1.1 What You’ll Learn
This page explains what CAD is, why it matters for rebreather building, and how the rebreathology CAD library works. By the end, you’ll know how to open, view, and use CAD files — even if you’ve never touched a CAD programme in your life.
You don’t need to become a CAD expert to build a rebreather. You just need to know enough to open a file, understand what you’re looking at, and send it to someone (or something) that can make the part for you.
What Is CAD?
CAD stands for Computer Aided Design. It’s the process of using software to create precise digital models of physical objects — the kind of models that can be spun around on screen, measured to the nearest hundredth of a millimetre, and sent directly to a machine that will manufacture the real thing.
If you’ve ever seen a 3D model of a car engine, a building, or a jet turbine rotating on a screen, you’ve seen CAD in action. The difference between CAD and, say, a 3D model in a video game is precision. A game model only needs to look right. A CAD model needs to be right — down to exact dimensions, material properties, and manufacturing tolerances.
In the context of rebreather building, CAD is how we design every custom part: scrubber canisters, head plates, mouthpiece housings, counterlung frames, and dozens of other components. A well-made CAD model isn’t just a pretty picture — it’s a complete, unambiguous specification that tells a manufacturer exactly what to make.
Why CAD Matters for This Project
In the old days, if you wanted to build a rebreather part, you’d sketch it on paper, hand it to a machinist, and hope they interpreted your drawing the way you intended. The result depended heavily on the skill of both the designer and the machinist, and there was always room for miscommunication.
CAD changes that equation completely. When you design a part in CAD:
- Every dimension is exact — no ambiguity, no guesswork
- The model can be checked for errors before anything is manufactured
- You can test how parts fit together digitally, catching problems before they become expensive mistakes
- The file can be sent directly to a CNC machine, 3D printer, or online manufacturing service — no human interpretation required
- Designs can be shared, modified, and improved by the community
That last point is the one that matters most for rebreathology. Because CAD files are digital, they can be shared as easily as a photograph. And because they contain exact specifications rather than approximate sketches, someone on the other side of the world can manufacture your part to the same precision as if they were standing next to you.
Think of a CAD file as a recipe for a physical object. Just as a recipe tells you exactly what ingredients to use and in what quantities, a CAD file tells a manufacturer exactly what shape to cut, what dimensions to hit, and what tolerances to hold.
The Rebreathology CAD Library
One of the core resources on this site is the rebreathology CAD library — a growing collection of 3D models for rebreather parts, components, and assemblies. Every file in the library is free to download, modify, and use under our open-source licence.
The library is designed around three principles:
Modular
Parts are designed to work together. A scrubber canister designed in the library will mate correctly with head plates, breathing hoses, and other components from the same library. This isn’t a random collection of files — it’s a coordinated system.
Parametric
Where possible, parts are built with adjustable parameters. Want a longer scrubber canister? Change the length value and the entire model updates — wall thickness, O-ring grooves, thread engagement, and all. You don’t need to redraw anything from scratch.
Manufacturing-ready
Every part in the library is designed to be manufactured. That means real-world tolerances, standard thread sizes, materials that are actually available, and features that can be machined or printed without heroic effort. If it’s in the library, it can be made.
You don’t need to buy expensive CAD software to use the library. All files are provided in standard formats that can be opened with free tools. We’ll cover exactly which tools to use in the next section.
CAD File Formats You’ll Encounter
CAD files come in many formats, and it can be confusing at first. Here are the ones you’ll see most often in the rebreathology library:
STEP (.step or .stp)
The universal exchange format for CAD. Almost every CAD programme can open a STEP file, which makes it the lingua franca of the engineering world. When we say a file is “manufacturing-ready,” we usually mean it’s available as a STEP file. This is the format you’ll send to online manufacturing services.
STL (.stl)
The standard format for 3D printing. An STL file describes the surface of a 3D object as a mesh of tiny triangles. It’s less precise than STEP (it loses some dimensional information), but it’s what every 3D printer expects to receive. If you’re printing a part, you’ll use this format.
Native formats (.f3d, .dwg, .ipt, etc.)
These are the working formats of specific CAD programmes — Fusion 360, AutoCAD, Inventor, and so on. Native files preserve all the parametric intelligence of the model (the adjustable parameters, the design history, the relationships between features). If you want to modify a design rather than just manufacture it as-is, native files give you the most flexibility.
STEP files contain exact mathematical surface definitions (NURBS curves and surfaces), which is why they’re preferred for precision manufacturing. STL files approximate those surfaces with flat triangles — fine for 3D printing, but not ideal for CNC machining where exact geometry matters.
How To View CAD Files (For Free)
You don’t need to buy software to view or work with CAD files. Here are the best free options:
Autodesk Fusion (formerly Fusion 360)
The most capable free option. Fusion is a full-featured CAD programme that’s free for personal, non-commercial use. It can open STEP, STL, and many native formats. It runs in a web browser or as a desktop app. If you want to modify designs, create your own parts, or generate technical drawings, this is the tool to learn.
FreeCAD
A fully open-source CAD programme. It’s rougher around the edges than Fusion, but it’s genuinely free (no licence restrictions at all), runs on Windows, Mac, and Linux, and has a passionate community. It handles STEP files well and is improving rapidly.
Online viewers
If you just want to look at a file without installing anything, several free online viewers exist. Autodesk Viewer and 3D Viewer Online both let you upload a STEP or STL file and spin it around in your browser. Good for a quick look, but limited if you want to measure or modify anything.
If you’re completely new to CAD and want a single recommendation: start with Autodesk Fusion. It’s free, powerful, well-documented, and has thousands of tutorials on YouTube. You can be productive within an afternoon.
How the CAD Library Fits Into Your Build
Here’s the typical workflow for using CAD files from the rebreathology library:
1. Browse and download. Find the part or assembly you need in the library. Download the STEP file (for manufacturing) or the native file (if you want to modify the design).
2. View and check. Open the file in Fusion, FreeCAD, or an online viewer. Spin it around. Check that the dimensions and features match what you need. If the part is parametric and you have the native file, adjust any parameters to suit your build.
3. Generate drawings (if needed). Some manufacturing methods require a 2D technical drawing in addition to the 3D model. CAD software can generate these automatically from the 3D model. We’ll cover technical drawings in detail on the next page.
4. Manufacture. Send the STEP file to an online manufacturing service (covered in Page 1.6), to your local machine shop, or slice the STL for your own 3D printer.
5. Verify. When the physical part arrives, measure it against the CAD model to confirm it’s been made correctly. A set of digital callipers and the CAD file are all you need.
The CAD library is a living resource. As the community grows, so does the library. If you design a part that isn’t already covered, consider contributing it back so others can benefit too.
What You Don’t Need To Know (Yet)
This page has given you the conceptual foundation — what CAD is, why it matters, and how to use the files. You don’t need to know how to create CAD models from scratch to start building. The library exists precisely so that you can download proven designs and get them manufactured without needing to model them yourself.
If you do want to learn CAD modelling — and it’s a genuinely rewarding skill — we’ll provide resources and tutorials as the project grows. But it’s absolutely not a prerequisite for building your rebreather.
Next up: Page 1.2 — Technical Drawings. How to read the 2D drawings that accompany CAD models — the language that tells a manufacturer exactly what to make.