ThePirateCaptain's Official Advice for importing STL files:
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1. Make absolutely sure you are starting with an unmodified .stl that is untouched by F360.
2. Import the .stl file to the cloud.
3. Immediately, before you do anything else, change the mode to model (It will be in Sculpt by default, I believe)
4. Immediately, before you do anything else, select the new components in the browser, and mouse down on it. Select convert to prep.
5. Check the units and size. Weird things happen all the time here.
6. TEST that you can make an important edit, e.g. change the appearance or material.
7. Save this file immediately!!!
8. You can import into another design or start a new design from this file. (On Mac, ctrl-click on the design listed in your project folder in data pane).
* The tricky part here is that when I did not do these things immediately after import, I was never able to convert or edit them.
* This advice is appropriate for people who work in Model mode and design with simple geometry but want to import something simple, like simple geometric primitives from OpenScad. For those of you who sculpt and do meshes, I have no idea about these things, they are mysteries for me.
Comments
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Complex meshes, e.g. from scan data or stuff you pull off the web at random can be too complex for conversion to prep. You will get an error message about "too many [faces or triangles or something, I forget which]", and it will warn you that you are going to get 1e5 or 1e6 and that's just a mess. HOWEVER, I got a warning on some of my imports and they worked fine anyway. Try it out, be prepared to give up and kill the process if it takes too long.
There are many youtube videos on how to edit the surfaces of the converted brep objects. You may want to do this to proceed. (basically, they just delete lots of the lines (erm...whathcamacall em .. face boundries? they are lines so they can't be vertices) in the faces, and usually that turns out ok, sometimes not, so you have to go line-by-line)
***Importing stuff into F360, moving files back-and-forth is a huge weakness in CAD and 3D printing and scanning. Hopefully you people out there smarter than me will build on this to write much better, more widely applicable documents, and hopefully the F360 folks will improve this situation with more converters.
2016 Dec. 07-09 I believe some of these features simply did not work correctly due to problems with the cloud. It could be worth waiting a couple days if things are acting unpredictably.
-TPC