Boolean Difference/Split Problem

In the attached file I have three surfaces (Gray, Purple, Red). Red started out as Purple with the ends cut down.

If I do a boolean difference of Gray - Purple it works.
But, if I do Gray - Red it fails. In fact I cannot do any split between

If I do a intersection the cap is here:

Problem Differnce.3dm (3.4 MB)

But it is not clear to me why there is a gap here at all.

I have to doubt it has to do with tolerance because I have changed that up and down with no effect and it works with the purple.

What is preventing the difference? Or Split?

Hello - the problem is the end cap of the red guy - this surface is, compared to the trimmed face you are using, gigantic -

Delete this and Cap the red extrusion and it should all work.

RH-59650 Shrinking misses the edges.


1 Like


Is this then a natural behavior of using BooleanIntersect then? I was using frame from another part that is much larger. I see that untrimall gives the original. Is there some better operation to use that might give smaller underlying surfaces?

Hi -

That is the natural behavior for any splitting operation.
It is up to the user to decide if a smaller underlying surface is desired and that has to be done as a post-trim operation.

Hello - Split has a Shrink=Yes/No option.


I do not see a Shrink option Is it possible that is on the Windoze version and did not make it to Mac?

Hi - I see that option here:

This is what I get:

I cannot type Split Shrink=YES because as soon as i hit the space the command executes.

If you don’t preselect your objects you might see the option…

I don’t get it that way either.

what version of rhino are you running? Rhinoceros>about rhino

also you have to run split, then pick a surface as the object to split before the option shows up

Version 6 (6.28.20199.17132, 2020-07-17)

make a surface and a line to split it with.

pick the split command, then pick the surface to split. Hit enter…this is when the shrink option shows up. then pick the line to split and complete the command with the shrink option selected.