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Angle Constraint

In one sentence: Fix the angle between two lines (or between a line and the sketch X-axis) to a specific value, in driving or reference mode.

Overview

Workbench: Sketcher · Menu: Sketch → Sketcher constraints → Angle Dimension
Toolbar: Sketcher Constraints · Shortcut: K, A

Angle removes 1 degree of freedom: the relative direction between two lines, or the absolute orientation of one line with respect to the sketch X-axis.

Intuition

Think of a protractor measuring the angle between two edges meeting at a point. Angle Constraint is the protractor that also locks the measurement: the angle can no longer change freely once the constraint is applied.

When to use it

  • Forcing two lines to meet at a specific angle (e.g. 60° for an equilateral triangle, 45° for a chamfer).
  • Fixing the absolute orientation of a single line relative to the sketch X-axis (e.g. a slanted profile edge at 30°).
  • Setting the included angle of an arc.
  • Constraining a regular polygon's orientation by fixing one side's angle.

When NOT to use it

  • You want exactly 90° — use Perpendicular; it is cleaner and does not require entering a number.
  • You want exactly 0° or 180° (lines parallel or collinear) — use Parallel or Tangent.
  • You want a line horizontal or vertical — use Horizontal/Vertical.

Step-by-step walkthrough

Angle between two lines

  1. Select two lines (Ctrl-click each).
  2. Press K, A or activate Angle Dimension.
  3. Click to place the angle arc label at the intersection.
  4. Enter the angle value in degrees and click OK.

Tip

The angle is measured from the first selected line to the second selected line, in the counterclockwise direction. If the result is the supplement of what you expected, select in the opposite order or add 180°.

Absolute angle of a single line

  1. Select one line.
  2. Press K, A.
  3. The angle is measured from the sketch X-axis to the line direction (CCW positive).
  4. Enter the value (e.g. 30 for 30° above horizontal).

Parameters

Parameter Type Default Description
Angle Degrees Current angle The angle to enforce. Range 0°–360°.
Mode Driving / Reference Driving See Dimension → Driving vs Reference.

Common mistakes and pitfalls

Angle constraint gives the supplement angle instead of intended

Cause: The angle is measured between the line directions (vectors), not between the lines themselves. Selecting lines in a different order or with different directions can flip whether you get θ or 180°−θ.
Fix: If you get 150° when you expected 30°, change the value to 30° — the solver will flip the geometry accordingly — or reselect in the other order.

Angle in reference mode reads differently than expected

Cause: The reference angle is computed from the line vectors as drawn; if the lines have been trimmed, the vector direction may be opposite to what you expect.
Fix: Check the arc direction marker (the small arc shown with the label) to confirm which angle is being read.

Python API

Minimal example

import FreeCAD as App
import Sketcher
import Part
import math

doc = App.activeDocument()
sketch = doc.getObject("Sketch")

line0 = sketch.addGeometry(
    Part.LineSegment(App.Vector(0, 0, 0), App.Vector(10, 0, 0)), False)
line1 = sketch.addGeometry(
    Part.LineSegment(App.Vector(0, 0, 0), App.Vector(7, 7, 0)), False)

# Angle of 45° between the two lines (value in radians internally)
sketch.addConstraint(Sketcher.Constraint("Angle", line0, 2, line1, 1, math.pi / 4))
doc.recompute()

Method signatures

# Absolute angle of a line from X-axis
sketch.addConstraint(Sketcher.Constraint("Angle", geoId, math.radians(angle_degrees)))

# Angle between two lines at a shared vertex
sketch.addConstraint(Sketcher.Constraint(
    "Angle", geo1, vertex1, geo2, vertex2, math.radians(angle_degrees)))

Note: the solver stores angles in radians even though the UI shows degrees.

See also