Vacuum Adapter Question.3dm (835.3 KB)
I need to fillet the inside corners of the groove. I am a real beginner at this! Looked at the other questions about fillets and they are so complex. I feel a little silly asking about something that looks so simple.
Hi Kurt,
welcome to the Discourse and into the Wild adventure in discovering how Rhino’s live. 
Here’s my file with both solutions: _FilletEdge and _ChamferEdge.
For 3DP is always better to use a Chamfer instead of a Fillet to improve the quality.
Vacuum Adapter Question.3dm (1.2 MB)
Following the steps you have to use to achieve them.
I hope this help you.
Riccardo
If you’re new to 3D surface modelling, I’d lay off the “joining” and “booleans” and “fillet edge” right away and use FilletSrf which works just great. Fillets are transitional surfaces between surfaces, they have nothing to do with “edges”. That said, your surfaces have a tangent discontinuity, because on one side, your inner surface is bent, curved, hence the fillet arrives in another location, and you can’t just have a revolve like on the other side (or a miter corner). Is this bent surface intentional, or an error?
For a miter corner, you could extend the vertical fillet and cut it with an extruded surface along the miter line, where you have the tangent discontinuity. Then you can build a Sweep2 that is tangent to the two groove surfaces. Then I’d remove the revolve on the opposite side and just have a miter corner there, too.
Yes, the one side of the groove slips over a very slightly curved edge for a better fit.
Then you could use the second suggestion.
