Difference boolean on non-intersecting objects

How come i cannot boolean difference two boxes (one smaller inside bigger one), i want to get smaller box with thickness outwards. i can do it with offsetsrf from the small one but why boolean operation does not produce the same thing?

in rhino 7

actually i cant get desired object with offsetsrt because it only produces another closed surface not a solid object

Select the two boxes and run the command NonManifoldMerge with the DeleteInput=Yes option. Select the new object and run the command CreateRegions. Select the resulting inner Surface (not the Polysurface) and delete. You now have a hollow box.


p.s. This used not to be possible in Rhino, which was designed as a surface modeller rather than a solid modeller. But by popular demand (and possibly the advent of 3d printing) …

but software evolves it is a good thing to be able to work with solids :slight_smile:

btw i would expect offset srf command with create solid/yes option to create such hollow box right away, or at least have that suboption inside the command suggesting that something will happen when offseting closed surface

Hi -
Note that the trick that Jeremy described will make an object that will cause many downstream commands to fail. If a hollowed object is your required final output, this process might be fine…

ok having surfaces which are enclosed defining a solid is easy.

hollow objects are fundamentally different because not a single polysurface defines a solid.

when i followed jeremy process what did i actually do? how does rhino now know that it that those two polysurfaces are a hollow solid?

should not this be handled in elegant way (not causeing downstream issues) within the kernel? i mean hollow objects are pretty common

Actually, before 3d printing came along they weren’t, in part because they were pretty impossible to make and in part because they were not often useful.

If you are worried about downstream problems (@wim, helpful to hear what those are) then revert to the “traditional” Rhino hollow box. “Drill” a small hole into the outer box, long enough to reach into the inner box. Then boolean difference the inner box from the outer. Effectively introducing a pipe connects the inner and outer surfaces. This works even if you make the pipe microscopic so you don’t see it in renders.

they are handled in the slicing process of the printers, as well as their fill-ins. for any other printing technology like sla or slm you need that hole that @jeremy5 described anyway.

i didnot model for manufacturing. But in case of steel capped rectangular profiles its easier to model as a hollow box rather than a profile with separate caps (for just conceptual geometry). Now i wonder what would happen if i unioned profile with separate caps.

ok it doest not work this way either. it creates some strange objects

Hi Jeremy -

There is no list over commands that are involved here. Things like filleting edges will make those objects fall apart again.

My recommendation is not to look for shortcuts but to model as a profile with separate caps.

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i will not from now on to try go againts programs will :smiley:

just for information is this a common thing that cads have problems with hollow solids or is this specific to rhino? i mean common engineering cads like autocad, microstation (parasolid), sketchup, …