Patent Publication Number: US-8972040-B2

Title: Control system for a machine tool

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a control system for a machine tool in which an optimum feed rate of cutting of the machine tool is calculated at the outset, a tool path as well as an optimum feed rate of cutting calculated is directly output to a driving unit of the machine tool, and in which a work and the tool are relatively moved along the tool path at an optimum cutting feed rate at each part of the tool path. 
     2. Description of Related Art 
     A conventional machine tool includes a CNC controller that controls the driving of the driving motor based on NC (numerical control) data composed of NC programs termed ‘G code’. If, in such conventional machine tool, a free curved surface, for example, is to be cut, an operating command is issued at each of a number of extremely short line segments, as shown in Patent publication 1. The CNC controller is instructed by NC data so as to render the cutting feed rate constant. On receiving the NC data, the CNC controller of the conventional machine tool actuates a driving motor, via a motor amplifier, in accordance with input NC data. 
     However, in the CNC controller of a conventional machine tool, the G code is pre-read, at the time of the machining operation, so that the cutting feed rate is slowed down from the command value in such a manner that the values of the acceleration as well as those of the speed, allocated to the respective driving shafts, will not exceed respective marginal values thereof. The reason may be such that the CNC controller of a conventional machine tool uses an interpreter system in which an input NC program is sequentially analyzed and executed. On the other hand, even though the limits of the acceleration and the speed of the driving unit that actuates the respective driving shafts may be known beforehand, there lack data on the mass weight or the inertial force of a moving object, such as a work, during the machining operations. It is thus not possible to calculate the limit of the torque generated with acceleration beforehand. 
     Thus, in the conventional machine tool, the cutting feed rate is dropped by a value more than is necessary than the command value. 
     When e.g., a corner of a work being machined is cut, the acceleration will theoretically become infinitely large unless the work is brought to a standstill. For this reason, a CNC controller of a conventional machine tool provides for a cutting mode of starting the operation of the next driving shaft before the outstanding operation (mode G64) is brought to a standstill at the corner of the work. That is, the corner of the work is rounded in a compromising fashion in order to raise the speed of the operations. 
     In such cutting mode, an R not inherently present in an engineering drawing is formed only in a compromising fashion by the CNC controller in order to speed up the operation. Hence, the finishing shape of the work tends to deviate from what has been intended by the designer. In addition, since the R produced at the corner of the work is set in a sloppy manner, there persists a problem that the final shape may not be surmised until the time the work has ultimately been cut. 
     RELATED TECHNICAL PUBLICATION 
     Patent Publication 
     [Patent Publication 1] 
     
