Patent Publication Number: US-10786857-B2

Title: Wire electric discharge machine

Description:
RELATED APPLICATIONS 
     The present application claims priority to Japanese Patent Application Number 2015-189482, filed Sep. 28, 2015, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wire electric discharge machine, and particularly to a wire electric discharge machine having a function of measuring a workpiece remain immersed in a working fluid with a touch probe. 
     2. Description of the Related Art 
     In a wire electric discharge machine, upon completion of a machining operation of an object to be machined (hereinafter referred to as a “workpiece”), it is important to accurately measure the dimensions of the workpiece and to determine whether it has been machined properly. The measurement of the dimensions of the machined workpiece are generally effected after detaching it from the machine, and then the dimensions are measured by means of a special measuring machine such as a coordinate measuring machine (CMM) or a measuring microscope. 
     However, when dimensional error is found after measurement, it is necessary to rework the part for the correction of the error. Hence, it is also necessary to remount the once detached workpiece accurately to the wire electric discharge machine, and a reference starting position needs to be determined. This process requires many man-hours and skill. If remount of the detached workpiece for rework is failed, the dimensions cannot be corrected accurately by the rework. 
     In this connection, a method for measuring the workpiece upon completion of the machining operation without detaching it from the machine by employing a contact measurement device (hereinafter referred to as a “touch probe”) attached to a body or upper guide part of a wire electric discharge machine has been proposed (see Japanese Patent Laid-Open No. 60-85829). 
       FIGS. 18 and 19  are flowcharts showing a conventional flow of working and measurement. 
     [Step sh 01 ] Read the position of upper guide. 
     [Step sh 02 ] Fill work tank with working fluid until upper guide is submerged. 
     [Step sh 03 ] Detect level of working fluid to determine whether the filling operation is completed, and if completed (YES), proceed to Step sh 04 , and if not completed (NO), return to Step sh 02 . 
     [Step sh 04 ] Start machining workpiece. 
     [Step sh 05 ] Check whether work program is completed, and if completed (YES), proceed to Step sh 06 , and if not completed (NO), wait until completion of work program. 
     [Step sh 06 ] Perform work tank drainage process, and proceed to Step sh 07 . 
     [Step sh 07 ] Determine whether measurement program is linked (whether measurement program is to be executed after completion of work program), and if linked, measurement program is started, and if not linked, terminate the program. 
     [Step si 01 ] Determine whether the drainage of work tank is completed, and if completed (YES), proceed to Step si 02 . 
     [Step si 02 ] Read the set value of workpiece thickness. 
     [Step si 03 ] Move upper guide to measuring height. 
     [Step si 04 ] Lower touch probe. 
     [Step si 05 ] Start measurement. 
     [Step si 06 ] Determine whether the measurement program is completed, and if completed, proceed to Step si 07 , and if not completed, wait for completion of the measurement program. 
     [Step si 07 ] Raise the touch probe, and terminate the process. 
     A method for correcting a measured value of the dimensions of the workpiece in view of an amount of thermal displacement caused by the temperature variation of the workpiece, and a set value of the temperature of the measuring environment has also been proposed (Japanese Patent Laid-Open No. 2009-279727). However, as mentioned in the problem to be solved by the invention, neither of the methods accurately obtains a measured value of the dimensions of the workpiece in the same environment as during the machining process. 
     A wire electric discharge machine generally performs machining within the working fluid, to prevent splashing of the working fluid, prevent corrosion of an upper surface of the workpiece while machining, and to suppress temperature variation. As has been described in the background art, conventionally, when measuring the workpiece without detaching it from the table, a work tank is completely drained of a working fluid, and then the workpiece is measured by a touch probe with removing the working fluid from the workpiece. 
       FIG. 20  is a diagram illustrating the principle how an error occurs when a workpiece is measured within a work tank vacant of a working fluid. If a work tank  23  is completely drained of a working fluid (water) at the time of the on-machine measurement, the working fluid on a workpiece upper surface  26   a  and around a machine table (hereinafter referred to as a “table”)  24  starts to evaporate. At this time, heat of a workpiece  26  and the table  24  of metal material contacting the working fluid is deprived of as heat of evaporation, and thus the material contracts. As a result, the position and dimensions at the time of measurement are deviated from those obtained by machining in the working fluid, because of the temperature variation. This hinders accurate dimension measurement by a sensor  3   a  (of a probe  3 ). 
     SUMMARY OF THE INVENTION 
     In view of the above problem inherent in the prior art, an objective of the present invention is to provide a wire electric discharge machine having a function of measuring a workpiece remain immersed in a working fluid by means of a touch probe, and capable of a more accurate dimension measurement by causing the contraction and expansion of a workpiece and a table of metal material due to temperature variation hard to occur, so that there is substantially no deviations between the position and dimensions obtained by machining in a working fluid, and those at the time of measurement. 
     A wire electric discharge machine of the present invention includes a work tank containing a working fluid, a table provided in the work tank and on which a workpiece is mounted, an upper guide and a lower guide supporting a wire electrode, and a measurement device detachable from or movable relative to an upper guide part having the upper guide, and including a sensor, the wire electric discharge machine being configured to machine the workpiece immersed in the working fluid, and measure the workpiece by means of the measurement device after machining, and characterized by including: plate thickness acquisition means for acquiring a plate thickness of the workpiece, level detection means for detecting a level of the working fluid in the work tank, and level adjustment means for adjusting the level of the working fluid when measuring the workpiece with the measurement device on the basis of the relative position of the upper guide and a measurement device body, a position of the upper guide, and the plate thickness, to the level not lower than a height of an upper surface of the workpiece and not higher than the measurement device body. 
