John: Created page with "{{Tutorial |name=Mechanical Design |competency=Mechanics |difficulty=Beginner |time=3-5 hours (split across multiple sessions) |prerequisites=Basic familiarity with 3D printing and CAD software |materials=Calipers ($15-30), CAD software (FreeCAD or Fusion 360), ruler, existing robot parts to measure |next_steps=Design custom parts for your robot, study SimpleBot CAD files, Gear Ratio Calculation }} '''Mechanical Design''' is your introduction to designing 3D..."

2025-10-11T20:15:59Z

Created page with "{{Tutorial |name=Mechanical Design |competency=Mechanics |difficulty=Beginner |time=3-5 hours (split across multiple sessions) |prerequisites=Basic familiarity with 3D printing and CAD software |materials=Calipers ($15-30), CAD software (FreeCAD or Fusion 360), ruler, existing robot parts to measure |next_steps=Design custom parts for your robot, study SimpleBot CAD files, Gear Ratio Calculation }} '''Mechanical Design''' is your introduction to designing 3D..."

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{{Tutorial
|name=Mechanical Design
|competency=[[Mechanics]]
|difficulty=Beginner
|time=3-5 hours (split across multiple sessions)
|prerequisites=Basic familiarity with 3D printing and CAD software
|materials=Calipers ($15-30), CAD software (FreeCAD or Fusion 360), ruler, existing robot parts to measure
|next_steps=Design custom parts for your robot, study [[SimpleBot]] CAD files, [[Gear Ratio Calculation]]
}}

'''Mechanical Design''' is your introduction to designing 3D-printable robot parts using CAD (Computer-Aided Design) software. This tutorial covers the essential skills for creating mounts, brackets, and structural parts for robots like [[SimpleBot]].

By the end of this tutorial, you'll understand:
* How to measure existing components with calipers
* How to design parts that fit together (tolerances)
* How to design mounting holes and fastener clearances
* How to create assemblies with multiple parts
* How to design for 3D printing (overhangs, bridges, support)

This tutorial is '''hands-on'''. You'll design a sensor mount for [[SimpleBot]] as a practical example.

== Part 1: CAD Software Setup ==

=== Choosing CAD Software ===

For robotics, you need '''parametric CAD''' software that can create precise mechanical parts:

* '''FreeCAD''' (Free, open-source)
** Pros: Completely free, runs on Linux/Mac/Windows, extensible
** Cons: Steeper learning curve, less polished UI
** Best for: Open-source advocates, Linux users, learning fundamentals

* '''Fusion 360''' (Free for hobbyists, students, startups under $100k)
** Pros: Professional-grade, excellent tutorials, cloud storage
** Cons: Requires account, online-only, subscription after trial
** Best for: Beginners, professional workflow, collaboration

* '''OnShape''' (Free for public projects)
** Pros: Browser-based, collaborative, parametric
** Cons: Projects are public unless you pay
** Best for: Collaboration, Chromebook users

'''This tutorial uses FreeCAD''' (works on all platforms), but concepts apply to any CAD software.

=== Installing FreeCAD ===

# Download from [https://www.freecadweb.org/ FreeCAD.org]
# Install (Linux: use package manager, Windows/Mac: run installer)
# Launch FreeCAD and create a new document (File → New)
# Select '''Part Design''' workbench (dropdown at top)

'''Key FreeCAD concepts:'''
* '''Sketch''' - 2D drawing on a plane (constrained shapes)
* '''Pad''' - Extrude sketch into 3D solid (add material)
* '''Pocket''' - Cut sketch from 3D solid (remove material)
* '''Body''' - Container for features (one part = one body)

== Part 2: Measuring with Calipers ==

Before designing parts, you need accurate measurements. '''Digital calipers''' are essential.

=== Caliper Basics ===

Calipers measure:
* '''Outside dimensions''' - Outer diameter, width (large jaws)
* '''Inside dimensions''' - Inner diameter, hole size (small jaws)
* '''Depth''' - How deep a hole or recess is (depth probe)
* '''Step height''' - Difference in height between surfaces

'''Accuracy:''' Digital calipers measure to 0.01mm (0.0005 inches)

=== Measuring Technique ===

# '''Zero the calipers''' - Close jaws and press "zero" button
# '''Place part squarely''' - Jaws perpendicular to surface
# '''Gentle pressure''' - Don't squeeze too hard (deforms plastic)
# '''Read display''' - Switch between mm and inches with "mm/inch" button

'''Exercise:''' Measure these dimensions on your [[Infrared Line Detector]] sensor:
* Width of PCB
* Height of PCB
* Mounting hole diameter
* Distance between mounting holes (center-to-center)
* Height of sensor above PCB bottom

Write down measurements - you'll use them to design a mount!

