How to Constrain Parts in FreeCAD Assembly 4

This guide explains how to constrain two parts together using Local Coordinate Systems (LCS) in the FreeCAD Assembly 4 workbench. Unlike traditional constraint solvers based on geometric mates like coincidence or tangency, Assembly 4 relies entirely on matching coordinate systems between components. By creating an LCS on each part and defining their relative orientation and offsets, you can assemble complex mechanical models with high stability and fast recalculation times.

Step 1: Create a Local Coordinate System (LCS) on Each Part

Before attaching parts together, each component must have an explicit coordinate system that serves as an attachment point.

  1. Open or activate the first part file (or PartDesign Body).
  2. In the Assembly 4 workbench, click the New Coordinate System button (the icon displaying red, green, and blue axes).
  3. In the attachment dialog, choose the geometry where you want to anchor the coordinate system, such as a circular edge, a vertex, or a planar face.
  4. Set the attachment mode (for example, Concentric for holes or Plane for flat surfaces) to position the LCS precisely.
  5. Click OK and rename the LCS to something descriptive (e.g., LCS_Mount_Base).
  6. Repeat this process for the second part, creating a corresponding LCS on the mating surface (e.g., LCS_Mount_Pin).

Step 2: Insert the Parts into the Assembly

  1. Create a new document and initialize it as an Assembly 4 model by clicking New Model.
  2. Save the assembly file in the same directory as your parts.
  3. Click the Insert Part button.
  4. Select the first part to insert it into the assembly tree. When prompted, attach its LCS to the assembly’s default LCS_0 origin.
  5. Click Insert Part again to bring in the second component.

Step 3: Constrain the Second Part to the First

  1. In the tree view, select the second part you just inserted.
  2. Click the Edit Placement button (the puzzle piece with coordinate axes) on the Assembly 4 toolbar.
  3. In the task panel:
    • Under Parent, select the first part (or the assembly root if connecting directly to the base).
    • Under Parent LCS, select the LCS you created on the first part (LCS_Mount_Base).
    • Under Part LCS, select the mating LCS on the second part (LCS_Mount_Pin).
  4. The second part will instantly snap so that its LCS aligns with the parent LCS.

Step 4: Fine-Tune the Alignment

  1. If the part is oriented backwards or inverted, check the Flip X, Flip Y, or Flip Z boxes in the task panel.
  2. Use the Rotations fields (Yaw, Pitch, Roll) or the Offsets fields (X, Y, Z) to apply micro-adjustments or dynamic clearances if the parts need a specific gap or angle.
  3. Click OK to confirm the attachment.

The two parts are now fully constrained. Any future modifications to the dimensions or positions of the original parts will automatically propagate to update the assembly alignment.