Ultimaker Cura is adding redundant top/bottom layers I'm trying to print a supporting base which will house the spindle for an electrostatic rotor. It's basically just a truncated cone with a hole down it's center to house the spindle.

For reasons that I cannot fathom, Ultimaker Cura keeps on adding an unrequested top/bottom layer (color-coded yellow in the screenshots) inside this hole, so instead of a single hollow cylinder of 10&nbsp;mm depth, the result is a hole only a few millimeters deep with another hollow cylinder behind it.

Here is the intended model, note the open space for the hole at the top.



Here is the inner view of the hole being printed as expected:



Finally, here is a layer view of the print a few millimeters from the final top layer with the unrequested top/bottom layer that covers the spindle hole:



The STL file is on Github (with a built-in viewer).

Can anyone help me understand why this is happening?
It would appear that your model does not conform properly to STL standards. I base this conclusion on a couple of factors. When I loaded the model into Simplify3D slicer, it displayed fine, but when sliced, displayed nothing. Using the onboard repair feature, it presented the entire model as being composed of non-manifold surfaces.

Meshmixer's Analysis/Inspector feature also highlighted the entire model as flawed.

Another observation is that there is an extraordinary amount of facets/triangles/faces to this model. Nearly three-quarters of a million triangles for something that should be much simpler.

The most recent version of Prusa Slicer 2.0 presents an error message indicating that no layers were detected. This is peculiar indeed.

All of the above points to a problem with the source file or the software used to create it.

Please consider to add to your post the program you used or the source of the model.