Geneva Mechanism

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Master parametric CAD design by creating a Geneva mechanism: the precision indexing system that powers film projectors, machine tool changers, and precise rotary positioning. Learn FreeCAD through intermittent motion engineering. #FreeCAD #GenevaDrive #IntermittentMotion #IndexingMechanism

🎯 Learning Objectives

By the end of this lesson, you will be able to:

1. Design polar geometry with angular patterns and radial features
2. Create parameter-driven angular features with precise indexing
3. Apply circular patterns for repeated slot features
4. Build intermittent motion mechanisms with locking geometry
5. Generate indexed position documentation and timing diagrams

🔧 Engineering Context: Why This Mechanism Matters

Real-World Applications

The Geneva mechanism appears everywhere precise indexing is needed:

The Engineering Problem

Design Challenge: Given continuous rotational input, how do we create intermittent, precise angular output with exact dwell periods and self-locking to prevent drift between indexes?

📚 Mechanism Fundamentals

Components and Motion

A Geneva mechanism consists of three key elements working together:

Mathematical Relationships

For an external Geneva mechanism with n slots:

🎯 What You’ll Build

By completing this lesson, you’ll create:

🚀 Part 1: Getting Started with FreeCAD

Installing FreeCAD

1. Download FreeCAD 0.21 or later

   Visit www.freecad.org

   Free & Open Source Cross-Platform

2. Install and launch

   Follow the installer for your operating system

3. Create new document

   File → New (Ctrl+N)

   Save as “GenevaMechanism.FCStd”

Understanding Workbenches for This Project

💡 Part 2: Parametric Design Strategy

Our Design Approach

Our Geneva Configuration

We’ll design a 6-slot external Geneva mechanism:

📊 Part 3: Creating the Parameter Spreadsheet

Building Your Parameter Table

1. Ensure you’re in Part Design workbench

   Use the workbench dropdown at top

2. Insert a spreadsheet

   Insert → Spreadsheet

   A “Spreadsheet” object appears in the left tree

3. Double-click to open the spreadsheet

   Click on “Spreadsheet” in the tree

Entering Parameters with Formulas

Close the spreadsheet when done.

Your intelligent parameter foundation is ready!

🔩 Part 4: Creating the Driver Wheel

Design Intent

Step-by-Step: Driver Wheel

1. Create a Body

   • Ensure you’re in Part Design workbench
   • Click Create Body button
   • A “Body” object appears in the tree
   • Right-click → Rename → type Driver

2. Create a Sketch

   • Select the “Driver” body in tree
   • Click Create Sketch button
   • Dialog asks: Choose a plane
   • Select XY_Plane
   • Click OK

   You’re now in Sketcher workbench (automatic switch)

Drawing the Driver Profile

Creating 3D: Pad Operation

1. Close the sketch

   Click the Close button in the toolbar

2. Pad the profile

   • Select the sketch in the tree (if not selected)
   • Click Pad tool in Part Design toolbar
   • In the Pad panel (left side):
     • Type: Dimension
     • Length: Click ƒx button
     • Type: Spreadsheet.WheelThickness
     • Click OK

You now have a parametric 3D driver wheel!

Press V then F to fit view and admire your work.

🔗 Part 5: Creating the Geneva Wheel (Driven Wheel)

Design Intent

Step-by-Step: Geneva Wheel

1. Create new Body

   • Part Design workbench
   • Create Body
   • Rename: GenevaWheel

2. Create Sketch

   • Select GenevaWheel body
   • Create Sketch → XY_Plane

Drawing the Geneva Wheel Profile

Creating 3D: Pad Operation

1. Close the sketch

   Click Close button

2. Pad the profile

   • Select sketch
   • Click Pad tool
   • Length: Click ƒx button
   • Type: Spreadsheet.WheelThickness
   • Click OK

The Geneva wheel is complete!

Rotate the view to see the 6 radial slots cutting through the wheel!

🏗️ Part 6: Creating the Base Frame

Design Intent

The frame provides the fixed reference for both rotating wheels:

Creating the Frame

1. Create new Body

   • Part Design workbench
   • Create Body
   • Rename: Frame

2. Create Sketch

   • Select Frame body
   • Create Sketch → XY_Plane

3. Draw base plate

   • Rectangle tool
   • Draw centered rectangle
   • Width: 200 mm
   • Height: 120 mm
   • Use Symmetric constraints to center about origin

4. Add driver shaft hole

   • Circle at origin
   • Radius: 12 mm (shaft radius + clearance)

5. Add Geneva wheel shaft hole

   • Circle to the right of origin
   • Horizontal constraint with origin
   • Distance constraint from origin: Click ƒx → Spreadsheet.DriverCenterDistance
   • Radius: 12 mm

6. Add mounting holes (optional)

   • Four circles at corners
   • Radius: 6 mm (for M10 bolts)
   • Position symmetrically

7. Close and Pad

   • Close sketch
   • Pad: 20 mm

Frame complete!

