Patent Publication Number: US-2006011002-A1

Title: Machine tool with dimensional change compensation

Description:
RELATED APPLICATIONS  
      This application claims the benefit of U.S. Provisional Application No. 60/587,357, filed Jul. 13, 2004, hereby fully incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to machine tools, and more specifically to computer controlled machine tools with dimensional change compensation.  
     BACKGROUND OF THE INVENTION  
      Precision machine tools, such as CNC tools, are widely used in industry to make machine parts and components. The degree of machining accuracy required for these machine parts and components is often high, requiring that the moving parts of the machine tool, especially those for positioning a tool relative to a workpiece, be capable of extremely precise, predictable, and repeatable movement. Such a CNC machine tool is disclosed in U.S. Pat. No. 6,325,697, hereby fully incorporated herein by reference.  
      One assembly commonly used in machine tools for positioning a tool is known as a ball screw assembly. Generally, the ball screw assembly includes an elongate screw rotatably mounted in a pair of spaced apart bearings. A machine slide, to which the tool is coupled, is engaged with a ball screw nut on the screw and moves axially along the screw upon rotation of the screw. The direction of travel of the machine slide depends on the direction of rotation of the screw. The screw or the servo motor driving the screw typically has a rotary position device (rotary counter) which positions the screw to a specific point of rotation. By rotating the screw to a specific point of rotation positions, the slide driven by the ball screw nut to position to a precise linear position. One of the bearings is generally a thrust bearing, which locates the screw relative to a machine axis and restricts axial movement of the screw, while the other bearing enables the end of the screw to “float” axially to account for thermal expansion of the screw.  
      Thermal expansion of components of the screw and other components of the ball screw assembly may affect the position of the machine slide and tool. Rotation of the screw in the bearings and the movement of the ball screw nut on the screw create friction, resulting in heat transfer to the screw, and a consequent change in length of the screw due to thermal expansion or contraction with the temperature change. Heat can also be introduced to the screw from friction in other parts of the ball screw assembly, such as motors, drive pulleys, belts, and gears, as well as from the spindle of the tool. Unless compensated, thermal expansion of the screw will cause the rotary position device, which tracks only rotary position of the screw, to inaccurately position the slide.  
      Previous approaches have compensated for thermal expansion of the screw by sensing the temperature of the ball screw nut and making assumptions as to what temperature the ball screw is to calculate the change in length of the screw using the thermal expansion coefficient of the screw material. This calculated screw length is then used as an input to adjust the position of the machine slide and tool.  
      A disadvantage of this approach is the ball screw nut temperature may not represent the temperature of the ball screw over its full length. Bearings supporting the ball screw can produce heat into the ball screw that does not get transferred to the ball screw nut. If the travel of the ball screw nut only occurs over a small area of the ball screw, again the ball screw nut temperature will not reflect the temperature along the entire length of the ball screw. As a result, compensation approaches in which the ball screw nut temperature is relied on are typically inaccurate. These methods have prevailed because the ball screw nut does not rotate and the thermal measurement device can be mounted to it easily.  
      Another previous method uses a linear positioning counter connected to the slide itself which forces the machine tool computer to an exact point (count) of the linear scale. This approach, however, may add undesirable complexity and additional cost to the machine tool.  
      What is needed in the industry is a low-cost device and method for directly compensating for thermal expansion of a machine tool component such as a ball screw.  
     SUMMARY OF THE INVENTION  
      The present invention addresses the need of the industry by providing a low-cost system and method for directly compensating for thermal expansion of a machine tool component such as a ball screw. Two styles can be applied which both offer direct measurement of growth rather than the measurement of temperature and consequential estimates of thermal expansion. According to an embodiment of the invention, a linear voltage to distance transducer (LVDT), or a hall effect linear transducer, is mounted at the “floating” end of the ball screw to directly sense the thermal expansion of the screw. The LVDT includes a slug portion which travels within a coil. The slug is affixed to the floating end of the screw and the coil is affixed to the bearing block in which the screw is rotatably mounted. Thermal expansion of the screw causes the slug to move axially within the coil, in turn causing the coil to produce an electrical signal with a varying voltage proportional to the change in position of the slug within the coil.  
