How to Create a Subtractive Loft in FreeCAD

A subtractive loft in FreeCAD allows you to carve a complex, transitioning cavity out of an existing solid body by interpolating between two or more cross-sectional profiles. This guide covers the essential steps to prepare your base geometry, position your sketch profiles, and use the Subtractive Loft tool in the Part Design workbench to generate a clean, customized cut through your model.

Prerequisites

Before generating a subtractive loft, ensure you meet the following conditions:

  • You are working within the Part Design workbench.
  • You have an active Body containing a base solid (such as a Pad or Revolution) to cut into.
  • You have at least two closed, non-intersecting Sketches positioned on different planes or faces along the intended path of the cut.

Step 1: Prepare the Cross-Section Sketches

  1. Create or select your first sketch on a face or datum plane where the cut will begin. Ensure the sketch geometry is fully closed.
  2. Create a second sketch (and any subsequent sketches) along the path where the loft should travel. You can place these sketches on parallel faces, offset datum planes, or angled datum planes.
  3. Exit the sketcher for all profiles so that only the 3D model view is active.

Step 2: Initialize the Subtractive Loft

  1. In the Tree View, select the first sketch you want to use as the start of the cut.
  2. In the top toolbar, click the Subtractive Loft icon (represented by an orange transitional shape being cut away from a block), or go to Part Design > Subtractive Loft from the top menu.
  3. The Loft Parameters dialog will appear in the Tasks panel on the left.

Step 3: Add Profile Sections

  1. Under the Sections list in the Tasks panel, click the Add Section button.
  2. In the 3D view, click an edge of the second sketch you want the loft to transition into. Its name will now appear in the list.
  3. Repeat this process for any additional sketches along the path, selecting them in sequential order.
  4. If you accidentally select the wrong sketch, highlight it in the list and click Remove Section.

Step 4: Configure Loft Settings and Finalize

  1. Ruled Surface: Check this option if you want straight, linear transitions between sketches instead of smooth, curved transitions.
  2. Closed: Check this if you are creating a closed-loop cut that loops back from the final sketch to the first.
  3. Preview the cut in the 3D view to verify that the geometry does not self-intersect or invert.
  4. Click OK to apply the feature.

Best Practices for Error-Free Lofts

  • Vertex Counts: Lofts generate most reliably when the connecting sketches share a similar number of vertices and a similar overall shape.
  • Orientation: Ensure that sketches are oriented logically along the direction of the cut; extreme twisting between profiles can cause geometry kernel errors.
  • Datum Planes: Use datum planes rather than model faces to attach your secondary sketches to prevent topological naming issues if the underlying base solid changes.