All three parts are now ready for assembly!

🧩 Part 7: Assembly

Assembly Strategy

Creating the Assembly

Part 8: Technical Drawing

Creating Geneva Mechanism Drawing

1. TechDraw Workbench Switch to TechDraw workbench from the dropdown

2. Create Page Insert → New Page and select A3_Landscape template

3. Add Top View Insert view showing both wheels in plan view with clearly visible pin, slots, and center distance

4. Add Multiple Position Views Create 3 additional views showing different phases:

   • View 1: Driver at 0° (pin entering slot 1)
   • View 2: Driver at 60° (mid-index, Geneva rotating)
   • View 3: Driver at 120° (pin exited, dwell begins)

5. Dimension Key Features Add dimensions for center distance, slot angle (60°), pin diameter, and number of slots

6. Add Timing Diagram Sketch a diagram showing X-axis as driver rotation (0° to 360°), Y-axis as Geneva wheel position (discrete steps at 0°, 60°, 120°, 180°, 240°, 300°), and mark dwell periods (horizontal lines) and index periods (vertical jumps)

7. Export to PDF File → Export → Select PDF format

Part 9: Testing Parametric Control

Change Number of Slots

1. Open Spreadsheet
2. Change NumSlots from 6 to 4
3. Recompute

All geometry updates:

• Slots now 90° apart
• Index angle = 90°
• Driven radius recalculated
• 4 slots appear on Geneva wheel

Test different values:

• NumSlots = 8 (45° indexing)
• NumSlots = 5 (72° indexing)

Change Center Distance

• Increase DriverCenterDistance to 100mm
• Larger mechanism
• All ratios maintain

Learning Outcomes

By completing this lesson, you have:

• ✅ Designed with polar geometry and angular patterns
• ✅ Created parameter-driven angular features
• ✅ Applied circular patterns for repeated features
• ✅ Designed intermittent motion mechanism
• ✅ Used angular constraints precisely
• ✅ Generated indexed position documentation
• ✅ Understood locking geometry principles

Design Verification

1. Do all slots align radially from center?
2. Are slots evenly spaced (60° apart for 6-slot)?
3. Does driving pin fit into slots with clearance?
4. When NumSlots changes, does geometry update correctly?
5. Is the center distance appropriate for smooth pin engagement?

Mathematical Background

Geneva Wheel Radius Calculation

For an external Geneva mechanism:

Where:

• = Geneva wheel radius (center to slot entry)
• = Driver radius (center to pin)
• = Number of slots

For n = 6:

If , then .

Dwell vs. Index Time

Dwell angle:

Index angle:

For 6-slot Geneva:

• Dwell: of driver rotation
• Index: of driver rotation

Challenges for Further Practice

1. Design an internal Geneva mechanism (pin inside driven wheel)
2. Add a safety guard covering moving parts
3. Create a double-Geneva (two Geneva wheels on same driver)
4. Design a spherical Geneva (3D version for spatial indexing)
5. Add position sensors (mounting for optical sensors to detect index positions)
6. Model wear analysis (contact stress at pin/slot interface)

Common Issues and Solutions

“Slots not evenly spaced”

• Cause: Polar pattern not set to correct angle or count
• Solution: Verify SlotAngle = 360 / NumSlots
• Fix: Check pattern constraints reference spreadsheet parameters

“Pin doesn’t fit in slot”

• Cause: Slot width too narrow
• Solution: SlotWidth should be 1.2-1.5 × PinDiameter
• Fix: Adjust SlotWidth parameter

“Geneva wheel radius seems wrong”

• Cause: Incorrect TAN formula or units
• Solution: Verify formula: =B8*TAN(180°/B2)
• Fix: Check that TAN function uses degrees (or convert with RADIANS())

“Mechanism binds during motion”

• Cause: Center distance incorrect or slot angle wrong
• Solution: Recalculate center distance using correct geometry
• Fix: Verify all geometric relationships

Next Steps

In the final lesson on Scotch Yoke Mechanism, we’ll explore:

• Slot-driven motion
• Sinusoidal motion profiles
• Sliding contact geometry
• Clearance considerations
• Stroke-based parameterization