=== Common Measurement Mistakes ===

* '''Tilted calipers''' - Jaws not perpendicular gives false reading
* '''Measuring over obstacles''' - Measure at narrowest point (not over screw heads)
* '''Forgetting to zero''' - Calipers drift; re-zero before each measurement
* '''Confusing inside/outside''' - Check which jaws you're using
* '''Rounding too aggressively''' - Keep 0.1mm precision, don't round to whole mm

== Part 3: Designing a Sensor Mount ==

Let's design a mount for an [[Infrared Line Detector]] sensor. This mount will:
* Hold the sensor at correct height (3-5mm from ground)
* Attach to SimpleBot chassis with M3 screws
* Keep sensor level and aligned

=== Step 1: Create a Sketch (Base Plate) ===

In FreeCAD Part Design workbench:

# Create a new '''Body''' (right-click Body in tree, select "Create body")
# Create a '''Sketch''' on XY plane
# Draw a '''rectangle''' - 30mm × 20mm (base plate for mount)
# '''Constrain dimensions''' - Select each edge, press "C" (constrain), enter value
# Close sketch (click "Close" in toolbar)

'''Tip:''' Press "V" then "F" to fit sketch to screen.

=== Step 2: Extrude Base Plate (Pad) ===

# Select the closed sketch
# Click '''Pad''' tool (extrude icon)
# Set length to 2mm (base plate thickness)
# Click OK

You now have a 30mm × 20mm × 2mm base plate!

=== Step 3: Add Mounting Holes for Chassis ===

The base plate needs holes to attach to the robot chassis with M3 screws.

# Select top face of base plate
# Create new '''Sketch''' on that face
# Draw two '''circles''' - 3.2mm diameter, 20mm apart horizontally, centered vertically
# Close sketch
# Select sketch and click '''Pocket''' tool (cut icon)
# Set depth to "Through All"
# Click OK

'''Why 3.2mm holes for M3 screws?'''
* M3 screw is 3.0mm diameter
* 0.2mm clearance (3D printing tolerance) = 3.2mm hole
* This allows screws to pass through freely ('''clearance hole''')

=== Step 4: Add Vertical Wall for Sensor ===

The sensor mounts vertically on a wall:

# Select front face of base plate
# Create new '''Sketch''' on that face
# Draw a '''rectangle''' - 25mm wide × 15mm tall
# Close sketch
# '''Pad''' forward 2mm (wall thickness)

Now you have a base plate with a vertical wall!

=== Step 5: Add Sensor Mounting Holes ===

The sensor PCB attaches to the wall with two M3 screws:

# Select front face of vertical wall
# Create new '''Sketch'''
# Measure your sensor: holes are (example) 20mm apart, 8mm from bottom
# Draw two '''circles''' - 2.8mm diameter, positioned to match sensor holes
# Close sketch
# '''Pocket''' depth "Through All"

'''Why 2.8mm holes for M3 screws in plastic?'''
* M3 screw is 3.0mm diameter
* '''Thread-forming screws''' cut threads in plastic as they're screwed in
* 2.8mm hole (0.2mm undersized) allows screw to cut threads tightly
* Alternative: Use 3.2mm holes and M3 nuts on backside

=== Step 6: Add Chamfers (Optional) ===

'''Chamfers''' are angled edges that improve appearance and printability:

# Select top edges of base plate
# Click '''Chamfer''' tool
# Set distance to 0.5mm
# Click OK

This removes sharp corners and helps the print look professional.

== Part 4: Design for 3D Printing ==

Not all CAD designs can be 3D printed successfully. Follow these rules:

=== Overhang Rule: 45-Degree Maximum ===

FDM printers struggle with overhangs steeper than 45 degrees:

* '''OK:''' Walls at 90 degrees (vertical), floors at 0 degrees (horizontal)
* '''OK:''' 45-degree chamfers and slopes
* '''NOT OK:''' Upside-down 45-degree slopes (use support)
* '''NOT OK:''' Horizontal holes (print vertically or use support)

'''Fix:''' Orient part so overhangs are under 45 degrees, or add supports.

