Abstract:
A worked surface of a workpiece is evaluated on the basis of how the surface is actually perceived by a person&#39;s (observer&#39;s) eyes (vision) or fingers (touch), and a work process whereby a workpiece is worked is changed on the basis of the evaluation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Phase patent application of International Patent Application No. PCT/JP2013/059729, filed on Mar. 29, 2013, which is hereby incorporated by reference in the present disclosure in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method of evaluating a surface of a workpiece processed by a machine tool, a controlling apparatus a machine tool using the evaluating method and a machine tool with the controlling apparatus. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the case of a workpiece processed by a machine tool, generally, the criteria for evaluating the machined surface is surface accuracy, in particular surface roughness of the machined surfaces of the workpiece. Conventionally, the lower the surface roughness, the more it is regarded as good machining. However, recently, due to strips, caused by cusps formed on the machined surfaces, observed during a visual inspection by an observer or gritty-texture sensed by an observer when he touched the surfaces with his finger, there are times when the processed workpiece is rejected, even if the surface roughness of the processed workpiece is lower than acceptable by a user. For this reason, technologies have been proposed to measure the shiny appearance or the texture of a machined surface in addition to the simple measurement of the surface roughness. In this context, it should be noted that cusp means unprocessed micro portions on the machined surfaces of the workpiece or micro protrusions and concaves formed in the machined surfaces by vibrations of the machine tool induced by servo-control when a workpiece is cut with a rotating tool along a tool path including pick feeds or scraped with a spring-necked bit. 
         [0004]    Patent Literature 1, for example, describes a method for evaluating surface property, relative to criteria of a plurality of measuring items, including specularity, by projecting a cross-stripes pattern, displayed on a display, to a surface to be measured, capturing the reflected image by an image pick up, and measuring the image data by a computer relative to specularity, the dispersion of the specularity, glossiness, sharpness, undulation or form error, contrast and opacity sequentially. Further, Patent Literature 1 describes, as a prior art, a method for measuring surface property by using a specular gloss meter for measuring the glossiness based on the amount of light received. 
         [0005]    Further, Patent Literature 2 describes a method for measuring surface texture by combining maximum height of a shape (Rz), mean wave length of the shape (RSm) and surface texture parameter.
   Patent Literature 1: Japanese Unexamined Patent Publication No. 2012-215486   Patent Literature 2: Japanese Unexamined Patent Publication No. 2003-500677   
 
       SUMMARY OF THE INVENTION 
       [0008]    The inventions described in Patent Literatures 1 and 2, evaluate the surface property relative to the criteria of a plurality of measuring items by projecting a check pattern to a surface to be measured. However, these methods are not sufficient for an evaluating method, since it is not clear how the surface property thus evaluated is actually sensed by the human (observer) eyes (visual perception) and fingers (tactile perception). 
         [0009]    The invention is directed to solve the above-described problem of the prior art, and the objective of the invention is to provide a method of evaluating a machined surface of a workpiece similar to that an observer actually evaluates with human sense, and a controlling apparatus and a machine tool using the evaluating method. 
         [0010]    In order to achieve the above objective, the invention provides a method of evaluating a machined surface of a workpiece which uses evaluating criteria based on the characteristics of human sense. 
         [0011]    Further, according to the invention, there is provided a control device for a feed axis of a machine tool, comprising a machined surface shape simulating section for simulating the machined surface of a workpiece by a calculation; 
         [0012]    a contrast calculating section for calculating the maximum minimum luminance of reflecting light on the machined surface whereby the contrast is calculated based on the maximum minimum luminance; a spatial frequency calculating section for calculating spatial frequency of the machined surface; 
         [0013]    judging section for determining whether or not the contrast of the machined surface can be visibly detected by the human visual perception, by using a contrast sensitivity function, based on the contrast and the spatial frequency; and a machining process modifying section for modifying a machining process, when the contrast of the processed surface is detectable by human visual perception, so that the contrast of the processed surface cannot be detected by human visual perception. 
         [0014]    According to another feature of the invention, there is provided a control apparatus for a feed axis of a machine tool, comprising a machined surface shape simulating section for simulating a machined surface of a workpiece by a calculation; a vibration stimulus frequency calculating section for calculating vibration stimulus frequency transmitted to a finger of an observer when the observer fingers the machined surface simulated by the machined surface simulating section; a judging section for determining whether or not the amplitude of the vibratory stimulation, transmitted to the finger of the observer, can be detected by human tactile sense, relative to the frequency of the vibratory stimulation calculated by the vibration stimulus frequency calculating section, the amplitude of the vibratory stimulation being obtained based on the distance between peaks of cusps formed on the machined surface simulated by the machined surface simulating section, and the speed of the finger of the observer; and a process modifying section for modifying a machining process, when the amplitude of vibratory stimulation can be detected by human tactile sense, so that the amplitude of vibratory stimulation cannot be detected by human tactile sense. 
