Abstract:
A machine tool includes a rotational drive unit that rotationally drives a main spindle to which a tool is attached, a moving unit that relatively moves the tool with respect to the workpiece, and a rotational speed adjustment unit that adjusts a rotational speed of the main spindle by controlling the rotational drive unit. The machine tool also includes a machining start determination unit that determines that machining of the workpiece by the tool is started with the rotational drive unit rotationally driving the main spindle. The rotational speed adjustment unit exponentially raises the rotational speed of the main spindle after reducing the rotational speed to a predetermined rotational speed set in advance, on condition that the machining start determination unit determines that the machining is started, and causes the rotational speed of the main spindle to reach a value of the rotational speed before being reduced.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application claims the benefit of Japanese Patent Application Number 2013-150812 filed on Jul. 19, 2013, the entirety of which is incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a machine tool in which the rotational speed of a main spindle to which a tool configured to machine a workpiece is attached is adjustable to a predetermined rotational speed, and to a control method for the machine tool. 
       DESCRIPTION OF RELATED ART 
       [0003]    Japanese Patent Application Publication No. 2004-322246 (JP 2004-322246 A), for example, discloses a machine tool including a rotational drive unit that rotationally drives a main spindle to which a tool configured to machine a workpiece is attached, and a moving unit that relatively moves the tool with respect to the workpiece. In the machine tool, the rotational speed of the main spindle is gradually raised linearly to a final rotational speed, or the moving speed of the tool is gradually raised linearly to a final moving speed, in a machining start period, which is a predetermined period since machining of the workpiece by the tool is started, within one cycle in which the workpiece is machined by the tool. If the rotational speed of the main spindle is gradually raised linearly to the final rotational speed as in the machine tool according to JP 2004-322246 A, an impact applied to the tool during machining of the workpiece can be relieved compared to a case where the rotational speed of the main spindle is abruptly raised to the final rotational speed. Therefore, it can be expected that occurrence of chipping of the tool is suppressed. 
         [0004]    Even in the case where the rotational speed of the main spindle is gradually raised linearly to the final rotational speed as described above, however, an impact force applied to the tool may not be reduced in the case where it is necessary to deeply cut the workpiece which is a difficult-to-machine material using the tool, for example. Thus, occurrence of chipping of the tool may not be suppressed sufficiently, and the life of the tool may be shortened. 
       SUMMARY OF THE INVENTION 
       [0005]    In view of such circumstances, an object of the present invention is to provide a machine tool capable of suppressing occurrence of chipping of a tool to extend the life of the tool, and a control method for the machine tool. 
         [0006]    A first aspect of the present invention provides a machine tool including a rotational drive unit, a moving unit, and a rotational speed adjustment unit. The rotational drive unit rotationally drives a main spindle to which a tool configured to machine a workpiece is attached. The moving unit relatively moves the tool with respect to the workpiece. The rotational speed adjustment unit is capable of adjusting a rotational speed of the main spindle by controlling the rotational drive unit. The machine tool includes a machining start determination unit that determines that machining of the workpiece by the tool is started with the rotational drive unit rotationally driving the main spindle. In the machine tool, on condition that the machining start determination unit determines that the machining is started, the rotational speed adjustment unit gradually raises a rising rate of the rotational speed of the main spindle after reducing the rotational speed to a predetermined rotational speed set in advance, and causes the rotational speed of the main spindle to reach a value of the rotational speed before being reduced. 
         [0007]    A second aspect of the present invention provides the machine tool according to the first aspect, in which the rotational speed adjustment unit exponentially raises the rotational speed of the main spindle after being reduced to the predetermined rotational speed, and causes the rotational speed of the main spindle to reach the value of the rotational speed before being reduced. 
         [0008]    A third aspect of the present invention provides a control method for a machine tool in which a rotational speed of a main spindle to which a tool configured to machine a workpiece is attached is adjustable by rotationally driving the main spindle to relatively move the tool with respect to the workpiece. The control method includes a machining start determination step and a rotational speed adjustment step. The machining start determination step is a step of determining that machining of the workpiece by the tool is started with the main spindle rotationally driven. The rotational speed adjustment step is a step of, on condition that it is determined in the machining start determination step that the machining is started, gradually raising a rising rate of the rotational speed of the main spindle after reducing the rotational speed to a predetermined rotational speed set in advance, and causing the rotational speed of the main spindle to reach a value of the rotational speed before being reduced. 
         [0009]    A fourth aspect of the present invention provides the control method for a machine tool according to the third aspect, in which the rotational speed adjustment step includes exponentially raising the rotational speed of the main spindle after being reduced to the predetermined rotational speed, and causing the rotational speed of the main spindle to reach the value of the rotational speed before being reduced. 
