This video is in response to a question that we received on our question line from Ignacio.  Ignacio asked us if it is a bad practice to reference three datums in a single drawing callout for the parallelism symbol.  The short answer is yes, that is a bad practice. 

To explore this in more detail, we will first consider the parallelism of a surface.  The feature control frame for this callout will contain the parallelism symbol, the tolerance amount, and a single datum plane reference.  For example, think of one surface of a part, and impose a parallelism constraint of 0.030, with respect to a datum plane “A.”  The tolerance zone in this example is defined by two parallel planes 30 microns apart.  The parallel planes that form the tolerance zone boundaries are also perfectly parallel to datum plane “A.” As you can see, datum plane “A” provides all the information we need to define the parallelism constraint.  Any additional datum planes referenced would not be parallel to plane “A” and would only serve to confuse the reader.   

A single datum plane reference is sufficient for a parallelism constraint for a surface because parallelism does not require us to constrain all six degrees of freedom.  Parallelism controls the orientation of a feature, but it does not control location.  Going back to our parallel surface example, we can allow the surface to be translated parallel to the datum plane or rotated about an axis perpendicular to the datum plane and still accurately define the desired parallelism tolerance zone.

Consider a second example, where the orientation of a hole is controlled with a parallelism constraint.  The feature control frame for this callout will contain the parallelism symbol, a diameter symbol followed by the tolerance amount, and two datum plane references.  The diameter symbol in this feature control frame indicates that the tolerance applies to a feature of size.  However, we are not controlling size or location with this constraint, only orientation.  Visualize three mutually perpendicular datum planes A, B, and C located along the coordinate system X, Y, and Z axes.  Now, imagine that the hole axis must be parallel to one of these planes, B.  Note that the axis can still be rotated in a plane parallel to datum plane B.  If we want to lock down the orientation of the axis fully, we must add a reference to a second datum plane, C.  Now, the axis cannot rotate, and the orientation is fully constrained.  The axis cannot be parallel to the third datum plane, and it is, in fact, perpendicular.

To summarize, the parallelism callout is used with one or two datum references, depending on the type of feature to be controlled.  Parallelism should never reference three datums, and this is because parallelism only controls orientation, not location.

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