     Also, a wire electric discharge machine of the present invention includes a work tank containing a working fluid, a table provided in the work tank and on which a workpiece is mounted, an upper guide and a lower guide supporting a wire electrode, and a measurement device detachable from or movable relative to an upper guide part having the upper guide, and including a sensor on its tip end, the wire electric discharge machine being configured to machine the a workpiece immersed in the working fluid, and measure the workpiece by means of the measurement device after machining, and characterized by including: plate thickness acquisition means for acquiring a plate thickness of the workpiece, or plate thickness setting means; and level adjustment means for adjusting the level of the working fluid when measuring the workpiece with the measurement device on the basis of the level detected by level detection means for detecting a level of the working fluid in the work tank, to the level not lower than a height of an upper surface of the table on which the workpiece is mounted, and not higher than a height of an upper surface of the workpiece. 
     According to the wire electric discharge machine of the present invention as described above, even when the workpiece is immersed only on the lower surface thereof, the measurement accuracy can more stably be achieved with respect to that not immersed at all and started to dry. The object of the present invention can partially be achieved only by immersing to the upper surface of the table, since the table will dry and surely be deformed unless the leg or legs of it is immersed. 
     Also, a wire electric discharge machine of the present invention includes a work tank containing a working fluid, a table provided in the work tank and on which a workpiece is mounted, an upper guide and a lower guide supporting a wire electrode, and a measurement device detachable from or movable relative to an upper guide part having the upper guide, and including a sensor on its tip end, the wire electric discharge machine being configured to machine the workpiece immersed in the working fluid, and measure the workpiece by means of the measurement device after machining, and characterized by including: level detection means for detecting a level of the working fluid in the work tank; and level adjustment means for adjusting the level of the working fluid when measuring the workpiece with the measurement device on the basis of a position of the upper guide, and a distance from the position, to a tip end of the sensor of the measurement device at a position where the sensor is attached to the upper guide, or at a position to which the sensor is lowered while being fixed in a vertically movable manner, by controlling water supply to the work tank and water discharge from the work tank, so that the level detected by the level detection means varies in conjunction with the position of the tip end of the sensor of the measurement device at the position where the sensor is attached to the upper guide, or at the position to which the sensor is lowered while being fixed in a vertically movable manner. 
     Also, a wire electric discharge machine of the present invention includes a work tank containing a working fluid, a table provided in the work tank and on which a workpiece is mounted, an upper guide and a lower guide supporting a wire electrode, and a measurement device detachable from or movable relative to an upper guide part having the upper guide, and including a sensor on its tip end, the wire electric discharge machine being configured to machine the workpiece immersed in the working fluid, and measure the workpiece by means of the measurement device after machining, and characterized by including: level detection means for detecting a level of the working fluid in the work tank; and level adjustment means for adjusting the level of the working fluid when measuring the workpiece with the measurement device, by obtaining a distance from a preset upper guide position to a tip end of the sensor, and a distance from an upper surface of the table to the body of the sensor, and controlling water supply to the work tank and water discharge from the work tank, so that the level detected by the level detection means varies in conjunction with the position of the sensor of the measurement device, and also adjusting such that the level is not lower than the height of the upper surface of the table on which the workpiece is mounted, and not higher than a body of the measurement device. 
     The level detection means may use a pressure sensor to detect hydraulic pressure near the bottom of the work tank, or pneumatic pressure into which the hydraulic pressure is converted, and calculates a height of the level according to the detected pressure. 
     Also, a wire electric discharge machine of the present invention includes a work tank containing a working fluid, a table provided in the work tank and on which a workpiece is mounted, an upper guide and a lower guide supporting a wire electrode, and an optical measurement device detachable from or movable relative to an upper guide part having the upper guide, and including a camera and having a measuring function of recognizing an image of a measuring part and detecting an edge part, the wire electric discharge machine being configured to machine the workpiece immersed in the working fluid, and measure the workpiece by means of the measurement device after machining, and characterized by including: any one of plate thickness acquisition means for acquiring a plate thickness of the workpiece, and setting means; and level adjustment means using level detection means for detecting a level of the working fluid in the work tank, to adjust the level of the working fluid when measuring the workpiece with the measurement device, to a level not lower than a height of an upper surface of the table on which the workpiece is placed, and not higher than a height of an upper surface of the workpiece. 
