FreeCAD Path Lead-In and Lead-Out Moves

This guide provides an overview of configuring lead-in and lead-out strategies within the FreeCAD Path (CAM) workbench. You will learn why these cutting transitions are critical for surface finish, how to locate and modify entry and exit properties in 2.5D profile operations, and how to verify toolpaths using the built-in simulator to prevent workpiece gouging or tool breakage.

Purpose of Lead-In and Lead-Out Moves

Directly plunging an end mill onto a cut line or retracting it straight out creates localized stress, surface blemishes (witness marks), and premature tool wear. Lead-in and lead-out moves introduce the cutter tangentially or at a controlled angle outside the finished part boundary. This ensures the tool reaches full cutting depth in waste stock or open air before engaging the critical edge.

Enabling Leads in FreeCAD Operations

Lead transitions are most commonly configured within Profile and Contour operations.

  1. Open the Operation Settings: In the Tree view, double-click your existing Path operation (e.g., ProfileBasedOnEdge or ProfileBasedOnFace), or select it and inspect the Data tab in the Property view.
  2. Locate Start and End Controls: Switch to the operation's parameters dialog. Look for the tabs labeled Lead or check the property list under the Path category.
  3. Set the Start Point: Enable a defined start point if you need the cut to begin at a specific coordinate rather than an automatically generated edge vertex.

Key Lead-In and Lead-Out Properties

FreeCAD manages lead moves via specific properties located in the Data panel:

  • Lead In Style / Lead Out Style: Specifies the path geometry taken by the cutter.
    • None: The tool engages directly at the contour boundary.
    • Line (Linear): Approaches the contour along a straight path at a defined angle.
    • Arc (Circular): Approaches the contour along a smooth circular tangent, which is ideal for avoiding dwell marks.
  • Lead In Length / Lead Out Length: The distance the tool travels during a straight-line approach before contacting the part geometry.
  • Lead In Radius / Lead Out Radius: The radius of the circular arc path when an arc transition is chosen. Set this to at least the radius of the active cutting tool to provide adequate clearance.
  • Lead In Angle / Lead Out Angle: The angular relationship between the lead move and the cut direction. A 90-degree angle provides a perpendicular entry, while angles between 30 and 45 degrees provide smoother cutting-force transitions.

Overlap Settings

When machining closed profiles, setting an Overlap value works alongside lead moves to eliminate seams:

  1. Select the Profile operation in the Tree view.
  2. Under the Data tab, locate Overlap.
  3. Enter a small distance (typically 1–2 mm, or slightly larger than the tool radius).
  4. The tool will cut past the original entry point by this distance before executing the lead-out move, ensuring no material tab remains.

Verifying and Troubleshooting

  • Check for Interferences: Arc and linear leads require open space. If your lead radius is larger than the surrounding stock margin or an adjacent feature, the tool may gouge adjacent surfaces.
  • Toolpath Visibility: Toggle the toolpath visibility on (Spacebar) and zoom in to confirm that the green (lead-in) and red (lead-out) directional vectors smoothly meet the profile geometry.
  • CAM Simulator: Click the Cam Simulator tool in the Path toolbar to step through the operation. Confirm the tool descends outside the cut boundary, smoothly transitions into the material, and exits completely before retracting along the Z-axis.