PURPOSE:To maintain the accuracy of a working shape and to shorten the number of correction times and correction hours by correcting the dividing position in the axial direction of a grindstone rotating spindle on each work or each several pieces according to the wear speed in the rotary axial direction of a cup like grindstone tip part and maintaining the absolute position of the grindstone tip part. CONSTITUTION:The error in the curvature depth of a worked lens is measured and the correction amt. of the index position stopping the cutting operation of a diamond grindstone 9 is calculated from this measured value. The curvature shape of the lens is corrected by adding this correction amt. to the NC data of the feeding stop position in the direction (d) of an NC device. In this case, the center thickness error of the lens is corrected by the following correction due to the error in the curvature depth and that in the center thickness of the lens to be worked by the wear of this grindstone 9 being proportional. The wear speed in the rotary shaft direction of the grindstone 9 is calculated from the relation of the number of continuous working pieces to the curvature depth error, the grindstone wear per working is found from this calculation value and this grindstone wear is added on each working to the feeding stop position in the direction (d) of the NC data.
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