      In an alternative embodiment, a magnetic Hall effect linear transducer system includes a magnet affixed to the floating end of the screw and a Hall effect linear transducer affixed to the bearing block in which the screw is rotatably mounted. Thermal expansion of the screw causes the magnet to move axially toward or away from the Hall effect linear transducer in turn causing the Hall effect transducer to produce an electrical signal with a varying voltage proportional to the change of distance between the magnet mounted on the end of the ball screw and the Hall effect linear transducer pick-up device. The signals from the coil or the Hall effect transducer may be digitized and provided as an input to the computer controlling the machine tool. The machine tool computer may use this input to accurately determine the position of the tool accounting for the thermal expansion of the screw or other components in the ball screw assembly.  
      In an embodiment of the invention, a machine tool includes a pair of spaced apart support structures and an elongate element having a pair of opposing ends and presenting a length dimension. The element extends between the support structures and is selectively rotatably mounted thereby so that one of the pair of opposing ends of the element is constrained from longitudinal movement relative to the support structures and the other of the opposing ends is free to move longitudinally relative to the support structures in response to changes in the length dimension of the element. The system further includes a length change sensing apparatus with a first portion of the apparatus fixedly coupled with the support structure proximate the free end of the element, and a second portion of the apparatus operably coupled to the free end of the elongate element so as to move relative to the first portion of the apparatus with changes in the length dimension of the element. The apparatus produces a signal having a parameter that varies linearly with changes in the length dimension of the element. This signal may be coupled to the computer control of the machine tool, where can be used to adjust the position of a tool positioned along the length of the element for the thermal growth of the element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a fragmentary exploded view of a ball screw assembly of a machine tool according to an embodiment of the present invention;  
       FIG. 2  is an axial cross-sectional view of the ball screw assembly and LVDT temperature compensation apparatus depicted in  FIG. 1 ; and  
       FIG. 3  is a fragmentary exploded view of a ball screw assembly of a machine tool according to an alternative embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Ball screw assembly  10  of a machine tool (not depicted) generally includes screw  12  which is rotatably mounted in thrust bearing  14  and bearing  16 , which is received in bearing block  17 . Thrust bearing  14  axially locates screw  12  at distal end  18 , while proximal end  20  of screw  12  “floats” axially in bearing  16 . A ball screw nut (not depicted) is threadably received on screw  12 , and moves axially along screw  12  as screw  12  rotates in order to drive a linear slide for positioning a tool (not depicted).  
      In the embodiment of  FIGS. 1 and 2 , measuring temperature compensation apparatus  22  generally includes LVDT slug  24 , LVDT coil  26 , slug mount  28 , and coil mount  30 . Slug mount  28  has an axially projecting threaded portion  32  and is fixed on proximal end  20  of screw  12  with set screw  34 . LVDT slug  24  defines axial bore  36 , which is internally threaded and dimensioned so that LVDT slug  24  may be threaded onto threaded portion  32  of slug mount  28 .  
      Coil mount  30  generally includes cylindrical body portion  38  and flange portion  40 . Cylindrical body portion  38  defines bore  42  for receiving LVDT coil  26  therein. LVDT coil  26  is fixed within bore  42  with set screw  44 . Flange portion  40  of coil mount  30  may have a plurality of apertures  46  defined therein, corresponding to apertures  48  in bearing block  17 . Fasteners (not depicted) extending through apertures  46  and into apertures  48  may be used to secure coil mount  30  to bearing block  17 . LVDT coil  26  is coupled to the machine tool computer control (not depicted) through one or more wires  49 .  
      LVDT slug  24  travels within axial bore  50  of LVDT coil  26 . As screw  12  rotates, LVDT slug  24 , which is fixed to screw  12  with slug mount  28 , rotates within axial bore  50  of LVDT coil  26 , which is fixed to bearing block  17 . Further, as the axial length of screw  12  changes with temperature, LVDT slug  24  travels axially within axial bore  50 . As the axial position of LVDT slug  24  changes within LVDT coil  26 , the output voltage of LVDT coil  26  changes in linear proportion to the distance LVDT slug  24  moves.  
      LVDT slug  24  and LVDT coil  26  may be selected from commercially available LVDT components. It will be appreciated that slug mount  28  and coil mount  30  may be fashioned from any suitable material, such as for example, metals including cast iron or steel.  
