Some considerations about Part Design Fillet.
Fillet is a useful tool, very agile for certain operations, but subject to certain limitations.
The examples are based on a cube 10x10x20.
First case. Fillet on an edge.
In appearance this tool acts on an edge, but in reality creates a curved surface that joins two other surfaces. Therefore it uses two faces as references, for example the side face and the upper face of a cube. At the edge that separates them can be applied a rounding with a radius equal to the limit dimension of the smaller face less a "residual safety value" of 0.0001 (according to the value of decimal set). The "security value" serves to maintain existing a small surface, in this case large 0.0001.
When one of the two surfaces that are connected disappears completing the operation fails because it lacks one of the references used by the function, and the result is unpredictable.
Second case. Fillet applied on two opposite edges of the same face.
In this case, there is not a limit radius, but the limit is (R1 + R2 <thickness-0.0001). The limit is valid if R1 or R2 are not superior to others thicknesses involved in the operation (eg. The height of the cube). The "residual safety value" is positioned at the junction of the fillets.
It is recommended to use this function only when modeling is ended, to eliminate "sharp corners", and instead use alternative tools to produce the model.
Alternative methods of rounding.
A simple method to create the rounded surfaces in a continuous manner, without the "surface residue" is, for example, to create a "cleaner" sketch on the front face as in this figure
and then apply a Pocket -> Through all. It returns a completely clean solid, without any interruption of the rounding surface. Adequately shaping the sketch you can create any profile, even eccentric.
Even the Boolean operations allow to obtain the same result, but the dependence of the elements is different.
Since version 0.16_pre 6692 FreeCAD has been equipped with an extra check if fillet and chamfer operation returns valid shape.
Lines created by a PartDesign Fillet can not be selected for an additional fillet or a chamfer.
PartDesign Chamfer has the same behavior.
Lines created by a Chamfer can be selected for an additional chamfer or fillet.
PartDesign fillet and chamfer on tangent edges..........
Part .......... many things are similar.