     In accordance with the present invention, a wire electric discharge machine can be provided having a function of measuring with a touch probe a workpiece remain immersed in a working fluid, and capable of a more accurate dimension measurement by causing less contraction and expansion of a workpiece and a table of metal material under the effect of temperature variation, so that there is little deviation between the position and dimensions obtained by machining in a working fluid, and those at the time of measurement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned and other objectives and characteristics of the present invention will be made clear from the following description of examples, which will be given with reference to the accompanying drawings. Of the drawings, 
         FIG. 1A  is a schematic diagram illustrating a wire electric discharge machine of the present invention; 
         FIG. 1B  is a schematic diagram illustrating a controller for controlling a wire electric discharge machine body; 
         FIG. 2  is a diagram illustrating the principle of the reduction of the occurrence of an error while the measurement of the workpiece is effected under the fully immersed condition within the working fluid contained in the work tank; 
         FIG. 3  is a graph showing a relation between the detected pressure and the level; 
         FIG. 4  is a flowchart illustrating the level adjustment of an embodiment of the present invention; 
         FIG. 5  is a diagram illustrating the principle for controlling the level upon machining a workpiece by a wire electric discharge machine; 
         FIG. 6  is a diagram illustrating the principle for controlling the level upon measuring in Embodiment 1; 
         FIG. 7  is a flowchart showing the process of work program when machining; 
         FIG. 8  is a flowchart showing the process of measurement program when measuring; 
         FIG. 9  is a diagram illustrating the principle for controlling the level upon measuring in Embodiment 2; 
         FIG. 10  is a flowchart showing the process of measurement program when measuring; 
         FIG. 11  is a diagram illustrating the principle for controlling the level upon measuring in Embodiment 3; 
         FIG. 12  is a flowchart showing the process of measurement program when measuring; 
         FIG. 13  is a diagram illustrating the principle for controlling the level upon measuring in Embodiment 4; 
         FIG. 14  is a flowchart showing the process of measurement program when measuring; 
         FIG. 15  is a diagram illustrating the principle for controlling the level upon machining, by means of a pressure sensor placed outside of a work tank; 
         FIG. 16  is a diagram illustrating the principle for controlling the level upon measuring, by means of the pressure sensor placed outside of the work tank; 
         FIG. 17  is a flowchart showing the process for automatically adjusting the level of working fluid inside the work tank in conjunction with a height position of an upper guide, depending on a pressure detected by a level detector such as a hydraulic pressure sensor; 
         FIG. 18  is a flowchart (No. 1) showing a conventional measurement flow; 
         FIG. 19  is a flowchart (No. 2) showing the conventional measurement flow; and 
         FIG. 20  is a diagram illustrating the principle of the occurrence of an error while the measurement of the workpiece is effected within the work tank vacant of working fluid. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention will now be described with reference to the drawings. 
       FIG. 1A  is a schematic diagram illustrating a wire electric discharge machine of the present invention. A wire electric discharge machine  1  includes a wire electric discharge machine body  40  and a controller  50  for controlling the body  40 . A wire bobbin  11  around which a wire electrode  2  is wound is given a predetermined low torque, which is directed in the opposite direction of a pullout direction of the wire electrode  2 , by a delivery torque motor  10 . 
     The wire electrode  2  pulled out from the wire bobbin  11  is threaded over multiple guide rollers (not shown), and its tension between a brake shoe  13  driven by a brake motor  12  and a feed roller  19  driven by a wire electrode delivery motor (not shown) is adjusted by the brake shoe  13 . A tension detector  20  is a detector for detecting the magnitude of tension in the wire electrode  2  running between an upper guide  14  and a lower guide  15 . Also, as shown in  FIG. 5 , an upper guide part  21  accommodates the upper guide  14 . 
     The wire electrode  2  having passed the brake shoe  13  passes the upper guide  14 , the lower guide  15 , and a lower guide roller  16 , is pinched by a pinch roller  18  and the feed roller  19  driven by the wire electrode delivery motor (not shown), and is collected into a wire electrode collection box  17 . 
     A holding part  22  holding a touch probe  3  is attached to the upper guide part  21 . The holding part  22  has a function of projecting and retracting the touch probe  3 . In other words, the touch probe  3  is attached to the upper guide part  21 , through the holding part  22  that has the projecting or retracting function. The touch probe  3  includes a sensor  3   a  having a tip end of spherical configuration. The touch probe  3  is attached such that the projecting and retracting function allows it to move vertically, parallel to the running direction of the wire electrode  2 . The touch probe  3  is a sensor that outputs a signal upon contacted the tip end thereof with an object to be measured (workpiece). When not measuring, the touch probe  3  is raised to a retracted position. 
       FIG. 1B  is a schematic diagram illustrating the controller  50  for controlling the wire electric discharge machine body  40 . When measuring a part of the workpiece  26  to be measured with the touch probe  3 , the touch probe  3  can be lowered to position the tip end of the sensor  3   a  of the touch probe  3  in a predetermined position. Additionally, the upper guide  14  may include a U-axis drive mechanism and a V-axis drive mechanism (not shown) to adjust the XYZ axis positions thereof. With this mechanism, the taper machining of the workpiece (cutting tool) can also be effected. 
     The wire electric discharge machine body  40  machines a workpiece under the control of the controller  50  shown in  FIG. 1A . The controller  50  includes a processor (CPU)  51 , a memory  52  such as a RAM and a ROM, a display interface  53 , a display device  54 , a keyboard interface  55 , a keyboard  56 , a servo interface  57 , a servo amplifier  58 , and an input/output interface  60  for exchanging signals with external devices. The above elements are mutually connected through a bus  61 . 
