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Ch75-I044963.fm  Page 372  Tuesday, August 1, 2006  5:58 PM
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               372    Page 372  Tuesday, August  1, 2006  5:58 PM
               The  sequential  two  points  method  (STPM)  was  developed  to  evaluate  the  straight  form  error  of
               machine  tools.  By  this  method  it  is  possible  to  identify  both  error  forms  for  the  tool  post  and  the
               machined  part  by  a  single  feed  traverse  of  the  tool  post  in  terms  of  simple  algorithm  for  the  data
               (Tozawa  et  al.  (1982),  Tanaka  &  Sato  (1986)).  The  method  has  advantages  in  that  any  standard  of
               specific  straightness  is not  required, the  algorithm  is simple,  repetiive measurement  accuracy  is high,
               and the result is stable even under vibration  environment.
               In this  investigation,  the  following  targets  are  set to  evaluate  the  straightness  error movement  of the
               probe of the CMM, and  consequently  its error space, which  makes  it possible to compensate the error
               for the work space and to improve performance  in accuracy:
               1. Identify  the straightness error motion  of the probe in the CMM,
               2.  Identify  the  planelike  error  form  which  is composed  of  a bundle  of the  straightness  error  motions
                 for the objective plate due to term 1,
               3. Identify  the 3D error space and develop a compensation method  for 3 axes.


               EXPERIMENTAL   FACILITIES

               Principle  of STPM
               In Figure  1 schematic view of the STPM is illustrated. When an engine lathe is supposed, two  sensors
               which  can measure the relative distance to whatever objective  or machined part are located on the tool
               post  with  the  distance  L  in-between.  When  the  tool  post  is  fed  and the  data  are  acquired  stepwise  at
               positions  of L multiplied by integers, two data  series  for the individual  sensors are obtained.  The data
               are processed by the following  equations,
                                       X/,  -  -^st-1 + D tA  D {k_ i)B                (1)
                                                                                       (2)
                                        Y k=X k+D kA-D 0J!
                                        X 0=Q,X }=0,Y 0=Q                              (3)

               where D^A and D^B are the measured data by the sensors A and B, X^ and Yt are the straightness error
               motion  for the tool post and  form  for the objective  respectively.  It is shown that  error  forms  could be
               simultaneously and independently  derived.













                                                    Trajectory  of the sensor head
                                 Figure  1: Principle  of sequential two points method

               Application  of  the  STPM  to  the  CMM  is  attempted  to  calibrate  and  compensate  the  movement
               accuracy  of the  probe  in the  following,  by which  machining  error  in the  production  system  could  be
               improved.
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