Measuring methods for use on machine tools

Abstract

On a machine tool, a program 12 receives data from a scanning or analogue probe P, measuring a feature of a workpiece W. This data is combined with assumed machine position data during the scanning movement. This avoids having to break into the servo feedback loop 24 to get actual measured machine position data. The assumed machine position data can be derived from a part program 20 which controls the scanning movement. Several ways are described for compensating for errors between the assumed machine position values and the actual values.

1 . A method of measuring workpieces on a machine tool, comprising the steps of: 
 causing relative movement between an analogue or scanning probe and the workpiece, along a pre-defined path comprising nominal positions of the probe relative to the workpiece;    during the movement, taking measurements of the position of the workpiece surface relative to the probe, using an output of the probe; and    combining said position measurements from the output of the probe with data based upon said pre-defined path.    
     
     
         2 . A method according to  claim 1 , wherein the result of said combining step gives a dimension of the workpiece.  
     
     
         3 . A method according to  claim 1 , wherein the data based upon the pre-defined path comprises at least one target or nominal probe output reading.  
     
     
         4 . A method according to  claim 1 , wherein the result of said combining step gives an error value for a dimension of the workpiece.  
     
     
         5 . A method according to  claim 1 , including a step of compensating for a difference between the nominal position in the pre-defined path and the actual position of the probe relative to the workpiece.  
     
     
         6 . A method according to  claim 5 , wherein the compensating step uses data from an artefact of known dimensions and form.  
     
     
         7 . A method according to  claim 6 , wherein the artefact has similar dimensions to the feature of the workpiece being measured.  
     
     
         8 . A method according to  claim 6 , wherein the artefact has a dissimilar dimension to the feature being measured, or is measured at a dissimilar speed, and the compensation includes interpolation to correct for such dissimilar dimension or speed.  
     
     
         9 . A method according to  claim 5 , wherein the compensating step uses data from reference points measured on the workpiece surface.  
     
     
         10 . A method according to  claim 5 , wherein the compensating step uses an error map determined by comparing scans of the workpiece surface at slow and fast speeds.  
     
     
         11 . A controller or probe interface for a machine tool, programmed to perform a method according to  claim 1.