     The servo amplifier  58  drives a servo motor  62 . “Servo motor  62 ” refers to servo motors corresponding to the respective drive shafts of the X-axis, Y-axis and Z-axis, and indicates servo motors of a number corresponding to the required drive shafts. The servo motor  62  provided in each shaft includes an unillustrated position detector for detecting positions. Position detection signals detected by the position detectors of the servo motor  62  are fed back to the controller  50 . In the embodiments of the present invention, the servo motor  62  also drives the driving part of the holding part  22 , which projects and retracts the probe  3 . 
     The wire electric discharge machine body  40  including a machining power source is controlled through an interface  59 . When a work program is started, an instruction to turn on the machining power source is given through the interface  59 . When it is intended to turn off the machining power source, an instruction is also provided to the wire electric discharge machine body  40  through the interface  59 . An input/output device  63  exchanges input/output signals through the input/output interface  60 . 
     In the wire electric discharge machine  1 , the level of a working fluid (level) upon machining is normally automatically adjusted in cooperation with the height of the upper guide  14 , by detecting the level of the water surface (the level of the working fluid  4 ) inside the work tank  23  by a level detector (float switch) (not shown) attached to the upper guide part  21 . This level is normally set in a level where the working fluid  4  does not splash and can absorb the heat energy provided by a current supply part (i.e. electrode pin) of the upper guide part  21 . The level is generally adjusted to that 50 to 100 mm above the upper surface of the workpiece. 
     As shown in  FIG. 6 , when measuring, the sensor  3   a  of the touch probe  3  detects the worked surface below a workpiece upper surface and above a workpiece lower surface, at a height between a position about 10 mm lower than the workpiece upper surface  26   a  and about the center of the thickness of the workpiece. Hence, the sensor is adapted to be positioned lower than a nozzle lower surface of the upper guide part  21 , which comes into close contact with the workpiece  26 . 
     In order to prevent the sensor  3   a  of the touch probe  3  from interfering with the workpiece upon machining, in some cases the touch probe  3  is adapted to be retracted upward by a raising and lowering system (i.e. holding part  22  having function of projecting and retracting the probe  3 ) when machining, and is lowered when measuring to place the sensor  3   a  below the upper guide part  21 . In other cases, the touch probe  3  can be detached by a detachable fixing system. In this situation, the level control scheme on machining raise the level to submerge the upper guide part  21  and also the body of the touch probe  3 . This may lead to the malfunction of the expensive detector for measurement. 
     Hence, a mechanism is used for adjusting the level in conjunction with the height of the upper guide  14  (upper guide part  21 ). In this mechanism, the level may be measured by using a pressure sensor for detecting hydraulic pressure of the working fluid  4  instead of the float switch, as a method for detecting the level, and the height of the upper guide part  21  may be read with a numerical control device through the motor and position detector. A hydraulic pressure sensor  25  is adapted to measure that near the bottom of the work tank  23 . When measuring the dimensions of a workpiece with the touch probe  3 , in accordance with an embodiment of the present invention, the level is controlled to a height obtained by subtracting a distance to the tip end of the sensor  3   a  of the touch probe  3 , from the submerged position of the upper guide part  21 . 
     When machining, a level detector such as a level sensor  25  for detecting pressure automatically adjusts the level in conjunction with the height of the upper guide  14 , to a level where the upper guide part  21  can be sufficiently submerged in the working fluid  4  to prevent splashing of the working fluid  4  and to cool the power feeder element or the feeder of the upper guide part  21 , while the upper guide part  21  is brought close to the workpiece upper surface  26   a . At this time, the touch probe  3  is positioned in a raised position by the holding part  22 , and therefore does not interfere with the workpiece  26  and is not submerged. The touch probe  3  may be detached. 
     When measuring, the upper guide part  21  is raised, and the touch probe  3  is positioned in a lowered position, or is attached to the upper guide part  21 .  FIG. 2  is a diagram illustrating the principle of the reduction of the occurrence of an error while the measurement of the workpiece is effected under the fully immersed condition within the working fluid  4  contained in the work tank  23 . When measuring, the working fluid  4 , the temperature thereof is adjusted to the same temperature as on the machining, is remain occupied the tank to suppress the temperature variation of the workpiece  26  and the machine table. The same system as that used when machining is adopted, as a mechanism for controlling the level of the working fluid  4  (level sensor  25  and components such as an unillustrated valve opening and closing switch and pump or so). 
     A level sensor  25  that detects hydraulic pressure near the bottom of a work tank, and has a processor for calculating a level depending on the hydraulic pressure may be used. Unlike a float switch fixed to the upper guide part to detect the level, this level sensor  25  can adjust the level to the height of the touch probe  3  when the touch probe is lowered, even when the upper guide  14  is raised and the float switch leaves the water surface. By performing control to maintain a level where the touch probe body is not submerged and the workpiece upper surface  26   a  is immersed, an optimal level can be maintained. 
       FIG. 3  is a graph showing a relation between detected pressure and the level (the level of working fluid  4 ). The level inside the work tank  23  can be controlled based on a pressure Pa detected by the level sensor  25 . 
       FIG. 4  is a diagram illustrating the flowchart of the embodiment of the present invention for adjusting the level inside a work tank. A description of the flowchart will be given below, according to the steps. 
     [Step sa 01 ] Read the position of upper guide. 
     [Step sa 02 ] Read the pressure represented by the level sensor. 
     [Step sa 03 ] Determine whether pressure at measuring position exceeds appropriate range, falls below appropriate range, or is within appropriate range, and if it exceeds appropriate range, proceed to Step sa 04 , if it falls below appropriate range, proceed to Step sa 05 , and if it is within appropriate range, terminate processing of level adjustment.
 