=== Bridging Rule: Short Gaps Only ===

Printers can ''bridge'' gaps (print unsupported horizontal lines) up to ~20mm:

* '''OK:''' 10mm horizontal hole (bridges nicely)
* '''NOT OK:''' 50mm horizontal hole (sags in middle)

'''Fix:''' Keep bridged gaps under 20mm, or use support.

=== Minimum Feature Size ===

FDM printers have limits on small features:

* '''Minimum wall thickness:''' 1.2mm (2× nozzle diameter for 0.6mm nozzle)
* '''Minimum hole diameter:''' 2mm (smaller holes often close up)
* '''Minimum gap:''' 0.4mm (smaller gaps fuse together)

'''Fix:''' Make features at least 2mm thick for reliability.

=== Tolerances for Fit ===

3D printed parts shrink slightly and have rough surfaces. Add clearance:

* '''Press fit:''' 0.0 to 0.1mm clearance (tight, may need force)
* '''Sliding fit:''' 0.2 to 0.3mm clearance (moves smoothly)
* '''Loose fit:''' 0.5mm+ clearance (rattles slightly)

'''For our sensor mount:'''
* Chassis mounting holes: 3.2mm (clearance fit for M3 screws)
* Sensor mounting holes: 2.8mm (thread-forming fit)

== Part 5: Assemblies (Multiple Parts) ==

Real robots have many parts. CAD software can show how they fit together:

=== Creating an Assembly in FreeCAD ===

# Create new document
# Insert parts: File → Insert → Part (select your sensor mount .FCStd file)
# Insert sensor model (if available) or create a simple box to represent it
# Use '''Transform''' tool to position parts
# Add '''Constraints''' to lock parts in place

'''Assembly shows:'''
* Do parts collide or interfere?
* Are mounting holes aligned?
* Is sensor at correct height?

=== Common Assembly Mistakes ===

* '''Over-constrained:''' Too many screws in slotted holes (thermal expansion causes binding)
* '''Under-constrained:''' Part wobbles or shifts (add more screws or pins)
* '''Interference:''' Parts overlap (check clearances)
* '''Wire routing:''' No space for wires (leave gaps and channels)

== Part 6: Practical Design Exercise ==

Let's design a motor mount bracket for SimpleBot:

=== Requirements ===
* Mount a DC motor to acrylic chassis
* Motor diameter: 24mm (measure with calipers)
* Motor shaft length: 12mm (protrudes from motor)
* Chassis mounting: Two M3 screws, 20mm apart
* Motor output: Wheel must attach to shaft

=== Design Steps ===

# '''Sketch base plate''' - 30mm × 30mm × 3mm thick (stronger for motor vibration)
# '''Add motor clamp''' - Half-circle cutout 24.2mm diameter (0.2mm clearance)
## Pad walls on both sides to clamp motor
## Add M3 screw hole through walls to tighten clamp
# '''Add chassis mounting holes''' - Two 3.2mm holes, 20mm apart
# '''Add wire routing''' - Small channel for motor wires to exit
# '''Check clearances''' - Wheel must not hit chassis (leave 2mm gap)

=== Design Checklist ===

Before printing, verify:
* ☐ All holes are correct size (calipers + tolerance)
* ☐ Mounting points are spaced correctly (measure chassis)
* ☐ No overhangs steeper than 45 degrees (or add supports)
* ☐ Minimum wall thickness 1.2mm
* ☐ Clearance for moving parts (wheels, gears)
* ☐ Space for wires to route through
* ☐ Chamfers on sharp edges (optional, looks better)

== Part 7: Exporting for 3D Printing ==

=== Export STL File ===

# Select the '''Body''' in FreeCAD tree
# File → Export → Select "STL Mesh" format
# Save as "sensor_mount.stl"

'''STL''' (STereoLithography) is the universal format for 3D printing - it stores the outer shell as triangles.

