TinkerCad is nice. All of my high school students love it. I prefer Google SketchUp though which is also free. It does require a few extensions to be added but it is pretty simple. There are lots of YouTube videos on how to use it as well.
Tinker Cad is fine to get a feel of it all. Sketchup is a more flexible program but you need careful choosing of addons to handle stl files easily. The list of free programs are a google search away. The most powerful of which imo is Blender. But the learning curve for Blender is like running into a brick wall, fifty times, after that your good.
Thanks Fella's,
I do have Sketchup pro, had it for things before 3D printing, so what are the extensions I would need.
As for blender I have tried it , but as you say the learning curve is very high, and at my age the brain does not work as well as it used to.
The two extensions I use are the 'Solid Inspector 2' and the 'SketchUp STL' extension, which are pretty much required. Perhaps other users can point out other helpful ones that they use because I know there are lots.
Before you dive in and start making things though it is worthwhile to research how to make solid objects in SketchUp if your not already familiar.
For something that is still simple/easy to use, but more advanced than Tinkercad (which I myself use from time to time), I'd recommend Autodesk 123D Design. It's good for hard-surface\product design stuff. If you're looking to go the more organic route, Autodesk Meshmixer has been coming along nicely, and has hooks directly into 3d printing.
If you want a step up from 123D Design in the hard-surface\product design category, both Autodesk Fusion 360 & OnShape are gaining some traction, and both currently free I believe.
I don't recommend Blender unless you're ready to really 'dive in': While it's the best free 'general purpose cg package' out there, it's not a 'solid modeler' at heart (like the previous software I listed is): Meaning, it's easy to make stuff that won't print without cleanup.
If you want to start paying cash money:
* Hard surface : Autodesk Inventor / Solidworks
* Organic: Z-Brush / Mudbox
* General CG: Autodesk Maya / Autodesk 3DS Max
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