# Trimming an Object That Intersects Itself (?)

Hi everybody,
I’m currently working on this “Infinite Loop” ring on which i’m unable to trim the intersecting parts on the infinites. I made the loops using the “Sweep 1 Rail” command so everything is basically one big surface. Is it possible to somehow trim a surface with itself?
I don’t know how to proceed from here.

IL1.3dm (3.3 MB)

Hello- for the purposes of building it it may help to just split it into pieces.

-Pascal

Some of your surfaces are inside-out. When I split the figure 8 surface in its intersection, I got over 2000 surfaces. It seems that you have to make new figure 8 surface in several parts. I would start with curves defining the intersection.

Hi @pascal,

I don’t think that is sufficient in this case. If you do that and then try to trim out the intersection you get some really nasty surfaces, like @andrew.nowicki says. I’d speculate that this has something to do with the fact that your split pieces carry the baggage of the common untrimmed surface.

Splitting the single figure-of-eight surface, radially near the widest points, and at each end, allows you to copy the profile and edges to make six new surfaces using sweep2’s. The cross-sections should be rebuilt first - 12 points works well for a trivial deviation. The shallower crossing piece will trim the deeper one cleanly, Untrimming the top surfaces will give two clean intersections with the new surface which, in conjunction with the edges from the previous trim, will give a clean trim of the deeper piece. The six parts can then be joined to create a complete surface.

IL1 001.3dm (5.2 MB)

HTH
Jeremy

Fillet Edge opens my closed polysurface! - Rhino / Rhino for Windows - McNeel Forum

I mean something like this -

Does that make sense here?

-Pascal

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Hi @pascal,

Unfortunately, after splitting out the intersection (on this occasion along isocurves), when you trim the higher of the two channels with the lower and its associated top surfaces, it fragments thus;

(n.b. Top surfaces hidden in this view)

IL1 split trimmed fragmented.3dm (7.3 MB)

Regards
Jeremy

Yes, I ended up using this very method to solve the problem even though the surfaces don’t join anymore…maybe because they became trimmed surfaces(?)

Hi Jeremy - you need to shrink the split surfaces.

-Pascal

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Thanks, helpful to know. If you can afford the time, any chance of of a layman’s explanation why the unshrunk surface fragments and the shrunk one doesn’t?

I have not dug into that yet, but I will - it looks somewhat buggy to me…

-Pascal