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Filets on polygons - how to do it properly?

hlavachlavac Member Posts: 4
Okay, so I was doing a knob. I made it using 10-sided Inscribed polygon.
To constrain it, I had to give dimension to the inscribed circle, and construct a vertical line to the midpoint of one of the sides to give it orientation.

Now, since it is a 3D printable part, I don't want sharp edges for several reasons (printability, print speed). So I went for the Sketch fillet tool and stared filleting the corners.
Oh boy!
Suddenly the shape is unconstrained again except the one side the dimension is touching.
I found out the sides gained a new degree of freedom in the length department!
I had to go and manually set all the sides to be equal length to make it constrained again.

Is it normal for fillet tool to break constraints this way? Or should I expect it to not unconstrain it? Is this normal behavior or should I file it as a bug?
Should I constrain the initial polygon in some other way that will not be broken by fillets?


  • paul_chastellpaul_chastell Onshape Employees Posts: 121
    This sounds like a bug that we identified a week or so back and which I expect you will see fixed when 1.28 goes live. What we intend, and you should expect, is that adding fillets doesn't remove constraints. Indeed we will transfer certain constraints to virtual sharps to maintain pre-existing design intent. That ought to happen in this case: that it didn't is a bug.

    It is the pattern constraint that is being lost: you may find that another set of constraints will survive the fillet and maintain your intent. 

    In general I would recommend contacting support with such cases that you believe ought to work. I would also suggest you do so with this case so that we can be sure it is indeed resolved. I'll post again shortly with our internal bug number so you can share that with support and help us join the dots. 
    Paul Chastell / VP of R&D / Onshape Inc.
  • paul_chastellpaul_chastell Onshape Employees Posts: 121
    If it is the same issue as the one we fixed then I expect you will find that the corner with the problems is adjacent to the line that has the midpoint and that the pattern constraint is still present after the fillet. If this is the case it may be that you can improve the behavior here by deleting the midpoint before applying the fillet. It is also possible that this issue is not the same, and if it isn't I would like to track it down and fix it. If you do contact support, please refer them to BEL-18411.
    Paul Chastell / VP of R&D / Onshape Inc.
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