Sketches and Constraints
This chapter is about the parametric world — FreeCAD, Fusion, Onshape, SolidWorks. There are no scripts to run here; instead, you're learning the ideas those programs are built on, so the first time you open one, it feels familiar instead of alien.
The sketch: a drawing with rules
A sketch is a 2D drawing on a flat plane — usually one of the three principal planes (XY, XZ, YZ), or the face of something you've already built. You draw rough shapes: rectangles, circles, lines, arcs. Rough is fine, because the magic isn't the drawing — it's what comes next.
Constraints: telling the sketch what you mean
A constraint is a rule you attach to the sketch. Instead of carefully drawing a 40 mm line, you draw any line and add a dimension constraint: "this is 40 mm." Instead of eyeballing two lines parallel, you add a geometric constraint: "parallel."
The common ones:
| Constraint | What it says | Example |
|---|---|---|
| Dimension | "This length/radius is exactly N" | Hole diameter = 4.5 mm |
| Horizontal / Vertical | "This line is exactly horizontal/vertical" | Bracket edges |
| Coincident | "These two points touch" | Line endpoint on a circle |
| Parallel / Perpendicular | The obvious | Bracket sides |
| Concentric | "Same center" | Bolt circle holes |
| Symmetric | "Mirror of each other about this line" | Two holes mirrored |
Fully defined: the goal
A sketch is fully defined (some programs say "fully constrained") when every point's position is nailed down by constraints — nothing can wiggle. Programs usually color-code this: blue/under-defined (can still move), black or green/fully defined.
Why it matters: an under-defined sketch is a landmine. You change one dimension and a hole slides somewhere unexpected, because it was never actually told where to be — it just happened to look right. Fully defining is the parametric equivalent of writing down your measurements instead of trusting your memory.
The habit: when a sketch looks right, try dragging its points. If anything moves that shouldn't, add constraints until it can't. Five minutes here saves a reprint later.
The parametric superpower: edit, don't redraw
Remember the bracket that's 62 mm instead of 60? In a parametric
modeler you open the sketch, change the dimension 60 to 62, and
everything built on that sketch — holes positioned from its edges,
the upright sitting on its face — updates. The model remembers
how it was built.
This is also why parametric models beat raw STLs: an STL has no dimensions to edit, just triangles. The recipe (sketch + constraints + features) is the valuable part. Same rule as What Is CAD?: keep the recipe.
How this maps to OpenSCAD
OpenSCAD doesn't have interactive sketches — but the idea is identical, and you'll use it in every script from here on:
- A dimension constraint becomes a variable:
hole_d = 4.5;used everywhere the hole appears. Change it once, every hole updates. (See OpenSCAD in Depth.) - A geometric constraint becomes math in the script:
translate([x, base_w/2, -1])is a "centered" constraint, written as arithmetic.
So when a FreeCAD tutorial says "constrain the hole 10 mm from the
edge," your OpenSCAD brain hears translate([10, ...]). Same
thought, different buttons — exactly as promised.
Which parametric tool should you try?
Honest, short guidance:
- Tinkercad (free, browser): the shallow end. Great for a first afternoon, outgrown quickly.
- FreeCAD (free, open source, all platforms): the real thing, zero cost. Rougher edges than commercial tools, fully capable.
- Fusion (free tier for hobbyists): polished, widely tutorialized. Cloud account required; check the current free-tier limits before investing heavily.
- Onshape (free tier): runs in a browser, excellent for Chromebooks. Free-tier designs are public.
Any of them will teach you sketches and constraints. Pick the one whose installer annoys you least.
Next: From Sketch to Solid