Edge alignment method

An edge alignment method for a disc-shaped workpiece, comprising:

• a holding step of holding the workpiece on a chuck table,
• an imaging step of imaging at least a portion of the workpiece at a plurality of different locations in a peripheral direction of the workpiece on an outer peripheral portion of the workpiece,
• a coordinate calculation step of calculating coordinates of a plurality of points having a possibility of corresponding to an edge of the workpiece at the plurality of different locations imaged during the imaging step, wherein the coordinates are calculated based on images obtained during the imaging step,
• an approximate circle forming step of forming an approximate circle by using a least squares method on all the coordinates calculated in the coordinate calculation step,
• a false detection position excluding step of calculating deviations between the approximate circle formed in the approximate circle forming step and respective ones of all the points, and in a case where plural ones of the points have deviations greater than or equal to a preset threshold, respectively, determining the point, the deviation of which is greatest, to be a false detection position, and excluding from consideration candidates the point determined to be the false detection position,
• a machining area deriving step of, after the false detection position excluding step, estimating a position of the edge of the workpiece from the coordinates of three or more of the points still remaining without exclusion, and based on the estimated position of the edge, deriving a machining area at the outer peripheral portion of the workpiece,
• wherein the method further comprises:
• an additional approximate circle forming step of, after the exclusion of the point the deviation of which is the greatest in the false detection position excluding step, for ming an approximate circle again by using the least squares method on the coordinates of all the points still remaining without exclusion from the consideration candidates in the false position excluding step, and
• an additional false detection position excluding step of calculating deviations between the approximate circle formed in the additional approximate circle forming step and the respective ones of all the points still remaining without exclusion from the consideration candidates in the false detection position excluding step, and if plural ones of the points have deviations greater than or equal to the preset threshold, respectively, determining the point, the deviation of which is greatest, to be a false detection position, and excluding from consideration candidates the point determined to be the false detection position,
• wherein in a case where no point is determined to have a deviation greater than or equal to the preset threshold after calculating the deviations between the approximate circle formed in the additional approximate circle forming step and the respective ones of all the points remaining without exclusion from the consideration candidates in the false detection position excluding step, the machining area deriving step is performed again.