difficult to fillet

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 From:  wimverbe
6986.1 
Hi,

I am trying to fillet a quite simple shape, there are no intersections that make the filleting difficult, and yet it is impossible to make a larger fillet than 0.7 cm. even that, after 3 of the 8 edge sets MoI refused to do the rest properly. I need a fillet of at least 1cm.
any suggestions here?
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 From:  bemfarmer
6986.2 In reply to 6986.1 
It seems to be trying to fillet.

I tried this simpler joined surface. Maximum successful fillet radius was 6.87.

Scaling up by 10 made a slight improvement.
Scaling up by 100 was much worse.

- Brian
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 From:  Michael Gibson
6986.3 In reply to 6986.1 
Hi Wim, it seems to be having some difficulty generating the fillet surface in these areas here:



But I don't see any particular problem in the shapes themselves that should be problematic, so I think it's some kind of bug in the fillet processing.

Since the cuts are symmetrical and fillets are being generated on other areas (once I replaced them with some analytic planes using Construct > Planar), it should probably be possible to mirror one of the ok fillet pieces and then manually trim the fillet into place, I'll see if I can do that for you.

- Michael

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 From:  Michael Gibson
6986.4 In reply to 6986.3 
Hi wim, it looks like it will not be as simple as mirroring some of the generated pieces to make the missing fillet pieces, I will take more of a look at this but I won't be able to get to it until a ways later today.

- Michael
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 From:  DannyT (DANTAS)
6986.5 
Seems to work out with a 0.9cm 'Const Dist' fillet.

-
~Danny~
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 From:  Michael Gibson
6986.6 In reply to 6986.1 
Hi Wim, I've been trying to fillet your same shape in some other CAD programs and they seem to have problems too getting a fillet radius of 1 unit there.

I think it's related to a bit of a tighter than you'd think amount of curvature in the bottom semi-circle shape (screenshots here from Rhino):



That semi-circle like shape is only an exact circular arc on this left part here:



On the right part it's a general curve and right near the end where the control points are kind of close together, the curvature of the curve in that end spot is actually 0.7 units, a fair amount more tightly curved than the rest of the curve:




I had to measure it with the Curvature command in Rhino to really see what the curvature at the end was. This tight curvature at the very end is what is likely limiting the fillet radius as well, because just in general if you try to offset something (fillets involve offset surfaces as part of their calculation) of a distance that is greater than the radius of curvature at a particular spot, it results in a bunching effect like this:



So anyway I think it's this that is causing your fillet difficulty, a kind of hard to see tightening of the curvature right on the back end of the bottom semi-circular shape.

So probably the easiest way to solve it is to make the bottom an exact semi-circle rather than half circle and half general curve, that will change the shape somewhat is that acceptable for what you need?

I have attached a 3DM file with a version where I have done that - replaced the bottom part with a revolved exact semi-circle, this version should fillet for you at radius = 1 without any problems. Again, note that this is a slightly different shape in the bottom rounded area than your original model. If that existing shape is important to you, moving that second control point in the current tightly curved end just downwards by 0.15 units is enough to make the curvature over 1 unit at the end of the curve so that would be a pretty minimal change.

- Michael

EDITED: 20 Oct 2014 by MICHAEL GIBSON


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 From:  wimverbe
6986.7 
it must be something at my side, I tried with your file Michael, and it works splendidly. but if I generate my own perfectly circular shapes, it fails. I even simplified the shapes, I use quarter and half circular arcs only, but still get no further than .6 cm.

o, btw, I just discovered these answers now and by accident, the number after my title in the left column of the forum in my browser (firefox) still said [1], which made me think that there were no answers yet. I opened my thread by accident and saw all these entries. strange...
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 From:  wimverbe
6986.8 
screenshot of browser
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 From:  wimverbe
6986.9 
Michael,
I just saw that V3 is officially available. could it be that it is some instability of my beta jun-27-2014 version?

I'll upgrade this week.

wim
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 From:  Michael Gibson
6986.10 In reply to 6986.9 
Hi Wim,

> could it be that it is some instability of my beta jun-27-2014 version?

No, it doesn't seem to be related to that. And I can't see anything wrong with your object, this one looks like a fillet bug in MoI. I am able to fillet this one in other software, I have attached one version here with the fillet completed so you can have it finished up.

re: forum in web browser - the forum does not automatically refresh so if you've had the same page open for a while that's normal that the left pane will stay the same, you can hit F5 or click the "Discussions" link on the top bar or also the "Topics" text at the top of the threads list to get a refreshed topics pane.

- Michael
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 From:  Michael Gibson
6986.11 In reply to 6986.9 
The fillet bug itself is rather mysterious I'm afraid, it seems to be some failure in dealing with the inside vertical wall against the curved top arch.

- Michael
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 From:  wimverbe
6986.12 
Thanks Michael, I'll use your file for the moment. this job is just for illustration, no accuracy required ;-)

rgds
wim
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