You can select a 3D curve such as a 3D line, a 3D spline, or a 3D arc and then append (untrim) it either on both ends or any one end as shown in the following images.
You cannot select a 3D circle to append as there are no open ends to append.
1. Click 3D Geometry and then, in the 3D Curve group, click More.
2. Click Untrim Curve in the Tools section. The Untrim dialog box opens.
Alternatively, you can also select a 3D line, a 3D spline, or a 3D arc in the viewport and click on the Command Mini Toolbar (CMT) to open the Untrim dialog box.
3. In the viewport, click Edge and select the 3D curve that you want to append (untrim).
4. To append the selected 3D curve at both ends, click the -t and +t check boxes. This is the default selection. To append only one end of the curve click either -t or +t check box.
◦ Select an Enlarge Type. Each of these types will continue the curvature of the original 3D curve:
▪ Full: Continues the 3D curve in a circular manner for an arc and connects both the open ends of the arc to complete the circle.
▪ Linear: Continues the 3D curve in a linear manner.
▪ Circular: Continues the curvature by appending circular arcs that have the same curvature radius as the original curve at its end points. This type of enlargement is limited to a full circle.
As shown in the following image, appending circular arc does not complete the circle unless the +t and -t values are large enough to complete the circle.
▪ Flat End: Begins with the curvature of the original 3D curve and gradually straightens until it becomes almost linear at the outer edges.
◦ Length: Type the append length for the 3D curve.