Part Modeling > Datum Features > Datum Axes > To Create a Datum Axis by Selecting a Circular Curve or Edge
  
To Create a Datum Axis by Selecting a Circular Curve or Edge
1. Click Model > Axis. The Datum Axis dialog box opens.
2. In the graphics window, select a circular edge or curve, a datum curve, or the edge of a cylindrical surface that is coplanar as the placement reference for the datum axis. A preview of the datum axis is displayed.
In the Datum Axis dialog box, the selected reference appears in the References collector. The constraint type for the selected reference is Center by default.
 
• If the constraint type is Center, the datum axis is created through the center of the selected circular edge or curve and normal to the plane on which the selected curve or edge lies.
• If you change the constraint type of the selected reference to Tangent, you must select an additional reference, such as a vertex or datum point with Through as its constraint.
• If the constraint type is Tangent, with Through as the constraint for the vertex or datum point that is the additional reference, the datum axis is created as constrained tangent to the curve or edge and through the vertex or the datum point.
3. To adjust the length of the datum axis outline, click the Display tab, select the Adjust outline check box, and select one of the following options from the list:
◦ Size—Makes the axis a specific size. Type a value in the Length box.
◦ Reference—Fits the axis to a selected reference. Select a reference to which to fit the axis.
4. Click OK.