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Basic design conceptsBeginner18 min

From a sketch to a solid

Constrain a sketch, change its dimensions, then extrude or revolve it. Build a small plate to a brief.

DESIGN LAB · Learn by doing

Most mechanical parts begin with a deliberate 2D sketch. You specify not just how it looks, but what must stay true when a dimension changes.

Sketch → dimensions → solid

Start with the rectangle. Anchor it, set its dimensions, and close its outline. Then pull it straight or turn it around its left edge.

  1. 1 · Sketch
  2. 2 · Dimensions
  3. 3 · Solid feature
Sketch profile32 mm20 mm0

Under-constrained: shape or position can still change

This rectangle already has horizontal and vertical constraints. Dimensions control its size; the origin constraint controls its position. Editing a dimension is allowed even in a fully constrained sketch.

Feature

Feature history: edit an earlier dimension and the dependent solid rebuilds.

Your turn

Make an anchored, fully constrained 40 × 24 mm rectangle and extrude it 12 mm.

  • Closed profile
  • Origin and dimensions constrained
  • 40 × 24 × 12 mm extrusion

Keep experimenting. The checklist updates as you work.

Dimensions say how much; constraints say how

A rectangle tool usually adds horizontal, vertical and coincident relationships automatically. Equal keeps lengths equal; parallel keeps directions aligned; tangent joins a line and curve smoothly. A dimension sets a length or angle. Fixing the origin and size removes the remaining freedom in this exercise. Adding conflicting dimensions can over-constrain a sketch; remove the conflict rather than adding more locks.

Straight path or circular path?

Extrude sweeps a profile in a straight direction: plates, brackets and boxes. Revolve turns a profile about an axis: knobs, wheels and turned parts. Here a rectangle touching its axis makes a solid cylinder at 360°. A partial revolution leaves a sector. The angle controls how far it turns, not the cylinder’s diameter. Width becomes the radius in this example.

A real FreeCAD sketch: dimensions and geometric relationships describe the plate before it becomes a solid. Screenshot from 2017; the interface may differ today.Ex.ungue.leonem · CC0 1.0 · Wikimedia Commons
The same plate as a solid: its outline becomes side faces, and the inner circle becomes a through-hole. Notice the rounded corners.Ex.ungue.leonem · CC0 1.0 · Wikimedia Commons

Words you can now use

Sketch plane
The flat reference surface where your 2D geometry lives.
Closed profile
A complete loop without gaps. These exercises use it to define a solid region.
Constraint
A relationship such as coincident, equal, horizontal or tangent.
Feature history
The sequence of modelling steps and dependencies you can return to and edit.

Use it in your CAD

In your CAD, create a sketch on a principal plane, draw a rectangle, add dimensions, finish the sketch, then choose Extrude/Pad or Revolve. Select the closed region rather than a single edge when making a solid. Reopen the sketch to change a dimension and inspect the rebuilt result.

Check yourself

Pick an answer for each question.

1. Your rectangle has width and height dimensions but can slide. What is missing?

2. Which feature makes a solid cylinder from a rectangle touching its axis?

3. How should you change an editable plate from 40 mm to 45 mm wide?

Sources (9)

Figures such as temperatures and drying times are typical ranges. Always check the label on your own spool and your printer manufacturer's guidance.