Lei Feng Fab Academy 2025

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Mechanical & Machine Design ​

Course Overview ​

This section focuses on the core principles and practical methods of mechanical design and machine automation. The course is divided into two main parts: the first introduces the design principles and key components of mechanisms; the second explores the drive, control, and integration technologies required to turn mechanisms into automated machines. This week's assignment is a group project, requiring students to collaborate to design and build a functional machine, demonstrating the full process from mechanism design to automation control.

Detailed Course Content ​

1. Basic Concepts and Principles ​

1. Definition of a Machine ​

A complete machine consists of four key elements:

  • Mechanism: The structural part that enables motion
  • Actuation: The power source that provides motion
  • Automation: The system that controls motion
  • Application: The actual use of the machine

2. Fundamentals of Material Mechanics ​

  • Stress-strain curve: The behavior of materials under force
    • Stress: The external force applied ("stress sounds like press")
    • Strain: The material's response ("strain sounds like pain")
    • Linear region: Elastic deformation, reversible
    • Nonlinear region: Plastic flow, permanent deformation
    • Failure point: Ultimate strength of the material
  • Key parameters:
    • Stiffness: Slope of the curve
    • Strength: Failure point
    • Toughness: Area under the curve

3. Structural Design Principles ​

  • Constraints and Degrees of Freedom (DoF): According to James Clark Maxwell's theory
    • Structures need appropriate constraints to prevent unwanted motion
    • At the same time, retain necessary degrees of freedom
    • Example: A bookshelf wobbles without cross-bracing; adding bracing stabilizes it

4. Backlash and Hysteresis ​

  • Hysteresis and Backlash:
    • Gaps in mechanical joints cause positional inaccuracy when changing direction
    • Example: The position error when a nut rotates back and forth on a screw

5. Force Loops ​

  • Force Loops: The path of force transmission in a machine
    • Example: In a milling machine, from the cutting tool to the workpiece through all connections
    • Precision depends on the cumulative error in the force loop
    • Design principle: Minimize the force loop to reduce error accumulation

6. Kinematic Coupling ​

  • Kinematic Coupling: High-precision positioning method
    • Uses combinations of spheres and grooves to achieve micron-level positioning
    • Three balls and three grooves yield only one stable position, ensuring repeatable alignment

7. Precision and Accuracy ​

  • Precision: Repeatability, how closely results cluster
  • Accuracy: How close results are to the target value
  • The ideal machine has both high precision and high accuracy

2. Machine Materials and Components ​

1. Common Materials ​

  • Plastics: e.g., HDPE
  • Metals: e.g., aluminum profiles
  • Rubber and foam: For energy absorption
  • Garolite: Machinable PCB material
  • Wood: Not recommended for precision machines (sensitive to temperature/humidity)
  • Cement: Provides stiffness and mass
  • Ceramics: For high-hardness parts

2. Fasteners ​

  • Nuts and bolts:
    • Plain washer: Distributes load
    • Lock washer: Prevents loosening
    • Lock nut: Built-in elastomer prevents loosening
  • Other fastening methods:
    • Heat set inserts: Threaded connections in 3D-printed parts
    • Rivets: Quick, permanent connections
    • Pins: Prevent movement, limit travel

3. Frame Design ​

  • Profile frames:
    • Aluminum profiles: Low cost, high stiffness
    • T-slots: Easy to mount components
    • Accessories: Sliding nuts, angle brackets, etc.
  • Self-aligning connections:
    • Machined parts can be designed with self-aligning features
    • Snap-fit connections enable accurate assembly
    • Advantages: Detachable, low assembly error

3. Transmission Systems ​

1. Gear Systems ​

  • Involute gears:
    • Most common gear type
    • Feature: Single-point contact during meshing, smooth motion
    • Use gear generators for accurate tooth profiles
  • Other gear types:
    • Cycloidal gears: Easier to machine but less efficient
    • Helical gears: Smoother, quieter operation
    • Herringbone gears: Self-aligning, eliminate axial forces
  • Reduction systems:
    • Planetary gears: Compact reducers
    • Harmonic drives: High reduction ratio, used in robot joints

2. Linear Transmission ​

  • Rack and pinion:
    • Converts rotary motion to linear motion
    • Can be custom-made to any length
  • Lead screw:
    • Ordinary lead screws have backlash
    • Anti-backlash nut: Spring preload reduces gap
    • Ball screw: Circulating balls reduce friction, smoother motion
  • Non-threaded linear drive:
    • Uses three heads on hardened rails for stable linear motion
  • Belt drive:
    • Reinforced timing belts are inextensible but flexible
    • Used to distribute force in machines
  • Capstan drive:
    • Uses wire or fishing line as the transmission element
    • Tensioning and winding increase friction
    • Advantages: Flexible shape, easy to customize

3. Guide Systems ​

  • Guide shafts: Hardened steel rods for sliding guidance
  • Guide rails: Linear bearings move along rails
  • Slides: Simple guides for low-precision applications

