How to Create a Slot Milling Toolpath in FreeCAD

This guide explains how to generate a slot milling toolpath using FreeCAD's Path (CAM) workbench. You will learn how to set up a machining job, select the correct geometry, configure tool parameters and cutting depths, and simulate the generated toolpath to prepare your project for CNC milling.

1. Set Up the Path Job

Before creating any cutting operations, you must define the machining environment:

  1. Switch to the Path (or CAM in FreeCAD 0.21+) workbench via the workbench dropdown menu.
  2. Select your solid 3D model in the tree view.
  3. Click the Create Job button (represented by a yellow folder with a gear icon).
  4. In the Job setup dialog, define your stock boundaries, set the machining origin (work coordinate system), and assign an end mill from your tool library that matches the slot width or is smaller than the slot width.
  5. Click OK to create the job container.

2. Select the Geometry

Locate the slot feature on your 3D model:

  • If the slot goes completely through the stock, select the bottom face or bottom contour edges of the slot.
  • If the slot is a blind slot (has a floor), select the bottom planar face of the slot.
  • Ensure you select only the geometry that defines the slot boundaries to prevent unnecessary tool movements.

3. Apply the Slot Operation

FreeCAD provides a dedicated slotting feature as well as profile-based slotting methods:

Using the Dedicated Slot Tool

  1. With the geometry selected, click the Slot operation icon from the Path workbench toolbar (or navigate to Path > Slot).
  2. The operation editor will open in the Task panel.

Alternative: Using Profile or Pocket

If the standard Slot tool does not fit your specific geometry, use the Pocket Shape or Profile tool:

  • Use Pocket Shape if the cutter diameter is smaller than the slot width, requiring multiple step-overs to clear the material.
  • Use Profile (set to "Inside") if you want the tool to trace the internal perimeter of the slot.

4. Configure Depths and Heights

Navigate to the Depths tab in the operation properties panel:

  • Start Depth: Set this to the top surface of the stock or the top of the slot (usually 0.0 mm if measured from the stock top).
  • Final Depth: Specify the bottom depth of the slot (e.g., -5.0 mm). FreeCAD often detects this automatically if a bottom face was selected.
  • Step Down: Enter the maximum depth the tool should cut per pass. Avoid setting this deeper than your tool's recommended depth of cut.

5. Set Operation Parameters

Switch to the Operation tab to adjust cutting behavior:

  • Tool Controller: Confirm that the correct milling cutter is assigned.
  • Cut Mode: Choose between Climb (recommended for modern CNCs) or Conventional milling.
  • Pattern: If clearing a wide slot, select an appropriate clearing pattern such as Line or ZigZag.
  • Extend At Start / End: Adjust extensions if you want the cutter to enter or exit beyond the material boundary when cutting through-slots.

6. Verify and Simulate the Toolpath

  1. Click Apply, then OK to compute the toolpath. Green lines indicating the cutting paths and rapid movements will appear in the 3D viewport.
  2. Select the Job in the tree view and click the CAM Simulator button.
  3. Play the animation to verify that the tool cuts to the correct depth, clears the slot properly, and does not collide with adjacent walls or clamps.
  4. Once verified, click the Post Process button to export the G-code suitable for your CNC machine's controller.