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Thinking in 3D

Before you model anything, you need the mental furniture: where things are, what unit you're in, and the one big idea every CAD tool is built on.

Coordinates: X, Y, Z

Every point in your model has an address made of three numbers: (x, y, z). The conventions:

  • X — left/right (width)
  • Y — front/back (depth)
  • Z — down/up (height)

Z is up. This trips up everyone exactly once — in school math, Y was up. In CAD and 3D printing, Z is up, because that's the direction the printer builds. A point at (10, 20, 5) is 10 mm right, 20 mm back, 5 mm up from the origin.

The origin (0, 0, 0) is the corner of your universe. In OpenSCAD, cube([20, 20, 10]) puts one corner at the origin and extends 20 in X, 20 in Y, 10 in Z. In parametric modelers, you usually sketch centered on the origin so the part grows symmetrically.

The habit to build: before modeling a real part, decide where the origin goes. A good default: a corner or the center of the part's bottom face, so Z = 0 is the surface that sits on the print bed. Your future self, orienting the STL in the slicer, will thank you.

Units: millimeters, always

The 3D-printing world runs on millimeters. Slicers assume millimeters. Filament is 1.75 mm. Nozzles are 0.4 mm. If you model in inches or centimeters and export without converting, your 20 mm bracket arrives as a 20-inch bracket (or a 2 mm speck).

The rule: model in millimeters, export in millimeters, and distrust any file that doesn't say. The units trap is common enough to get its own section in Exporting and Sharing.

The one big idea: sketch → feature

Every CAD tool, GUI or code, works the same way underneath:

  1. Sketch — draw a 2D shape (a rectangle, a circle, a profile).
  2. Feature — do something 3D with it: pull it into a solid (extrude), spin it (revolve), cut it out of something else.

A box is a rectangle, extruded. A pipe is a circle, extruded, with a smaller circle cut out. A vase is a curved profile, revolved. A bracket is rectangles, extruded, with holes cut out. That's about 80% of functional CAD: 2D shapes, given thickness.

In OpenSCAD the "sketch" and the "feature" happen in one line — linear_extrude(height = 10) square([20, 20]); — but it's the same idea. In FreeCAD you'd draw the square in the sketcher, then hit Pad. Different buttons, same thought.

Measuring the real world

CAD models real objects, so it starts with real measurements. The tool is a digital caliper (about $20–30, worth every cent):

  • Measure twice, in two orientations. Plastic parts are rarely perfectly square.
  • Measure the mating part, not your memory of it. "M4 screw" means the screw is ~4.0 mm, but the hole you need is bigger — see Measuring and Tolerances.
  • Write measurements down with a sketch, even a rough one on paper. "40 wide, 5 thick, holes 10 from each end" is already a design.

Careful: calipers have an inside-measurement jaw and an outside-measurement jaw — using the wrong one is a classic 2 mm error. The small jaws on top measure insides (holes); the big jaws measure outsides.

Think in features, not triangles

Beginners try to build shapes triangle-by-triangle in their head. Don't. Think in features: "a base plate, an upright, four holes, a gusset." Each feature is one sketch plus one operation. The L-bracket you'll build in the project chapter is exactly four features — and you'll be able to change any dimension in one place.

Next: Your First Model