         [0010]    With the machine tool according to the first aspect of the present invention and the control method for a machine tool according to the third aspect of the present invention, an impact applied to the tool during machining of the workpiece can be relieved by reducing the rotational speed of the main spindle, to which the tool is attached, to the predetermined rotational speed immediately after machining of the workpiece by the tool is started. Hence, occurrence of chipping of the tool can be suppressed. Further, an impact applied to the tool can also be relieved by suppressing an abrupt rise in rotational speed of the main spindle since machining of the workpiece is started until the rotational speed of the main spindle reaches the value of the rotational speed before being reduced. Therefore, occurrence of chipping of the tool can be suppressed. As a result, the life of the tool can be extended. 
         [0011]    According to the second and fourth aspects of the present invention, an abrupt rise in rotational speed of the main spindle can be suppressed until the rotational speed of the main spindle which has been reduced to the predetermined rotational speed reaches the value of the rotational speed before being reduced. Consequently, an impact applied to the tool can be relieved until the rotational speed of the main spindle reaches the value of the rotational speed before being reduced. Thus, occurrence of chipping of the tool can be suppressed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic diagram of a machining center according to an embodiment of the present invention. 
           [0013]      FIG. 2  is a flowchart of a process in which a rotational speed of a main spindle is raised exponentially in the machining center. 
           [0014]      FIG. 3  is a graph illustrating the proportion of the main spindle rotational speed during gradual increase adjustment to the main spindle rotational speed at the time of starting with respect to the cutting length. 
           [0015]      FIG. 4  is a graph illustrating the size of chipping with respect to the number of cuttings. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0016]    An embodiment of the present invention will be described with reference to  FIGS. 1 to 4 . As illustrated in  FIG. 1 , a machining center  1  includes a base  2 , a table  3 , a column  4 , a spindle head  5 , and a control device  6 . The machining center  1  is an example of the machine tool according to the present invention. 
         [0017]    The table  3  and a first drive motor  8  are provided on the upper surface of the base  2 . A ball screw  9  that extends in the front-rear direction (Y direction) of the base  2  is concentrically coupled to a rotary shaft of the first drive motor  8 . A nut member (not illustrated) fixed to the table  3  is threadably engaged with the ball screw  9 . When the ball screw  9  is rotated by driving the first drive motor  8 , the nut member is moved in the Y direction. This allows the table  3  to move in the Y direction. A workpiece W is removably mounted on the upper surface of the table  3 . 
         [0018]    The column  4  is provided to stand upright in rear of the table  3  (on the left side in  FIG. 1 ) on the upper surface of the base  2 . A drive shaft  11  that extends in the left-right direction (X direction) as the column  4  is viewed from the front is rotatably supported on the front surface of the column  4  (on the right side in  FIG. 1 ). The drive shaft  11  is concentrically coupled to a rotary shaft of a second drive motor (not illustrated). A saddle  12  is disposed on the front side of the drive shaft  11  (on the right side in  FIG. 1 ). A nut member (not illustrated) fixed to the saddle  12  is threadably engaged with the drive shaft  11 . The saddle  12  is provided with the spindle head  5  located over the table  3 . A tool spindle  13 , to which a tool  14  is mounted, is supported by spindle head  5  so as to be directed downward. A main spindle motor M configured to rotate the tool spindle  13  is built in the spindle head  5 . A nut member (not illustrated) fixed to the spindle head  5  is threadably engaged with a ball screw  15  that extends in the up-down direction (Z direction) of the saddle  12 . The ball screw  15  is concentrically coupled to a rotary shaft of a third drive motor  16  fixed to the saddle  12 . In the machining center  1 , when the drive shaft  11  is rotated by driving the second drive motor, the nut member of the saddle  12  is moved in the X direction. As a result, the tool  14  can be moved in the X direction together with the saddle  12 . When the ball screw  15  is rotated by driving the third drive motor  16 , meanwhile, the nut member of the spindle head  5  is moved in the Z direction. As a result, the tool  14  provided at the spindle head  5  can be moved in the Z direction. To perform cutting on the workpiece W by the tool  14 , the tool  14  is moved in the Z direction while rotating the tool  14  by rotating the tool spindle  13  by the main spindle motor M to cut into the workpiece W, the tool  14  is moved in the X direction, and the table  3  is moved in the Y direction. The first and third drive motors  8  and  16 , the second drive motor, the ball screws  9  and  15 , the drive shaft  11 , and the respective nut members of the table  3 , the spindle head  5 , and the saddle  12  are an example of the moving unit according to the present invention. The main spindle motor M is an example of the rotational drive unit according to the present invention. 