         [0015]    According to another feature of the invention, there is provided a machine tool having at least three orthogonal feed axes of X-, Y- and Z-axes for relatively moving a tool to a workpiece, comprising a drive mechanism for driving the three feed axes; an NC device for controlling the drive mechanism; and an evaluating apparatus for simulating a machined surface formed on the workpiece; 
         [0016]    wherein the evaluating apparatus comprising a machined surface shape simulating section for simulating the machined surface of a workpiece by a calculation; a contrast calculating section for calculating the maximum minimum luminance of reflecting light on the machined surface whereby the contrast is calculated based on the maximum minimum luminance; a spatial frequency calculating section for calculating spatial frequency of the machined surface; a judging section for determining whether or not the contrast of the machined surface can be visibly detected by the human visual perception, by using a contrast sensitivity function, based on the contrast and the spatial frequency; and a machining process modifying section for modifying a machining process, when the contrast of the processed surface is detectable by human visual perception, so that the contrast of the processed surface cannot be detected by human visual perception, whereby a tool path and a machining condition are sent to the NC device, based on the modified machining process. 
         [0017]    According to another feature of the invention, there is provided a machine tool having at least three orthogonal feed axes of X-, Y- and Z-axes for relatively moving a tool to a workpiece, comprising a drive mechanism for driving the three feed axes; an NC device for controlling the drive mechanism; and an evaluating apparatus for simulating a machined surface formed on the workpiece; 
         [0018]    wherein the evaluating apparatus comprising a machined surface shape simulating section for simulating a machined surface of a workpiece by a calculation; a vibration stimulus frequency calculating section for calculating vibration stimulus frequency transmitted to a finger of an observer when the observer fingers the machined surface simulated by the machined surface simulating section; a judging section for determining whether or not the amplitude of the vibratory stimulation, transmitted to the finger of the observer, can be detected by human tactile sense, relative to the frequency of the vibratory stimulation calculated by the vibration stimulus frequency calculating section, the amplitude of the vibratory stimulation being obtained based on the distance between peaks of cusps formed on the machined surface simulated by the machined surface simulating section, and the speed of the finger of the observer; and a process modifying section for modifying a machining process, when the amplitude of vibratory stimulation can be detected by human tactile sense, so that the amplitude of vibratory stimulation cannot be detected by human tactile sense, whereby a tool path and a machining condition are sent to the NC device, based on the modified machining process. 
         [0019]    According the invention, the machining surface of a workpiece is evaluated based on criteria how it is actually sensed by human (observer) eyes (visual perception) and fingers (tactile perception), enabling objective evaluation based on the human sense, as well as production of workpieces, which are evaluated good by the human sense and accomplishes higher customer satisfaction. Further, there is provided a control device and a machine tool using the control device, which predicts the property of a machined surface by a simulation before the actual machining, automatically evaluates the property of the machined surface based on the criteria of human sense, and if it is predicted that there is a disturbance, which can be detected by the human sense, in the machined surface, then modifies the machining process so as to make the disturbance unable to be detected, enabling reduction in machining defect and increase in productivity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a schematic block diagram of a machine tool according to a preferred embodiment of the invention. 
           [0021]      FIG. 2  is a block diagram of a machined surface evaluating apparatus according to a first embodiment of the invention. 
           [0022]      FIG. 3  is a schematic illustration for explaining contrast. 
           [0023]      FIG. 4  is a schematic illustration for explaining luminance ratio. 
           [0024]      FIG. 5  is a chart for explaining a contrast sensitivity function. 
           [0025]      FIG. 6  is a schematic illustration for explaining a method for modifying the process. 
           [0026]      FIG. 7  is a chart showing an example of the process modification. 
           [0027]      FIG. 8  is a chart showing another example of the process modification. 
           [0028]      FIG. 9  is a block diagram of a machined surface evaluating apparatus according to a second embodiment of the invention. 
           [0029]      FIG. 10  is a chart for explaining characteristics of frequency threshold of human sense. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    With reference to the attached drawings, a preferred embodiment will be described below. 
