A base flange is the first feature in a new sheet metal part. When you add a base flange feature to a SolidWorks part, the part is marked as a sheet metal part. Bends are added wherever appropriate, and sheet metal specific features are added to the FeatureManager design tree.
Some additional items to note about a base flange feature:
The Base-Flange feature is created from a sketch. The sketch can be a single open, a single closed, or multiple-enclosed profiles.
There can be only one base flange feature in a SolidWorks part.
The thickness and bend radius of the Base-Flange feature become the default values for the other sheet metal features.
To create a Base-Flange feature:
Create a sketch that meets the requirements above. Alternatively, you can select the Base-Flange feature before you create a sketch (but after you select a plane). When you select the Base-Flange feature, a sketch opens on the plane.
Click Base-Flange/Tab 
on the
Sheet Metal toolbar, or click Insert,
Sheet Metal, Base
Flange.
The controls on the Base
Flange PropertyManager update according to your sketch. For example,
the Direction 1 and Direction
2 boxes do not appear for a sketch with a single closed profile.
If necessary,
under Direction 1 and Direction
2, set the parameters for the End
Condition and Depth 
.
Under Sheet Metal Parameters:
Set
a value for Thickness 
to specify the sheet metal thickness.
Select Reverse direction to thicken the sketch in the opposite direction.
Set
a value for Bend Radius 
.
Under Bend Allowance, select a bend allowance type.
If you selected K-Factor, Bend Allowance, or Bend Deduction, type a value.
If you selected Bend Table, select a bend table from the list, or click Browse to browse to a bend table file.
Under Auto Relief, select a relief type. If you selected Rectangular or Obround:
Select Use relief ratio and set a value for Ratio.
- or -
Clear
Use relief ratio and set a value
for Relief Width
and Relief Depth 
.
Click
OK 
.