I tried carving the attached stl file. I started from the front; the bowl (concave), carved with no problems. When the cnc got to the handle, it carved the back of the handle, from the front side. I contacted support for my cam program and this is what they told me:
"This simply is an invalid STL file, as the information about which side of the surface is on the inside is incorrect."
They also gave me a workaround.

I would like to know what I did wrong, and how to avoid the problem in the future. Bonus points for a means of detecting the problem, the model looks fine in Blender and MeshMixer.

Thanks,
Steve

Surface normals good for solid bowl.

The handle is not a solid.

- Brian

Hi Steve, the problem is that your object has some open surfaces in it, it isn't a closed solid.

To be able to be sliced by the 3d printer software the object needs to be a solid, where all of its edges are joined between 2 surfaces.

The bowl part is a solid but the handle isn't. You need to connect the handle to the bowl so it's just one combined object.

- Michael

You can use Boolean Union to combine the handle and the bowl.

First get the handle to be a solid with planar ends:


Then select the handle solid but for the bowl only select these 2 faces:


Then you can use Construct > Boolean > Union and it will only cut the handle with those selected faces on the bowl and not the inner face of the bowl. That's called a "subset boolean".

The result will then be a combined solid and can then be processed by the printer's slicing software.


- Michael

Thanks Michael, that's very clear. At one point I did try using Boolean, but did not figure out how to make use of it.
Much appreciated.

Steve

Hi Steve,

re:
> Bonus points for a means of detecting the problem, the model looks fine in Blender and MeshMixer.

To detect this problem in Blender, in Object Mode or Edit Mode go to the Overlays button here:



Enable "Face Orientation" :



Now forward facing polygons will be displayed in blue and backwards facing polygons will be displayed in red:



With a well formed solid object you should only be seeing all blue, if there are any red areas those don't have the proper orientation.

In MOI you should have just one object that selects as one piece and when it is selected the object type indicator in the upper right area of the window should read "Solid" and not "Surface" or "Joined srf".



- Michael

Hi Steve, another thing is your STL is kind of coarse, you probably want to increase the triangulation density.

You can do this when you generate the STL out from MOI by moving the slider all the way over to the "More polygons" side and additionally put in "Divide larger than : 0.2 so anything larger than 0.2 units will get subdivided some more:



- Michael

Hi Steve, you're welcome!

> At one point I did try using Boolean, but did not figure out how to make use of it.
> <...>

You probably got this unwanted piece of the handle coming through? :


The trick to get rid of that piece is to select just the outer faces of the bowl when you do the boolean.

- Michael

Excellent information, thank you.
This spoon is mostly my means of figuring out if I want to buy Moi (yes!). That and me carving it on a cnc router rather than 3d printer made me unconcerned about the triangulation, though I appreciate the details there for future use.