         [0031]    In  FIG. 1 , a machine tool  50  comprises a processing machine  60  and a machined surface evaluating apparatus  10  ( 30 ). The processing machine  60  comprises a bed  62  providing a base fixed to a flower of a factory, a table  64  mounted to a top of the bed  62  for attaching a workpiece W to a upper surface of the bed, a spindle head  68  for rotationally supporting a spindle  68  about a rotational axis O, the spindle  68  being adapted to receive a tool T facing the workpiece W attached to the table  64 , a drive mechanism  52  for reciprocally driving the spindle head  68  in orthogonal three axes of X-, Y- and Z-axes, an NC device  54  for controlling servomotors of the drive mechanism  52 , a tool path calculating device  56  for calculating and generating tool paths and sending data associated with the generated tool paths to the NC device  54 , and the machined surface evaluating apparatus  10  ( 30 ), provided between the tool path calculating device  56  and the NC device  54 , for correcting the tool path data sent to the NC device  54 . 
         [0032]    The drive mechanism  52  comprises for example X-, Y- and Z-axis ball screws (not shown), nuts (not shown) engaging the ball screws and X-, Y- and Z-axis drive motors Mx, My and Mz, each provided with a servomotor connected to one end of each X-, Y- and Z-axis ball screws, for rotationally driving the X-, Y- and Z-axis ball screws. Further, the machine tool  50  may include one or plurality of rotational feed axes such as an A-axis i.e., a rotational feed axis around the horizontal X-axis or a C-axis i.e., a rotational feed axis around the vertical Z-axis, in addition to the orthogonal three axes of X-, Y- and Z-axes. In such a case, the drive mechanism  52  includes servomotors for the feed axes i.e., A-axis and/or C-axis, in addition to the X-, Y- and Z-axis servomotors Mx, My and Mz. 
         [0033]    The tool path calculating device  56  may be configured by for example a CAM device connected to a CAD device  58  through a computer network  70  such as a LAN so as to calculate tool paths based on shape data generated by CAD device  58  and to generate data associated with the tool paths for the machined surface evaluating apparatus  10  ( 30 ). The tool path calculating device  56  may be incorporated in a control program stored in a machine control device (not shown) of the processing machine  60  or the NC device  54 . 
         [0034]    With reference to  FIG. 2 , the machined surface evaluating apparatus  10  according to the first embodiment comprises, as its main elements, an input section  12 , a tool action simulating section  14 , a feed axis action simulating section  16 , a machined surface shape simulating section  18 , a contrast calculating section  20 , a spatial frequency calculating section  22 , a judging section  24  and a process modifying section  26 . 
         [0035]    Input into the tool action simulating section  14  and the feed axis action simulating section  16  at the input section  12  are data  100  associated with the machining condition, such as the material of the workpiece used in the machining process, the rotational speed of the spindle  66 , the diameter, the length and the number of cutting edges of the tool T to be used, the cutting depth of the tool T into the workpiece W, the amount of the pick feed and the corrections of feeding speed of the respective feed axis. The input section  12  may be formed by, for example, a key board (not shown) or a touch panel of the NC device  54  or a machine controller (not shown) of the processing machine  60  or a database (not shown) stored in a server which is connected, along with the tool path calculating device  56 , to a computer network. 
         [0036]    The tool action simulating section  14  simulates, based on the machining condition which had been input through the input section  12 , the action of the tool T such as the inclination of the tool T and/or the rotating movement of the ends of the cutting edges. The simulation results or Data  104  associated with the action of the tool T, are sent to the machined surface shape simulating section  18 , as described below. 
         [0037]    The feed axis action simulating section  16  simulates the actions of the three orthogonal linear feed axes of X-axis, Y-axis and Z-axis, and the rotational feed axes of the A-axis and C-axis, based on the tool path data  102  from the tool path calculating device  56  and the machining condition  100  input through the input section  12 . The simulation results or data  100 , associated with the actions of the three orthogonal linear feed axes of X-axis, Y-axis and Z-axis and the rotational feed axes of the A-axis and C-axis, are sent to the machined surface shape simulating section  18 . 
         [0038]    In the machined surface shape simulating section  18 , the removal of chips from the workpiece W by the machining process is simulated, based on the data  104  associated with the action of the tool T and the data  106  associated with the actions of the feed axes, whereby the shape of the machined surface formed in the workpiece W, in particular the height of cusps, the interval between adjacent cusps, the inclination of the sides of cusps, etc. The calculation results or data  108  associated with the shape of the machined surface of the workpiece W are sent to the contrast calculating section  20  and the spatial frequency calculating section  22 . 
