280

Next, thermal displacement correction control executed by the numerical controller 50 will be described based on the flowchart of FIG. In the figure, Si (i = 1, 2,...) Indicates each step. However, this thermal displacement correction control will be briefly described because there are many overlapping parts with the contents described above. Also,
It is assumed that machining by numerical control for an actual workpiece is performed in parallel with this thermal displacement correction control.

  When this control is started, initial setting is first executed in S1. In this initial setting, first, processing such as setting a matrix necessary for calculation by the finite element method from setting data such as parameters, setting an initial temperature, and clearing a memory area related to the RAM 53 is executed. The Next, in S2, the ball screw shaft 81 is divided into six calculation sections 1 to 6 as shown in FIG.

Next, in S3, the counter I is set to 0, and in S4, the X-axis feed data and the detection signal of the encoder 41a are read. Next, in S5, the amount of heat generated every 50 ms in the calculation sections 1 to 6 is calculated and stored in the memory. In the next S6, the counter I is incremented by “1”. In S7, it is determined whether or not the counter value of the counter I is “127”. If the determination is No, the process returns to S4 and S4 to S6 are repeated. When the determination in S7 is Yes, the process proceeds to S8. In S8, the total heat generation amount Q _{1 to} Q ₆ and the total heat generation amount Q _T for 6400 ms in the calculation sections 1 to 6 are calculated and stored in the memory.

In S9, the calorific value Q _F of the aforementioned respective units, Q _N, Q _B is stored is computed in the memory, the distribution calorific value Q _N1 to Q _N6 arithmetic operation to the arithmetic section 1-6 was distributed to the calorific value Q _N And stored in the memory. Further, the distribution heat generation amount shown in FIG. 7 for the calculation sections 1 to 6 is also calculated and stored in the memory. In S10, the temperatures θ _{1 to} θ 6 of the calculation sections 1 to ₆ are calculated based on the distributed heat generation amount shown in FIG. 7 and stored in the memory.

  In S11, based on the above equation (1), the thermal displacement amounts at the calculation section break positions for the six calculation sections are calculated and stored in the memory. In S12, based on the [correction amount calculation formula], the correction amounts at the 16 correction section break positions are calculated as described above. Next, in S13, using the correction amount obtained in S12, correction processing for a pitch error correction amount set in advance for the 16 correction section break positions is executed, and the corrected pitch error correction amount is executed. Is executed. In S14, it is determined whether or not the thermal displacement correction process is to be terminated. If the determination is No, the process returns to S3 and is repeatedly executed after S3. When the determination in S14 is Yes, this control is terminated.

Next, the correction amount calculation processing for calculating the correction amount for correcting the pitch error correction amount in S12 will be described based on the flowchart of FIG. In the figure, Si (i = 20, 21...) Indicates each step. When this process is started, the counter n is reset to 0 (S20), and then the correction amount ΔM _n of the position Xn is calculated by the following equation (S21).

First, the correction amount ΔM ₀ of the position X0 is obtained from ΔM _n = D _F + ΔD _n × {(Xn−X _F ) / L _n } −ΔM _n−20, n = 0. Note that this equation simply represents [correction amount calculation equation].
Here, D _F : the total amount of thermal displacement generated in the calculation section on the fixed side from the position Xn,
ΔD _n : thermal displacement amount generated in the calculation section including the position Xn,
X _F : Left separation position of the calculation section including the position Xn,
L _n is the length of the calculation interval including the position Xn.
However, the .DELTA.M _-20 used when seeking .DELTA.M ₀ to 0.

After n is incremented by 20 in S22, it is determined whether or not n is 320 in S23. When n is not 320 (S23; No), it is determined that the calculation for the correction amount up to the position X300 is not completed, and the process returns to S21 to calculate the correction amount ΔM _n for the position Xn. Until obtaining a correction amount .DELTA.M ₃₀₀ position X300 is repeatedly executes S21 to S23. After obtaining the correction amount ΔM ₃₀₀ (S21), n = 320 in S22, and the determination in S23 is Yes, so this process ends and the process proceeds to S14 in FIG.

  The encoder 71a corresponds to “speed detection means”, the numerical control device 50 that executes S3 to S7 corresponds to “heat generation amount calculation means”, and the numerical control device 50 that executes S8 to S10 corresponds to “temperature distribution calculation means”. The numerical control device 50 that executes S11 corresponds to “thermal displacement amount calculation means”, and the numerical control device 50 that executes S12 corresponds to “correction amount calculation means”.

  Next, the operation and effect of the thermal displacement correction method for the machine tool M described above and the thermal displacement correction apparatus will be described. The entire length of the ball screw shaft 81 of the ball screw mechanism is divided into fixed length calculation sections longer than the correction section from the fixed side end 81e, and the length of the calculation section including the movable side end 81f is greater than or equal to a certain length. In order to set, the calorific value and temperature distribution of the plurality of calculation sections are calculated, and based on the calculation, the thermal displacement amount is calculated at the break position of the plurality of calculation sections, and the correction amount for correcting the pitch error correction amount is determined. Therefore, the following effects can be obtained.

  It is not necessary to match the dividing position of the calculation section with the dividing position of the correction section. Therefore, by setting the section length of the calculation section to be long, the calculation cycle for calculating the thermal displacement amount can be lengthened, and the calculation processing load in the numerical controller 50 can be reduced.