=== Slicing (Preparing for Print) ===

Use slicer software (Cura, PrusaSlicer, etc.):

# Import STL file
# Orient part (flat side down, minimize supports)
# Set print settings:
## '''Layer height:''' 0.2mm (good balance of speed and quality)
## '''Infill:''' 20% (functional parts), 10% (decorative)
## '''Supports:''' Enable if overhangs exceed 45 degrees
## '''Material:''' PLA (easy), PETG (strong), ABS (heat-resistant)
# Slice and generate G-code
# Send to printer or save to SD card

=== Print Orientation ===

Orientation affects strength and quality:

* '''Best strength:''' Forces perpendicular to layers (layers resist shear poorly)
* '''Best quality:''' Curved surfaces horizontal (layers follow curve)
* '''Minimize supports:''' Flat side down, overhangs under 45 degrees

'''For sensor mount:''' Print base plate down (vertical wall prints nicely, no supports needed).

== Part 8: Iteration and Testing ==

First prints rarely fit perfectly. Iterate:

=== Test Fit ===

# Print part
# Test fit with real components (sensor, screws, chassis)
# Check for issues:
## Holes too tight? Increase diameter by 0.1-0.2mm
## Holes too loose? Decrease diameter by 0.1-0.2mm
## Part warps? Add thicker base or print slower

=== Common Print Issues and Fixes ===

{| class="wikitable"
! Issue !! Cause !! Fix
|-
| Warping (corners lift) || Uneven cooling || Add brim or raft, lower bed temp
|-
| Holes too small || Printer over-extrudes || Increase hole diameter by 0.2mm
|-
| Weak parts || Under-extruding || Increase flow rate or check filament
|-
| Rough surface || Too fast, too hot || Slow down print, lower nozzle temp
|-
| Supports stuck || Too close to part || Increase support gap in slicer
|}

=== Design Iteration Loop ===

# Design part in CAD
# Export STL and slice
# Print part
# Test fit and identify issues
# Modify CAD design (change dimensions)
# Repeat until perfect fit

'''Tip:''' Print small test sections first (just one mounting hole) to check fit before printing the entire part.

== Part 9: Advanced Techniques ==

Once you're comfortable with basic design:

=== Parametric Design ===

'''Parameters''' are variables you can change to update the entire design:

* Create a parameter "motor_diameter = 24mm"
* Use "motor_diameter + 0.2mm" for clamp hole size
* Change motor_diameter to 20mm → entire mount resizes!

'''In FreeCAD:''' Use Spreadsheet workbench to create parameters.

=== Fillets vs Chamfers ===

* '''Chamfer''' - Angled edge (45 degrees typical), easier to print
* '''Fillet''' - Rounded edge, stronger, looks better, slower to print

Use fillets for stress concentration areas (corners that bear load).

=== Threaded Holes ===

Instead of thread-forming screws, design threads into plastic:

* M3 thread = 3.0mm outer diameter, 0.5mm pitch
* Add '''Helix''' feature (spiral) and '''Subtract''' to cut threads
* Requires high resolution (0.1mm layers) to print clearly

'''Easier alternative:''' Use heat-set inserts (metal threaded inserts melted into plastic).

=== Assemblies with Motion ===

Design parts that move (hinges, gears, linkages):

* Add clearance for rotation (0.3mm radial clearance for shafts)
* Use '''Snap fits''' - flexible tabs that lock parts together
* Use '''Living hinges''' - thin flexible sections (0.3-0.5mm thick)

=== Library of Standard Parts ===

Don't redesign screws and nuts - import models:

* [https://grabcad.com/ GrabCAD] - Free CAD library
* [https://www.mcmaster.com/ McMaster-Carr] - CAD models for every fastener
* FreeCAD Fasteners Workbench - Built-in screw library

== Part 10: Studying Existing Designs ==

Learn from [[SimpleBot]] CAD files:

# Download SimpleBot repository (symlinked at ./simplebot/)
# Open FreeCAD files (.FCStd) in CAD software
# Study how parts fit together
# Measure features with '''Measure''' tool
# Modify and remix designs for your own robot

=== What to Look For ===

* '''Hole placements''' - How are mounting holes spaced?
* '''Tolerances''' - What clearances are used for screws and shafts?
* '''Wall thicknesses''' - How thick are structural parts?
* '''Assembly method''' - How do multiple parts attach?
* '''Wire routing''' - Where do wires pass through?

