Today I Learned - Calypso - Form Datum
The following article is from GD&T and Calypso manual - View here
What the Form Datum does is establish a FUNCTIONAL Datum Reference Frame coordinate system, similar to if you were setting up the primary, secondary, and tertiary datums on a functional checking gage. This is ONLY for a datum reference frame and does not modify how the base and secondary alignments evaluate their respective coordinate system. In the screen shot below, you can notice the different origin points are for a base alignment and datum reference frame.
To set the Form Datum correctly click on Extras/ Settings/ Measurement/ Form Datum, check both the Outer Tangential Element and the Re. Calculation as per ISO 5459 boxes.
When we create our base alignment the alignment uses the intersection point of your features as below.
This is based on the measurement of the actual features of the part.
Form Datum uses the highest contact points that would touch a Datum Feature Simulator, in order of primary, secondary, and tertiary features, establish a real coordinate system. Calypso will establish the proper location through the form datum reference calculation per ISO 5459. Again this is ONLY used for a Datum Reference Frame and does not affect the Base Alignment.
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