It was a Siemens employee on their forums, but I don’t know if it was in the non-public section or not and their search is unfortunately so bad that I’m afraid it will be impossible to find it. 
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Personally, I always pipe trim this stuff and use blend surf -
If I was being super picky, I’d extract isocurves to match my blendsrf shapes to and dial them in very closely. upside is you can do g3 or g4 is you want to get crazy this way…
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Many Rhino users underestimate the power of “Loft” and its "Loose"option. It’s a great way to achieve clean surfaces with the minimum amount of control points and G1, G2, G3, G50 or whatever “G” you want, as long as there is a way to place profile curves at the right locations.
In this example, I extracted some isocurves from the cone surfaces, then I applied a new CPlane where they both intersect with each other (check the saved CPlanes to find the added Cross plane) and built the white ellipse. I skipped the blue curves to create this particular lofted surface, but you can use any combination to achieve different results. If you enable the History recording, you can also slide the extracted isocurves to modify the shape of the lofted surface afterwards.
Rhino 7 file:
simple variable pipe Bobi.3dm (1.1 MB)
Extra examples of solving tricky transitions with “Loft”:
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I think that so many users have a hang-up about fillets is because in most other CAD packages (and I’m including Alias here, so not just parametric solid modelers), it’s very easy to add, change and remove fillets whereas in Rhino, it’s an extremely time consuming hassle.
Also, fillets are quite often used in mechanical design whereas lofts are used for aesthetic design. With Push/Pull being a thing in V8 now, hopefully the next logical evolution are fillets which can adapt to Push/Pull changes. And then if the fillets could even survive the drafting stage, that would be heaven! 
But then they’d finally have to do something about this (which would again benefit us all)… 
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@Rhino_Bulgaria we could do that with Rhino 4 or earlier…
If we show we are happily able to use workarounds, we are sending a message opposite to this thread original post. (maybe look again the first post, no quick workarounds for that…)
Fillets. Not workarounds.
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I’d love this workaround, and I’m using it all the time because for me fillets is where I can add my designer touch against cad monkeyes.
But the question is: if is so simple to do fillets this way, why there isn’t any scripted/embedded official command to add fillets this way?
As shown before the Railtype isn’t working in this situations where it should be the more logical and simpler way to solve.
@Gijs, any hint?
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This is an amazing tricks but it has less control on the final shape and the outer radius results in a much rounded shape
Alternative solutions are a clever ploy to make Cad software appear to be what it is not: limited.
In my opinion, in Rhino, the resources on fillets are really small, inconsistent. I think it’s the company’s (McNeel) choice, otherwise, after decades of development, we would have had more robust and effective solutions.
For each version of Rhino they fix a few dozen cases and it goes on like this, nothing more. To have fillets like those of Plasticity (to name one at random), we would have to wait for the release of Rhino 50! (we are still at version 8, I would like to remind you…).
The road is still long and tortuous.
The input curves could be modified (either control point edited or replaced by rotated ellipses) to create a relatively the same blend radius along the area where both conesnlm intersect with each other.
I’m pretty sure that the Rhino developers are aware of the purpose of this topic. Some of them even showed workarounds above. Me adding yet another workaround is just a reminder that Rhino users who are stuck with difficult transitions could utilize various techniques to solve problems that the current Rhino 8 can’t. Not to mention that in the majority of cases, the Rhino users rely on fillets, because they are not aware about the alternatives.
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About others… I don’t know that. I can’t know that.
I myself know well the alternatives… but when I have to make a fillet, I think “should I try my luck with _FilletEdge ?” … and often I don’t even try (losing 3 seconds + undo) because my models are so “complex” for rhino that the fillet fails 9/10 times.
Sorry about the harsh response but Rhino and Rhino devs bounce too much around workarounds and half-solutions. It feels like it’s deviating from searching a real solution.
Even the last example with the 90 degree cone. It’s a single surface meeting with another single surface! Plenty of space for a fillet, no strange intersections.
I can understand (maybe?) rolling ball failing … but EVEN distbetweenrails fail! (which should be easier…?)
So
- _FilletEdge fail
- 1st alternative is distbetweenrail, still fail
- 2nd alternative, manual work, but the fillet surface still doesn’t reach the flip-flop between concave and convex, so extend+FitPt… trim… ok, done!
I think sometime we forget since how many years we are modeling and we use our experience as workarounds. We became used to use workarounds!
And we use the same “tricks” since… 2005? Probably earlier for many…
Sometime I would also like simple things to just work.
I was curious about other software in this simple case because… are there any other software that fail to do that fillet? Only Rhino fails it?
Please someone find another…
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FWIW, part of the issue here is that running pipe in an angle doesn’t produce an accurate enough result. If you create two lines of 20 and add a pipe from 5 to 1 to both, then intersect, the resulting intersection is clean and FilletSrf works without any manual intervention:
FilletSrf.3dm (311.5 KB)
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If you give me some example cases to work with I can see what can be done. Pipe trims need to be used with care as well, the resulting pipe is not the same as a constant distance fillet depending on the input. Maybe something can be done with Inset / OffsetCrvOnSrf. Often pipes also need to be extended to be able to trim.
Not for the other case. You found a case where it works… ok…
EDIT:
@Gijs what i’m doing wrong here?
8.8.24170.13001, 2024-06-18
And this is still just surfaces, the real deal is having it work as solid, FilletEdge … 
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I think what @skysurfer really is asking is: why is filletedge/blendedge with the “distance from edge” or “distance between rails” options not working (given that the respective manual workarounds are fairly trivial)
I hope I am not interpreting @skysurfer wrongly.
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I think that if there is no realistic prospect of there ever being a re-work of the fillet backend, it would be great if Pascal’s ExtendFromMid script was at least integrated into pipe as an Overshoot option.
It’s a little quality of life improvement, but it would be an extra step removed from the intersect procedure.
hence my fwiw 
looks like the seams on top as in my example works better
Hi everyone,
I haven’t been using Rhino since 2005 like some of you guys. I love how everyone is trying to find new workarounds. However, for new users, these workarounds can be very hard. I hope Rhino 9 can fix all of these issues without the need for a workaround.
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I vote for _PascalSuperFillet command. 
Jokes aside, from what I read in the other topics in this forum, it looks like many Rhino users still ignore the ! _FilletSrf command and instead rely on the flawed ! _FilletEdge counterpart.
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I had a feeling something was wrong with the input surfaces. FilletSrf is usually able to accurately handle making fillets between two cones even when the result is a pointy fillet.
Yes that is what is missing in this discussion → a view of the big picture…
Who gets paid to model just 2 surfaces connected by a transition?
What “downstream” means for me is that there is almost never ever just 2 surfaces connected by a transition surface. There is always more that has to meld well with those initial surfaces.
The majority of proposed solutions for this transition are just not going to hold up to any additional downstream work. With the possible exception of Bobi’s Loose Loft the other proposed solutions are just not accurate enough to hold up when things get more complicated.
Here is an example of what I mean:
FilletSrf_pipex.3dm (4.2 MB)
Even if by some miracle you manage to make all the transition surfaces with blends, the shell is almost certainly going to fail. With FilletSrf its all quick and easy and accurate.
Edit: Its important to note that you don’t need to trim or join the two cones to get FilletSrf to work. The problem with not trimming the cones is that FilletSrf will give you multiple fillets, most of which you don’t need.
If you make the cones shorter, it is easier to make the fillets and choose the ones you need.
choose_fillet.3dm (153.2 KB)
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