Fusion is great for creating toolpaths for large STL files, but before you start your CAM you must reduce your file size as much as possible to make the process smoother.
Step 1: Reduce in Mudbox
Ideally your geometry should be reduced as low as possible while still maintaining all your detail. If you reduce the poly count too much the faceting can appear in your final surface when milling. I usually like to look at the object on screen at its final size. So, if your model is larger than your screen size, choose a section and zoom in to it to size. Turn on wire frame (W) to see the polygons.
Step 2: Reduce Mesh
In the menu at the top choose mesh, reduce mesh. Set your mesh reduction. I like to go extreme at first so let's put in 90% and see what it looks like. You will be surprised to find that the file still looks great even though you just threw out 90 % of information. If your satisfied you can move to the next step otherwise try some other settings to see what they look like.
Step 3: Exporting an OBJ
Now that you have your reduced file, let's export as an obj to then import in Fusion. Select your object, file, export.
Step 4: Decimating in Zbrush
In Zbrush you will need to use the Zplugins folder. In the Zplugins folder you will find a tool called Decimation master.
Step 5: Wire Frame
Turn on Poly Frame (Shift + F)so you can see the polygons change from quads to triangles
Step 6: Decimation Master
In the Zplugins folder, Decimation Master choose pre-process current. Pre-processing prepares your file for decimation, this is a great feature because once pre-processed you can now change different settings on decimate to view the mesh at various densities.
Step 7: Decimate
Adjust the slider under decimate to choose the amount of reduction on the file, then click decimate. Toggle Shift +F (poly frame) to see what your geometry looks like. Make sure to zoom in to actual size to evaluate the surface.
Step 8: Export an Obj
Now that you are happy with the amount of decimation, export the file as an obj under the ztools menu.
Your files are now ready to upload into to Fusion for CAM. Some files may need some additional editing Nettfab is a great program for this. If you had multiple parts like the Escher Hands, it may be useful to stitch those together. I find that you can further reduce the file size by doing those, sometimes as much as half. Once again this all pays off later when you are manipulating the files in other programs.