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Parametric sketching with Apple Pencil

A parametric sketch is one where the sizes and relationships drive the shape. Change a number and the drawing follows.

Dimensions and constraints

120 120
Write a size and the line takes it.

A driving dimension sets a size; a constraint sets a relationship. Change the number and the shape follows.

Horizontal Vertical Parallel Perpendicular Equal Coincident On outline Tangent Concentric Midpoint Symmetry Angle Fix
The thirteen constraints, with PencilCAD’s glyphs.

Degrees of freedom

80 40 8 free 7 free 6 free 5 free 4 free 3 free 2 free Sketch fully defined
Shape takes 4 freedoms, sizes 2, the fixed corner the last 2.

Each constraint removes freedom. At zero the sketch is fully defined, and later edits behave.

Over-constrained

80 40 (80) 80 Already defined, so it’s a reference Drive instead Cancel
A size it already has lands as a reference: (80).

One constraint too many and they can’t all hold. Remove or replace the one you care about least.

A good order to work in

Shape first
30 75
Then sizes, from one edge
Then position
60 60
Don’t double up

Shape, then sizes from one edge, then position. Dimension equal lines once.

Parametric sketches in PencilCAD

Draw a mark Write a size Equal Parallel Perpendicular Coincident 120 3 1/2 2′6″ R20 ⌀40 45°
Marks and sizes it reads.

PencilCAD reads the marks you’d make on paper.

  1. Tap the toolset chip beside the page count and choose CAD, or use Settings → Tools, and sketch with Line, Rectangle or Circle.
  2. In CAD, write a size beside a line with Apple Pencil.
  3. Draw a mark, or open All constraints in the constraint column for the full list.
  4. Tap AutoConstrain to add what’s missing. Fully defined glows green.
  5. On a clash, Replace swaps out the one in the way.

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