Abstract:
CNC apparatus having a mechanism for controlling length variation of a lead screw due to thermal expansion and method therefore when the apparatus is rotating in high speed and/or when load is high are disclosed. The lead screw is supported by two spaced ball bearing sets and is hollow to permit cooling fluid to flow through. A deflection detecting unit is disposed proximate one ball bearing set for detecting its deflection. In one embodiment, an adjusting nut is operatively connected to one end of the lead screw and is adapted to pre-stress the ball bearing set. The method includes pre-stressing the ball bearing set for deflecting in one direction and in operation in response to detecting the ball bearing set deflected in an opposite direction cooling the lead screw for substantially maintaining its length unchanged with respect to a bed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The invention relates to devices for controlling length variation of a mechanical element and more particularly to a computer numerical control (CNC) machine for controlling length variations of the lead screw due to thermal expansion in high speed rotation and/or high load, thereby maintaining the length of the lead screw unchanged with respect to a bed. Moreover, the invention relates to a method for controlling such length variations in which pre-stressing, deflection detection and cooling are involved. 
         [0003]    2. Description of Related Art 
         [0004]    Length variation (e.g., elongation) due to thermal expansion is a potential problem for any mechanical device in operation. Thus, how to effectively control length variation of a device in operation is an important issue. 
         [0005]    Referring to  FIG. 1 , in a first typical arrangement a feed mechanism  12  is fixedly secured to a threaded carrier  11  which is again threadedly put on a lead screw  10 . The lead screw  10  has one end being supported by a first ball bearing  13  and secured by a nut  14 , and the other end being supported by a second ball bearing set  15  and secured by a nut  16 . Distance between the two nuts  14  and  16  cannot be changed since both the first and second ball bearing sets  13 ,  15  are fixed. That is, no axial movement (i.e., length increase or decrease) of the lead screw  10  and no radial movement thereof are allowed. Hence, the first ball bearing set  13  and/or the second ball bearing set  15  may be damaged because the lead screw  10  may increase its length due to thermal expansion when the feed mechanism  12  is rotating in high speed and/or when load is high. 
         [0006]    Referring to  FIG. 2 , in a second typical arrangement as an improvement of the first typical arrangement cooling fluid, as indicated by arrows, feeds through the hollow lead screw  10  for cooling. However, in practice it is found that temperature distribution along the lead screw can not be maintained evenly because some parts of the lead screw may receive more thermal load than others. Further, only one thermometer is placed in a predetermined point of the cooling path (i.e., only one control point). This is, however, not sufficient. Only the control point can be maintained close to the pre-determined reference temperature. As such, obtained temperature data of the arrangement is not correct, resulting in an insufficient cooling of the arrangement. The undesired insufficient cooling is particularly significant for an elongated lead screw  10 . Hence, the problem of length variation due to thermal expansion has not been solved. 
         [0007]    Referring to  FIG. 3 , in a third typical arrangement a feed mechanism  22  is fixedly secured to a threaded carrier  21  which is again threadedly put on a lead screw  20 . The lead screw  20  has one end being supported by a first ball bearing  23  set and secured by a nut  24 , and the other end being supported by a second ball bearing  25  set and secured by a nut  26 . The rows  251 ,  252  of balls of the second ball bearing  25  are positioned by rings disposed in the same direction (i.e., the second ball bearing  25  is adapted to deflect relative to a underlying bed (not numbered)) and the rows of balls of the first ball bearing  23  are positioned by rings disposed in opposing directions (i.e., the first ball bearing  23  is not adapted to deflect). Thus, the lead screw  20  is not allowed to extend leftward. Further, the lead screw  20  is only allowed to extend toward right (i.e., rightward axial movement) due to thermal expansion when the machine is rotating in high speed and/or when load is high. As such, both the ball bearings  23  and  25  are (i.e., the feed mechanism is) prevented from being damaged. However, positioning accuracy of the feed mechanism  22  degrades significantly. Moreover, axial stiffness of the feed mechanism  22  is lowered undesirably, as compared to first and second typical arrangements, in the rotating operation. 
         [0008]    Referring to  FIG. 4 , in a fourth typical arrangement as an improvement of the third typical arrangement a deflection detection unit  27  is provided proximate the second ball bearing  25  at the other end of the lead screw  20 . The deflection detection unit  27  is adapted to detect movement of the lead screw  20  (i.e., elongation) due to thermal expansion and sends the displacement data to a control unit  28 . The control unit  28  obtains data of the length variation of the lead screw  20  and the current position of the feed mechanism  22  from a rotary encoder  29  attached to a motor  30 , and then performs extensive calculation based on the data received. The calculation is summarized as following: 
         [0000]    
       
