How to Model Screw Threads in FreeCAD

Creating thread representations on a bolt or screw in FreeCAD can be done using standard parametric tools or specialized add-on workbenches. This guide covers the two most common and effective methods: using the Fasteners Workbench for fast, automated standard hardware with toggleable real threads, and using the Part Design Workbench to model custom helical threads from scratch for 3D printing or custom manufacturing.

The Fasteners Workbench is an official FreeCAD add-on that provides pre-modeled, standard screws, bolts, and nuts with an option to generate real 3D threads.

  1. Install the Workbench: Open FreeCAD, navigate to Tools > Addon Manager, locate Fasteners, install it, and restart FreeCAD.
  2. Insert a Fastener: Switch to the Fasteners workbench via the workbench dropdown. Click on any bolt or screw type (such as an ISO 4017 Hex Head Bolt) to drop it into your document.
  3. Configure the Fastener: Select the generated screw in the tree view. In the Data tab of the Property View below:
    • Set diameter (e.g., M6, M8).
    • Set length.
  4. Enable Thread Representation: Find the property labeled thread in the Data tab. Change its value from false to true. FreeCAD will compute and display the fully modeled thread profile on the screw body.

Note: Real threads consume significant computing resources. Keep thread set to false during general assembly design and toggle it to true only when exporting for 3D printing or rendering.

Method 2: Manually Modeling Threads with Part Design

For non-standard, custom, or specialized threads, you can manually cut a thread using the Subtractive Helix tool.

1. Create the Cylinder

  1. Switch to the Part Design Workbench and create a new Body and Sketch.
  2. Draw a circle matching the nominal major diameter of your bolt (e.g., 8 mm for an M8 bolt) and pad it to your desired screw length.

2. Sketch the Thread Profile

  1. Create a new sketch on a plane perpendicular to the face of the cylinder (typically the XZ or YZ plane).
  2. Draw the triangular or trapezoidal profile of the thread cut. For standard ISO metric threads, this is a 60-degree triangle.
  3. Align the profile to the outer edge of the cylinder, ensuring it slightly overlaps the outer boundary to avoid zero-thickness geometry errors.
  4. Position the start of the profile slightly above or below the cylinder end face to ensure clean entry and exit.

3. Generate the Subtractive Helix

  1. Select the thread profile sketch and click the Subtractive Helix tool.
  2. In the parameters panel:
    • Set the Pitch (e.g., 1.25 mm for a standard M8 coarse thread).
    • Set the Height to cover the length of the threaded section.
    • Ensure the Axis is set to the central axis of the bolt cylinder.
  3. Click OK to compute the cut.

Method 3: 2D Symbolic Representation for Technical Drawings

If you only need to represent threads on a 2D technical drawing rather than a 3D model:

  1. Model the bolt as a simple, smooth cylinder corresponding to the nominal outer diameter.
  2. Switch to the TechDraw Workbench and create a projection view of the part.
  3. Use the Cosmetic Thread or Cosmetic Line tools in TechDraw to manually add dashed or offset lines according to technical drafting standards (such as ISO 6410 or ASME Y14.6), avoiding the performance overhead of full 3D geometry.