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Slot / Arc Slot

In one sentence: Draw a closed slotted profile — two parallel straight ends joined by two semicircles (Slot) or a profile bounded by two concentric arcs (Arc Slot) — in a single operation.

Overview

Workbench: Sketcher · Menu: Sketch → Sketcher geometries → Slot
Toolbar: Sketcher Geometries (Slot dropdown) · Shortcut: G, S (Slot) / G, A, S (Arc Slot)

Tool Shortcut Description
Slot G, S Straight slot: two semicircles joined by two parallel lines
Arc Slot G, A, S Curved slot: two concentric arcs joined by two radial lines

Intuition

A slot is what you get when you drag a circle along a straight path — the circle sweeps out a "stadium" shape with flat sides and rounded ends. Slot creates this shape in one step.

An Arc Slot is the curved version: imagine dragging a circle along an arc instead of a straight line. The result is a profile bounded by two concentric arcs and two radial end-walls — the shape of a curved slot in a flanged plate or a radial adjustment slot on a mounting bracket.

When to use it

  • You need an elongated hole that allows a bolt to slide (straight slot).
  • You need an adjustment slot that follows a bolt circle pattern (arc slot).
  • You are modelling a track, guide channel, or oblong pocket.

When NOT to use it

  • You need an oblong shape that is not aligned to an axis — Slot is easiest when oriented naturally. For an angled slot, use Slot then rotate/mirror it, or draw manually with lines and arcs.
  • You need the slot to be a different shape at each end — draw individual lines and arcs and constrain them manually.

Step-by-step walkthrough

Straight Slot

  1. Press G, S or select Slot from the dropdown.
  2. Click the centre of the first semicircle (one end of the slot).
  3. Click the centre of the second semicircle (the other end). The slot appears spanning between the two centres with a width equal to twice the default radius.
  4. Constrain the slot:

Arc Slot

  1. Press G, A, S or select Arc Slot from the dropdown.
  2. Click the arc centre (the pivot of the curvature).
  3. Click the centre of the first end-cap.
  4. Click the centre of the second end-cap. The arc slot sweeps around the pivot.
  5. Constrain:
    • Inner and outer radius (use Radius/Diameter on the inner and outer arcs).
    • Angular span with Angle.
    • Pivot position.

Parameters

Slot

Parameter Type Applied via
End-cap radius Distance Radius/Diameter on either semicircle
Centre-to-centre length Distance Distance between the two semicircle centres
Orientation Angle Horizontal/Vertical or Angle
Position Coordinate Lock Position or Coincident

Arc Slot

Parameter Type Applied via
Inner radius Distance Radius/Diameter on the inner arc
Outer radius Distance Radius/Diameter on the outer arc
Angular span Angle Angle on the bounding radii
Pivot position Coordinate Lock Position on the pivot point

Common mistakes and pitfalls

Slot not fully constrained after setting radius and length

Cause: The slot still has a rotational degree of freedom (it can pivot about its own centreline) and a positional degree of freedom (it can translate).
Fix: Add a Horizontal or Angle constraint to fix orientation, and a Lock Position or Coincident to fix position.

Arc slot inner and outer radii are equal

Cause: The third click was not far enough from the second, resulting in a zero-width slot.
Fix: Delete and redraw with a greater separation between the inner and outer arcs.

Python API

Minimal example

import FreeCAD as App
import Sketcher
import Part

doc = App.activeDocument()
sketch = doc.getObject("Sketch")

# A horizontal slot: two arcs + two lines
# This is the low-level equivalent of what the Slot tool creates.
# For simplicity, use the GUI tool and constrain programmatically.

# Fix a slot's semicircle radius
# (geo index of first arc is idx_arc0, etc.)
# sketch.addConstraint(Sketcher.Constraint("Radius", idx_arc0, 5.0))
# TODO: verify — the exact GeoId layout produced by the Slot tool
print("Use the Slot tool interactively; constrain via Python after creation.")

See also