Help with offsetting surface needed


I am trying to give this form a 50mm solid wall thickness, the protruding part shown in the screenshot needs to be removed flush with the top surface and which ever way I try it simply won’t work. Is there a way of producing curves at these intersections?

The main issue is getting a 50mm wall thickness, so I need the surface to offset inwards, again, whatever methods I try don’t seem to work.

Any help would be very very much appreciated.

Many thanks Si.

Forum drawing.3dm (6.6 MB)

Moved to Rhino category

Yeah… tough to trim on a surface self-intersection. You may need to split the object into two pieces using Split > Isocurve > Shrink=Yes , then rejoin after the trim.


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Thanks Pascal, heres the updated drawing, the method you describe is what I have been using but it seems to create a manner of unwanted issues as I then begin to try and offset, extend, split etc.

For example, if I create a dup edge on the freshly split surface and then try to offset on the surface, when prompted to choose base surface it will only highlight parts of the split even after joining.

The real issue is getting this neat and tidy 50mm solid offset (to the inside of the form), any help here?

Forum drawing 2…3dm (12.4 MB)

An alternative method is to duplicate the object and split the duplicate. Intersect the two split parts which results in a curve. Then use the intersection curve to trim the original object.

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Thanks David, I tried this method several times but the curve created to split with won’t work, its says they do not intersect.

Hi Simon - heh - what is inside?? But as I think you mean it, ofsetting 50mm gets you past the edges:

The other way might be made to work but seems less like what I would call the inside… it is also messy…


But… does it work, here?


When I say inside the form i mean when I choose offset I flip the arrows to point inward rather than outward. The resulting offset results in various issues like odd turn ups at bottom edge and then when I try to offset curves on srf it will only allow me to highlight the split parts and not the whole even though it’s joined.

Plus when I make any offset of the srf it takes about 30 seconds to carry out the command.

Will keep trying.

So, since the form is self-intersecting, inside or outside is whatever you say it is, there is no logical outside… I think though that you mean my upper picture in the previous post - but keep in mind that if you offset, to the inside, to a distance larger than the radius of curvature of the surface, weird things happen:



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Yes, thats whats happening, not able to fix that without it causing further problems, am going to try scaling and see if that yields any results. Essentially I am just trying to create a backfill.

yeah - you need to either relax the outer radius there somewhat - greater than 50, or offset somewhat less. I do not know how VariableOffsetSrf will do with this thing - my guess is it will be slow - that is a very dense surface and hard to interact with but it may be worth a look.