         
           
             Compensation 
             = 
             
               
                 
                   
                     
                       Length_Variation 
                        
                       
                         _Dectected 
                         · 
                       
                     
                   
                 
                 
                   
                     
                       Current_Position 
                        
                       _of 
                        
                       _Feed 
                        
                       _Mechanism 
                     
                   
                 
               
               
                 Total_Length 
                  
                 _of 
                  
                 _Lead 
                  
                 _Ccrew 
               
             
           
         
       
     
         [0000]    The compensation value is then feedback to the motor  30  as command. The motor  30  drives the lead screw  20  to counteract the effect of thermal expansion on the positioning of feed mechanism  22 . As a result, precision (i.e., positioning accuracy) is improved greatly. However, there are still some very small positioning errors due to uneven temperature distribution of the lead screw  20 . The compensation calculation assumes heat is evenly distributed on the lead screw  20 , i.e., temperature is constant along the lead screw  20 . Furthermore, the fourth typical arrangement does not solve the problem of lower axial stiffness of the feed mechanism  22 . The axial stiffness of the feed mechanism  22  decreases as it travels away from the motor  30 . 
         [0009]    Referring to  FIG. 5 , in a fifth typical arrangement, as another improvement of the third typical arrangement, a linear scale  28  is mounted parallel to the axial direction of the lead screw  20 . The linear scale  28  is adapted to precisely feedback the current position of the feed mechanism  22  to the controller. Thus, the length variation of the lead screw  20  due to thermal expansion, when the machine is rotating in high speed and/or when load is high, does not affect the positioning accuracy of the feed mechanism  22 . However, the cost of the linear scale  28  is relatively high as compared to that of a rotary encoder. Furthermore, the fifth typical arrangement does not solve the problem of lower axial stiffness of the feed mechanism  22 . The axial stiffness of the feed mechanism  22  decreases as it travels away from a motor (not numbered). Thus, a need for improvement exists. 
       SUMMARY OF THE INVENTION 
       [0010]    It is therefore one object of the invention to provide a CNC apparatus with a mechanism for controlling length variations of the lead screw due to thermal expansion in high speed rotation and/or high load so as to maintain the length of the lead screw substantially unchanged with respect to a bed, have a desired positioning accuracy, have a strong axial stiffness similar to that of the first typical arrangement mentioned in prior art, and prevent the apparatus from being damaged. 
         [0011]    It is another object of the invention to provide a method for controlling length variation of a lead screw of a CNC apparatus due to thermal expansion. The method comprises pre-stressing, deflection detection and cooling so as to maintain the length of the lead screw substantially unchanged with respect to a bed, have an acceptable positioning accuracy, have a sufficient axial stiffness, and prevent the apparatus from being damaged in high speed rotation and/or when load is high. 
         [0012]    The above and other objects, features and advantages of the invention will become apparent from the following detailed description taken with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a longitudinal sectional view of a first conventional arrangement of a lead screw based feed mechanism; 
           [0014]      FIG. 2  a longitudinal sectional view of a second conventional arrangement of a lead screw based feed mechanism with hollow cavity in the lead screw for cooling and controlling length variation of a lead screw of a feed mechanism due to thermal expansion when the machine is rotating in high speed and/or when load is high; 
           [0015]      FIG. 3  is a longitudinal sectional view of a third conventional arrangement of a lead screw based feed mechanism; 
           [0016]      FIG. 4  is a longitudinal sectional view of a fourth conventional arrangement of a lead screw based feed mechanism with a displacement sensor for thermal distortion feedback; 
           [0017]      FIG. 5  is a longitudinal sectional view of a fifth conventional arrangement of a lead screw based feed mechanism with a linear scale for accurate positional feedback; 
           [0018]      FIG. 6  is a longitudinal sectional view of a first preferred embodiment of CNC apparatus according to the invention for controlling length variation of its lead screw due to thermal expansion when the apparatus is rotating in high speed and/or when load is high; 
           [0019]      FIG. 7  is a fragmentary view of the CNC apparatus of  FIG. 6  where a pre-stressing is performed; 
           [0020]      FIG. 8  is a view similar to  FIG. 7  where length variation (e.g., elongation) of the lead screw due to thermal expansion is illustrated; 
           [0021]      FIG. 9  is a view similar to  FIG. 6  where cooling of the lead screw is illustrated; 
           [0022]      FIG. 10  is a longitudinal sectional view of a second preferred embodiment of CNC apparatus according to the invention for controlling length variation of its lead screw due to thermal expansion when the apparatus is rotating in high speed and/or when load is high; 
           [0023]      FIG. 11  is a fragmentary view of the CNC apparatus of  FIG. 10  where length variation (e.g., elongation) of the lead screw due to thermal expansion is illustrated; and 
           [0024]      FIG. 12  is a view similar to  FIG. 11  where cooling of the lead screw is illustrated. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Referring to  FIGS. 6 to 9 , a CNC apparatus in accordance with a first preferred embodiment of the invention is shown. The CNC apparatus comprises a bed  30 , a first ball bearing set  31  provided on the bed  30  and including an annular housing  311  provided on the bed  30 , and two rows  312 ,  313  of balls positioned in the housing  311  by two inner races (not numbered) and two outer races (not numbered) disposed in opposing directions (i.e., the housing  311  is not adapted to deflect), a spaced second ball bearing set  32  provided on the bed  30  and including an annular housing  321  provided on the bed  30 , and two rows  322 ,  323  of balls positioned in the housing  321  by two inner races  3231  and two outer races  3223 ,  3233  disposed in the same direction (i.e., the housing  321  is adapted to deflect), a lead screw  33  interconnected the ball bearing sets  31  and  32  and having an axial channel  331  for flowing cooling fluid, a first nut  37  for securing one end of the lead screw  33  to the first ball bearing set  31 , a second nut  38  for securing the other end of the lead screw  33  to the second ball bearing set  32 , a threaded carrier  34  threadedly put on the lead screw  33 , a feed mechanism  35  fixedly secured to the threaded carrier  34 , a drive unit (e.g., motor)  36  spaced from the first ball bearing set  31  and operatively connected to the first ball bearing set  31 , an end cap  39  with rotary joint provided at the other end of the lead screw  33  with the second nut  38  concealed therein, a pipe  40  passing through the end cap  39  into the channel  331 , a drain  41  at the other end of the lead screw  33  through the end cap  39  and being in fluid communication with the pipe  40 , and a deflection detection unit  42  proximate the housing  321  (e.g., spaced from the housing  321  by a predetermined distance (e.g., 1 mm)) for measuring the deflection of the housing  321 . 
         [0026]    By configuring as above, the lead screw  33  is adapted to rotate and extend axially in either direction (e.g., toward right as shown in  FIG. 8 ) but being restricted in radial movement in a rotational operation. 
         [0027]    Referring to  FIG. 7  specifically, a pre-stressing is illustrated. Clockwise turning (i.e., tightening) the second nut  38  will urge an inner race  3231  of the row  323  of balls against balls  3232  which in turn urge an outer race  3233  of the row  323  of balls against an outer race  3223  of the row  322  of balls. As a result, the inner race  3231 , the balls  3232 , and the outer races  3233  and  3223  shift a very small first distance leftward with the housing  321  being deflected counterclockwise about the bed  30 . 
         [0028]    Referring to  FIG. 8  specifically, the lead screw  33  may elongate a minute amount due to temperature rise when the apparatus is rotating in high speed and/or when load is high. The increased length of the lead screw  33 , as indicated by rightward arrows, can decrease the force exerted upon the second ball bearing set  32  by the second nut  38 . As a result, the second ball bearing set  32  deflects clockwise to return to its original position before the pre-stressing. Also, length of the bed  30  increases. Further, the deflection detecting unit  42  is adapted to measure the predetermined distance between itself and the housing  321  (i.e., the deflection data) in order to determine whether there is a change. If so (i.e., there is change (e.g., lead screw elongation)), the defection data is then sent from the deflection detecting unit  42  to a control unit (not shown). 
         [0029]    Referring to  FIG. 9  specifically, length of the lead screw  33  increases due to thermal expansion if the apparatus continues to rotate in high speed and/or when load is high. It is contemplated by the invention in response to pre-stressing the lead screw  33 , the subsequent rotational movement of the lead screw  33 , and the clockwise deflection of the housing  321  the deflection detecting unit  42  immediately sends a signal to inform the control unit to open a valve of the pipe  40  for flowing cooling fluid (e.g., cooling water) through the channel  331  and eventually the cooling fluid is carried off by the drain  41 . Moreover, the degree of opening the valve of the pipe  40  is precisely controlled by the control unit. The opening control of the valve of the pipe  40  (i.e., cooling the lead screw  33 ) aims at maintaining the predetermined distance between the deflection detecting unit  42  and the housing  321  in a minimized range when the apparatus is rotating in high speed and/or when load is high. 
