Joining Surfaces, Separating a Part

Hi, I’m new to Rhino and would like some help if you have time.

I have four surfaces at the top of a cube. How can I join those surfaces together, so that I can then separate that part of the cube that is inside their intersection

Welcome @michaelperry
Post the file… it’s hard to guess without it!

thank you - I will do that shortly

BoxCutting.3dm (2.8 MB)

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Join - to connect the chamfered edges into complete extrusions
Trim - and select the four cutting planes
and then the cuboid

Invert - Hide to view the finished piece

I end up with a top surface, two chamfered edges and an open bottom surface.

I had hoped for a closed object with two chamfered edges and two simple routed edges

Hello - I can’t tell yet what you want to end up with here…


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Thank you for looking. Sorry for not explaining well - I’m trying to extract the top of the hollow cuboid as a separate component. It will have two opposite and chamfered edges and two opposite and “routed” edges.

This thing?


exactly so!

would you be able to tell me how you did that please?

I can’t even split 2D closed curves without them becoming open curves, so I hope your solution will help me with that too.

Hi Michael - there are a few ways to get that shape - am I correct that you want the corresponding cuts to be made to the top edge of the box?
One easy way out, once you solve the puzzle is to create all the bounding surfaces for your lid - all of these are in the file:

Select all of these and CreateSolid

Repeat for the bottom:

Any luck?


Thank you - I can create the top this way, though the top of the inner volume remains, whereas yours is already hollow. Is there some other setting that I should have selected.3dview.pdf (271.7 KB)

Hi Michael - actually it is easier - I forgot that you can do it all in one go and CreateSolid will find all possible solids at once. Select the cutting surfaces and the two boxes:

and CreateSolid


Thank you again. After reading your earlier post, that is exactly what I did - selected all six surfaces and then typed CreateSolid - but that leaves a surface at the top of the inner volume.

I wonder if there is something awry in the preferences for Rhino that makes your copy behave subtly differently to mine. Am using
OSX 10.14 and Rhino 7.

So, the way to do this ‘right’ in one step is

  1. Keep the boxes as solid boxes. My forgetting that CreateSolid makes more than one solid if that is what it finds made me suggest extracting surfaces - it used to be able to do one volume only, actually quite a long time ago, but I am living in the past.
  2. Select both boxes and all four cutters at the same time.



I can’t get such a clean answer as you - it seems impossible not to create an upper surface for the inner box, also a strange line appears at the front and back of my model. Some artefact front the CreateSolid I suppose.

I shall try again tomorrow



Try with this file -
BoxCutting_PG.3dm (89.9 KB)

Run MergeAllCoplanarFaces after…


Thank you, that worked - it created an inner box that I could delete.

Onwards and upwards…