         
         Japanese Laid-Open Patent Application 2006-11808 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     The present invention has been made to overcome the above mentioned status of the related technique. It is an object of the present invention to provide a control system for a machine tool in which an optimum cutting feed rate for a machine tool is calculated beforehand based on a tool path and data regarding the driving capability of the driving unit of the machine tool. The tool path and the optimum cutting feed rate calculated are directly output to the driving unit of the machine tool. The work and the tool are relatively moved along the tool path at a cutting feed rate optimum for each part of the tool path. The time taken by the machining operations is to be shorter and the accuracy in the machining operations is to be higher than in the conventional system. 
     Means to Solve the Problem 
     To accomplish the above object, the present invention provides a control system for a machine tool in which data driving a driving unit of the machine tool that causes relative movement between a work and a cutting tool is generated and output to the driving unit to control the machine tool. The control system includes a CL data generating unit that generates CL data including a tool path in a work coordinate system based on shape data regarding a post-machining shape of the work. 
     The control system also includes a CL data memory that stores the CL data generated by the CL data generating unit, and a driving capability data memory that memorizes driving capability data regarding the driving capability of a driving unit of the machine tool from the outset. The control system further includes a cutting feed rate data generation unit that, based on the CL data stored in the CL data memory and on the driving capability data stored in the driving capability data memory, generates cutting feed rate data in each part of the tool path of the CL data. The control system further includes a cutting feed rate data memory that memorizes the cutting feed rate data generated by the cutting feed rate data generation unit, and a controller that outputs the CL data stored in the CL data memory and the cutting feed rate data stored in the cutting feed rate data memory to the driving unit of the machine tool to cause relative movement of the work and the cutting tool at the cutting feed rate in each part of the tool path along the tool path of the CL data. 
     Meritorious Effect of the Invention 
     In the machine tool control system according to the present invention, the CL data inclusive of the tool path is generated by the CL data generating unit based on the shape data regarding the post-machining shape of the work. The cutting feed rate data in each part on the tool path of the CL data is then generated by the cutting feed rate generating unit based on the CL data and the driving capability data regarding the driving capability of the driving unit of the machine tool. The controller then directly outputs the CL data and the cutting feed rate data to the driving unit of the machine tool. It is thus possible to cause relative movement between the work and the tool at an optimum cutting feed rate at each part of the tool path along the tool path. 
     Thus, with the control system for the machine tool according to the present invention, the operating speed may be higher than in the conventional system to reduce the machining time. Moreover, in the machine tool control system of the present invention, the as-machined work, for example, its corner, may be machined as intended by the designer, thereby improving machining accuracy of the machine tool. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a machining system provided with a control system according to the present invention. 
         FIG. 2  is a diagrammatic view showing a tool path. 
         FIG. 3  is a diagrammatic view showing the relationship between X-coordinate values Xj(S) of pj(s) of the equation (1) and a parameter s. 
         FIG. 4  is a diagrammatic view showing the relationship between Y-coordinate values Yj(S) of pj(s) of the equation (1) and a parameter s. 
         FIG. 5  is a diagrammatic view showing the relationship between tj(s) of the equation (2) and a parameter s. 
         FIG. 5  is a graph showing the relationship between tj(s) and the parameter s of the equation (2). 
         FIG. 6  is a graph showing the values of dt/ds. 
         FIG. 7  is a flowchart showing the sequence of operations to find the cutting feed rate. 
         FIG. 8  is a graph showing the speeds of movement Vx, Vy in the X-axis and Y-axis directions, accelerations Ax, Ay in the X-axis and Y-axis directions and the cutting feed rate V. 
         FIG. 9  is a graph showing the speeds of movement Vx, Vy in the X-axis and Y-axis directions, accelerations Ax, Ay in the X-axis and Y-axis directions and the cutting feed rate V in case a cutting tool moves along a tool path at a constant cutting feed rate V as conventionally. 
         FIG. 10  is a graph showing the speeds of movement Vx, Vy in the X-axis and Y-axis directions, accelerations Ax, Ay in the X-axis and Y-axis directions and the cutting feed rate V in case the radius of curvature of the tool path is changed and the deceleration is in a decreased state. 
         FIG. 11  is a block diagram of a modification of a machining system provided with a control system according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The machine tool control system according to the present invention will now be described in detail with reference to the drawings. 
       FIG. 1  depicts a block diagram showing an arrangement of a system for machining operations  1  provided with the machine tool control system according to the present invention. Referring to  FIG. 1 , the system for machining operations  1  includes a machine tool  4 , and a control system  5  that controls the machine tool  4 . In the machining system  1 , a work  2  as an object of the machining operations is cut to a desired shape with a cutting tool  3  as the work  2  and the cutting tool  3  are moved relative to each other. 
