How to Position a Datum Axis in FreeCAD

A Datum Axis in FreeCAD is an essential reference geometry tool used in the PartDesign Workbench to guide modeling operations, create rotational features, and position complex objects. This guide explains what a Datum Axis is, why it is critical for robust parametric modeling, and the step-by-step process to create, align, and position one accurately using references and attachment offsets.

What is a Datum Axis?

A Datum Axis is an independent, non-physical reference line tied to an active body in FreeCAD's PartDesign workbench. Unlike physical edges, a Datum Axis extends infinitely in parametric space, though it is visualized as a bounded line segment in the 3D viewport.

Engineers and modelers use Datum Axes as:

  • Rotation axes for Revolutions and Grooves.
  • Central reference lines for Polar Patterns.
  • Direction vectors for linear extensions, lofts, or sweeps.
  • Alignment references to avoid the Topological Naming Problem (TNP), ensuring that downstream features reference stable parametric geometry rather than fragile generated edges.

How to Create a Datum Axis

Before positioning a Datum Axis, you must initialize it within your active working body:

  1. Open your project in the PartDesign Workbench.
  2. Ensure the target Body is active (double-click the Body in the tree view so its text turns bold).
  3. Optionally, select existing geometry (a vertex, edge, or face) in the 3D view to use as an initial reference.
  4. Click the Create a datum axis button (represented by a yellow line with an arrow) on the PartDesign toolbar, or navigate to Part Design > Create a datum axis from the top menu.

Positioning the Datum Axis Using References

Once created, FreeCAD opens the Datum Axis parameters dialog in the Combo View Tasks panel. The axis positions itself automatically based on the selected references and the chosen Attachment Mode.

You can select up to four references (faces, edges, vertices, or coordinate planes) to determine its placement:

  • Two Vertices: Selecting two points draws the axis directly through both vertices.
  • Cylindrical Face: Selecting a cylinder or hole automatically aligns the axis along the centerline/axis of revolution of that feature.
  • Planar Face: Selecting a flat plane creates an axis perpendicular (normal) to that surface.
  • Edge: Selecting a straight line or circular edge aligns the axis either collinearly along the edge or concentric to the circle.

If FreeCAD does not choose your intended orientation automatically, open the Attachment mode dropdown in the task panel and manually select the appropriate alignment method (such as Two points, Concentric, or Normal to face).

Fine-Tuning Position with Attachment Offsets

If your reference geometry places the axis nearby but not at the exact target location, use Attachment Offsets:

  1. In the Datum Axis parameters panel, expand the Attachment Offset section.
  2. Enter precise numeric adjustments for translation along the local X, Y, and Z axes to shift the axis into place.
  3. Enter angular values for Yaw, Pitch, and Roll to tilt or rotate the axis relative to its attached position.
  4. Click OK to confirm and lock the positioning.

To modify the position later, double-click the Datum Axis in the Tree View to reopen the parameters panel, or modify the Attachment Offset values directly under the Data tab in the Property View.