I have looked a Fusion 360 but I believe this is a online software only and is not free to use either? Is this true? For 'square' straight edge stuff it will be fine but not for circles or round objects. It won't how ever allow me to make 'perfect circles for 3D printing and or CNC so isn't good for my designs I now am designing. I use it for designing timber and steel designs (furniture, guitars, speaker cabinets, work benches/tables, etc) and just use it to make the plan of every piece/component I need to make by hand and fit them together, ie using it to make a plan only.

I have SketchUp Make which is easy as to use and great for designing things that I am not going to print in 3D or CNC etc. I am a hobbyist at the moment and only doing this in my spare time which is why I want freeware, if I get some good designs happening I will then look at buying a program (like rhino or something like that) as I am hopeing to turn this into a business eventually, but that is just a dream at the moment.

so I can work on my prototypes safely offline and save only to my offline computer. I am trying to find 3D 'CAD' modelling software, for Mac OS X, that has a free version and can be used offline so my files aren't saved on the 'cloud' or owned by the software company etc.

Designspark mechanical for mac