A geometric or dimensional rule applied to a 2D sketch that restricts the degrees of freedom of lines, points, or arcs, ensuring the sketch shape is fully defined.
A 2D sketch element has inherent degrees of freedom (DOF): a line has 4 DOF (two endpoints × x,y). Constraints reduce DOF. Geometric constraints include: coincident (two points share location), horizontal/vertical, parallel, perpendicular, tangent, concentric, equal length, and symmetric. Dimensional constraints assign numeric values (length, angle, radius). A fully defined sketch (all DOF = 0, shown in black in most CAD tools) produces stable, predictable geometry that updates correctly when parameters change.
Always fully define sketches before extruding or revolving: under-constrained sketches (shown in blue/yellow) produce geometry that may drift when the model regenerates. In Fusion 360, the sketch DOF count appears in the status bar. Good practice: fix one reference point to the origin or a datum plane first, then add constraints in logical order (geometry constraints before dimension constraints). Use symmetric constraints to avoid hardcoding mirror geometry.
Over-constraining a sketch (conflicting constraints) causes a red sketch error: CAD will not allow redundant constraints. Under-constrained sketches produce non-deterministic models that may behave unexpectedly when opened on a different machine or after software update. Driven (reference) dimensions are shown grayed out and cannot be edited: don't confuse them with driving (active) dimensions.
Related terms: Parametric Modeling, Datum, Extrude
| Field | 3D design, engineering |
|---|---|
| Also called | geometric constraint, parametric constraint, fully defined sketch |
Engineer, author of The Big Book of 3D Printing and additive manufacturing expert