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)