     The machine tool  4  is a vertical type machining center having three linear driving axes, that is, X-, Y- and Z-axes, perpendicular to one another, as shown in  FIG. 1 . Specifically, the machine tool  4  includes a machine table  11 , a machining head  13  and a main spindle  14 . The machine table  11  is movably carried on a head  10 , as a support block, for movement in two directions, that is, in an X-axis direction and a Y-axis direction, perpendicular to each other within a horizontal plane. The machining head  13  is carried by a column  12 , mounted upright on one end of the head  10 , for facing an upper part of the machine table  11  for movement in a plumb-line direction, that is, along the Z-axis direction. The main spindle  14  is mounted depending from the machining head  13 . The cutting tool  3 , such as an end mill, is detachably fitted to the main spindle  14 . 
     The machine table  11  is driven in the X-axis direction and in the Y-axis direction, in response to rotation of a feed screw shaft, not shown, driven by table feed motors  15   x ,  15   y . The machining head  13  is moved in the Z-axis direction in response to rotation of a feed screw shaft, not shown, driven by a cutting tool feed motor  15   z . Further, the main spindle  14  is connected to a main spindle motor  15   s  and is run in rotation about the Z-axis, along with the cutting tool  3  mounted to the lower end of the main spindle, in response to rotation of the main spindle motor  15   s . For example, the table feed motors  15   x ,  15   y , tool feed motor  15   z  and the main spindle motor  15   s  are servo-motors. Note that the table feed motors,  15   x ,  15   y , tool feed motor  15   z  and the main spindle motor  15   s  are also referred to below simply as driving motors  15 . 
     The machine tool  4  also includes a motor amplifier  16  that drives the driving motor  15 . The motor amplifier  16  directly inputs a driving command from the control system  5  via an input/output interface, not shown. On receiving the driving command, the motor amplifier  16  converts acceleration/deceleration data, as later explained, into a driving current to amplify the current to drive the driving motors  15 . The driving motors  15  and the motor amplifier  16  are also referred to below collectively as a driving unit  6 . 
     In the machine tool  4 , designed and constructed as described above, the cutting tool  3  is run in rotation by the main spindle motor  15   s  in response to the driving command received by the motor amplifier  16  from the driving system  5 . At the same time, the machine table  11  is moved by the table feed motors  15   x ,  15   y  in the X-direction and/or in the Y-direction, while the machining head  13  is moved by the tool feed motor  15   z  in the Z-axis direction. The machine tool  4  thus causes relative movement between the work  2  and the cutting tool  3 , mounted on the machine table  11 , such as to cut the work  2  to a desired shape by the cutting tool  3 . 
     Referring to  FIG. 1 , the control system  5  is comprised of, for example, a general-purpose computer which is distinct from the machine tool  4  and which includes a CPU  20 , a ROM  21 , a RAM  22  and so forth. The control system further includes a shape data memory  30 , a tool/machining data memory  31 , a CL (Cutter Location) data generation unit  32 , a CL data memory  33 , a driving capability data memory  34 , a cutting feed rate data generation unit  35 , a cutting feed rate data memory  36 , an acceleration/deceleration data generation unit  37 , an acceleration/deceleration data memory  38 , a machining simulation unit  39 , a display  40 , an input unit  41  and a controller  42 . 
     The CL data generation unit  32 , cutting feed rate data generation unit  35 , acceleration/deceleration data generation unit  37 , machining simulation unit  39  and the controller  42  may be implemented by the CPU  20  of the general-purpose computer. The shape data memory  30 , tool/machining data memory  31 , CL data memory  33 , driving capability data memory  34 , cutting feed rate data memory  36  and the acceleration/deceleration data memory  38  are made up by a memory of the general-purpose computer and by an external memory. 
     On receiving shape data from an external device  50 , distinct from the machine tool  4  and the control system  5 , and operating as a computer-aided design system (so-called CAD device), the shape data memory  30  transiently holds input shape data. The shape data, generated by the external device  50 , may, for example, be data on the ultimate shape, size and finishing surface accuracy of the work  2 , obtained by the machining operations, material quality of the work  2 , shape of the work  2  before the machining operations, or on the mass weight of the work  2 . These shape data are delivered to the shape data memory from a recording medium, such as a magnetic disc, an optical disc, a magneto-optical disc or a semiconductor memory, or from the external device  50  over e.g., a network. 
     In the tool/machining data memory  31 , data on machining conditions are stored via a variety of recording mediums or over a network, as in the above mentioned shape data memory  30 . Examples of the data on machining conditions include data on the machining mode, such as contour line machining, scanning line machining, linear interpolation, arcuate interpolation or the operation of evading pneumatic cutting, data on sorts of cutting tools, such as types or materials of the cutting tools. Other examples include diameters of the cutting tools, cutting speeds, as set from one cutting tool sort to another depending on the material type of the work  2 , amounts of cut per revolution or tolerances as set from one tool sort to another depending on the material types of the work  2 . 
     The CL data generation unit  32  reads out the shape data stored in the shape data memory  30  and the data on machining conditions stored in the tool/machining data memory  31  to generate CL data inclusive of a movement path of the cutting tool  3  on the work  2 . The CL data generation unit  32  outputs the so generated CL data to the CL data memory  33  to store the data temporarily in the CL data memory  33 . 