4. Bearings and Couplers ​

  • Bearing types:
    • Rotary bearing: Most common
    • Thrust bearing: For vertical loads
    • Linear bearing: For linear motion
    • Turntable bearing: For rotary loads
    • Sleeve bearing: For light loads
  • Bearing preload:
    • Apply slight axial force to bearings
    • Ensures good contact between balls and grooves, reduces noise and vibration
  • Couplers:
    • Connect motors to transmission components
    • Accommodate slight angular misalignment
    • Reduce vibration and noise

4. Mechanism Design ​

1. Flexure Mechanisms ​

  • Flexures:
    • Use material elasticity for smooth motion
    • Suitable for precision applications with limited travel
    • Example: Fine focusing mechanism of open-source microscopes

2. Series Elastic Transmission ​

  • Series elastic:
    • Add spring elements between motor and load
    • Control force instead of position for smoother motion
    • Similar to the way human muscles work

3. Linkage Mechanisms ​

  • Various linkage combinations: Convert between different motion types
  • Pantograph: Amplifies or reduces motion

4. Special Mechanisms ​

  • Delta robot: Three linear motions combine to create 3D movement
  • Hexapod: Six linear actuators enable full 6-DoF movement
  • CoreXY: Two fixed motors coordinate XY-plane motion
    • Advantage: Low moving mass, fast speed
    • Principle: Both motors rotate in the same direction for X, opposite for Y
  • Folding mechanisms (SARS): Use folding plates for linear motion
  • Hang printer: Uses cables for 3D spatial positioning
  • Art and mechanism design:
    • Chuck Hoberman's deployable structures
    • Theo Jansen's wind-powered Strandbeests

5. Machine Automation ​

1. Drive and Control ​

  • Motor types:
    • Stepper motor
    • Servo motor
    • Geared motor
  • Control methods:
    • Open-loop control: No feedback, position estimated by motor steps
    • Closed-loop control: Encoder feedback for actual position

2. Control Theory ​

  • Bang-bang control:
    • Simple on/off control, motion is not smooth
    • Sudden acceleration/deceleration, prone to overshoot
  • PID control:
    • Proportional: Error correction
    • Integral: Corrects accumulated error
    • Derivative: Suppresses rapid changes
  • Model predictive control:
    • Uses machine models to predict future behavior
    • Can use physical models or machine learning
    • Suitable for complex, high-precision systems

3. Machine Networking ​

  • Centralized control: Single controller manages all motors and sensors
    • Advantage: Simple
    • Disadvantage: Poor scalability, hard to modify
  • Distributed control:
    • Each part of the machine is networked in real time
    • Each motor is independently controlled and coordinated via network
    • Advantage: Modular, easy to expand and modify

4. Machine Instructions and Interfaces ​

  • G-code: Traditional CNC machine instructions
    • Long history, still widely used
    • Interpreters convert G-code to motor control signals
  • User interfaces:
    • Functions: Visualize G-code, edit instructions, real-time control, set origin
    • Open-source options: UGS, CNC.js, Chili Pepper, Candle, etc.
  • Path planning:
    • Converts design files to machine instructions
    • Includes edge detection, tool diameter compensation, direction control, etc.

6. Modular Machine Design ​

  • Modular concept:
    • Independent functional modules combine to form different machines
    • Interchangeable end effectors
  • Kinematic coupling for tool changing:
    • Use balls and grooves for precise tool changes
    • One machine can perform multiple functions (printing, cutting, milling, etc.)

7. Open-Source Machine Examples ​

  • Open-source machines in Fab Labs:
    • Rumbo: Simplified machine control, motors connect directly to USB
    • Modular Things: Modular system based on real-time networking
    • Jubilee: Modular machine with tool changing
    • Fellow Machines: Series of open-source machines
    • Open Lab Starter Kit: Complete open-source Fab Lab machine kit
  • Commercial spin-offs:
    • Shaper Tools, Ultimaker, Form Labs, etc.
    • Startups originating from Fab Labs

Assignment Requirements ​

This week's assignment is a group project. Requirements:

  1. Machine Design & Automation:
    • Design mechanical mechanisms and implement automation control
    • Test mechanisms manually before adding motor control
  2. Teamwork:
    • Collaborate on mechanism design, motor control, end effectors, user interface, etc.
    • Each lab can build a machine, or collaborate across labs
  3. Documentation:
    • Create a group page documenting the entire machine project
    • Personal pages document individual contributions
    • Include demo videos and detailed explanations
  4. Project Presentation:
    • Prepare a short demo (~1 minute)
    • Plan for a global presentation in two weeks

Learning Resources ​

Suppliers & Materials ​

  1. Mechanical parts suppliers:
  2. Motors & Control:
    • Stepper and servo motor suppliers
    • Controllers: PLUS, Rumba, Modular Things, etc.

Reference Projects ​

  1. Mechanical references:
    • Kentucky Lab's clock project
    • Nadia and Jonathan's self-aligning joints
    • Yen's custom rack and pinion
    • Quentin's capstan drive
  2. Open-source machine projects:
  3. Control system references:
  4. Creative references:

Further Learning Resources ​

With these resources, students can design and build their own machines, from simple mechanisms to complex automated systems, realizing the Fab Labs' core goal of "making your own tools."