Anyone wanting to chamfer the Hexgon Corners of a Nut in Fusion 360 will find, it cannot be done the conventional way ... after extruding the Hexagon. Because Fusion 360 is parametric modelling software it needs to be tackled as a Solid. After a lot of searching, I was able to work it out.

It is assumed that the user is familiar with the basics of Fusion 360

Step 1: Sketch Circle and Extrude Blank & Chamfer

Sketch a circle with diameter dimensioned the same as the across corner measurement of the nut. In the example it is 21mm for a M12x 1.75 Nut. The A/F (Across Flat) of the Hexagon is 17mm.

Extrude the circle to the thickness of the Nut eg.12.

Select the corners on both sides and Chamfer.

Step 2: Create Thread

Sketch a Circle in the centre of the Chamfered Blank.

Use Create, Thread and select the Sketched circle.

Enter the Thread Designation in the Thread pane.

Make sure the Modeled Box is ticked. (For 3D Printing, if the box is not ticked the STL files does not contain the thread and it will not be printed.)

Step 3: Sketch the Hexagon & Circumscribed Circle

Select the original face and use Sketch, Centre Diameter Circle to sketch a circle the same diameter as the extruded blank. (This is required for creating the 6 segments, that will be extruded in the final step)

Use Sketch, Circumscribed Polygon to draw the Hexagon.

Make sure that the Corners snap to the Circle.