PRIME MOTIONConstraints
A drawing is a set of lines that happen to be where you put them. A constrained sketch is a set of lines that know why they are there, and that stay right when something changes. Draw a four-sided outline from independent lines and stretch one side: nothing holds. Constrain it, sides horizontal and vertical, one dimension on the width and one on the height, and changing the width gives a wider rectangle.
All constraint tools are in the sketch rail's Constrain family, which starts on Dimension.
Dimensions
Dimension — tap an entity to lock its size, radius, or angle. Tap a line for its length, a circle for its diameter, an arc for its radius, or two lines for the angle between them. The Dimension keypad opens beside the drawer; type the value, or ƒx a parameter, and ✓.
Distance — tap two points, or a point and a line (perpendicular), then enter the distance.
The sketch origin is a real point pick. With Dimension, tap the origin first and then an endpoint or corner to set their distance; you do not need to draw a helper point at zero.
If ✓ cannot accept the entry, a red line under the field says why: Not a valid expression, Must be greater than zero, or Must be zero or more, depending on the field. Correct the entry and try ✓ again; the message clears when you type the next key.
Reference (driven) in the keypad's panel makes the dimension a read-out: it shows the size and updates with the sketch, but does not drive it.
An unfinished expression such as 7÷ shows Not a valid expression under the field when you tap ✓. The message clears on the next key. Correct the expression before committing it.
To change a dimension, tap its value label. Drag a selected dimension's label to move it out of the way.
Geometric constraints
Each says something about the shape, without a number. Most take two picks: tap the first (it stays highlighted), then the second.
| Constraint | Tap | Says |
|---|---|---|
| Coincident | Two points, or a point then a curve | These points are the same point, or this point lies on this curve |
| Collinear | A line, then another | These lines lie on the same line |
| Concentric | A circle, arc or ellipse, then another | These share a centre |
| Tangent | A line or curve, then a curve | They meet smoothly, with no kink |
| Smooth | A spline, then another | They meet with matching curvature too |
| Equal | An entity, then another of the same kind | Same length, or same radius |
| Parallel | A line, then another | Parallel |
| Perpendicular | A line, then another | At right angles |
| Horizontal | A line | Horizontal |
| Vertical | A line | Vertical |
| Symmetric | Two points or two curves, then the mirror line | Mirrored about the line |
| Midpoint | A line, then a point | The point sits halfway along the line |
| Fix/Lock | A point or entity | It does not move |
When the pen hovers over geometry a constraint can take, it tints green.
Use the origin and construction geometry
For Coincident, pick a point and then the origin, or pick a construction line to place the point on that line. Construction endpoints are point picks too. Dashed reference geometry stays outside the filled profile while still guiding its position.
For Symmetric, pick the two points or curves, then tap the actual sketch X or Y axis as the mirror line. The origin and its axes provide fixed references without extra drawn geometry.
Constrain projected body edges
In a sketch on a body face, Project the body edge first. A circular bore edge remains a circle: use Concentric between it and a new circle, then Dimension the new diameter. A projected straight edge remains a line: Distance from a circle’s centre to that edge sets the perpendicular distance. Concentric refuses a line with an explanation; choose a circular edge for that relation.
Tangent and Smooth are not the same
Tangent means two curves leave each other in the same direction. Smooth also matches how sharply they bend. The difference is invisible on a drawing and obvious on a machined, finished surface: a tangent join catches the light in a line; a smooth one does not.
When a constraint cannot be applied
Some combinations make no sense, and the banner says why, for example Equal requires same-kind entities — both lines, both circles, or both arcs or Can't be Horizontal — this line already has a Vertical constraint.
A constraint that would contradict the others is undone straight away: Over-constrained — Type reverted. A value typed in the keypad that would over-constrain the sketch shows Over-constrained under the field.
Seeing and removing constraints
Constraints show as small badges on the geometry while the Constraint filter in Sketch Options is on. Tap a badge to select it, then Delete to remove it.
Working practice
Constrain before you dimension. Say what the shape is: this is symmetric, these are equal, this is tangent. Then put numbers on the few sizes that are genuinely free. A sketch built that way needs fewer dimensions and behaves better when one changes.
A sketch that can still move will move. Dimension and constrain until the shape is fully defined.
Locking a whole sketch
A sketch's menu in the tree sets it Unrestricted, Locked (your edits are refused) or Frozen (its shape cannot change at all). See The design tree.