Dimension Flattening (angle)
You can dimension the face (2D) of a sheet metal part, beginning from the center point of the arc, using the specified angle.
You can define the angle or number of dimensions. The dimension will not act as a geometric constraint.

- Select Drawing | Dimensions |  Fast Dimensioning > Fast Dimensioning > Set Dimensions for Flattened Sheet. Set Dimensions for Flattened Sheet.
- Select the dimension properties.  Dimension Properties (2D) Dimension Properties (2D)
- Select the polylines between which the dimensions will be added.- Select the lines on the first (1st) side, and select Confirm. 
- Select the lines on the second (2nd) side and confirm the selection.
 
- Select the lines on the first (1st) side, and select Confirm.
- Define the dimensioning.- Angle a30, for example.
- Number 10, for example.
 
- Select OK.
