Ages ago I was on here asking about CAD programs.
I just finished CAD-ifying a set of drawings for a 1/10th scale Laird Solution.
I used QCAD, because it's the only CAD program that Ubuntu lists as being 'native'. I couldn't get my head wrapped around using it (I suspect it was a problem with CAD in general, not QCAD). I found a tutorial on the web on how to do one specific thing which opened the window, now I'm charging along with it.
It's only 2-D, but it's free, it makes DXF files, and it's available for both Windows and Linux.