The Files That Make Things

A model has to leave the program somehow

Your model lives inside a design program. A 3D printer in the corner of the room cannot open that program; neither can a laser cutter, or a factory across the ocean. So the model has to be exported — translated into a file format that machines agree on. Two formats do most of the work in a school shop, and once you know what is inside them, a lot of mysterious problems stop being mysterious.

STL: a bag of triangles

An STL file is the file 3D printers eat. Open one up and you will not find "a cylinder, 1 inch wide." You will find a long list of triangles — thousands of tiny flat facets, each described by its three corners. Together they form a mesh, a skin stretched over the shape. Flat faces need only two triangles. Curved surfaces need lots of them, and that is why a printed circle, looked at closely, is really a polygon with many short straight sides. The number of triangles is the file's resolution: more triangles, smoother curves, bigger file.

Two more things about STL files matter every single day. First, the mesh must be watertight: a closed skin with no holes or gaps, or the slicer cannot tell inside from outside and the print comes out with missing walls. Second — and this one has ruined more prints than any other — an STL has no units. It stores numbers, not inches or millimeters. The program that reads it has to guess, and most guess millimeters. Export a 2-inch block as "2" and the printer makes a 2-millimeter speck. Export it as 50.8 and it comes out right. When a print is 25 times too small, this is almost always why.

SVG: lines you can scale forever

A laser cutter does not want triangles. It wants to know where to drive its beam, so it eats SVG files: vector drawings made of paths — lines, arcs and curves defined by math, not by dots. Because a vector is math, you can scale it up to the size of a wall and the edge stays perfectly crisp. The laser follows each path as a cut line, or fills a region with back-and-forth passes to engrave it.

Compare that with a photo. A photo is a raster image: a grid of colored pixels. Zoom in and it turns to blocks, because there is nothing between the pixels to look at. A raster image can be engraved like a stamp, but it cannot be *cut* along an edge — there is no edge, only a fuzzy change from dark to light. That is why a logo drawn in a vector program cuts cleanly and the same logo saved as a JPG does not.

Which file for which machine

A 3D printer wants a solid: export an STL (or its newer cousin, 3MF, which does carry units). A laser cutter or vinyl cutter wants outlines: export an SVG. A machine shop or another engineer wants the full model with its real geometry: send a STEP file, the format CAD programs exchange. Photos and screenshots — PNG, JPG — are raster, fine for a worksheet, useless for cutting.

An STL is a shape described by triangles with no units. An SVG is a drawing described by math with no pixels. Know which one you are holding and half of fabrication's surprises disappear.

Three things went wrong in the school makerspace this week. Use what you know about files to sort out which file was which, and what happened.

1. 1

   Sort each fact: is it true of an STL file or of an SVG file?

   💡 STL = triangles for printers. SVG = paths for cutters.

   For each card, tap the bin it belongs in.

   Stores no units — just numbersSTLSVG

   What a 3D printer's slicer opensSTLSVG

   Scales to any size with crisp edgesSTLSVG

   A list of thousands of tiny trianglesSTLSVG

   Lines and curves defined by mathSTLSVG

   What a laser cutter follows as cut linesSTLSVG

   STL

   SVG
2. 2

   A 2-inch block came out of the printer 2 millimeters wide. What went wrong?
3. 3

   Put the path from a finished model to a running 3D printer in order.
4. 4

   Sort these files: raster (pixels) or vector (math)?

✅ Check: The Files That Make Things