Question about ShrinkTrimmedSrf

What is the difference between ShrinkTrimmedSrf and ShrinkTrimmedSrftoEdge,
and which one should i be using on my surfaces

Thanks in advance


Hi Richie- ShrinkTrimmedSrf shrinks the surface to any non-isocurve edge with a small margin- the idea being that arbitrarily shaped trim curves, being only accurate to tolerance, might conceivably fall outside surface it is trimming, microscopically, and that can be bad, so we leave a margin. Occasionally you want the full shrinkage, and ShrinkTrimmedSrfToEdge takes care of that. Either shrinks to edges that are isocurves, with no margin since that is not dependent on tolerance. In general there is no need for ShrinkTrimmedSrfToEdge.

ShrinkTrimmedSrfToEdge.3dm (57.9 KB)


Hey Pascal,

I wonder if you could give us some tips, on when it is worthwhile to run either ShrinkTrimmedSrf and ShrinkTrimmedSrftoEdge? Does filletedge work better with it?

I would appreciate it.

Thank you.


Hi Cosmas - in general, I would avoid ShrinkTrimmedSrfToEdge. Shrinking can be useful for a few reasons - as you may know, trimming a surface does not remove the part of the surface that is trimmed away, it merely hides it from the user. If the underlying surface is a lot larger than the visible trimmed face (I don’t know what minimum ratio is a red flag exactly, but certainly 100 to 1 is something I’d pay attention to), it is possible for some operations not to work as well, including, sometimes, filleting or intersecting. In general, even though it should work regardless, it is best if a trimmed face is not too much different from its surface.

Apart from that, you may save a fair amount of file ‘weight’ or complexity, especially with dense surfaces, if you shrink them down - you can jettison, potentially, a lot of surface data that you are not using.

And, if you need to use MatchSrf, it is often possible to shrink, then untrim a surface and match it- (trimmed edges cannot be matched) where the original underlying surface’s natural edge might be way too far away from the target edge to match.


Hey Pascal,

While we’re talking about this, is the only reason that surfaces don’t automatically shrink when you trim them, so that you can have the option of untrim them to their original shape-- or is there some other “maybe cryptic” reason for it?

Thank you.




If that is true, Pascal, why is it that you can’t untrim surfaces you have applied a fillet to (or chamfer)?


Hi Cosmas- You can untrim if the surface is not joined to any others… right? (Explode or ExtractSrf)


Hi Cosmas,

In fact you can; however the trim before the fillet is not remembered.
Meaning that if I have a trimmed surface that I trim even shorter (my means of a fillet of chamfer), the base surface gets a new trim curve. However, the previous trim curves are discarded.
So if you then untrim the surface it will not go back to the trim before the fillet, but to an untrimmed state.

This of course does not apply when the surface is not rimmed before the fillet. like in a box object.

And like Pascal points out when the fillet or chamfer in question is set on a polysurface there are corner situations to take into account and it immediately starts to get complicated.

I have heard rumors though that in V6 there will be a feature to “Unfillet” and “Unchamfer”…
We just have to wait for the first beta to be released (rumoured to be about a week away(and that was 5 weeks ago :wink: )

Thank you both.