Abstract:
A warm-up control system for a machine tool drives a spindle motor for rotating a main spindle of the machine tool before starting machining, thereby performing a warm-up operation, and acquires a spindle motor load value that is variable by heat generated during the warm-up operation. The warm-up operation is ended when the acquired spindle motor load value becomes substantially equal to a preset working reference load value. The cutting capacity of the main spindle can be improved by this warm-up operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a warm-up control system for a machine tool. 
         [0003]    2. Description of the Related Art 
         [0004]    Conventionally, if a machine tool starts machining in a cold state, a shaft is thermally displaced so that the machining accuracy is degraded as the temperature of a spindle motor increases. Before starting actual machining, therefore, it is necessary to previously perform a warm-up operation by repeating a normal operation so that the machine temperature is stabilized, thereby maintaining temperature consistency. 
         [0005]    A spindle motor controller for a machine tool is disclosed in Japanese Patent Application Laid-Open No. 2003-199378. This motor controller comprises an operation mode switching circuit for switching between a warm-up operation mode in which an warm-up operation is carried out and a normal operation mode in which a normal operation is carried out. In the case of the warm-up operation mode, the warm-up operation time is expected to be reduced by superimposingly applying high-harmonic voltage of a higher frequency than the drive frequency to a motor. 
         [0006]    Various means have been devised to make the most of the cutting performance of a machine tool for cutting work. In general, a method is used to improve the cutting performance by increasing the mechanical rigidity, based on the machine design, main spindle design, and other factors. 
         [0007]    In the warm-up control of the machine tool described above, the amount of thermal displacement of the machine tool, due to heat generation by rotational drive means, such as a motor, heat generation of a main spindle bearing, etc., is stabilized by idling a main spindle that is attached to the machine tool. This warm-up control, like the technique disclosed in Japanese Patent Application Laid-Open No. 2003-199378 described above, is used as a means for stabilizing the amount of thermal displacement of the machine tool due to the heat generation, and has never been used to improve the cutting capacity of the main spindle. 
       SUMMARY OF THE INVENTION 
       [0008]    Accordingly, the object of the present invention is to provide a warm-up control system for a machine tool, capable of improving the machining capacity of the main spindle of the machine tool by a warm-up operation. 
         [0009]    A warm-up control system for a machine tool according to the present invention comprises a warm-up operation executing unit configured to drive a spindle motor for rotating a main spindle of the machine tool before starting machining, thereby performing a warm-up operation, a spindle motor load value acquiring unit configured to acquire a spindle motor load value variable by heat generated during the warm-up operation of the warm-up operation executing unit, a comparison unit configured to compare the spindle motor load value acquired by the spindle motor load value acquiring unit with a preset working reference load value, and a warm-up operation ending unit configured to end the warm-up operation when it is determined as a result of the comparison by the comparison unit that the difference between the spindle motor load value and the preset working reference load value is within a predetermined range. 
         [0010]    The warm-up operation may comprise acceleration and deceleration of the main spindle and steady-state rotation of the main spindle achieved by driving the spindle motor. 
         [0011]    The spindle motor load value may be a value of current or a load meter value obtained from the spindle motor. 
         [0012]    The warm-up control system may further comprise a reference load value setting unit capable of setting a plurality of the working reference load values and a reference load value selection unit configured to select one reference load value to be used for the comparison in the comparison unit, from among the reference load values set in the reference load value setting unit. 
         [0013]    The warm-up control system may further comprise a warm-up operation end informing unit configured to inform of an end of the warm-up operation when the warm-up operation is ended by the warm-up operation ending unit. 
         [0014]    The warm-up control system may further comprise a machining starting unit configured to automatically start the machining when the warm-up operation is ended by the warm-up operation ending unit. 
         [0015]    According to the present invention, machining can be performed in a state where output of a main spindle is improved, by determining an end of a warm-up operation based on a spindle motor load value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The above and other objects and features of the present invention will be obvious from the ensuing description of embodiments with reference to the accompanying drawings, in which: 
           [0017]      FIG. 1  is a block diagram illustrating a principal part of a numerical controller for controlling a machine tool; 
           [0018]      FIG. 2  is a graph illustrating an example of transition of a spindle motor load value during a warm-up operation; 
           [0019]      FIG. 3  is a flowchart illustrating processing from the start to the end of the warm-up operation; 
           [0020]      FIG. 4  is a flowchart illustrating processing after the end of the warm-up operation; and 
           [0021]      FIG. 5  is a diagram illustrating set items for operating conditions. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]      FIG. 1  is a block diagram showing a principal part of a numerical controller  10  for controlling a machine tool. 
