How to create a nut with curvature on the top or bottom
You can easily use the feature of body to create the nut with the fillet as the above figure
First of all create a hexagon and a circle in the sketch on XY plane & Pad it to look like this.
After that go to INSERT -> BODY.
Now sketch a circumscribed circle for the hexagon on the XY Plane
Pad the circle to become a cylinder for the same pad length as the hexagon
Now create a fillet on the cylinder for the required dimension like below.
Go to INSERT -> BOOLEAN OPERATORS -> INTERSECT as shown in figure
Select the cylinder first, then after select the paded hexagon, & then click ok to look like this.
Your required Hexagon nut looks like this. See here.