I also thought a little bit about the FreeCAD logo. With the suggestion for a gear for the C and a more technical F, I came to this first draft:
The lightning is not OK and such, but I stopped at this point. It is true, that it does not work very well for a logo for several reasons. Like kkremitzki said, the gear looks ugly at small scales.
Then I reminded about a book about logo design I read some time ago. First, a logo should be easy. You should easily be able to sketch it with some lines on a sheet of paper. Then it should not include any characters. The F in the current logo looks "strange" for people who don't use the Latin alphabet. That almost all of the big, commercial CAD programs use Latin characters in their logos doesn't make it better.
With this in mind I came to a completely new design. It is easy to remind, can be scaled down to small sizes, so it is also usable in the menu bar and hopefully is unique.