         [0039]    Input into the contrast calculating section  20  through input section  12  are data  110  including the data associated with the material of the workpiece W, the data associated with the light source, such as the intensity of illumination, the wave length and the coordinate of the light source, and the data associated with the observer, such as the angle of the line of sight when a possible observer takes a look at the workpiece. In case of the workpiece W is a metallic mold used in an injection molding machine, it is preferable to input the data associated with the plastic material molded in the metallic mold, instead of the data associated with the material of the workpiece W. The contrast calculating section  20  calculates the maximum luminance Lmax and the minimum luminance Lmin of the light reflected on the workpiece W, based on the data  108  associated with the shape of the machined surface of the workpiece W from the machined surface shape simulating section  18  and the data  110  input through the input section  12 , whereby the contrast of the surface of the workpiece W is calculated. The calculation results or the contrast m is sent to the judging section  24 . 
         [0040]    With reference to  FIG. 3 , the contrast m is obtained by the following formulation. 
         [0000]        m =( L max− L min)/( L max+ L min)
 
         [0000]    where:
 
Lmax: Maximum Luminance (cd/m2)
 
Lmin: Minimum Luminance (cd/m2)
 
         [0041]    Further, the luminance is obtained by the following formulation. 
         [0000]        L =β(θ i,θr )·(ρ/π)· E  
 
         [0000]    where: 
       β: Luminance Ratio 
       [0042]    θi: Angle of Incidence (deg)
 
θr: Angle of Reflection (deg)
 
       ρ: Reflectance 
       [0043]    E: Intensity of Illumination of the Incident Light from the Light Source S (1×) 
         [0044]    With reference to  FIG. 4 , the luminance ratio β is the ratio of the luminance L(θr) on a sample surface in the diction of the angle of reflection (θr) among the diffuse reflection, shown by the broken line, on an actual reflecting surface relative to the luminance Lpd (shown by lines OP), which is constant in the all directions, as shown by two-dot chain line, on a perfect diffusively reflecting surface. On a perfect diffusively reflecting surface, the luminance is constant in all directions, while the luminance varies depending on the reflection angle θr on an actual surface. An example of a method for obtaining the luminance ratio β may include previously measuring the luminance of the reflected light, with respect to a variety of material, by a measuring device, such as a luminance meter, at a plurality of reflection angles θr, with the incident angle θr being changed, whereby the luminance ratios β are stored in the contrast calculating section  20  in the form of a table (database) or approximation formulas, associated with the incident angle θi and the reflection angle θr, as parameters, with regard to a variety of material for the workpiece W. 
         [0045]    The contrast calculating section  20  can lock for the contrast m by calculating the incident angle θi to the machined surface and the reflection angle θr on the machined surface, based on the data input through the input section  12 , in particular the material of the workpiece W, the distance between the workpiece W and the eyes of the observer, and the data  108  associated with the machined surface from the machined surface simulating section  18 , obtaining the luminance ratio β(θi, θr) by referring to the table (database) in association with the parameters, i.e., the material of the workpiece W, the incident angle θi and the reflection angle θr, and obtaining the maximum luminance Lmax and the minimum luminance Lmin based on the luminance ratio β(θi, θr). 
         [0046]    The spatial frequency calculating section  22  calculates the spatial frequency ω based on the data  108 , associated with the shape of the machined surface, from the machined surface shape simulating section  18 . With reference to  FIG. 3 , the spatial frequency ω is defined by the reciprocal of the angle of view α, which is the angle between the adjacent cusps projected to the eye E 0  of the observer, or the number of the cusps per the angle of view α=1 degree. The calculation results of the spatial frequency ω are sent to the judging section  24 . The maximum luminance Lmax and the minimum luminance Lmin are determined by securing the line of sight and the machined surface, locking for the angle of view α at which the luminance is maximum with the angle of view getting widen, and setting the luminance as the maximum luminance Lmax and the luminance, at the side opposite to the angle of view α, as the minimum luminance Lmin. 