     In the driving capability data memory  34 , as in the above mentioned shape data memory  30  and in the above mentioned tool/machining data memory  31 , there are stored driving capability data concerning the driving capability of the driving motors  15  of the driving unit  6  via a variety of recording mediums and over networks. These driving capability data may, for example, be data on the torques of the driving motors  15 , such as the starting torque, stalling torque (maximum torque) or the rated torque of each of the table feed motors  15   x ,  15   y , tool feed motor  15 Z or the main spindle motor  15   s.    
     The cutting feed rate data generation unit  35  reads out the CL data, stored in the CL data memory  33 , the mass weight of the work being cut  2  mounted on the machine table  11  stored in the shape data memory  30  and the driving capability data stored in the driving capability data memory  34  to generate the cutting feed rate data that will give the maximum speed at each part of the tool path of CL data. The cutting feed rate data generation unit  35  outputs the so generated cutting feed rate data to the cutting feed rate data memory  36  to store the data temporarily in the cutting feed rate data memory  36 . The sequence for the cutting feed rate data generation unit  35  to generate the cutting feed rate data that will provide the maximum speed at each part of the tool path of the CL data will be explained subsequently. 
     The acceleration/deceleration data generation unit  37  reads out the CL data stored in the CL data memory  33  and the cutting feed rate data stored in the cutting feed rate data memory  36 . Based on the CL data and the cutting feed rate data, the acceleration/deceleration data generation unit generates acceleration/deceleration data representing the relative acceleration or deceleration between the work  2  and the cutting tool  3 . The acceleration/deceleration data generation unit  37  outputs the so generated acceleration/deceleration data to the acceleration/deceleration data memory  38  to store the data temporarily in the acceleration/deceleration data memory  38 . 
     The machining simulation unit  39  reads out the CL data stored in the CL data memory  33  and the cutting feed rate data stored in the cutting feed rate data memory  36  or the acceleration/deceleration data stored in the acceleration/deceleration data memory  38  to perform machining simulation of causing relative movement between the work  2  and the cutting tool  3 . The machining simulation unit  39  calculates the time needed in machining the work  2  by the cutting tool  3  to a desired shape, and outputs the result of the machining simulation and the machining time to the display  40 . 
     The display  40  is made up of, for example, a CRT display or a liquid crystal display, and demonstrates the result of the machining simulation, carried out by the machining simulation unit  39 , machining time, the above mentioned cutting feed rate data or the acceleration/deceleration data. 
     The input unit  41  includes a keyboard, a mouse or a touch panel, operated by an operator of the machine tool  4 . With the input unit  41 , the operations of selecting desired data from the data stored in the respective memories, allowing each generation unit to generate data, allowing startup of machining by the machine tool  4  or editing stored or generated data, are carried out by the operator. 
     If the result of the simulation, machining time and so forth, indicated on the display  40 , is conformant to the designer&#39;s intention, and the input unit  41  has carried out the operation of allowing startup of the machining by the operator, the controller  42  reads out the acceleration/deceleration data stored in the acceleration/deceleration data memory  38 . The controller  42  directly outputs the acceleration/deceleration data and a driving command to the motor amplifier  16  of the driving unit  6 . 
     In the control system  5 , designed and constructed as described above, the CL data generation unit  32  generates the CL data based on shape data and machining condition data. The cutting feed rate data generation unit  35  generates cutting feed rate data, which becomes a maximum speed at each part of the tool path of the CL data, based on the CL data, mass weight of the work being cut  2  mounted on the machine table  11  and on the driving capability data. The acceleration/deceleration data generation unit  37  generates acceleration/deceleration data based on the CL data and the cutting feed rate data. The controller directly outputs the acceleration/deceleration data and a driving command to the motor amplifier  16  of the driving unit  6 . The controller thus causes relative movement between the work  2  and the cutting tool  3  on the machine tool  4  at a cutting feed rate which becomes a maximum speed at each part of the tool path, along the tool path, in order to cut the work  2  to a desired shape by the cutting tool  3 . 
     The sequence of operations in which the cutting feed rate data generation unit  35  of the control system  5  generates the cutting feed rate data in such a manner that the cutting feed rate will become maximum at each part of the tool path of the CL data will now be explained with reference to  FIGS. 2 to 7 . 
     Such an example case in which a corner part of the work  2 , mounted on the machine table  11 , is machined in a cutting mode (mode G64) of starting the machining operation by one of the X-axis driving shaft and the Y-axis driving shaft before cessation of the other driving shaft, as shown in  FIG. 2 , is explained. It is noted that, although the machine tool  4  has three driving shafts of the linear movement of X-axis, Y-axis and Z-axis driving shafts, the following explanation will be made in terms of two dimensions of the X-axis and Y-axis directions. 
       FIG. 2  shows a curvilinear tool path generated by the CL data generator  32 . It is noted that points entered on the curve represent control points. The curve is a third-order splined curve, for example, and may be differentiated with second-order differentiation. The curve may also be a Nurbs curve or a B-splined curve. 
     Such third-order splined curve p j (s), where s denotes a parameter, may be represented by the following equation (1):
 