         [0019]    As illustrated in  FIG. 1 , the control device  6  includes a computation device  20 , a storage device  21 , a main spindle torque measurement device  22 , a main spindle rotational speed adjustment device  23 , and a drive motor control device  24 . The storage device  21  is connected to the computation device  20 . The storage device  21  stores radius data on the respective radii of various types of tools  14  that can be mounted to the tool spindle  13 , threshold torque data for the tool spindle  13 , a calculation formula for calculating a command rotational speed for controlling the rotational speed of the main spindle motor M for the purpose of suppressing occurrence of chipping of the tool  14 , command rotational speed data for controlling the rotational speed of the main spindle motor M at the time of starting of the main spindle motor M, command rotational speed data for controlling the respective rotational speeds of the first and third drive motors  8  and  16  and the second drive motor, and a numerical control (NC) program. As discussed later, the threshold torque data are used to detect whether or not cutting performed on the workpiece W by the tool  14  has been started. 
         [0020]    The main spindle torque measurement device  22  is connected to the computation device  20 . The main spindle torque measurement device  22  is disposed inside the spindle head  5  and outside the tool spindle  13 . By way of example, the main spindle torque measurement device  22  measures torque to be applied to the tool spindle  13  on the basis of an electromotive force generated in a coil of the main spindle torque measurement device  22  during cutting performed on the workpiece W by the tool  14 . The computation device  20  can receive measured torque data for the tool spindle  13  from the main spindle torque measurement device  22 . 
         [0021]    The main spindle rotational speed adjustment device  23  is connected to the computation device  20 . The main spindle rotational speed adjustment device  23  is connected to the main spindle motor M. As discussed later, in the case where it is determined that the measured torque data are more than the threshold torque data, the computation device  20  calculates the command rotational speed for controlling the rotational speed of the main spindle motor M using the calculation formula stored in the storage device  21 . When a rotational speed command matching the command rotational speed is received from the computation device  20 , the main spindle rotational speed adjustment device  23  transmits the rotational speed command to the main spindle motor M. 
         [0022]    The drive motor control device  24  is connected to the computation device  20 . The drive motor control device  24  is connected to the first and third drive motors  8  and  16  and the second drive motor. When cutting is performed on the workpiece W by the tool  14  on the basis of the NC program, the drive motor control device  24  receives the command rotational speed data stored in the storage device  21  from the computation device  20 . When the command rotational speed data are received, the drive motor control device  24  transmits a rotational speed command matching the command rotational speed data to the first and third drive motors  8  and  16  and the second drive motor. This causes the rotary shafts of the first drive motor  8  etc. to rotate at a predetermined speed. As a result, the tool  14  is moved in the X direction and the Z direction, and the table  3  is moved in the Y direction. 
         [0023]    Next, a method for the control device  6  to adjust the rotational speed of the tool spindle  13  in a range where the cutting length of the workpiece W to be cut by the tool  14  is less than the radius of the tool  14  will be described with reference to the flowchart of  FIG. 2 . When power for the control device  6  is turned on, the computation device  20  executes a tool radius acquisition process (S 1 ). In the tool radius acquisition process (S 1 ), the radius data for the tool  14  mounted to the tool spindle  13  are acquired from the storage device  21 . 
         [0024]    After the tool radius acquisition process (S 1 ), the computation device  20  executes a main spindle torque acquisition process (S 2 ). In the main spindle torque acquisition process (S 2 ), measured torque data for the tool spindle  13  measured by the main spindle torque measurement device  22  are acquired when cutting is performed on the workpiece W by the tool  14  rotated at a predetermined rotational speed by the main spindle motor M on the basis of a rotational speed command matching the command rotational speed data at the time of starting of the main spindle motor M stored in the storage device  21 . 
         [0025]    After the main spindle torque acquisition process (S 2 ), the computation device  20  reads the threshold torque data from the storage device  21 . The computation device  20  then compares the measured torque data acquired in the main spindle torque acquisition process (S 2 ) and the threshold torque data to determine whether or not the measured torque data are more than the threshold torque data (S 3 ). In the case where cutting is not performed on the workpiece W by the tool  14  and it is determined in S 3  that the measured torque data are less than the threshold torque data, the process returns to the main spindle torque acquisition process (S 2 ). In the case where cutting performed on the workpiece W by the tool  14  is started with the tool  14  rotated at a predetermined rotational speed by the main spindle motor M and it is determined in S 3  that the measured torque data are more than the threshold torque data, on the other hand, the computation device  20  executes a main spindle rotational speed changing process (S 4 ). The computation device  20  repeatedly executes the main spindle rotational speed changing process (S 4 ) until it is determined in S 5  that the cutting length is more than the radius of the tool  14 . In the embodiment, it is determined that cutting performed on the workpiece W by the tool  14  is started in the case where it is determined in S 3  that the measured torque data are more than the threshold torque data. This allows the main spindle rotational speed changing process (S 4 ) to be executed from a correct machining start point. 
