I was using TurboCAD in support of my home machining, and found it difficult to learn and use, buggy, and not inexpensive. Their documentation and tutorials are poor. Still, I was getting engineering drawings created and printed, so it was getting the job done. (And TurboCAD's customer support is good.)
In December, I discovered Autodesk's Fusion 360 and worked through many of its tutorial lessons. It looks like an excellent tool and a worthy replacement for TurboCAD, but I haven't yet used it in my project work. But that I'm ready to use it, I need to redo the lessons! That's on tap for today and this weekend.
Earlier this week, however, I discovered Tinkercad and gave it a look. Yes, it's free and easy. I went through several of its lessons yesterday, enough to roughly understand its capabilities and limitations. For 3D-printing, this looks a very promising. And a nearby local library now has a 3D printer for public use, you pay so much per gram of material used. This has me interested in the 3D printing technology, although I wonder whether it is "cheating" when it comes to my model engineering activity. That's a subject for a separate thread. Anyhow, I will use TinkerCAD to make some simple test piece(s) and run it to the library for printing.
On the limitations, correct me if I'm wrong, but TinkerCAD is not capable of producing dimensioned 2-D drawings such as we need for our machine shop use. And it appears to lack the precision drawing tools like angular and distance key-ins. It doesn't do layers, it doesn't do lines, etc. I suspect there are workarounds, but it's not the right tool for the job.
For my French 75 mm cannon project, I think I am going to need to design and make some small miter gears. I don't think TinkerCAD can do that.
Bottom line: TinkerCAD is good for simple 3D models, period. Fusion 360 will do 3D, as well as the 2D-CAD we need for our lathes, mills, etc.
Greg