[Step sa 04 ] Discharge water, and return to Step sa 02 .
 
[Step sa 05 ] Supply water, and return to Step sa 02 .
 
     Embodiment 1 
     (When Machining) 
       FIG. 5  is a diagram illustrating the principle for controlling the level upon machining a workpiece  26  by a wire electric discharge machine  1 . 
     The level is automatically adjusted in conjunction with a upper guide height L 1  upon machining, to a level (working fluid level L 3  measured by hydraulic pressure sensor  25 ) where an upper guide part  21  can be lowered (see reference numeral  29 ) to be sufficiently submerged in a working fluid  4  to prevent splashing of the working fluid  4  and to remove the heat energy produced by supplying the electricity to the upper guide part  21  (i.e. for cooling the feeder), while the upper guide part  21  is brought close to a workpiece upper surface  26   a . To avoid interference of the sensor of a touch probe  3  with the workpiece  26  and submergence of the body, the touch probe  3  is driven by a holding part  22  to move in a probe rising direction  30 , and is positioned ascendant with respect to the upper guide part  21 . The touch probe  3  may be detached from the wire electric discharge machine  1 . 
     (When Measuring) 
       FIG. 6  is a diagram illustrating the principle for controlling the level upon measuring in Embodiment 1. The upper guide part  21  is raised (see reference numeral  31 ), and the touch probe  3  is positioned in a lowered position (see reference numeral  32 ). A level detector such as a hydraulic pressure sensor  25 , and a working fluid supply device  91  and a discharge device  92  automatically adjust the level of a working fluid, to a position where the upper surface  26   a  of the workpiece  26  is submerged in the working fluid  4  and the body of the touch probe  3  is not immersed. That is, by using preset information on a table upper surface height position L 2  of a table  24  on which the workpiece  26  is mounted, and information on a workpiece thickness L 5 , as well as the information of the position of the upper guide obtained from a numerical control device, the level is set not lower than the height of the upper surface of the workpiece and not higher than the body of the touch probe  3 .
 
