Datum
Datum Feature
Shaft labeled as a datum feature
Datum features are the actual, physical features on the part. They are not perfect and will always have manufacturing variation.
Imperfect Part
How is a theoretical perfect datum established from a physical imperfect datum feature? The answer is through a true geometric counterpart (TGC). This TGC is a perfect inverse shape and contacts the highest points of the datum feature. The perfect datum is derived from the TGC. The TGC is the connection between the theoretical datum reference frame and physical part.
The drawing specification defines the theoretical true geometric counterpart to establish the datum (no measurement uncertainty). In real inspections, using a TGC is not possible. Instead, a physical datum feature simulator is used to establish a simulated datum. This could be either a piece of hard tooling (collet, chuck, surface plate, angle plate) that engages the feature or soft tooling (CMM or OMM algorithms) used on the measurement points.
Physical datum feature simulator (smallest circumscribed cylinder
algorithm)
Finite number of data points
from CMM probe or scan
Simulated datum axis
Physical datum feature simulator (chuck)