Sweep spiral self intersecting, trim away inside

Hi all,

i’m trying to sweep a circle around a flat spiral like curve. My aim is to create a self intersecting solid and somehow trim away the inside. Is that possible at all?

From my experience you can’t trim an object against itself. Split it along by isocurve. As to “why?”…

i don’t really get what you’re saying. The second object is there to show what i mean by “insides”. I would like to put a fillet on the concave corners and export the object for machining.

If I get what you’re saying, you can do this - you first need to split the pipe section along isocurves lengthwise and then you can get the intersections between the halves, and split/delete/join. See attached…

SpiralPipeOverlap.3dm (2.7 MB)



thank you! :smile:

Returning to this old post with a similar question, but perhaps a bit more messy.

In this case it seems like a long and complicated thing to split and trim at intersections,
probably due to the sweep1 resulting Poli-Srf not being that great to begin with.

Perhaps my first question should be: how to make this a cleaner Srf/Poli-Srf,

The goal is similar as in the original post, to Trim the self-intersecting portions in order to make filets or blends

with thanks

self Intersecting sweep1.3dm (4.4 MB)

Hi Akash - the best would be not to have a polysurface at all - you can do this if you place all the section curves (circles and ellipses) with the orientation the same so that the points line up, like so:

And make sure to align the arrows in Sweep1 snapping to Knot,to hit the corresponding part of each curve.

self Intersecting sweep1_PG.3dm (668.5 KB)

And work from there.

1 Like

Yes thank you Pascal.
you made a perfect sweep here.

important lesson for me here with sweep1

all the best


Dear Pascal
i did the sweep 1 according to your instructions, and it came out good as well.
split with shrink the sweep at both directions. and run intersect.
this gave a few trim-able intersections but didn’t work for most places?
It also seems to be a very long process, looking for additional valid splitting ways…

it would seems that a simpler shorter way is very much needed in this case?

here is split model each split piece is on it’s own layer for clarity.
self-intersecting sweep intersected.3dm (1.7 MB)

thanks a lot

Hi Akash,

Split the surface by Isocurve and then intersect to get the curves you need for trimming and self intersections.

Work with two models - have one that you split up to create trimming curves with and have the original single surface to copy over the curves and trim. That way you’ll end up with a really nice surface without all the splits you’ve created to achieve the result you want.

self-intersecting sweep intersected_01.3dm (922.9 KB)


Thank you Andy
I’ll do as you say,

good learning for me here

Hi Andy
Following your method, it has worked beautifully to a point…
i was able to trim a couple of intersections, and then, attempting to trim the others result in good parts being removed…
and i have no idea why.

  • a couple of intersections were not closed, [i joined them] but the trim is failing also with the originally closed ones…?

self-intersecting sweep 2 halfs.3dm (5.8 MB)

with thanks

Take another look at the detail… What needs cutting and how will it be cut?

You either need to extract another isocurve or join the elliptical curves and create a face:

Having said that though, due to the way the surface has been constructed (and as someone stated earlier) there are problems with self intersecting surfaces - even by pulling the curves to the surface, extruding curves into surfaces that bisect the area you want to trim, it still creates issues:

I’m afraid I don’t know any way around it - you’re going to have to split the model up a little but maybe not to the full extent of your first example.

Sorry I couldn’t find the solution.


One split: