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Joe Ardent 2023-01-10 13:49:43 -08:00
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@ -54,22 +54,44 @@ shop:
>
> *Me*: That's totally fair! I'll get you some files in a few days.
As you can see, I leaned even harder into the bluff; my next communication with the shop was nearly
four weeks later. But that's getting ahead of things.
"I'll get you some files in a few days," was an even harder lean into the bluff; my next
communication with the shop was nearly four weeks later. But that's getting ahead of things.
# Meshes and solid bodies
First off, let's talk about file formats and how to represent shapes with a
computer.[^math-computers] I said I could provide an *STL
file*. [STL](https://en.wikipedia.org/wiki/STL_(file_format)) is a pretty bare-bones format that
describes the outside surface of a shape as a set of many, many triangles, each of which is described
by three 3D points. This format is popular with 3D printers, which is how I became familiar with
it.
describes the outside surface of a shape as a set of many, many triangles, each of which is
described by three 3D points, forming a "mesh" in the shape of the thing you're modeling. This
format is popular with 3D printers, which is how I became familiar with it.
This type of representation is easy to create and read, but it's not great for manipulation.
This format is simple to implement and easy for a computer to read, but if you have a model in that
format that you need to manipulate, you won't have a good time. In order to actually do things like
change the shape of the model, it needs to be converted into a CAD program's native representation
of a "solid body", which is pretty much what it sounds like: a shape made of a finite volume of
"stuff", instead of an infinitesimally thin shell enclosing an empty volume, which is what the mesh
is.
In order for the CAD program to convert a mesh into a solid body, the mesh must be *manifold*,
meaning, no missing faces (triangles), and with a clearly-defined interior and exterior (all
triangles are facing in one direction relative to their interior). When there are no missing faces,
it's called "water tight". You can still have "holes" in a mesh, like if you have a model of a
donut, but the surface of the donut can't have any missing faces.
The CNC shop had requested a model in a format called
[ST**P**](https://www.fastradius.com/resources/everything-you-need-to-know-about-step-files/). `.stp`
is the extension for a "STEP" file; STEP is supposed to be short for "standard for the exchange of
product data", so someone was playing pretty fast and loose with their initialisms, but I
digress. But the main thing about STEP files is that CAD programs can really easily convert them
into their native internal solid body representation, which allows easy manipulation.
But so far, I had nothing at all. Time to get some data and see if I can turn it into a model.
# Public data
## From space?
## Thanks, California state!