Hi,
Any guidance to why ExtendSrf successfully “shrinks” three sides of my polysurface with an End osnap, but not the fourth one ?
I tried exploding, rebuilding edges, shrinking, nothing works.
I guess I’ll have to trim it with a line 
FailExtendSrf.3dm (285.0 KB)
is this, what you re after ?
some additional tools that might help:
_showEdges (in the screenshot, will show if edges are fractionated )
_splitEdge
_mergeEdge
_untrimAll (KeepTrimObjects = yes) … work on the trimmed curves
_simplifyCrv
_planarSrf
for a 100% orthogonal part
_split Isocurve shrink = yes
and yes - draw a line, _trim , optional _mergeEdge
my guess: rhino does not like that shrinking a trimmed surfaces will result in the loose of one additional edge - therefore change the typology… this might be combined with some microscopic tolerance issues …
Hi Tom, thanks for your answer.
Yes, I’d like to trim that excess portion that for some reason FilletEdge refuses to remove.
I wasn’t 100% clear, sorry. This is meant as a beginner exercice for a Rhino training, so I’d like to avoid at all costs all those workarounds you mention (and which I’m aware of).
So if I sum up this, if by any chance some fillet gurus like @pascal read this :
I would LOVE a command option for FilletEdge to choose if the fillet is minimum, maximum or smooth from one edge length to the other. Rhino is capable of producing the correct “smooth” fillet with FilletSrf and the Extend=Yes option, but this is for a single edge at a time, why FilletEdge can’t ?
Since this is never going to happen untli the next time the Halley comet is in our sight 
, I would definitely expect ExtendSrf to work here, edges are perpendicular straight lines ! It “works” on 3 cases, but not the fourth for no reason, even lowering the tolerance.
And it leaves that nasty 0-length edge, which I saw and thought… yeah, no one will notice 
. But neither RebuildEdges or MergeEdge can remove it anyway, so untimately I have to abandon this method, it’s not clean enough.
I’m going to go for 4 FilletSrf’s then FilletEdge then (!) delete the extra pieces it misses to remove 
, but I’m already hearing the complaints of the students that this is convioluted, and they would be right.
Prosecution rests.
Dear @magicteddy
I think it s a quite special usage-case for _extendSrf
(shrinking a planar trimmed surface and changing the topology from “L” to “I” - while the surface is part of a polysurface)
and therefore not a good learning example
for a beginner course I would recommend to draw the required line and use trim.
(at least for the first corner)
this is much closer to the concept of trimmed surfaces that students have to understand.
and a more general idea of how to get trimmed planar surfaces - the trim curves have to be drawn.
as a further step - show that the line is not needed:
_trim
I think this workflow is a nicer learning-outcome.
(understand the option of trim and the general concept of trimmed planar surfaces)
…and less questions that will get you of the learning path…
kind regards -tom
I really like the workflow with Extend cutting lines, never paid attention to that option.
Sub-object selection is also a good thing to practice, especially with the upcoming PushPull, so that’s a great use case.
Thanks a lot, and now let’s wait until Halley returns and we get better fillet tools 
@magicteddy Another option that you can use is to Explode first and then use ReplaceEdge with Mode=ExtendSideEdges, dragging around the two edges to remove.
Anyhow, the fail is noted, and seems to fail on all the extensions in V8
RH-76287 ExtendSrf fails