huangyhg

<< Don Turcotte -- 10/28/08 08:38:36>>
PC-DMIS uses a datum fitting algorithm that emulates the function of a hard gauge. Datums constrain degrees of freedom (translation in X,Y,Z and rotation about X,Y,Z). The primary datum constrains as many of the 6 degrees of freedom as the datum feature allows. So a primary plane constrains translation in Z and rotation about X and Y. The secondary datum constrains as many of the remaining degrees of freedom as the datum feature allows. So a secondary line constrains translation in Y and rotation about Z. The tertiary datum constrains the remaining degrees of freedom. So in this case the tertiary circle constrains only translation in X. Since the tertiary datum is at MMC, the datum fitter can slide the datum reference frame alignment in X to the extent of the bonus on the tertiary datum. The datum fitter tries to reduce the deviation on the considered feature as much as possible. The TP line reports the available bonus on the considered feature plus the remaining bonus on the datum after fitting. In this case, the fitter used all the available datum bonus trying to bring the considered feature within tolerance so the bonus on the TP line is only the bonus on the considered feature.

In your example, if you wanted the circle to constrain both translation in X and Y, then the circle should be the secondary datum @ MMC and the line should be the tertiary datum in which case the line would constrain only rotation about Z.
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