As promised, here's some more complex ways of cutting models up in Fusion 360. I'm going to demonstrate with a Nova-class frigate. This ideally needs the use of the STEP files, which are much easier to work with than STLs. STLs store geometry as triangles while STEP files store the geometry as faces forming a body. This means when you import it you get a lovely clean model to work with. I must admit however that TinkerCAD does a pretty spectacular job of making STLs easy to work with.
You can get the STEP file from https://cults3d.com/en/3d-model/game/space-marine-frigate-1 or from https://www.thingiverse.com/download:7980881 if Thingiverse is playing ball. Next import it into Fusion 360, or your CAD software of choice. The steps are also shown in the slides below:
As you can see, the imported STEP file looks identical to the original CAD image. This will make life much easier! Looking at the model, all the overhangs and wings will need a lot of support. This will make printing it very difficult, especially in FFF. Trust me, I tried! Eventually I got the best result by cutting the wings and antenna off, and slicing the main hull in half.
The tools we'll need are lines in sketches and the split body tool. Split body lives under "Modify" in the design space. You should also enable recording the design history, right click on the root of the file in the browser and select "Capture Design History". See the slides above if you're having trouble finding it. As you can see from the image of the sketch, I've put lines at the best place to cut the wings and antenna, and also down the centre of the model. Let's take a look at each in turn.
The cuts for the wings and antenna are aligned with what I want the bottom surface of the prints to be. There is a little detail on the inboard side which I don't want on the main hull. The hull lines also give some way to locate the parts when gluing the printed model together which makes life easier. The hull-dividing line is a litte more complex. It's best to refer lines to the geometry so if the model gets moved the lines go too. I've added a pair of construction lines, locked them to a convienient bit of hull, and used an equal constraint on them to make sure the line is down the middle. You will also probably need to combine the turrets with the main hull (Modify -> combine, select the main hull and then drag a selection box over everything).
Now you can start splitting the body up. To prevent slicing up the wrong parts uncheck the "Extend Splitting Tool(s)" option. This limits the cut to the length of the line. The first time you do a cut it'll hide the sketch, you can make it visible again under sketches, click on the eye icon in the browser. Once you've split it up the models you can use the "Move" tool to spread the parts out a bit. Save each part as an STL for printing by right clicking on the body in the browser (not the root) and selecting "Save as STL".
Now you can insert it into your slicing software. Don't forget to add in mirrors of all the parts. Printing them out looks like an old model kit! A bit of careful knifework on the brim and it goes together beautifully.
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