         [0023]    A processor (CPU)  11  of the numerical controller  10  reads system programs stored in a ROM  12  through a bus  21  and generally controls the numerical controller according to the system programs. A RAM  13  is loaded with temporary calculation data, display data, various data that are inputted by an operator through an LCD/MDI (manual data input) unit  70 , etc. 
         [0024]    An SRAM  14  is a nonvolatile memory that is backed up by a battery (not shown) such that it can maintain its memory state even after the numerical controller  10  is switched off. In the SRAM  14 , a program for the measurement of an initial position, a program for thermal displacement correction of the machine tool, a machining program (described later) read through an interface  15 , a machining program input through the LCD/MDI unit  70 , etc are stored. Further, the ROM  12  is preloaded with various system programs for the execution of edit mode processing required for the creation and editing of the machining programs and processing for automatic operations. 
         [0025]    The interface  15  is an interface for an external device which can be connected to the numerical controller  10  and is connected with an external device  72 , such as an external storage device. The machining programs, thermal displacement measurement program, etc., are read from the external storage device. A programmable machine controller (PMC)  16  controls auxiliary devices or the like on the machine tool side according to sequential programs in the numerical controller  10 . Specifically, necessary signals on the auxiliary device side are converted according to these sequential programs, based on M-, S-, and T-functions commanded by the machining programs. The converted signals are output to the auxiliary device side through an input-output unit  17 . The auxiliary devices, e.g., various actuators, are activated by these output signals. When signals are received from various switches of a control panel on the main body of the machine tool, moreover, they are processed as required, and the result of the processing is delivered to the processor  11 . 
         [0026]    Image signals representative of the respective current positions of the individual axes of the machine tool, alarms, parameters, image data, etc., are transmitted to the LCD/MDI unit  70  and displayed on its display. The LCD/MDI unit  70  is a manual data input device provided with the display, a keyboard, etc. An interface  18  receives data from the keyboard of the LCD/MDI unit  70  and delivers it to the processor  11 . An interface  19  is connected to a manual pulse generator  71 . The manual pulse generator  71  is mounted on the control panel of the machine tool and used for precisely positioning of movable parts of the machine tool by respective-axes control with distributed pulses based on manual operation. 
         [0027]    X- and Y-axis control circuits  30  and  31  for moving a table T of the machine tool and a Z-axis control circuit  32  receive move commands for the individual axes from the processor  11  and output the commands to servo amplifiers  40  to  42 . On receiving these commands, the servo amplifiers  40  to  42  drive servomotors  50  to  52  for the individual axes of the machine tool, respectively. Position sensors are incorporated individually in the servomotors  50  to  52 . Position signals from these position sensors are fed back as pulse trains. 
         [0028]    A spindle control circuit  60  receives a spindle rotation command for the machine tool and outputs a spindle speed signal to a spindle amplifier  61 . On receiving this spindle speed signal, the spindle amplifier  61  rotates a spindle motor  62  of the machine tool at a commanded rotational speed, thereby driving a tool. A position detector  63  is coupled to the spindle motor  62  by gears, a belt, or the like. The position detector  63  outputs feedback pulses in synchronism with the rotation of a main spindle, and the feedback pulses are read through an interface  20  by the processor  11 . A clock circuit  65  is a clock device adjusted so as to synchronize with the current time. 
         [0029]    It is experimentally confirmed that the value of the spindle motor load changes based on warm-up control by the numerical controller  10 . In performing the warm-up control by the numerical controller  10  that drivingly controls the machine tool, the temperature of the spindle motor is also increased by a warm-up operation. As disclosed in Japanese Patent Application Laid-Open No. 2003-199378 mentioned above, therefore, the warm-up operation may possibly be controlled based on the spindle motor temperature. Since the spindle motor temperature is based on a plurality of temperature factors, such as the temperatures of a winding and rotor that influence the output of the spindle motor, however, whether or not the spindle motor output is improved cannot be determined based on the temperatures of the winding and rotor. Since a spindle motor load value is directly influenced by a spindle motor output, in contrast, the spindle motor output can be accurately determined. The value of current that flows through the spindle motor, acquired by the numerical controller  10 , or a load meter value can be used as the spindle motor load value. 
         [0030]      FIG. 2  is a graph illustrating an example of transition of the spindle motor load value during a warm-up operation. 