         [0047]    The judging section  24  determines, based on the contrast m from the contrast calculating section  20  and the spatial frequency ω from the spatial frequency calculating section  22 , whether or not human eyes can recognize the cusps on the surface of the workpiece W, machined by the machining process, as strips by using a contrast sensitivity function. With reference to  FIG. 5 , the human visual perception cannot in general discriminate the difference in luminance when the contrast is lower. The minimum contrast, at which the difference in contrast can be discriminate, is referred to as contrast threshold, and the reciprocal of contrast threshold is referred to as contrast sensitivity. Contrast sensitivity varies depending on spatial frequency, and the changes in contrast sensitivity relative to spatial frequency is generally referred to as contrast sensitivity curve or contrast sensitivity function Fcs. It is generally known that contrast sensitivity function Fcs has a peak at a spatial frequency of 4 cycle/degree, and that when the spatial frequency is higher or lower than it, contrast sensitivity is reduced. 
         [0048]    In this embodiment, the judging section  24  determines, based on the contrast m from the contrast calculating section  20  and the spatial frequency ω from the spatial calculating section  22 , whether or not the contrast of the stripes of the cusps on the machined surface of the workpiece W formed by the machining process is at the higher side (the strips are visible) or the lower side (the strips are invisible) relative to the contrast sensitivity function Fcs. While contrast sensitivity function Fcs varies from person to person, a preliminary experiment may be carried out whereby a suitable contrast sensitivity function Fcs is stored in the judging section  24 . Alternatively, a plurality of sensitivity functions Fcs may be stored in the judging section  24 , allowing a user to choose one advantageously. 
         [0049]    When the judging section  24  judges (Yes) that the contrast m from the contrast calculating section  20  and the spatial frequency ω from the spatial frequency calculating section  22  fall in the upper region (the strips are invisible) higher than the contrast sensitivity function Fcs in  FIG. 5 , the machined surface evaluating apparatus  10  sends the machining condition input through the input section  12  and the data  118  associated with the tool path generated by the tool path calculating device  64  to the NC device  62 . 
         [0050]    When the judging section  24  judges (No) that the contrast m from the contrast calculating section  20  and the spatial frequency ω from the spatial frequency calculating section  22  fall in the lower region (the strips are visible) lower than the contrast sensitivity function Fcs in  FIG. 5 , the machined surface evaluating apparatus  10  commands at least one of the input section  12  (command  120   1 ), the tool action simulating section  14  (command  120   2 ), the tool path calculating device  64  (command  120   3 ) and the feed axis action simulating section  16  (command  120   4 ) so as to modify the machining process. The process modification may include changing in the rigidity of the bearings for supporting the spindle  66  for rotation and the tool path generated by the tool path calculating device  56 , as well as changing in the machining condition, such as the rotational speed of the spindle  66 , the feeding speed of the respective feed axes, the diameter, the length and the number of the cutting edges of the tool T to be used, the cutting depth of the tool T into the workpiece W, the amount of pick feed and/or the correction value for the feeding speed of the respective feed axis. 
         [0051]    With reference to  FIG. 6 , if the spatial frequency ω from the spatial frequency calculating section  22  is larger than the spatial frequency ωp, providing the peak P of the contrast sensitivity function Fcs, as shown by point A in  FIG. 6 , then the process modifying section  26  modifies the machining process so as to reducing the contrast m (a 1 ) or so as to reduce the contrast m and increase the spatial frequency ω (a 2 ). On the other hand, if the spatial frequency ω from the spatial frequency calculating section  22  is smaller than the spatial frequency ωp, providing the peak P of the contrast sensitivity function Fcs, as shown by point B in  FIG. 6 , then the process modifying section  26  modifies the machining process so as to reducing the contrast m (b 1 ) or so as to reduce the contrast m and the spatial frequency ω (b 2 ). 
         [0052]    The process modifications a 1 , b 1  for reducing the contrast m can be carried out by, for example, outputting the command  120   1 , for reducing the diameter of the tool T to be used, to the input section  12 . Alternatively, when the bearings for rotationally supporting the spindle in the spindle head  68  include a magnetic bearing using magnetic force, then the command  1202 , for reducing the magnetic force of the magnetic bearing, may be sent to the tool action simulating section in order to reduce the stiffness of the bearing so that the size of the entire cusps increases, whereby the local cusps become less noticeable and the contrast m is reduced. Further, the command  120   4 , for reducing the feeding speed of the feed axes, may be to the feed axis simulating section  16 , as shown in  FIG. 7 , in order to reduce the height of the cusps, which can be generated based on the positions errors during the acceleration and deceleration of the feed axes, whereby the contrast m is reduced. 
