You can select three nodes that lie on a circular arc to define a coordinate system that is attached to your model. This method is useful when the model does not contain any nodes along the axis of a hollow cylinder or at the center of a hollow sphere, but you would like to define a coordinate system at those locations. The center of the circle defined by the three nodes is the origin of the coordinate system. The line from the origin to node 1 defines the X-axis (for a rectangular coordinate system) or the R-axis (for a cylindrical or spherical coordinate system); thus, the order in which you select the nodes determines the direction of increasing 
. Three colinear nodes are not valid. The normal to the circle is parallel to the Z-axis (for a rectangular or cylindrical coordinate system) or the 
-axis (for a spherical coordinate system). The motion of the coordinate system is defined by the translational motion of the three nodes.
The visual display of coordinate systems that follow nodes on your model is updated to reflect any deformation scale factors applied; however, result transformations based on these coordinate systems use the actual, not the scaled, deformations.
To create a coordinate system following three nodes on a circle:
From the main menu bar, select Tools
Coordinate SystemCreate.
The Create Coordinate System dialog box appears.
Tip:
You can also create a coordinate system by using the 
tool in the toolbox or the Session Coordinate Systems container in the Results Tree for the current output database.
In the Create Coordinate System dialog box:
Accept the default name for the coordinate system, or enter the name of your choice in the text field.
Toggle on System following 3 nodes on a circle to specify the motion of the coordinate system.
Select one of the following as the coordinate system type:
Rectangular: The X-, Y-, and Z-axes are interpreted as the 1-, 2-, and 3-axes, respectively.
Cylindrical: The R-, -, and Z-axes are interpreted as the 1-, 2-, and 3-axes, respectively.
Spherical: The R-, -, and -axes are interpreted as the 1-, 2-, and 3-axes, respectively.
Click Continue.
Abaqus/CAE displays prompts in the prompt area to help you define the coordinate system axes.
Select three nodes in the viewport that lie on a circular arc.
The coordinate system appears in the viewport, in the list in the Coordinate System Manager, and under the Session Coordinate Systems container in the Results Tree for the current output database. Abaqus saves user-defined coordinate systems only until the output database is closed unless you save them to the output database file. You cannot edit a coordinate system once it has been created; you can rename or delete coordinate systems that have not been saved to the output database.