 P   j ( s ):( x   j ( s ) ,y   j ( s ))  (1)
 
where
 
 x   j ( s )= a   xj   +b   xj ( s−s   j )+ c   xj ( s−s   j ) 2   +d   xj ( s−s   j ) 3  
 
and
 
 y   j ( s )= a   yj   +b   yj ( s−s   j )+ c   yj ( s−s   j ) 2   +d   yj (( s−s   h ) 3  
 
     Note that ax j , ay j  denote constants at an initial value S 0 , and bx j , by j  denote constants at the first-order differentiation. That is, these are constants obtained on partial differentiation with x and y, c xj  and c yj  are constants in the second-order differentiation and d xj , d yj  are constants in the third-order differentiation. 
     Hence, a curve between a jth control point and a (j+1)st control point, out of the control points, is expressed by p j (s), and may be shown by an X-coordinate value x j (S) shown in  FIG. 3  and a Y-coordinate value y j (S) shown in  FIG. 3 . 
     Given that, as a presupposition,
 
[Equation 1]
 
 x   j ( s   j )= x   j   ,y   j ( s   j )= y   j :  (1)
 
passing through a control point;
 
 x   j ( s   j+1 )= x   j+1 ( s   j+1 )= x   j+1 ,
 
 y   j ( s   j+1 )=y j+1 ( s   j+1 )= y   j+1 :  (2)
 
continuous
 
 x   j ′( s   j+1 )= x   j+1 ′( s   j+1 ),
 
 y   j ′( s   j+1 )= y   j+1 ′( s   j+1 ):  (3)
 
first derivative value is continuous
 
 x   j ″( s   j+1 )= x   j+1 ″( s   j+1 ),
 
 y   j ″( s   j+1 )= y   j+1 ″( s   j+1 ):  (4)
 
second derivative value is continuous
 
 x   0 ″(0)= x   n-1 ″( sn )=0,
 
 y   0 ″(0)= y   n-1 ″( sn )=0:  (5)
 
second derivative values at beginning and terminal points are 0,
 
     it is possible to find constants ax j , bx j , cx j , dx j , ay j , by j , cy j  and dy j  in each domain. 
     Since the speed of movement is expressed by time changes of the parameter s, its inverse function t(s) may be defined by a curve that may be differentiated by second-order or higher-order differentiation, as indicated by the following equation (2), in the same way as by the above equation (1):
 
[Equation 2]
 
 t   j ( s )= a   sj   +b   sj ( s−s   j )+ c   sj ( s−s   j ) 2   +d   sj ( s−s   j ) 3   (2)
 
