From Sketch to Solid

Sketches are flat. Parts aren't. This chapter is the bridge: the handful of operations that turn 2D shapes into 3D solids. Learn these five and you can build most functional parts.

Extrude: pull the sketch into a solid

Extrude takes a 2D shape and gives it thickness — like pushing dough through a cookie cutter. It's the single most-used CAD operation.

// A 30 x 20 mm rectangle, pulled 8 mm tall.
linear_extrude(height = 8)
    square([30, 20]);

In FreeCAD this is Pad; in Fusion, Extrude. Same thought: pick a sketch, pick a distance, get a solid.

Extrudes also cut: sketch a circle on the face of a solid and extrude it into the part, and you've made a hole. (In OpenSCAD you'd more often use difference() with a cylinder, as in Your First Model — same result.)

Revolve: spin the sketch

Revolve spins a 2D profile around an axis. Knobs, pulleys, vases, bottles — anything round.

// A knob: profile points are [radius, height], spun around Z.
rotate_extrude($fn = 64)
    polygon(points = [[0, 0], [12, 0], [12, 4], [6, 10], [0, 10]]);

Read the profile: start at the axis (radius 0, height 0), out to radius 12, up 4 mm, taper in to radius 6, up to height 10, back to the axis. Spun 360°, that's a knob 24 mm across and 10 mm tall with a tapered top.

Careful: the profile must stay on one side of the axis (all radii ≥ 0) and should touch the axis at the top and bottom if you want a solid rather than a hollow shell. If your revolve looks like a weird funnel, check for negative radii.

Most holes are one of three:

Through hole — all the way through. The plate.scad hole from Your First Model: a cylinder taller than the part, difference()d out.
Blind hole — partway. A cylinder that stops inside the part (a pocket for a magnet, a screw boss). In a parametric tool: extrude-cut with a depth instead of "through all."
Countersunk / counterbored — a hole with a cone or wider cylinder at the top so a screw head sits flush:

// M4 countersunk hole in a 6 mm plate: 4.5 mm shaft, cone to 9 mm.
difference() {
    cube([30, 30, 6]);
    // straight shaft, all the way through
    translate([15, 15, -1])
        cylinder(d = 4.5, h = 8, $fn = 32);
    // cone at the top for the screw head
    translate([15, 15, 3])
        cylinder(d1 = 4.5, d2 = 9, h = 3, $fn = 32);
}

d1 is the bottom diameter, d2 the top — so this cone widens upward, meeting the shaft at z = 3 and opening to 9 mm at the top face (z = 6). The screw head hides inside.

Fillets and chamfers: removing the sharp edge

Fillet = round an edge. Chamfer = cut it at 45°. Both do three jobs: they remove stress risers (sharp inside corners crack), they're kinder to fingers, and on printed parts they hide the layer-line stair-step on angled edges.

In parametric tools this is one click on an edge. In OpenSCAD it's awkward — honest limitation — so the pragmatic move is:

Design the chamfer into the profile (draw the polygon with the corner already cut, like the knob's tapered top above), or
Add a 1–2 mm 45° chamfer only where fingers or stress demand it, and skip the rest. Printed parts rarely need jewelry-grade fillets.

Patterns: don't place sixteen holes by hand

Pattern (array) repeats a feature: four mounting holes, a row of vents. In OpenSCAD it's a for loop — see OpenSCAD in Depth. In parametric tools it's the Pattern tool. Either way, the idea is the same: define it once, repeat it with spacing.

The feature list of a real part

Look at the L-bracket you'll build in the project chapter through this lens:

Base plate — rectangle, extruded 5 mm.
Two holes — circles, cut through.
Upright — rectangle, extruded 5 mm, joined to the base.
Two more holes — cut through the upright.
Gusset — triangle, extruded 5 mm.

Five features. That's a Tuesday evening, not a thesis.

Next: OpenSCAD in Depth