Choose Home tab→Annotation group→Centerline 
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On the Centerline command bar, select the Arc centerline type.
From the Placement Options list, select By 2 Arcs.
Click the first arc, and then click a second, concentric arc.
The arc centerline appears midway between the two arcs.
Tip:
If you select By Center Point from the Placement Options list, you can place an arc centerline by specifying a center point (1), a start point (2), and an end point (3). You can click keypoints on geometry or points in free space.
You can modify the sweep of an arc centerline by dragging the edit handles on the centerline. You can also cut, copy, or paste a centerline.
You can change the appearance of the centerline by changing its properties in the Center Line and Mark Properties dialog box. Select the centerline and then click Properties on the shortcut menu.