      It will be appreciated by those of skill in the art that the output voltage of LVDT coil  26  may be calibrated to represent a specific distance increment that LVDT slug  24  moves. For example, LVDT coil  26  may be calibrated to produce a 0.05 volt output voltage change for each 0.001 inch axial movement of LVDT slug  24  within axial bore  50 . This voltage change signal from LVDT coil  26 , which is representative of the overall length change of screw  12  due to thermal expansion, may be provided to the control computer of a CNC machine tool. The control computer may be programmed to apportion the overall thermal change distance equally over the length of screw  26 . The control computer may then determine the appropriate modification for the position of the linear slide by adding or subtracting the appropriate portion of the thermal change distance depending on the axial position of the linear slide relative to the length of screw  12 . Those of skill in the art will appreciate that this position correction algorithm for the axial slide may be implemented in the position loop of the CNC control computer. Essentially the computer that drives the position servo loop (servo motor) has a plus or minus correction to the position loop over many points based on the magnitude of the growth or shrinkage of the length of screw  12 , which is represented by the travel of LVDT slug  24 .  
      A specific operational example may serve to illustrate the operation of the present invention. During operation of a machine tool, ball screw  12 , which has a 10 inch overall initial length, may absorb heat from friction, causing it to lengthen by 0.01 inch overall. LVDT slug  24  correspondingly moves axially 0.01 inch within axial bore  50  of LVDT coil  26 . The axial movement of LVDT slug  24  causes a proportional variation in the output voltage of LVDT coil  26 , which is communicated to the control computer of the machine tool. The control computer would calculate a 0.0001 inch position correction for each 0.1 inch of linear distance that linear slide is spaced apart from an index position on screw  12 , which for convenience may be where it is axially located by thrust bearing  14 . For instance, if the linear slide is positioned at a point one-half the length of screw  12 , this position would be calculated as 5.005 inches from the index position at thrust bearing  14 .  
      A further benefit of embodiments of the present invention is that the axial movement of screw  12  may be monitored as it changes from clockwise to counterclockwise rotation. Thrust bearing  14 , which locates and inhibit axial movement of screw  12 , will allow more and more axial movement as it wears. The control computer may be programmed to monitor the LVDT coil signal for a position change of LVDT slug  24  occurring at the instant screw  12  changes rotational direction. Any axial movement of screw  12  at the instant of direction change results in measurable change in LVDT coil output and gives a consequent quantifiable indication of thrust bearing wear or failure.  
      In the alternative embodiment of  FIG. 3 , ball screw assembly  52  of a machine tool (not depicted) generally includes screw  54  which is rotatably mounted in thrust bearing  56  and bearing  58 , which is received in bearing block  60 . Thrust bearing  56  axially locates screw  54  at distal end  62 , while proximal end  64  of screw  54  “floats” axially in bearing  58 . A ball screw nut (not depicted) is threadably received on screw  54 , and moves axially along screw  54  as screw  54  rotates in order to drive a linear slide for positioning a tool (not depicted).  
      In the embodiment of  FIG. 3 , temperature compensation apparatus  66  generally includes magnet  68 , magnetic mount  70 , transducer mount  72 , and Hall effect linear transducer  74 . Magnetic mount  70  is received on proximal end  64  of screw  54  and is secured thereto with set screw  76 . Magnet  68  has threaded portion  78 , which is received in threaded bore  80  of magnetic mount  70 . Transducer mount  72  is attached to bearing block  60  with fasteners (not depicted) through apertures  82 . Transducer  74  is received in bore  84  of mount  72  and is secured in place with set screw  86 .  
      In operation, a change in the length of screw  54  causes magnet  68  to move axially relative to transducer  74 , resulting in an air gap change. This air gap change results in a linearly proportional change in the output voltage of transducer  74 . Like LVDT coil  26 , the output voltage of transducer  74  may be calibrated to indicate the magnitude of axial travel.  
      It will also be appreciated that the present invention may be applied to determine thermal expansion or contraction of machine tool assemblies other than the ball screw assembly. For example, the spindle assembly of a machine tool is typically rotatably mounted in a cast iron housing and this housing is subject to thermal growth or shrinkage. As the cast iron changes temperature, it alters the location of the spindle centerline, which may then introduce a position error in the workpiece. An elongated rod made from a material having a temperature expansion coefficient significantly different from the material of the head assembly, may be affixed to a stationary point on the head assembly that houses the spindle. The opposite end of the elongated rod is allowed to float axially. An LVDT slug or magnet is fixed to the floating end of the rod. The LVDT slug or magnet is axially slidable in an LVDT coil or hall effect transducer, which is attached to another fixed structure. A signal is produced, as described above, proportional to the dimensional change in the spindle due to temperature change, and this signal may be used by the control computer to adjust tool position for dimensional change of the workpiece.