table upper surface height  L 2+workpiece thickness  L 5≤working fluid level on measurement  L 3′≤table upper surface height  L 2+upper guide height on measurement  L 1′−measurement device body position on measurement  L 4′   (Expression 1)
 
     Note that (upper guide height on measurement L 1 ′-measurement device body position on measurement L 4 ′) corresponds to “a relative position between an upper guide and a measurement device body” described in the claims. The measurement device body position on measurement L 4 ′ corresponds to a distance from the height of the upper guide when measuring, to a lowermost part of the casing of the holding portion  22  of the probe  3 . 
     Hence, when measuring, it is possible to prevent thermal displacement of the table  24  of stainless steel of high linear expansion coefficient on which the workpiece  26  is mounted, and an additional sub-table for supporting the workpiece  26 . It is also possible to prevent thermal displacement of the entire workpiece  26  caused by temperature variation. Since this can reduce the dimensional deviation between the machining position and the measuring position caused by temperature variation, the dimensions can be measured far more accurately than measuring after discharging the working fluid  4 . The thermal expansion or contraction of the workpiece  26  and table  24  of metallic material are less likely to be caused by temperature variation. There is little deviation between the position and dimensions obtained by machining in the working fluid  4  and those at the time of measurement, so that the dimensions can be measured more accurately. 
       FIGS. 7 and 8  are flowcharts illustrating the process to be effected in the Embodiment 1 of the present invention.  FIG. 7  is a flowchart showing the process for machining the workpiece through the machining program. The machining program will now be described with reference the following steps. 
     [Step sb 01 ] Read the position of upper guide. 
     [Step sb 02 ] Fill the work tank with the working fluid until upper guide is submerged. 
     [Step sb 03 ] Detect the level to determine whether filling is completed, and if completed, proceed to Step sb 04 . 
     [Step sb 04 ] Start machining. 
     [Step sb 05 ] Determine whether machining program is completed, and if completed, proceed to Step sb 06 . If not completed, wait until completion. 
     [Step sb 06 ] Determine whether the measurement program is linked, and if not linked, proceed to Step sb 07 , and if linked, start measurement program. 
     [Step sb 07 ] Perform work tank drainage process, and terminate the process. 
       FIG. 8  is a flowchart showing the process of a measurement program of Embodiment 1. 
     [Step sc 01 ] Read preset value of workpiece thickness. 
     [Step sc 02 ] Move upper guide to measuring height. 
     [Step sc 03 ] Adjust level until workpiece upper surface is submerged. 
     [Step sc 04 ] Determine whether the adjustment through level detection is completed, and if completed, proceed to Step sc 05 , and if not completed, return to Step sc 03 . 
     [Step sc 05 ] Lower touch probe. 
     [Step sc 06 ] Start measurement. 
     [Step sc 07 ] Determine whether the measurement program is completed, and if completed, proceed to Step sc 08  and if not completed, wait for completion of measurement program. 
     [Step sc 08 ] Raise touch probe, discharge the working fluid, and terminate the process. 
     The following is supplementary explanation about the flowchart described above. Step sc 01  corresponds to plate thickness acquisition means of the scope of claims. Step sc 03  corresponds to level adjustment means. 
     Embodiment 2 
     (When Machining) 
     The steps effected upon working are the same as those explained in the description of the first embodiment so that no explanation are made to avoid the duplication. 
     (When Measuring) 
       FIG. 9  is a diagram illustrating the principle for controlling upon measurement in Embodiment 2. When measuring, an upper guide part  21  is raised (see reference numeral  31 ), and a touch probe  3  is positioned in a lowered position (see reference numeral  32 ), or the probe  3  is attached to the upper guide part  21 . Then, a level detector such as a hydraulic pressure sensor  25 , and a working fluid supply device and discharge device automatically adjust the level of a working fluid, to a position where a lower surface of a workpiece  26  is immersed, and a table  24  and an unillustrated workpiece attachment jig attached between the workpiece  26  and the table  24  are completely submerged. That is, by using preset information on the high L 2  of the upper surface of the table on which the workpiece  26  is mounted, and information on a workpiece thickness L 5 , the level is set not lower than the table upper surface height L 2  and not higher than a workpiece upper surface height (table upper surface height L 2 +workpiece thickness L 5 ).
 
