Re: Void Normal Direction

From: Bill Richards (
Date: Sun Nov 09 2003 - 00:14:39 EET

Why bother creating a pattern that has an enclosed void? Think about it
for a moment. Until that void has a path to the outside, the object
should be considered a solid anyway. Here's my point:

        Take a balloon and inflate it. Tie off the neck so no air escapes. We
all know that the balloon is hollow. It's filled with air, and the
actual structure of the balloon is a skin that is stretched around this
        Truth is, that balloon is a solid object. Philosophically speaking, of
        Once you tied off the neck, creating a border between the outside air
and the air trapped inside, the inside air became part of the structure
of that balloon. If you took a cross-section of this new solid object,
it would have a surface of rubber a few mils thick surrounding a core
of pressurized air.
        Now take that balloon, attach it to the bottom of a bucket and pour in
plaster of paris. After the plaster sets, pop the balloon. Then remove
the rubber from the cavity you just created.
        Next, fill the cavity with molten metal. After the metal cools, break
away the plaster and study the resulting object.
        It's a very heavy, solid ball of metal. Did you really expect it to be
a thin shelled, hollow ball of metal?

        The point is, if that void has no exit to the outside of the object,
then as far as the software is concerned, this is a solid object, and
it won't bother with building the object as such. It may be deleting
the void as a modeling error.
        When you cut the quarter-section out of the model and it builds
properly, I'll bet you are penetrating to the void, giving it a
connection to the outside of the object. Therefore, it is no longer an
irrelevant void, so the software retains it as part of the model.
        If your intent was to build the object and then drill into the void or
cut it in half, why didn't you simply design the model with the drill
hole (or at least a pilot hole) already there, or already in two
        Assuming that there is a very valid reason for having a fully
contained void in the model -- how are you going to clean out the
support material/structures from inside this void? The object may very
well end up being a solid, anyway.

        - Bill

                Stress is when you wake up screaming and then you realize
                you haven't fallen asleep yet.

On Nov 7, 2003, at 10:52 AM, Jonathan Chertok wrote:

> List,
> I have a shape that has an enclosed void, which seems to want to print
> as a solid. If I take a quarter section out of the model it prints
> fine, however as a fully enclosed volume it prints as a solid.
> I am wondering if Magics Communicator Lite has a "direction" command
> that would show the normal directions for its X, Y and Z sections - as
> the model appears to be formed appropriately.
> I also have the demo version of STL Editor that works great and would
> be interested in knowing if there is a better way to inspect this
> model with STL Editor as well.
> Rhino seems to show the model with the void, but does not show any
> interior normal directions.
> Is there something tricky about this process that I should know?
> Cheers,
> Jonathan
> Universal Joint: Buildings + Digital Design
> Jonathan Chertok. Principal
> Austin, Texas [512] 407 9628
> Full Service Design and Construction
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