So you want to make a model for 3D Printing

You want me to slice a poly-what into a G-who?

You want me to slice a poly-what into a G-who?

 

Don’t worry, you don’t need to be as concerned as this cat. 3D modeling and 3D printing are both actually pretty simple in practice. I’ll spare you the backstory and promise of 3D printing – you can find that elsewhere. What you’re here to learn is the how, what, and why of 3D printing and modeling.

How does the 3D printer know what to make?

3D Printers use programmed instructions, called G-codeThese instructions are produced by a program called a slicer. These programs exist so that we don’t have to learn how to program to 3D print something – they do all the hard work for us! Slicers take 3D models and cut them up into layers; picture in your head an object that you want to 3D print. Then, starting from the bottom and going to the top, slice it into layers about .1 millimeter thick. What you’ll end up with is a series of cross-sections – that is exactly what a slicer program does. That same program then creates movement instructions for the 3D printer head, based on these cross sections.

These movement instructions are known as G-code, which tell the printer where to move the extruder (see sidebar), how fast to move it, and what path to move it through. The 3D printer completes these movements, goes up a layer, and then repeats the same process again. This happens over and over again until the print is done.

PrintHead

Extruders

Also known as “hot ends” or the “print head”, these are the parts in a 3D printer that melt the plastic and push it through a small nozzle onto the build surface. (Image from: Slic3r online manual)

3D Printers use slicing programs to cut 3D models up into 2D cross sections. Image credit: Stefan Sydow

An example of a print-ready 2D section of a 3D model. Image credit: Stefan Sydow

 

Ok, but what does the slicer slice up?

Good question! Slicers read and slice files called STLs. If you were to look at one of them in a folder on your computer, the extension (the type of file) will always be .STL, which stands for “STereoLithography”. This is a standard format, used throughout every industry that uses Computer-Aided Manufacturing (CAM) or Rapid Prototyping, which is also known as 3D Printing.

 

But what are  .STL files?

Another good question – .STL files are raw polygonal meshes that are generated from the surface normals of a 3D model.

What’s a polygonal mesh?

An example of a polygon mesh made predominantly of four side polygons, or "Quads".

An example of a polygon mesh made predominantly of four side polygons, or “Quads”.

A polygon mesh is a collection of multiple edges, faces, and vertices that are connected with each other.

What’s a polygon?

In geometry, a polygon is a closed 2D figure that is made up of a chain of straight line segments. In 3D modeling, polygons are 3D objects that are created by the intersection of line segments, which are referred to as edges. The points at which the edges intersect are called vertices (in the 3D world we call them “verts” for short), and the surface that is generated from these intersections forming a closed shape is called a face or plane. In the image below, we see that 3D models are built out of triangles, which together form polygons of four sides each.

Image courtesy of Wikipedia.

The different components of a polygon mesh. Image courtesy of Wikipedia.

While a polygon can have any number of sides, STL’s only use triangles are used because they are the simplest shape in 3D – 3 vertices, connected by 3 edges to create a flat face, nomatter where the vertices are in space. This is important because with enough triangles, we are able to precisely describe it’s form in three dimensions.

What’s a surface normal?

Surface Normals

The computer has to have some way of determining which way the polygon is facing, “in” or “out”. The angle that is perpendicular (opposite) to the face is called the “Normal” or “Normal Vector” and is what is used to determine which way the polygon is facing. If your surface normal is pointing towards you, you will see a face. If it is pointing away from you, the polygon will appear invisible.

 

Wrap-Up

So far, you’ve learned that you use programs called slicers to cut a 3D model up into layers to generate G-code, which are the instructions that 3D Printers need to know where to move the print head when they print a model. You also learned that these slicers use files called .STLs, which are made up of lots and lots of triangles, which is a type of polygon. These polygons are connected with each other to form polygon meshes. You also learned a little bit about how surface normals are used to calculate which way a triangle is facing so the slicing program knows which side of the model is “in” and which is “out”.

 

What’s Next?

In the next post, we’ll be going into different types of 3D data that 3D modelers use to create data for 3D printing, CAM, and computer graphics. We’ll also go over what you should consider when choosing a 3D software package for 3D printing and modeling, and some important things to keep in mind when creating a 3D-printable model.

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