     In the above mentioned machine tool  4 , the machine table  11  is driven in the X-axis and Y-axis directions. Hence, the speed of relative movement between the work  2  and the cutting tool  3 , the speed of movement of the machine table  11  in the X-axis direction Vx may be represented by the following equation (3): 
                   [     Equation   ⁢           ⁢   3     ]                           Vx   =           ⅆ       x   j     ⁡     (   s   )           ⅆ   s       ·       ⅆ   s       ⅆ   t         =         ⅆ       x   j     ⁡     (   s   )           ⅆ   s       /       ⅆ   t       ⅆ   s                   (   3   )               
and that in the Y-axis direction may be represented by the following equation (4):
 
     
       
         
           
             
               
                 
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     Therefore, the cutting feed rate V is represented by the following equation (5):
 
[Equation 5]
 
 V =√{square root over ( Vx   2   +V   y   2 )}  (5)
 
     Further, the acceleration Ax in the X-axis direction of the machine table  11  may be represented by the following equation (6), and the acceleration Ay in the Y-axis direction of the machine table  11  may be represented by the following equation (7): 
     
       
         
           
             
               
                 
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     In the machine table  4 , in which the machine table  11  is moved in the X-axis direction and/or the Y-axis direction at an X-axis movement speed Vx, a Y-axis movement speed Vy, an X-axis acceleration Ax and a Y-axis acceleration Ay, it is necessary for table feed motors  15   x  and  15   y  to develop a torque lesser than rated torques for fear of malfunctions or excess heating of the table feed motors  15   x  and  15   y . It is noted that the table feed motor  15   x  drives the machine table  11  in the x-axis direction, whilst the table feed motor  15   y  drives the machine table  11  in the y-axis direction. 
     If the tool path defined by Pj(s), shown in  FIG. 2 , that is, the post-machining shape of the work  2 , may not be changed, the shape of tj(s) of the equation (2), that is, the control point positions, may be changed to control the speed of movement Vx, Vy as well as the acceleration Ax, Ay. 
     Since t denotes time, the shape of tj(s) of the equation (2) is to be changed as long as tj(s) is a linear increasing function, shown in  FIG. 5 . To this end, it is easier to change the value of tj(s) of the equation (2) in a positive value range than changing tj(s) itself, shown in  FIG. 6 . 
     Based on the above concept, such a cutting feed rate V that will yield the maximum values of the speeds of movement Vx, Vy, with the acceleration values Ax, Ay being lower than that corresponding to the rated torque, are found. 
     The sequence of operations to find the cutting feed rate V will now be explained with reference to the flowchart of  FIG. 7 . 
     Referring to  FIG. 7 , in a step S 1 , the cutting feed rate data generation unit  35  reads out the mass weight of the work being cut, stored in the shape data memory  30 , and the rated torques of the table feed motors  15   x ,  15   y , stored in the driving capability data memory  34 . The cutting feed rate data generation unit then calculates marginal speeds Lv, −Lv and marginal accelerations La, −La. The marginal speeds Lv, −Lv and marginal accelerations La, −La are sometimes referred to below as marginal values. 
     Then, in a step S 2 , the cutting feed rate data generation unit  35  sets an initial value of tj(s) of the equation (2) so that dt/ds, referred to above, will be a constant value of a proper magnitude. This constant value is also referred to below as an initial constant value. Since dt/ds corresponds to a reciprocal of the speed, the initial constant value of dt/ds is set so that the speeds of movement Vx, Vy as well as the acceleration La, −La will not get to the marginal values of the table feed motors  15   x ,  15   y . That is, the initial constant value of dt/ds is set so that the speeds of movement Vx, Vy will be within the range of the marginal speeds Lv and −Lv and so that the accelerations Ax, Ay will be within the range from the marginal accelerations La and −La. 
     Then, in a step S 3 , the current value of the control point of tj(s) of the equation (2) is set so that the current value of dt/ds will be smaller on the whole by a preset value. By so doing, the inclination of a straight line in  FIG. 5  showing the relationship between t (vertical axis) and s (horizontal axis) in connection with tj(s) of the equation (2), becomes more moderate, that is, the speed of movement is increased. 
