Sketch fillet breaks constraints

[gouach]

Hello,
i'm running freecad 0.18-16110 and i try to fillet a rectangle whose size is constraint by distance constraints on its edges. I've noticed that when i use the fillet on this rectangle, it makes the edges disapear and thus the constraints on the size of the rectangle are broken (you can see that as you change the fillet radius, it changes the size of the rectangle).

Is this a bug or I'm i using freecad badly.
thanks for your help.
best

[chrisb]

Hi gouach, welcome to the forum.
This is a known issue. The point where the dimension is hooked to is removed and with it the dimension constraint.

[quick61]

Funny, but I had always looked at it as almost a feature. When one applies a fillet to a constrained sharp angle, FreeCAD has no idea what the radius of the fillet is going to be, so the constraint must be removed to accommodate it's placement, otherwise the radius would need to end up infinitely small in order to maintain the preexisting dimension, would it not?

Not arguing, just an observation from another perspective. I have always come at Sketcher as something that one first "sketches" out the complete rough shape, then applies the dimensioning to constrain. If a fillet is forgotten in a corner, FreeCAD is kind enough to apply it, (the fillet), and remove the constraint(s) that would otherwise be in the way. After all, how is the program suppose to know whether or not one wants a constraint on the dimension that comes before or after the fillet radius?

Mark

Oh, I say "FreeCAD is kind enough to apply it, (the fillet), and remove the constraint(s)" as sort of a personification. The constraint is lost because the coincident that it was attached to is removed/deleted and 2 new coincidents are created. With the law of averages on our side, if FreeCAD were to guess which new vertex to attach the preexisting constraint, 50% of the time it would get it wrong.

[HarryGeier]

idea:

sketch on middle plane of the element, constraints will stay when upper/ower faces get fillets. If the fillets need to be constructive and parallel to the plane of orign, use sketchers fillet instead of part designs fillet ( obviously you need constructions lines then )..which..could be the solution anyway..i will make a try..

it works, The referencing lines endpoints are not influenced by the fillets. The real edges are just constrained as beein vertically/horizontally to the helper lines

[chrisb]

In the case of a rectangle with horizontal/vertical lines there could be a sensible replacement for a removed vertical or horizontal constraint. But as soon as it is tilted it becomes clear that this is no longer possible.

So Mark is right, a general strategy can only be to delete the constraint; thus it is rather a feature.

Before filleting:

Bildschirmfoto 2019-05-09 um 23.04.03.png (10.44 KiB) Viewed 1826 times

.

After filleting, where should the constraint go?

Bildschirmfoto 2019-05-09 um 23.04.56.png (9.67 KiB) Viewed 1826 times

[openBrain]

IIRC in SolidWorks, the trimmed line segments are replaced by colinear construction ones so constraints are kept.
In FC world it would give something like that (left before filleting, right after) :

filleting.png (12.27 KiB) Viewed 1820 times

However this isn't IMO the worst case as it is understandable that a dimension constraint can be lost.
What I find a pity is that the sketcher isn't able to "explode" a point-point perpendicularity to keep it when filleting.
With pictures : in the case of a coincidence + line-line perpendicularity => perpendicular constraint is kept when filleting

filleting_llp.png (6.2 KiB) Viewed 1820 times

In the case of a point-point perpendicularity => perpendicular constraint is (unfortunately) lost

filleting_ppp.png (4.58 KiB) Viewed 1820 times