         [0053]    Furthermore, the command  1204 , for modifying the correction filter sent to the servomotors when the rotation of the feed axes is reversed, may be sent to the feed axis simulating section  16 . For example, by modifying the correction filter in to a small and smooth shape, as shown in  FIG. 8 , the shape of the machined surface of the workpiece W is smooth, while the absolute value of position error associated with quadrant change, whereby the contrast m is reduced and the spatial frequency ω is changed (directions a 2  and b 2 ). 
         [0054]    Accordingly, in this embodiment, until stripes on the machined surface of the workpiece W cannot be visibly detected by an observer or the judgment by the judging section  24  is Yes, the modification of the machining process is repeated. As described above, in this embodiment, it is estimated, using the contrast sensitivity function Fcs, whether or not strips can be visibly detected by an observer, based on the contrast m of the strips generated by the cusps formed on the machined surface of the workpiece and the spatial frequency ω, and the machining process is modified until the strips on the machined surface cannot be visibly detected by an observer. 
         [0055]    With reference to  FIG. 9 , a second embodiment of the present invention will be described. In the first embodiment, the human visual perception, in particular, the contrast sensitivity is used as a human-sense based evaluation criteria for a machined surface. In the second embodiment, the human tactile perception, in particular the characteristics of frequency threshold of the human tactile perception is used as the evaluation criteria. In  FIG. 9 , the elements similar to those in  FIG. 2  are indicated by the same references. 
         [0056]    With reference to  FIG. 9 , a machined surface evaluating apparatus  30  according to the second embodiment includes a vibration stimulus frequency calculating section  32 , instead of the contrast calculating section  20  and the spatial frequency calculating section  22  of the machined surface evaluating apparatus  10  of the first embodiment. In the second embodiment, the speed  130  of finger(s) of an observer sliding along the machined surface of the workpiece W (the relative speed to the machined surface) and the data  108  associated with the machined surface, in particular the distance between the adjacent cusps are input into the vibration stimulus frequency calculating section  32  through the input section  12  and the machined surface shape calculating section  18 , respectively. 
         [0057]    The vibration stimulus frequency calculating section  32  calculates the vibration stimulus frequency, which is transmitted to the finger(s) of the observer, and sent the data associated with the calculation results or the vibration stimulus frequency to the judging section  34 . Further, the data  108  associated with the machined surface, in particular the distance between the adjacent cusps are sent to the judging section  34  from the machined surface shape calculating section  32 . 
         [0058]    With reference to  FIG. 10 , it is generally known that human tactile perception has characteristics that at a higher vibration stimulus frequency, small amplitude of vibratory stimulation cannot be detected (the characteristics of frequency threshold of the human tactile perception). In this embodiment, the judging section  34  determines, based on the vibration stimulus frequency obtained from the vibration stimulus frequency calculating section  34  and the amplitude of the vibratory stimulation transmitted to the finger of an observer, which is calculated based on the distance between the peaks of the adjacent cusps and the speed of the finger of the observer, whether or not the observer can recognize the cusps on the surface of the workpiece W, machined by the machining process, by touching them with the finger. 
         [0059]    Although the curve or function Fts, representing the frequency threshold characteristics of human tactile perception, shown in  FIG. 10 , differs from person to person, a suitable frequency threshold function Fts can be previously stored based on an experiment. Alternatively, a plurality of frequency threshold functions Fts may be stored in the judging section  24  so as to allow a user to choose one. 
         [0060]    As described above, in this embodiment, the judging section  34  determines, based on vibration stimulus frequency and the distance between the peaks of adjacent cusps, whether or not the cusps on the machined surface of the workpiece W formed by the machining process falls in the higher side (the cusps are sensible) or the lower side (the cusps are sensible) relative to the frequency threshold function Fts of the human tactile perception, and repeats the modification of the machining process, until the cusps on the machined surface of the workpiece W cannot be detected by the tactile perception of an observer or the judgment by the judging section  24  is Yes. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  Machined Surface Evaluating Apparatus 
           12  Input Section 
           14  Tool Action Simulating Section 
           16  Feed Axis Action Simulating Section 
           18  Machined Surface Shape Simulating Section 
           20  Contrast Calculating Section 
           22  Spatial Frequency Calculating Section 
           24  Judging Section 
           26  Process Modifying Section 
           30  Machined Surface Evaluating Apparatus 
           32  Vibratory Stimulus Frequency Calculating Section 
           34  Judging Section 
           50  Machine Tool 
           56  Tool Path Calculating Section 
           62  Bed 
           64  Table