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Screw Joint

In one sentence: Link the rotation and axial translation of a part on the same axis by a thread pitch so that one full revolution advances the nut by exactly one pitch — the joint for lead screws and threaded fasteners.

Overview

Workbench: Assembly
Menu: Assembly → Joints → Screw
Shortcut: W

A Screw joint couples a Revolute and a Slider on the same axis by a pitch ratio. One full rotation advances the translating part by exactly one pitch length. This is a coupled joint — it does not add new geometric constraints but locks the ratio between the two existing DOF.

Intuition

A lead screw works by converting rotation into precise linear motion: turn the screw one revolution, the nut advances by one pitch. A Cylindrical joint also allows both rotation and translation on the same axis — but independently. The Screw joint removes that independence by saying "every degree of rotation corresponds to exactly this many mm of advance."

The physical analogy: a bolt being threaded into a nut. The bolt cannot rotate without advancing, and cannot advance without rotating.

When to use it

  • Lead screws in CNC machines, 3-D printers, and precision linear stages.
  • Worm drives (combine with a Gears coupled joint on the output shaft for the reduction ratio).
  • Any mechanism where rotation and linear advance are rigidly linked by a thread pitch.

When NOT to use it

  • Don't use Screw if rotation and translation are independent — use Cylindrical.
  • Don't use Screw if the linear motion is on a different axis from the rotation — use Rack and Pinion.

Prerequisites

The two parts must already have joints that establish their independent motion:

Step-by-step

  1. Press W or choose Assembly → Joints → Screw.
  2. Click a cylindrical face on the rotating part (screw axis).
  3. Click the corresponding axis geometry on the translating part.
  4. In the task panel, set the Pitch (mm advance per full revolution).
  5. Click OK.

Parameters

Parameter Type Default Description
Reference 1 Link — Axis geometry on the rotating part
Reference 2 Link — Axis geometry on the translating part
Pitch Length — Linear advance per full revolution (mm/rev)

Common metric pitch values:

Thread Pitch (mm/rev)
M2 0.4
M3 0.5
M4 0.7
M5 0.8
M8 1.25
T8×2 (lead screw) 2.0
T8×8 (4-start lead screw) 8.0

Python API

Minimal example

import FreeCAD as App

doc = App.ActiveDocument
asm = doc.getObject("Assembly")

joint = asm.Joints.newObject("Assembly::JointObject", "LeadScrew")
joint.JointType = "Screw"
joint.Reference1 = (doc.getObject("Screw"), ["Face3"])
joint.Reference2 = (doc.getObject("Nut"), ["Face1"])
joint.Pitch = 2.0   # 2 mm advance per revolution
doc.recompute()

Common mistakes and pitfalls

Adding Screw before the Revolute and Slider exist

The Screw joint requires the rotating part to have a Revolute joint and the translating part to have a Slider joint. Add those first.

Confusing Screw and Cylindrical

Both allow rotation and translation on the same axis. Cylindrical makes them independent; Screw locks them by a ratio. If you set up a Cylindrical joint and notice the simulated motion doesn't advance the nut, replace it with a Screw joint and set the pitch.

Pitch is per revolution, not per radian

FreeCAD's Pitch parameter is in mm per full revolution (360°), not per radian. A common mistake is entering the pitch in mm/rad which gives a ratio 2π times too small.

See also