     Then, in a step S 4 , the cutting feed rate data generation unit  35  calculates the speeds of movement Vx, Vy and the accelerations Ax, Ay. Then, in a step S 5 , the cutting feed rate data generation unit  35  compares the speeds of movement Vx, Vy and the accelerations Ax, Ay, as calculated, to the marginal values of the table feed motors  15   x ,  15   y  to decide whether or not the speeds of movement Vx, Vy as well as the accelerations Ax, Ay have exceeded the marginal values. 
     If, in the step S 5 , the speeds of movement Vx, Vy as well as the accelerations Ax, Ay are not in excess of the marginal values, processing reverts to the step S 3  to correct the position of the control point tj(s) of the equation (2) so that the value of dt/ds will be further smaller by the same preset value 
     If conversely the speeds of movement Vx, Vy as well as the accelerations Ax, Ay are in excess of the marginal values, in the step S 6 , the value of dt/ds in the vicinity of the point of exceeding the marginal value is restored to the directly previous value. Processing then comes to a close. 
     By the above sequence of operations, the cutting feed rate data generation unit  35  may get the cutting feed rate V of a maximum value for a torque not greater than the rated torques of the table feed motors  15   x ,  15   y , that is, with the speeds of movement Vx, Vy being within the range of the marginal speeds Lv, −Lv and with the accelerations Ax, Ay being within the range of the marginal accelerations La, −La. 
       FIG. 8  shows speeds of movement Vx, Vy, accelerations Ax, Ay and the cutting feed rate V. Note that, in  FIG. 8 , the acceleration Ax and the speed of movement Vx in the X-axis direction are indicated by solid lines, the acceleration Ay and the speed of movement Vy in the Y-axis direction are indicated by chain dotted lines and the cutting feed rate V is indicated by a double-chain dotted line. 
     It is seen from  FIG. 8  that, in the X-axis direction, deceleration is commenced at, for example, a third control point c 3 , and that, as from a fourth control point c 4  as far as the sixth control point c 6 , deceleration is continued at approximately the marginal acceleration −La. This enables the speed of movement Vx from the control point c 0 , that is, an initial value (0th control point), to the first control point c 1 , to be set at the marginal speed Lv. Furthermore, in the X-axis direction, acceleration is commenced at, for example, an 18th control point c 18 , and continued from the 19th control point down to the 22nd control point c 22  at approximately the marginal acceleration La. This enables the speed of movement Vx as from the 23rd control point c 23  to be set at the marginal speed Lv. 
     For comparison sake,  FIG. 9  shows the speeds of movement Vx, Vy and the accelerations Ax, Ay in the X-axis direction and in the Y-axis direction, as well as the cutting feed rate V, in case the work is moved along the tool path at a constant cutting feed rate, as in a conventional system. Note that, in  FIG. 9 , the acceleration Ax and the speed of movement Vx in the X-axis direction are denoted by solid lines, the acceleration Ay and the speed of movement Vy in the Y-axis direction are denoted by chain dotted lines and the cutting feed rate V is denoted by a double dotted chain line. 
     It is seen from  FIG. 9  that, although the accelerations Ax, Ay have reached the marginal values La, −La, the speeds of movement Vx, Vy are appreciably lower than the marginal values Lv, −Lv. 
     That is, as may be seen from  FIGS. 8 and 9 , it is possible with the control system  5  to set the integrated value of the cutting feed rate V so as to be appreciably larger than the integrated value of the conventional cutting feed rate V shown in  FIG. 9 . This provides for machining time with the control system  5  which is shorter than in the conventional system. 
     It is noted that, in the control system  5 , the machining simulation unit  39  is able to verify and evaluate the speeds of movement Vx, Vy, accelerations Ax, Ay and the cutting feed rate V at the outset. Thus, in case the shape of the as-cut work  2 , in particular the corner part of the work  2 , may be changed within the range of the tolerance, the control system  5  may cause the input unit  41  to change the radius of curvature from R 1  to a larger value of R 2 , as shown for example in  FIG. 10 . By so doing, a cutting feed rate with reduced deceleration may be generated to allow the machine tool  4  to perform machining operations at a higher speed. 
