The model is a recipe, not a shape

Why parametric CAD stores the steps that build a part rather than the part itself — and your first solid in Onshape.

Two ways a computer can hold a shape

When you download an STL from a model-sharing site, you get a mesh: a few hundred thousand triangles pretending to be a surface. It knows where every corner is and nothing at all about what the part is for. That is why editing one is miserable. There is no “make this hole 1 mm bigger” — there is only pushing triangles around and hoping.

Parametric CAD stores something completely different. It stores the ordered list of operations that would build the part, along with the numbers that drive them. Sketch a 60 × 40 rectangle. Extrude it 4 mm. Cut a 5 mm hole 10 mm in from that corner. Round these edges. The shape on screen is the output of running that list.

Why this matters more for makers than for anyone else

Professional engineers change models because a spec changed. You will change models because you printed the part and it was wrong. That is not a failure mode, it is the actual workflow: design, print, check, adjust, print again. Two or three iterations is normal for a part that has to fit something.

A recipe-based model makes iteration cost thirty seconds — change 4 mm to 4.4 mm, re-export, re-slice. A mesh makes it cost an evening. When people say CAD has a steep learning curve, they are describing the price of admission for that thirty seconds.

The four words Onshape will put in front of you

Document — the container, roughly “the project”. It holds everything related to one design and it versions itself, so there is no bracket-v7-FINAL-real.stl in your downloads folder.
Part Studio — where you model. One Part Studio can hold several parts that need to fit each other, which is exactly what you want for a two-piece enclosure.
Feature list — the recipe itself, down the left side. Every operation you perform lands here in order. You can double-click any line and change it, and everything after it rebuilds.
Sketch — flat 2D geometry on a plane. Nearly every feature starts as a sketch. This is where the next lesson lives.

The full vocabulary is on the glossary page. You do not need to memorise it; you need to stop being surprised by it.

Hands on

Build a plate, then prove it is a recipe

Done when: Changing one number in the feature list resizes the printed part, with no remodelling.

Sign in to Onshape and press Create → Document. Call it cad-course. You land in a Part Studio.
Click the Sketch tool, then pick the Top plane. The view rotates to look straight down at it.
Draw a rectangle roughly centred on the origin using the corner rectangle tool. Do not fuss over the size — it is wrong on purpose and you will fix it properly in the next lesson.
Press the green check to close the sketch, then click Extrude, select the sketch region, and type 4 mm. You now have a solid.
Here is the whole point. In the feature list, double-click Extrude 1 and change 4 mm to 10 mm. The part rebuilds instantly. Now double-click Sketch 1, drag a corner, and close it — the extrusion follows.
Right-click the part in the parts list and choose Export → STL. That is your slicer handoff, and you can regenerate it any time the recipe changes.

What you did not learn today

You made a solid by dragging, which means its dimensions are whatever your mouse happened to land on. A model built that way drifts every time you touch it. Fixing that — making the sketch hold still — is the single highest-leverage skill in CAD, and it is the next lesson.

Check your recall

Answer from memory — no scrolling back.

1. What does a parametric CAD file actually store on disk?
2. Why is an STL awkward to modify after the fact?
3. In Onshape, where does the recipe live on screen?