table upper surface height  L 2≤working fluid level  L 3′ on measurement≤table upper surface height  L 2+workpiece thickness  L 5  (Expression 2)
 
     Hence, even when measuring, it is possible to prevent thermal displacement of at least the table  24  of stainless steel of high linear expansion coefficient, and an additional sub-table for supporting the workpiece  26 . It is also possible to prevent a decrease in temperature of the workpiece lower surface due to heat of evaporation, to thereby suppress the variation in the temperature of the entire workpiece  26 . Since this can reduce the dimensional deviation between the machining position and the measuring position caused by temperature variation, the dimensions can be measured far more accurately than measuring after discharging all working fluid  4 . 
       FIG. 10  is a flowchart illustrating the process to be effected in a measurement program of Embodiment 2 of the present invention. The measurement program will now be described, according to the steps. 
     [Step sd 01 ] Read the preset value of workpiece thickness. 
     [Step sd 02 ] Move upper guide to measuring height. 
     [Step sd 03 ] Adjust the level to height where upper surface of table is immersed. 
     [Step sd 04 ] Determine whether the adjustment through level detection is completed, and if completed, proceed to Step sd 05 , and if not completed, return to Step sd 03 . 
     [Step sd 05 ] Lower the touch probe. 
     [Step sd 06 ] Start measurement. 
     [Step sd 07 ] Determine whether the measurement program is completed, and if completed, proceed to Step sd 08 , and if not completed, wait for completion of measurement program. 
     [Step sd 08 ] Raise the touch probe, discharge water, and terminate the program. 
     The following is supplementary explanation about the flowchart described above. Step sd 01  corresponds to plate thickness acquisition means or setting means of the claims. Step sd 03  corresponds to level adjustment means. 
     As another mode of Embodiment 2, an optical measurement device having a measuring function for detecting an edge part of a workpiece by recognizing an image of the part to be measured may be used, instead of the touch probe  3 . 
     Embodiment 3 
     (When Machining) 
     The steps effected upon working are the same as those explained in the description of the first embodiment so that no explanation are made to avoid the duplication. 
     (When Measuring) 
       FIG. 11  is a diagram illustrating how the level is controlled when measuring in Embodiment 3. 
     When measuring, an upper guide part  21  is raised (see reference numeral  31 ), and a touch probe  3  is positioned in a lowered position (see reference numeral  32 ), or is attached to the upper guide part  21 . Then, a level detector such as a hydraulic pressure sensor  25 , and an unillustrated working fluid supply device and discharge device automatically adjust the level of a working fluid to a position near the height of a sensor  3   a  on the tip end of the touch probe  3 . At this time, a preset upper guide height on measurement L 1 ′, and a distance from the upper guide height on measurement L 1 ′ to the sensor  3   a  on the tip end of the touch probe  3  (sensor position upon measurement L 6 ′) are used, to adjust the level in conjunction with the height position of the sensor  3   a  on the tip end of the touch probe  3 .
 
working fluid level on measurement  L 3′=table upper surface height  L 2+upper guide height on measurement  L 1′−sensor position on measurement  L 6′  (Expression 3)
 
     Hence, even when measuring, it is possible to prevent thermal displacement of at least the table  24  of stainless steel of high linear expansion coefficient, and an additional sub-table for supporting a workpiece  26 . It is also possible to prevent a decrease in temperature of the workpiece  26  due to heat of evaporation, to thereby suppress variation in the temperature of the entire workpiece  26 . Since this can reduce the dimensional deviation between the machining position and the measuring position caused by temperature variation, the dimensions can be measured far more accurately than measuring after discharging a working fluid  4 . 
       FIG. 12  is a flowchart illustrating the process to be effected in a measurement program of Embodiment 3 of the present invention. The measurement program will now be described, according to the steps. 
     [Step se 01 ] Read preset value of sensor distance (sensor position upon measurement L 6 ′). 
     [Step se 02 ] Move upper guide to measuring height. 
     [Step se 03 ] Adjust the level from upper guide position to sensor distance. 
     [Step se 04 ] Determine whether the adjustment through level detection is completed, and if completed, proceed to Step se 05 , and if not completed, return to Step se 03 . 
     [Step se 05 ] Lower the touch probe. 
     [Step se 06 ] Start measurement. 
     [Step se 07 ] Determine whether the measurement program is completed, and if completed, proceed to Step se 08 , and if not completed, wait for completion of the measurement program. 
     [Step se 08 ] Raise the touch probe, discharge water, and terminate the process. 
     Embodiment 4 
     (When Machining) 
     The steps effected upon working are the same as those explained in the description of the first embodiment so that no explanation are made to avoid the duplication. 
     (When Measuring) 
       FIG. 13  is a diagram illustrating the principle for controlling the level upon measurement in Embodiment 4. 
     When measuring, an upper guide part  21  is raised (see reference numeral  31 ), a touch probe  3  is positioned in a lowered position (see reference numeral  32 ), or is attached to the upper guide part  21 . Then, a level detector such as a hydraulic pressure sensor  25 , and an unillustrated working fluid supply device and discharge device automatically adjust the level of a working fluid when measuring, to a position between the height of a sensor  3   a  on the tip end of the touch probe  3  and the body (lowermost part of body) of the touch probe  3 . At this time, a preset upper guide height on measurement L 1 ′, and a distance from the upper guide height on measurement L 1 ′ to the sensor  3   a  on the tip end of the touch probe  3  (sensor position on measurement L 6 ′) are used, to adjust the level in conjunction with the height position of the sensor  3   a  on the tip end of the touch probe  3 .
 