     In the control system  5  for the machine tool, according to the present invention, the cutting feed rate data generation unit  35  is able to generate cutting feed rate data at the outset based on CL data, mass weight of the work  2  being cut and the values of the rated torque of the driving motors  15  of the driving unit  6  of the machine tool  4 . The cutting feed rate of the cutting feed rate data is of such a value that will provide the maximum speed for the torque of the table feed motors  15   x ,  15   y  not greater than their rated torque values, that is, for the speeds of movement Vx, Vy within the range between the marginal speeds Lv and −Lv for the work being cut, and for the accelerations Ax, Ay within the range between the marginal accelerations La and −La for the work  2  being cut. It is thus possible with the control system  5  for the machine tool of the present invention to perform machining operations at a higher speed and shorter machining time than with the conventional system. It is moreover possible with the control system  5  for the machine tool of the present invention to verify cutting feed rate data that will provide for the maximum speed of the machine tool  4 . 
     Moreover, in the control system  5  for the machine tool according to the present invention, the acceleration/deceleration data generation unit  37  generates acceleration/deceleration data based on the CL data and the cutting federate data. The controller  42  directly outputs the acceleration/deceleration data and a driving command to the motor amplifier  16 , which motor amplifier then actuates the driving motors  15  of the driving unit  6  of the machine tool  4  in accordance with the input acceleration/deceleration data and the driving command. Hence, in the machine tool  4 , the work  2  and the cutting tool  3  may be moved relative to each other along the tool path at the cutting feed rate V which will become a maximum speed at each part of the tool path as verified at the outset. It is thus possible with the control system  5  of the machine tool according to the present invention to cut the corner part of the work  2  to a shape a designer intended at the outset, and hence to improve the machining accuracy of the machine tool  4 . 
     Moreover, in the control system  5  for the machine tool according to the present invention, the machining simulation unit  39  is able to perform machining simulation in which the work  2  and the cutting tool  3  are relatively moved based on the CL data and the cutting feed rate data or on acceleration/deceleration data. The machining simulation unit then demonstrates the results of the simulation on the display  40  to allow the cutting feed rate data and the overall operation of the machine tool  4  to be verified extremely readily. 
     Referring to  FIG. 11 , the control system  5  for the machine tool according to the present invention may further be provided with an NC data generation unit  43  that generates NC data, composed of an NC program, termed ‘G code’, based on the CL data stored in the CL data memory  33 . Suppose that, like a conventional machine tool  60  shown for example in  FIG. 11 , a machine tool as an output destination includes a CNC controller  62  that controls a driving unit  61  based on NC data. In the control system  5 , the NC data generation unit  43  generates NC data, and the controller  42  outputs the NC data to the CNC controller  62  of the conventional machine tool  60 . On receiving the NC data, the CNC controller  62  actuates the driving motor  64 , via a motor amplifier  63 , in accordance with the input NC data. That is, the control system  5  for the machine tool is able to actuate and control both the machine tool  4  and the conventional machine tool  60 . The machine tool  4  actuates the driving motors  15  by the motor amplifier  16  in accordance with the input driving command, while the conventional machine tool  60  includes the CNC controller  62  that actuates the driving motor  64  in accordance with the input NC data. 
     Each of the machine tools  4 ,  60  of the machine tool control system  5  according to the present invention is not limited to the vertical type machining center including three linear-movement driving axes of X-, Y- and Z-axes perpendicular to one another. Each of the machine tools may thus be a horizontal type machining center including three linear-movement driving axes of X-, Y- and Z-axes. Each of the machine tools may also be a 5-axis machining centers including three linear-movement driving axes of X-, Y- and Z-axes perpendicular to one another and two rotational driving axes about two selected out of the three linear-movement driving axes as center of rotation. Each of the machine tools may further be a 5-axis control complex machining device including an NC lathe as a basic unit and a main spindle rotationally mounted on the NC lathe to perform milling operations. 
     It should be understood by those skilled in the art that various modifications, combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.