Fig.6.24 residual stress caused by cold alignment
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6.3 statistic analysis
6.3.l classification of machining error
Constant error：the size and direction of the machining error almost keep constant when continuously machining a batch of workpieces
System error
Variable system error：the machining error varies regularly according to the machining sequence.
Random errorthe size and direction of the error vary without law when machining a batch of workpieces.
6.3.2 Method of error distribution curve
6.3.2.1Equationofthenormaldistribution
It was proved from theory and practice that the error will follow the normal distribution law when the number of the workpieces is very big and the factors that influence the error have not any deviation（also called Gauss curve). The equation of the normal distribution is：
(X??)2?12?2Y?e?2?where Y ——the probability density of the normal distribution——the standard deviation of the normal distribution
（square root of the deviation）
??——the average value of the normal distribution.