         [0031]    As seen from the graph of  FIG. 2 , the spindle motor load value obtained when the spindle motor is in steady-state rotation ( 2 ) is lower than that obtained when the spindle motor is in steady-state rotation ( 1 ). Thus, the spindle motor load value measured during the steady-state rotation after the warm-up operation is lower than that measured during the steady-state rotation before the warm-up operation, which indicates that the spindle motor output is improved by the warm-up operation. Such improvement of output of a spindle motor is considered to be attributable to reduction of resistance due to increases of temperature of oil that lubricates friction spots inside the main spindle as well as to the characteristics of the motor used. The transition shown in  FIG. 2  is only an example, and it may alternatively be shown in a different diagram depending on the characteristics of the spindle motor and the like. The spindle motor load value is given by the load meter value or the value of current flowing through the spindle motor acquired by the numerical controller  10 . 
         [0032]    The warm-up control according to the present invention will now be described with reference to  FIGS. 3 to 5 . 
         [0033]    Before starting the warm-up operation, as shown in  FIG. 5 , necessary operating conditions for the warm-up control of the machine tool by the numerical controller  10  are manually set in the numerical controller  10 . An operating condition setting  100  comprises a setting  110  used during the warm-up operation and an operation selection  120  after the end of the warm-up operation. 
         [0034]    The setting  110  used during the warm-up operation comprises a setting  112  of one or a plurality of reference load values used to determine whether or not to end the warm-up operation and a setting  114  of a working reference load value (PmA) for selecting an actually used value, from among other reference load values set by the reference load value setting  112 . 
         [0035]    On the other hand, the operation selection  120  after the end of the warm-up operation comprises a setting  122  for automatic machining and a setting  126  for simply ending the warm-up operation. Further, the setting  122  for automatic machining comprises a setting  124  of the machining programs for machining, while the setting  126  for simply ending the warm-up operation comprises a setting  128  for informing an operator by means of audio-visual means, such as a lamp, buzzer, etc. 
         [0036]    A spindle motor load value determined experimentally, empirically, or by calculation is set as a reference load value used to determine whether or not to end the warm-up operation (see reference numeral  112  in  FIG. 5 ). A plurality of reference load values can be set, and that one of them which is to be actually used depends on the setting (see reference numeral  114  in  FIG. 5 ) of the working reference load value (PmA). In a warm-up control system according to the present invention, the reference load value setting  112  of  FIG. 5  corresponds to a reference load value setting unit capable of setting a plurality of working reference load values, while the setting  114  of the working reference load value (PmA) of  FIG. 5  corresponds to a reference load value selection unit configured to select one reference load value to be used for comparison in a comparison unit, from among the reference load values set in the reference load value setting unit ( 112 ). 
         [0037]    Warm-up control processing according to the present invention will now be described with reference to  FIGS. 3 and 4 . 
         [0038]    [Step SA 01 ] It is determined whether or not a warm-up operation mode is on. If this mode is on, the program proceeds to Step SA 02 . If not (NO), activation of the warm-up operation mode is awaited. 
         [0039]    [Step SA 02 ] The warm-up operation is started. Specifically, a program for performing the warm-up operation is read. 
         [0040]    [Step SA 03 ] The activity of the warm-up operation is read from the operating condition setting. [Step SA 04 ] The spindle motor load value is acquired. 
         [0041]    [Step SA 05 ] It is determined whether or not a current spindle motor load value (D) is substantially equal to the working reference load value (PmA) determined by the operating condition setting. If the values are substantially equal (YES), the program proceeds to Step SA 06 . If not (NO), the program proceeds to Step SA 07 . If the difference between the two values is within a preset range, the values are regarded as being substantially equal. 
         [0042]    [Step SA 06 ] The warm-up operation is ended. 
         [0043]    [Step SA 07 ] It is determined whether or not a predetermined time (T) has elapsed. If the lapse of the predetermined time is confirmed, the program proceeds to Step SA 04 . If not (NO), the lapse of the predetermined time (T) is awaited. 
         [0044]    [Step SA 08 ] An operation to be performed after the end of the warm-up operation is selected. If the setting of the operation is the setting  122 , the program proceeds to Step SA 12 . If the operation setting is the setting  126 , the program proceeds to Step SA 09 . 
         [0045]    [Step SA 09 ] It is determined whether or not the setting  128  is selected. If the setting  128  is selected (YES), the program proceeds to Step SA 10 . If not (NO), the program proceeds to Step SA 11 . 
         [0046]    [Step SA 10 ] The operator is informed by audio-visual means, such as a lamp, buzzer, etc. 
         [0047]    [Step SA 11 ] The warm-up operation mode is turned off. 
         [0048]    [Step SA 12 ] The warm-up operation mode is turned off. 
         [0049]    [Step SA 13 ] Machining is automatically started.