174 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
The location of the highest point is unknown,
so the workpiece is pushed against a simulator,
such as an angle block or perpendicular plate
while maintaining three point contact with the
primary plane and two point contact with the sec-
ondary plane. The workpiece will stop when con-
tact is made with the highest point. The location of
the high point cannot be predicted. Therefore, the
perpendicular plate must be at least as large as the
datum feature.
It is possible that more than one point will
contact the plate. Contact with more than one
point is acceptable.
All translational and rotational degrees of
freedom of the workpiece are constrained when
the primary, secondary, and tertiary datums
are established. See Figure 6-22. The shown tool
simulates three datum features and constrains all
degrees of freedom when the workpiece is placed
in the tool.
Datum Simulation of Flat Features
Theoretically, a fl at datum feature creates a
datum plane that coincides with a perfect simula-
tor of the feature. In real-world applications, theo-
retical datums cannot be contacted, and simulation
methods must be used to determine the datum
locations with a level of accuracy that is adequate
for the workpiece and the tolerances applied to it.
Simulation of datum features is required because
the parts produced and inspection results must
achieve the specifi ed tolerances in relationship to
the specifi ed datums. The simulation can be accom-
plished with a tool-quality piece of hardware, or
it may be done through computer calculations
based on data obtained with coordinate measur-
ing machines (CMM), numerical-controlled (NC)
machines, or other measurement equipment.
Datum simulation is a means of approximat-
ing, with a high degree of accuracy, the theoretical
location of the datums. Datum simulation is meant
to locate the datum on the basis of the workpiece
datum feature. See Figure 6-23. The datum feature
in the fi gure has variations (shown greatly exag-
gerated) that are caused by fabrication processes.
The simulation tool or process will also have varia-
tions, although very small. In the fi gure, the datum
plane is shown in contact with the high points on
the workpiece. The simulated datum is in con-
tact with the high points on the datum simulator.
When the workpiece is set on the datum simula-
tor, the datum and simulated datum may not be
in exactly the same place if the high points on the
workpiece do not coincide with the high points
on the simulator. Because of the variations in the
datum feature and the datum feature simulator,
the simulated datum is unlikely to be exactly the
same as the theoretical datum.
Variations in a datum simulation tool or pro-
cess can result in what is typically a small amount
of datum simulation error, and that can result in
some measurement error for any features mea-
sured from the datum. So, there is a theoretical
datum plane, and there is a simulated datum plane.
The simulated datum plane is the one achieved in
production or measurement. The theoretical plane
is the one that exists relative to the datum feature
without any simulation error. Since tolerances are
specifi ed relative to the theoretical datums, any
error in the simulation method must be accounted
for when measurements are made.
Goodheart-Willcox Publisher
Figure 6-22. The shown tool establishes all three
datum planes when the workpiece is inserted into it.
Workpiece
Simulated datum plane
Datum feature surface with
relatively large errors
Minor surface errors
Datum simulator
Datum
Goodheart-Willcox Publisher
Figure 6-23. Accurate tooling components placed
against the datum features on a workpiece simulate the
theoretical datums.