Blender is good, but not really CAD, more 3d modelling for animation though it does contain 3d printing tools and an stl exporter. Sketchup can be made to run under wine, and there is an export path from it to blender using a few free tools*, I do this often and it works very well. There is also freeCAD which I haven't personally used but has an interface supposedly similar to most of autodesk's (makers of autoCAD, inventor...) products. Autodesks's stuff apparently doesn't usually run well under wine.
Openscad does scripted models only, you don't model so much as type code which compiles to make shapes. It's ok for mathematically defined parts like worm gears but you'll probably want to export the stl files from it then import them into another tool to make all the other parts of your models.
*the export path I worked out is described at the link below, hopefully it works for later versions of sketchup than version 8 but I haven't upgraded sketchup beyond version 8, I just keep the old installer exe files to hand. http://www.instructables.com/id/Sketchup-3D-modelling-file-conversion-and-advice-/
scroll to near the bottom for the bit's you'll want. Apologies for the bad writing in it, I made that tutorial ages ago and in a rush at the time. For info on installing sketchup on ubuntu based (including mint) linux distros look at https://www.dedoimedo.com/computers/sketchup-ubuntu-xerus.html .
Edited by rp88, Yesterday, 05:24 PM.
Back on this site, for a while anyway, been so busy the last year.
My systems:2 laptops, intel i3 processors, windows 8.1 installed on the hard-drive and linux mint 17.3 MATE installed to USB