         [0026]    In the main spindle rotational speed changing process (S 4 ), a command rotational speed S C  for controlling the rotational speed of the main spindle motor M is consecutively calculated using the following formula (1) stored in the storage device  21  until the cutting length becomes more than the radius of the tool  14 . The computation device  20  consecutively transmits command rotational speed data matching the command rotational speed S C  to the main spindle rotational speed adjustment device  23  until the cutting length becomes more than the radius. 
         [0000]        S   C   =S   0 ×{0.8+0.2×exp[−7.5×( r−L )/ r ]}[min −1 ]  (1)
 
         [0027]    In the formula, S 0  is the command rotational speed [min −1 ] for the tool spindle  13 , r is the radius [mm] of the tool  14 , and L is the cutting length [mm]. 
         [0028]    In the main spindle rotational speed changing process (S 4 ), the main spindle rotational speed adjustment device  23  consecutively provides the main spindle motor M with a rotational speed command that matches the command rotational speed data consecutively received from the computation device  20 . The command rotational speed S C  that matches the command rotational speed data is reduced to 80% of the command rotational speed S 0  for the tool spindle  13  immediately after cutting performed on the workpiece W by the tool  14  is started. The command rotational speed S C  is then gradually increased exponentially along with an increase in cutting length (L) to reach a value corresponding to 100% of the command rotational speed S 0  when the cutting length (L) becomes equal to the radius (r) of the tool  14 . Therefore, upon receiving a rotational speed command matching the command rotational speed S C , the main spindle motor M exponentially raises the rotational speed of the tool spindle  13  after reducing the rotational speed to 80% of that at the time of starting immediately after cutting is started. Then, the rotational speed of the tool spindle  13  reaches a value corresponding to 100% of that at the time of starting when the cutting length (L) becomes equal to the radius (r, which is 25 mm in the example), as illustrated in  FIG. 3 . Herein, control in which the rotational speed of the tool spindle  13  is raised exponentially after being reduced to 80% of that at the time of starting is referred to as exponential control. An impact applied to the tool  14  during cutting can be relieved by reducing the rotational speed of the tool spindle  13  to which the tool  14  is mounted to 80% of that at the time of starting immediately after cutting is started through the exponential control. Hence, occurrence of chipping of the tool  14  can be suppressed. Further, an impact applied to the tool  14  can also be relieved by suppressing an abrupt rise in rotational speed of the tool spindle  13  since cutting is started until the rotational speed of the tool spindle  13  reaches the value of the rotational speed at the time of starting. This also makes it possible to suppress occurrence of chipping of the tool  14 . The computation device  20  and the main spindle rotational speed adjustment device  23  are an example of the rotational speed adjustment unit according to the present invention. 80% of the rotational speed of the tool spindle  13  at the time of starting is an example of the predetermined rotational speed according to the present invention. The main spindle rotational speed changing process (S 4 ) is an example of the rotational speed adjustment step according to the present invention. 
         [0029]    When the computation device  20  determines in S 5  that the cutting length is more than the radius of the tool  14 , the main spindle rotational speed changing process (S 4 ) is terminated, and cutting is performed on the workpiece W by the tool  14  on the basis of the NC program while rotating the tool spindle  13  at the rotational speed at the time of starting. In the embodiment, the main spindle rotational speed changing process (S 4 ) is executed only in a range where the cutting length is less than the radius of the tool  14 , and the time until the cutting length becomes equal to the radius is significantly short compared to the total time of cutting of the workpiece W. Thus, even if the rotational speed of the tool spindle  13  is reduced from that at the time of starting during execution of the main spindle rotational speed changing process (S 4 ), the total machining time for the workpiece W is extended only slightly and not significantly. 