== Part 11: Design Principles for Robotics ==

=== Minimize Part Count ===

Fewer parts = faster assembly, fewer failures:

* Combine multiple features into one part (mount + bracket = one piece)
* Use snap fits instead of screws where appropriate
* Design parts to be 3D printable in one piece

=== Design for Assembly ===

Make parts easy to put together:

* Use '''alignment pins''' or '''tabs''' to position parts correctly
* Add '''chamfers''' to screw holes (guide screws in)
* Leave '''clearance''' for screwdriver or hex key access
* Print '''assembly instructions''' into parts (arrows, labels)

=== Design for Serviceability ===

Robots need maintenance and upgrades:

* Don't trap components inside (leave access holes)
* Use standard fasteners (M3 screws are common)
* Make fragile parts replaceable (separate wheel mounts from chassis)
* Route wires through channels or clips (prevent tangling)

=== Design for Strength ===

Identify stress points and reinforce them:

* '''Thin sections''' break easily (use minimum 2mm thickness)
* '''Sharp corners''' concentrate stress (add fillets)
* '''Cantilever beams''' need support (add ribs or gussets)
* '''Print orientation''' affects strength (layers are weak in shear)

== Part 12: Resources and Next Steps ==

=== Learn More CAD ===

* '''FreeCAD Tutorials''' - [https://www.freecadweb.org/wiki/Tutorials Official wiki tutorials]
* '''Fusion 360 Tutorials''' - [https://www.autodesk.com/products/fusion-360/learn Autodesk Learning]
* '''YouTube Channels''' - Maker's Muse, Teaching Tech, 3D Printing Nerd

=== Practice Projects ===

# Design a custom sensor mount for your robot
# Design a battery holder for 4× AA batteries
# Design a motor mount with gear reduction
# Design a robot arm with 2 servos
# Study and remix [[SimpleBot]] chassis

=== Advanced Topics ===

* [[Gear Ratio Calculation]] - Design gear trains for speed/torque
* [[Kinematics Tutorial]] - Calculate robot motion from wheel speeds
* [[Inverse Kinematics]] - Multi-DOF robot arm calculations
* [[FEA Tutorial]] - Simulate stress in parts (advanced)

=== Related Competencies ===

* '''[[3D Printing]]''' - Fabricate your designs
* '''[[Mechanics]]''' - Understand forces and motion
* '''[[Electronics]]''' - Design mounts for sensors and motors
* '''[[Soldering]]''' - Assemble electronics that mount to your parts

== Troubleshooting ==

=== CAD Issues ===

* '''Sketch won't close?''' Check for gaps or overlapping lines
* '''Can't pad sketch?''' Sketch must be fully constrained
* '''Part disappeared?''' Check if hidden (press "Space" to toggle visibility)
* '''Changes don't update?''' Press "F5" to recompute all features

=== Print Issues ===

* '''Parts don't fit?''' Add 0.2mm clearance for sliding fits
* '''Screw holes too tight?''' Increase diameter by 0.2mm
* '''Parts warp?''' Increase bed adhesion (brim, glue stick, hairspray)
* '''Supports won't remove?''' Increase support gap to 0.2mm

=== Design Issues ===

* '''Part too weak?''' Increase wall thickness or add ribs
* '''Motor vibration causes loosening?''' Add threadlocker or locknut
* '''Wires tangled?''' Add wire routing channels or clips
* '''Parts misaligned?''' Add alignment pins or reference surfaces

== Summary Checklist ==

By now, you should be able to:

* ☐ Measure parts accurately with calipers
* ☐ Create sketches and extrude them into 3D parts
* ☐ Design mounting holes with correct clearances
* ☐ Add chamfers and fillets to edges
* ☐ Export STL files for 3D printing
* ☐ Design for printability (overhangs, bridging, supports)
* ☐ Test fit parts and iterate designs
* ☐ Create assemblies with multiple parts
* ☐ Design for strength and serviceability

If you can check most of these boxes, you're ready to design custom parts for your robot!

== See Also ==

* [[Mechanics]] - Full competency overview
* [[SimpleBot]] - Apply your skills to build a robot
* [[3D Printing]] - Fabrication techniques for your designs
* [[Gear Ratio Calculation]] - Design gear trains
* [[Robotics Ontology]] - How mechanical design fits into BRS

[[Category:Tutorials]]
[[Category:Mechanics]]
[[Category:Beginner]]

Mechanical Design - Revision history