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ACIS was designed around the concept that all parts are modeled in a singleunitless number space (object space). One could think of the numbers as being inches, or millimeters, or any other unit.
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Although
ACIS has historically been a unitless modeler, the sharing of model data between applications can be difficult if no units are applied, because different applications may
assume different units of measure. For example, application
M may assume the units are millimeters, while application
N assumes feet. If application
M reads in a model that was stored in an
ACISsave file (.sat or
.sab file) by application
N, there is an incompatibility.
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To avoid incompatibilities when applications share model data,
ACIS has chosen
millimeters as the default unit of measure. Internally,
ACIS does not apply any unit of measure to numbers. However, an underlying unit of measure is required for persons and applications to properly interpret data. When data is written to a save file,
ACIS assumes this underlying unit to be millimeters.
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To support the sharing of model data between various applications having different internal units, the save file format enables an application saving the model to indicate the units assumed when the model was created. This is done by indicating the number of millimeters (the default unit) represented by each unit in the save file data. The current value of
SPAresabs and
SPAresnor are also saved when a part is written to the file. Refer to the
Kernel Component Manual for information about how this data is saved in the file.
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ACIS takes no action upon seeing values for the millimeters per unit,
SPAresabs, or
SPAresnor in the save file that are different from those set in the restoring application, other than producing a warning message using
sys_warning. It is the responsibility of the application to take appropriate actions, such as setting a body transformation on the model via a scaling transformation. This unit and tolerance data in the save file simply enables the application restoring the model from the file to determine the units and tolerance in effect when the model was written to the file. The application can then compute an appropriate scaling factor.
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