Is this a bug? PartDesign + Fillet + Round

[ChemicalCoal]

Starting with the following model:
15mm cube, with R5mm circle extruded off of it, and a 2mm fillet.
Applying a fillet to the outer edge has some unexpected results.


2mm round (sharp corner round)


3mm round (rounded corner round)


5mm round (rounded corner round)


8mm round (??)


OS: Windows 10 Version 2009
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.24276 (Git)
Build type: Release
Branch: releases/FreeCAD-0-19
Hash: a88db11e0a908f6e38f92bfc5187b13ebe470438
Python version: 3.8.6+
Qt version: 5.15.1
Coin version: 4.0.1
OCC version: 7.5.0
Locale: English/United States (en_US)

[mendy]

Your Fillet001 is too large and runs into Fillet.
Works with 0.5mm for Fillet001

[ChemicalCoal]

Your Fillet001 is too large and runs into Fillet.

I understand that it is the contact between Fillet and Fillet001 that causes this to happen, but the 5mm fillet should be possible, and the 8mm fillet should error out.

By adding the 3rd edge, I can get fillets up to 7mm to work, so I don't think the issue is the 5mm fillet is too big.


I think that the issue is the tool switched from a sharp corner round to a rounded corner round when it shouldn't have. I'm not sure why the round doesn't error at 8mm (which should be an error) but errors at 7.5mm (I think this is correct) and 2.6mm (I think this shouldn't be an error, and this is around the point where it switched from sharp corner rounds to rounded corner rounds).
This is an example I found online.

[chrisb]

The limitation mentioned by mendy comes directly from the OCC geometric kernel. It is known that fillets have their limitations, this is one of them.