         [0030]      FIG. 4  illustrates an example in which the size of chipping of the tool  14  with respect to the number of cuttings was measured for different control methods for the rotational speed of the tool spindle  13 . Measurement conditions were as follows. The workpiece W was made of a titanium alloy. The tool  14  was a milling tool with a diameter of 50 mm. The cutting speed was 45 m/min. The feed speed was 0.1 mm/blade. The cutting depth in the axial direction of the tool was 15 mm. The cutting depth in the radial direction of the tool was 50 mm. Cutting of a groove was performed with four blades. The exponential control discussed above, linear control (see  FIG. 3 ), and no control were selected as the control methods for the rotational speed. In the linear control, the rotational speed was raised linearly after being reduced to 80% of that at the time of starting immediately after cutting of the groove was started, and the rotational speed was caused to reach a value corresponding to 100% of that at the time of starting when the cutting length became equal to the radius of the milling tool. With no control, the rotational speed was maintained at a value corresponding to 100% of that at the time of starting since immediately after cutting of the groove was started until the cutting length became equal to the radius of the milling tool. With any of the control methods, as illustrated in  FIG. 4 , chipping of the milling tool became larger along with an increase in number of cuttings performed by the milling tool. The measurement results indicated that when the number of cuttings was 15, the size of chipping with the exponential control was ⅛ of the size of chipping with no control, and 1/3.5 of the size of chipping with the linear control. Thus, it was found that occurrence of chipping was suppressed to the greatest degree when the exponential control, among the three control methods, was performed. This is because, when the exponential control is performed, the rotational speed of the tool spindle  13  is raised only slightly from 80% of that at the time of starting for a while since cutting performed on the workpiece W by the milling tool is started, and therefore chipping is less likely to occur if the rotational speed of the tool spindle  13  is reduced from that at the time of starting for a long time. Even in the case where it is necessary for the milling tool to deeply cut a difficult-to-machine material, occurrence of chipping of the milling tool can be suppressed when the exponential control is performed, compared to cases with no control or the linear control, as seen from the measurement results in  FIG. 4 . As a result, the life of the milling tool can advantageously be extended even for deep cutting. 
       EFFECTS OF EMBODIMENT 
       [0031]    With the machining center  1  according to the embodiment and the control method for the machining center  1 , an impact applied to the tool  14  during cutting can be relieved by reducing the rotational speed of the tool spindle  13 , to which the tool  14  is mounted, to 80% of that at the time of starting immediately after cutting performed on the workpiece W by the tool  14  is started. Hence, occurrence of chipping of the tool  14  can be suppressed. Further, an impact applied to the tool  14  can also be relieved by suppressing an abrupt rise in rotational speed of the tool spindle  13  since cutting is started until the rotational speed of the tool spindle  13  reaches the value of the rotational speed at the time of starting. Therefore, occurrence of chipping of the tool  14  can be suppressed. As a result, the life of the tool can be extended. 
         [0032]    The present invention is not limited to the embodiment discussed above, and the configuration of the embodiment may be partially modified as appropriate without departing from the scope and spirit of the present invention. Although occurrence of chipping of the tool  14  is suppressed by performing the exponential control in the embodiment discussed above, the present invention is not limited thereto. For example, in order to suppress occurrence of chipping, the rotational speed of the tool spindle  13  may be raised along a upwardly concave curve having a gradually increasing rising rate and defined by a quadratic function, a cubic function, or a hyperbolic function after being reduced to 80% of that at the time of starting, and caused to reach a value corresponding to 100% of the rotational speed of the tool spindle  13  at the time of starting. Consequently, an impact applied to the tool  14  can be relieved by suppressing an abrupt rise in rotational speed of the tool spindle  13  since cutting is started until the rotational speed of the tool spindle  13  reaches the value of the rotational speed at the time of starting as in the embodiment discussed above. Hence, occurrence of chipping of the tool  14  can be suppressed. 
         [0033]    Unlike the embodiment discussed above, the rotational speed of the tool spindle  13  may be raised exponentially after being reduced to an appropriate rotational speed that is less than 80% of that at the time of starting. Further, occurrence of chipping may be suppressed by performing the exponential control also in a range in which the cutting length is more than the radius of the tool while suppressing significant extension of the total cutting time for the workpiece W. In addition, unlike the embodiment discussed above, it may be determined that cutting performed on the workpiece W by the tool  14  is started when a command for feed in the X direction, the Y direction, and the Z direction is switched from a fast feed command for use to move the tool  14  closer to and away from the work (workpiece W) to a cutting feed command for use during machining, for example. Besides, it may be determined that cutting is started by executing a machining control program having a command code specifying a cutting start point at which cutting performed on the workpiece W by the tool  14  is started, and causing the computation device  20  to analyze the command code. 
         [0034]    It is explicitly stated that all features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original disclosure as well as for the purpose of restricting the claimed invention independent of the composition of the features in the embodiments and/or the claims. It is explicitly stated that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate entity for the purpose of original disclosure as well as for the purpose of restricting the claimed invention, in particular as limits of value ranges.