Why Moving a Sketch Element Distorts Others in FreeCAD

When you move a sketch element in FreeCAD and another part of the geometry shifts, twists, or collapses, the issue is almost always caused by an under-constrained sketch interacting with the software’s parametric solver. Because FreeCAD computes sketches based on mathematical relationships rather than static coordinate drawing, moving one entity forces the solver to recalculate the positions of all connected elements. Without enough geometric and dimensional constraints to lock their shapes, the solver adjusts unconstrained elements freely, leading to unwanted distortion.

How the Constraint Solver Works

FreeCAD’s Sketcher workbench relies on a solver that continuously balances every rule you apply to the geometry. When you drag a line, arc, or vertex, you are not simply translating a static drawing; you are feeding new coordinate data into the solver. The solver must keep all existing rules intact while accommodating the new position of your cursor. If the sketch has multiple "degrees of freedom" remaining, the solver finds the mathematically easiest way to fulfill the rules, which often stretches or rotates unsecured entities in unpredictable ways.

Shared Vertices and Coincident Constraints

By default, when lines are connected end-to-end, FreeCAD applies a coincident constraint to merge the vertices. This makes the elements behave like a physical linkage system or a chain. If two lines share a vertex and you drag one line, the shared vertex must move along with it. If the adjacent line does not have a fixed angle, length, or orientation, it will stretch, tilt, or swing around its other endpoint to maintain the connection.

Lack of Defined Dimensions and Orientations

A shape that looks rigid on your screen is not rigid to the solver unless specifically defined. For example, a drawn rectangle without horizontal or vertical constraints is technically just four connected lines that can deform into an irregular parallelogram. If you do not assign constraints such as:

  • Horizontal and Vertical: Restricting lines to purely horizontal or vertical axes.
  • Length and Distance: Explicitly defining how long a line is or the exact distance between two points.
  • Angles: Locking the angular relationship between intersecting elements.
  • Equality: Ensuring symmetrical lines maintain identical lengths.

Without these parameters, the solver treats the size and orientation of those entities as variable.

How to Prevent Sketch Distortion

To keep sketches stable while repositioning or shaping elements:

  1. Constrain Progressively: Rather than drawing an entire complex profile and attempting to reshape it later, apply basic geometric constraints (parallel, perpendicular, tangent, horizontal, vertical) as you draw.
  2. Anchor to the Origin: Constrain at least one critical vertex or center point directly to the sketch origin or axes using dimensional or coincident constraints. This anchors the sketch in 3D space.
  3. Monitor Degrees of Freedom: Check the solver messages in the Tasks panel. A fully constrained sketch turns green and has zero degrees of freedom, meaning no element can distort when selected.
  4. Use Temporary Lock Constraints: If you need to manipulate one section of a sketch without altering another, temporarily apply a Lock constraint (fixing X and Y coordinates) to the vertices you want to stay static, make your adjustments, and replace the lock with proper parametric dimensions afterward.