table upper surface height  L 2+upper guide height on measurement  L 1′−sensor position on measurement  L 6′≤working fluid level on measurement  L 3′≤table upper surface height  L 2+upper guide height on measurement  L 1′−measurement device body position on measurement  L 4′.   (Expression 4)
 
     Hence, even when measuring, it is possible to prevent thermal displacement of the table  24  of stainless steel of high linear expansion coefficient, and an additional sub-table for supporting a workpiece  26 . It is also possible to prevent a decrease in temperature of the workpiece  26  due to heat of evaporation, to thereby suppress variation in the temperature of the entire workpiece  26 . Since this can reduce the dimensional deviation between the machining position and the measuring position caused by temperature variation, the dimensions can be measured far more accurately than measuring after discharging a working fluid  4 . 
       FIG. 14  is a flowchart illustrating the processing of a measurement program of Embodiment 4 of the present invention. The measurement program will now be described, according to the steps. 
     [Step sf 01 ] Read set value of sensor distance. 
     [Step sf 02 ] Move upper guide to measuring height. 
     [Step sf 03 ] Adjust the level of working liquid on the basis of the level detector to that between the upper guide position upon measurement and the sensor position upon measurement. 
     [Step sf 04 ] Determine whether the adjustment on the basis of the level detector is completed, and if completed, proceed to Step sf 05 , and if not completed, return to Step sf 03 . 
     [Step sf 05 ] Lower the touch probe. 
     [Step sf 06 ] Start measurement. 
     [Step sf 07 ] Determine whether the measurement program is completed, and if completed, proceed to Step sf 08 , and if not completed, wait for completion of measurement program. 
     [Step sf 08 ] Raise the touch probe, discharge water, and terminate the process. 
     Next, another example of measuring the level of a working fluid  4  inside a work tank will be described.  FIG. 15  is a diagram illustrating the principle for controlling the level upon machining. Level detection means may be a sensor for detecting the hydraulic pressure near the bottom of the work tank  23 . The level detection means of a structure as shown in  FIG. 15  may also be used. In this structure, the hydraulic pressure is converted to the pressure of the air contained in the conduit  33 , and thus converted pressure is detected by the pressure sensor  34 . The level of the working liquid can be calculated from the detected pressure. This level measuring means is applicable to Embodiments 1 to 4.  FIG. 16  is a diagram illustrating the principle for controlling upon measuring. 
       FIG. 17  is a flowchart showing the process for automatically adjusting the level of a working fluid inside a work tank, in conjunction with the height position of an upper guide, and in dependence on a pressure detected by a level detector such as a hydraulic sensor. The process is applicable for the adjustment of the level of a working fluid automatically when measuring, in the aforementioned embodiments. 
     [Step sg 01 ] Read the position of upper guide. 
     [Step sg 02 ] Read pressure of level sensor. 
     [Step sg 03 ] Determine whether pressure at measuring position exceeds appropriate range, falls below appropriate range, or is within appropriate range, and if it exceeds appropriate range, proceed to Step sg 04 , if it falls below appropriate range, proceed to Step sg 05 , and if it is within appropriate range, terminate the process for adjusting the level.
 
[Step sg 04 ] Discharge working fluid from the work tank.
 
[Step sg 05 ] Supply working fluid into the work tank.
 
[Step sg 06 ] Determine whether there is change in upper guide position or pressure, and if there is change, return to Step sg 01 , and if there is no change, wait for change.
 
     While the present invention have been described above in terms of the preferred embodiments thereof, it is not intended to be so limited, and can be implemented in other modes by adding changes according to need.