         [0030]    It is envisaged by the invention that the length of the lead screw  33  is substantially unchanged with respect to the bed  30  when the apparatus is rotating in high speed and/or when load is high. For example, length of the lead screw  33  increases from 5 m to about 5.05 m and length of a section of the bed  30  corresponding to the lead screw  33  also increases from 5 m to about 5.05 m when temperature of the bed rises from 20° C. to 21° C. This characteristic can prevent the apparatus from being damaged due to uneven temperature distribution. Moreover, the invention can have a desired positioning accuracy and a strong axial stiffness. 
         [0031]    Referring to  FIGS. 10 to 12 , a CNC apparatus in accordance with a second preferred embodiment of the invention is shown. The characteristics of the second preferred embodiment are detailed below. The apparatus comprises a bed  50 , a first ball bearing set  51  provided on the bed  50  and including an annular housing  511  provided on the bed  50 , and two rows  512 ,  513  of balls positioned in the housing  511  by two inner races (not numbered) and two outer races (not numbered) disposed in opposing directions, (i.e., the housing  511  is not adapted to deflect), a spaced second ball bearing set  52  provided on the bed  50  and including an annular housing  521  provided on the bed  50 , and two rows  522 ,  523  of balls positioned in the housing  521  by two inner races  5221  and two outer races  5223 ,  5233  disposed in opposing directions (i.e., the housing  521  is not adapted to deflect), a lead screw  53  interconnected the ball bearing sets  51  and  52  and having an axial channel  531  for flowing cooling fluid, a first nut  57  for securing one end of the lead screw  53  to the first ball bearing set  51 , a second nut  58  for securing the other end of the lead screw  53  to the second ball bearing set  52 , a threaded carrier  54  threadedly put on the lead screw  53 , a feed mechanism  55  fixedly secured to the threaded carrier  54 , a drive unit (e.g., motor)  56  spaced from the first ball bearing set  51  and operatively connected to the first ball bearing set  51 , an end cap  59  with rotary joint provided at the other end of the lead screw  53  with the second nut  58  concealed therein, a pipe  60  passing through the end cap  59  into the channel  531 , a drain  61  at the other end of the lead screw  53  through the end cap  59  and being in fluid communication with the pipe  60 , and a deflection detecting unit  62  proximate the housing  521  (e.g., spaced from the housing  521  by a predetermined distance (e.g., 1 mm)) for measuring the deflection of the housing  521 . 
         [0032]    By configuring as above, the lead screw  53  is adapted to rotate but being restricted in both axial and radial movements in a rotational operation. 
         [0033]    Referring to  FIG. 11  specifically, the lead screw  53  may elongate a minute amount due to temperature rise when the apparatus is rotating in high speed and/or when load is high. The increased length of the lead screw  53 , as indicated by rightward arrows, can increase the force exerted upon the pre-stressed second ball bearing set  52 . As a result, the second ball bearing set  52  deflects clockwise to decrease the predetermined distance between itself and the deflection detecting unit  62 . Further, the deflection detecting unit  62  is adapted to measure the predetermined distance between itself and the housing  521  (i.e., the deflection data) in order to determine whether there is a change. If so (i.e., there is change (e.g., lead screw elongation)), the defection data is then sent from the deflection detecting unit  62  to a control unit (not shown). 
         [0034]    Referring to  FIG. 12  specifically, length of the lead screw  53  continuously increases due to thermal expansion if the apparatus continues to rotate in high speed and/or when load is high. It is understood that the lead screw  53  will deform and thus damages the first ball bearing set  51  and/or the second ball bearing set  52  since both the first and second ball bearing sets  51 ,  52  are not adapted to deflect. Advantageously, it is contemplated by the invention that after detecting any deflection of the second ball bearing set  52  the deflection detecting unit  62  sends a signal to inform the control unit to open a valve of the pipe  60  for flowing cooling fluid (e.g., cooling water) through the channel  531  and eventually the cooling fluid is carried off by the drain  61 . Moreover, the degree of opening the valve of the pipe  60  is precisely controlled by the control unit. The opening control of the valve of the pipe  60  (i.e., cooling the lead screw  53 ) aims at maintaining the predetermined distance between the deflection detecting unit  62  and the housing  521  in a minimized range when the apparatus is rotating in high speed and/or when load is high. 
         [0035]    It is envisaged by the invention that the length of the lead screw  53  is substantially unchanged with respect to the bed  50  when the apparatus is rotating in high speed and/or when load is high. For example, length of the lead screw  53  increases from 5 m to about 5.05 m and length of a section of the bed  50  corresponding to the lead screw  53  also increases from 5 m to about 5.05 m when temperature of the bed rises from 20° C. to 21° C. The second embodiment also has other beneficial advantages the same as the first embodiment. 
         [0036]    While the invention herein disclosed has been described by means of specific embodiments, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope and spirit of the invention set forth in the claims.