Cura thinks this flat surface is 2 surface.

[edta1 0]

I'm trying to print the attached sizing ring.  It should be a simple print.  It's just a ring.  Top and bottom should be flat.  However cura slices it and thinks the model isn't flat.  

 

 

 

What happened here?  What can I do to have cura slice it correctly?

Sizing.stl

[Dustin 160]

UltiMaker Cura thinks the model isn't flat, because the model isn't flat.. 
The red areas are because they need to be supported.
Its model isn't flat 

[edta1 0]

No way...I even used fusion 360 and cut off 1mm from the bottom just to get it "flush" and nothing changed.  I activated the tree supports and cura doesn't show the supports either.  

 

FWIW I'm using Cura 5.3.0

 

Thoughts and comments most welcome.  Thanks.

 

[43915 19]

If it's not flat and there's only one layer that's off it will not show any supports anyway, since there's not enough space for it. To put it into perspective, supports usually leave one "empty" layer between the support and the print, to prevent them from sticking together too much. If you have a one layer gap between the build plate and some areas of the print it won't be putting any supports there because that one layer gap would be left out between the support and the print anyway.

 

It's most definitely not flat, I've downloaded it and checked as well.

Edited August 9, 2023 by 43915