Every solid model begins with a 2D sketch, and every 2D sketch lives on a plane. Mastering where to sketch is the first real CAD skill. Here's the universal workflow, with specifics from OpenShape3D on iPad and iPhone.

1. Start on a world plane

With an empty scene, choose one of the three world planes — XY (top), YZ (right), or ZX (front). In OpenShape3D, tap the Sketch tool and pick one of the plane tiles shown at the origin. The camera animates to look at the plane head-on so you draw in true proportions.

Tip: think about which direction the part will "grow." A bottle sketches best on the top plane; a bracket profile often starts on the front plane.

2. Sketch directly on a model face

Once you have a solid, any flat face is a sketch plane. Enter the Sketch tool and tap the face — your new profile is automatically aligned and attached to it. Draw a circle on top of a box, pull it up, and you have a boss; push it down for a pocket. This "sketch on face" loop is the heartbeat of direct modeling.

3. Use offset construction planes

Need to sketch above a face — say, 20 mm over a surface to loft toward? Pull an offset construction plane off any face: in OpenShape3D, drag the offset arrow or type the exact distance, and the plane persists in your model for reuse.

4. Draw the profile

Use the standard toolkit: line (with chain continuation and close-the-loop), rectangle, circle, arc, ellipse, and polygon. Snapping to endpoints, midpoints, centers, and the grid keeps things tidy, and an Apple Pencil makes freehand-then-constrain workflows genuinely pleasant. Closed loops fill automatically, showing you what can be extruded.

5. Return and refine

A sketch isn't consumed when you extrude it. Re-enter the same plane and the same sketch continues — add profiles, trim segments, apply constraints and dimensions. The camera snaps head-on each time so you're never drawing at a skewed angle.

Common beginner mistakes

  • Unclosed profiles: a 0.5 mm gap means no fill and nothing to extrude. Use endpoint snapping and the close-the-loop hint.
  • Sketching on the wrong plane, fixing it with rotations: pick the right plane instead; transforms compound confusion.
  • Ignoring construction geometry: dashed reference lines that don't fill are perfect for symmetry axes and guides.