  Furthermore, by setting the section length of the calculation section so as not to become unnecessarily long, highly accurate thermal displacement correction can be performed. By setting the section length of the computation section to an appropriate size, it is possible to achieve the target machining accuracy while suppressing the computation processing load in the numerical controller 50.

Next, a modified example in which the above embodiment is partially modified will be described.
1] In the above embodiment, the case where the thermal displacement correction device and the thermal displacement correction method of the present invention are applied to the thermal displacement correction of the X-axis ball screw mechanism has been described. However, the Y-axis ball screw mechanism and the Z-axis ball screw have been described. It is also possible to apply the correction to the thermal displacement of the mechanism.
2] In the above-described embodiment, the front shaft portion 81a and the nut portion moving range 81b of the ball screw shaft 81 are divided into five calculation sections at intervals of 80 mm (four times the set length of the correction section), but longer than the correction section. Any calculation section may be used. For example, the calculation section may be divided into lengths other than four times, such as 1.5 times or three times the set length of the correction section.

3] In the above embodiment, the X-axis motor 71 is provided on the front shaft portion 81a side of the ball screw shaft 81. However, the X-axis motor 71 is also provided on the rear shaft portion 81c side of the ball screw shaft 81. In the same manner as described above, the thermal displacement amounts of the six calculation sections of the ball screw shaft 81 can be obtained.
4] In the above embodiment, the calculation period of 50 ms for calculating the calorific value has been described as an example, but this calculation period is not limited to 50 ms. The 6400 ms of the predetermined period is only an example, and the present invention is not limited to this. For example, the predetermined period may be set to an order of 20 to 30 s.

M Machine tool 8a Nut 50 Numerical control device 51 CPU
71 X-axis motor 71a Encoder 81 Ball screw shaft 81b Nut portion moving range 81e Fixed side end 81f Movable side end

Claims (4)

A method for correcting thermal displacement of a numerically controlled machine tool having a ball screw mechanism for feed driving, a servo motor for rotationally driving a shaft into which a nut of the ball screw mechanism is screwed, and a control means for controlling the servo motor based on control data In
A fixed side end connected to the servo motor on the shaft, and a movable side end opposite to the fixed side end are provided in advance,
When calculating the amount of thermal displacement of the shaft during operation of the machine tool,
The total length of the shaft is divided by a fixed length calculation section from the fixed side end, and the length of the calculation section including the movable side end is set to be equal to or greater than the fixed length, and the plurality of calculations A method of correcting a thermal displacement of a numerically controlled machine tool, characterized by calculating a calorific value and a temperature distribution of a section.   The nut movement range of the shaft is divided into a plurality of correction sections shorter than the calculation section, and a pitch error correction amount for correcting a pitch error for each of the plurality of correction sections is corrected using the thermal displacement amount. The thermal displacement correction method for a numerically controlled machine tool according to claim 1. A feed drive ball screw mechanism; a servo motor that rotationally drives a shaft into which a nut of the ball screw mechanism is screwed; and a control unit that controls the servo motor based on control data. In a thermal displacement correction method for a numerically controlled machine tool that divides the correction section into a plurality of correction sections and corrects pitch error for each of the plurality of correction sections
A fixed side end connected to the servo motor on the shaft, and a movable side end opposite to the fixed side end are provided in advance,
The total length of the shaft is divided into a calculation section having a fixed length longer than the correction section from the fixed side end, and the length of the calculation section including the movable side end is set to the fixed length or more. A first step for obtaining a calorific value generated in the plurality of calculation sections at predetermined time intervals based on the rotation speed of the servo motor and control data;
A second step of calculating a temperature distribution of the plurality of calculation sections for each predetermined period based on a total heat generation amount obtained by accumulating the heat generation amounts of the plurality of calculation sections for a predetermined period and an unsteady heat conduction equation;
A third step of calculating a thermal displacement amount of a plurality of calculation sections of the shaft from the temperature distribution for each predetermined period;
A fourth step of calculating a correction amount for correcting a pitch error correction amount of each of the plurality of correction sections based on the thermal displacement amounts of the plurality of calculation sections for each predetermined period;
A thermal displacement correction method for a numerically controlled machine tool, comprising: A feed drive ball screw mechanism; a servo motor that rotationally drives a shaft into which a nut of the ball screw mechanism is screwed; and a control unit that controls the servo motor based on control data. In the thermal displacement correction device for a numerically controlled machine tool that divides the correction interval into a plurality of correction intervals and corrects the pitch error for each of the correction intervals,
The shaft has a fixed side end connected to the servo motor, and a movable side end opposite to the fixed side end,
Speed detecting means for detecting the rotational speed of the servo motor;
The total length of the shaft is divided into a calculation section having a fixed length longer than the correction section from the fixed side end, and the length of the calculation section including the movable side end is set to the fixed length or more. A calorific value calculation means for obtaining a calorific value generated in the plurality of calculation sections at predetermined time intervals based on the rotation speed of the servo motor and control data;
Based on the total calorific value obtained by accumulating the calorific value of the plurality of calculation sections for a predetermined period and the unsteady heat conduction equation, temperature distribution calculating means for calculating the temperature distribution of the plurality of calculation sections for each predetermined period;
Thermal displacement amount calculating means for calculating the thermal displacement amount of the plurality of calculation sections of the shaft for each predetermined period from the temperature distribution;
Correction amount calculation means for calculating a correction amount for correcting the pitch error correction amount of each of the plurality of correction sections based on the thermal displacement amounts of the plurality of calculation sections for each predetermined period;
A thermal displacement correction device for a numerically controlled machine tool, comprising: