3D Printing for Robotics

3D Printing for Robotics
Competency	3D Printing
Difficulty	Intermediate
Time Required	6-8 hours (plus iterative design time)
Prerequisites	3D Printing Basics, basic CAD Design knowledge
Materials Needed	FDM printer, PLA/PETG filament, calipers, test hardware (screws, nuts, bearings, motor shafts)
Next Steps	CAD Design, design custom robot chassis, document implementation pages

3D Printing for Robotics teaches you how to design and print functional mechanical parts that work reliably in robots. This is not about making pretty objects - it's about creating parts that hold motors, position sensors, transfer forces, and survive repeated use.

You'll learn the engineering principles specific to FDM printing: layer orientation for strength, designing tolerances for fit, minimizing supports, embedding hardware, and iterating quickly.

By the end of this tutorial, you'll be able to:

Design parts that print successfully on the first try
Choose print orientation for maximum strength
Design holes, slots, and assemblies with correct tolerances
Eliminate or minimize support material
Embed hardware like nuts, bearings, and heat-set inserts
Iterate designs efficiently based on test fits
Design complete robot chassis from scratch

This tutorial assumes you've successfully printed parts designed by others (see 3D Printing Basics) and have basic CAD modeling skills.

Part 1: Design Principles for FDM

FDM printing has constraints that machined or molded parts don't have. Ignore these and your parts will fail.

Layer Orientation Determines Strength

FDM parts are anisotropic - strength varies dramatically with orientation:

Layers parallel to force - Very strong (plastic material strength)
Layers perpendicular to force - Weak (layers peel apart)

Example: Motor mount bracket

Bad orientation - Mount hole on side, forces pull perpendicular to layers → bracket snaps
Good orientation - Mount hole on top/bottom, forces compress layers together → strong

Rule of thumb: Orient parts so primary forces push along layers, not between layers.

The 45-Degree Rule

FDM cannot print in mid-air. Overhangs are limited:

0-45° overhang - Prints fine, no support needed
45-60° overhang - May print, depends on geometry and cooling
>60° overhang - Requires support material

Horizontal holes are problematic:

Top of hole is >45° overhang
Solution: Orient part vertically, or accept support inside hole

Tear-drop holes - Flatten top of circular hole to avoid overhang

Instead of perfect circle, use circle with flat top above centerline
Maintains most of the circular opening without support

Minimum Feature Sizes

FDM has resolution limits:

Minimum wall thickness - 2× nozzle diameter (0.8mm for 0.4mm nozzle)
- Thinner walls may not print reliably
Minimum hole diameter - 1mm (smaller may close up or need drilling)
Minimum gap - 0.4mm (0.2mm may fuse together)
Minimum text height - 3mm for raised text, 1mm for engraved
Finest detail - Limited by layer height and nozzle diameter

Bridging Capability

FDM can print horizontal spans between two support points:

Short bridges (<20mm) - Usually successful
Medium bridges (20-40mm) - May sag in middle
Long bridges (>40mm) - Will fail without support

Design strategy: Add intermediate support pillars for long bridges.

Part 2: Tolerances and Fit

Printed parts have dimensional variation. You must design for clearance.

Understanding Tolerance

Tolerance is the allowable variation from nominal dimension:

FDM accuracy - Typically ±0.1-0.2mm (but varies by printer, material, settings)
Holes print undersized - 0.1-0.2mm smaller than designed
Shafts print oversized - 0.1-0.2mm larger than designed

Why? First layer squish, elephant's foot, material shrinkage, thermal expansion.

Design Clearances for Assemblies

When parts must fit together, add clearance:

Fit Type	Clearance	Description	Robotics Use
Interference fit	-0.1mm	Parts pressed together, stay tight	Bearing press-fits
Press fit	0.0mm	Tight fit, may need mallet	Wheel hubs on shafts
Slip fit	+0.2mm	Parts slide together with friction	Hinges, sliding joints
Free fit	+0.5mm	Parts move freely, slight wobble	Rotating shafts in bearings
Loose fit	+0.8mm	Easy assembly, significant play	Clearance holes for screws

Example: M3 screw (3.0mm nominal diameter)

Threaded hole - Design 2.5mm, print will be ~2.6-2.7mm, tap with M3 tap
Tight clearance - Design 3.2mm, allows screw to pass with minimal wobble
Standard clearance - Design 3.5mm, easy assembly (most common)
Loose clearance - Design 4.0mm, allows adjustment during assembly

Testing and Iterating Tolerances

Every printer is different. You must calibrate:

Print a tolerance test piece

- Design: Rectangular block with holes of different sizes (3.0mm, 3.2mm, 3.5mm, 4.0mm)
- Test with M3 screw - which hole gives best fit?

Print a shaft test piece

- Design: Cylinders of different diameters (3.0mm, 3.2mm, 3.5mm, 4.0mm)
- Test fit in holes - which combination gives desired fit?

Document your printer's behavior

- "My printer needs +0.3mm for M3 clearance holes"
- "My printer needs -0.2mm for press-fit shafts"

Apply corrections to future designs

Tip: Print a "hardware calibration kit" with holes for all common screws (M2, M2.5, M3, M4) and slots for nuts. Test once, use forever.

Designing Holes for Hardware

Screw clearance holes:

M2 (2.0mm) → Design 2.3-2.5mm
M2.5 (2.5mm) → Design 2.8-3.0mm
M3 (3.0mm) → Design 3.3-3.5mm
M4 (4.0mm) → Design 4.3-4.5mm

Hex nut pockets:

M3 hex nut (5.5mm across flats) → Design 5.7-5.8mm hexagon
M3 hex nut (2.4mm thick) → Design 2.5-2.6mm deep pocket

Bearing press-fits:

608 bearing (22mm OD) → Design 21.8-21.9mm hole
Test fit and adjust by 0.1mm increments

Part 3: Support Strategies

Supports waste time and material. Minimize them through smart design.

Designing to Avoid Supports

Strategy 1: Orient for Printability

Rotate part so overhangs are <45°
Sometimes compromises strength - weigh trade-offs

Strategy 2: Split Parts

Design in two pieces that print flat
Assemble with screws or glue after printing
Example: Sensor bracket with 90° angle → print as two flat pieces, bolt together

Strategy 3: Add Chamfers

Chamfer bottom edges of holes to reduce overhang angle
45° chamfer = no support needed

Strategy 4: Use Tear-Drop Holes

Horizontal holes with flattened tops
Maintains circular opening at bottom (where precision matters)

Strategy 5: Add Self-Supporting Features

Design deliberate material under overhangs
Example: Ribs, gussets, or small supports that become part of the design

When Supports Are Necessary

Some geometries require supports. Optimize them:

Use tree supports (PrusaSlicer, Cura) - Less material, easier removal
Support enforcers - Add supports only where critical
Support blockers - Remove supports where they're not needed
Support interface layers - Dense layer between support and part (better surface finish)
Support Z-distance - 0.2mm gap allows easier removal

Design tip: Add small tabs or nubs on support surfaces - gives supports better attachment point and easier removal.

Part 4: Embedding Hardware

Robots need metal hardware (screws, nuts, bearings) integrated into plastic parts.

Heat-Set Inserts

Heat-set inserts are brass threaded inserts melted into plastic:

Advantages: Strong, reusable threads (screws won't strip plastic)
Installation: Use soldering iron with special tip, heat insert, press into plastic
Design:
- Hole diameter: Insert OD + 0.1-0.2mm (e.g., M3 insert needs ~4.0-4.2mm hole)
- Hole depth: Insert length + 0.5mm (allows insert to sit flush or slightly recessed)
- Boss diameter: At least insert OD + 3mm (provides material for insert to grip)

Common sizes for robotics:

M2.5 × 4mm length (small brackets, PCB standoffs)
M3 × 5mm length (general purpose, most common)
M4 × 6mm length (motor mounts, high-stress points)

Captive Nuts

Captive nuts are trapped in hexagonal pockets during printing:

Design:
- Hexagonal pocket: Nut size + 0.2mm across flats
- Pocket depth: Nut thickness + 0.1mm
- Pocket height above surface: At least 2 layer heights (0.4mm for 0.2mm layers)
Installation: Drop nut into pocket during print (pause print) or slide in from side
Retention: Hex shape prevents rotation, pocket prevents pull-out

Pause method:

Slice model, note layer number where pocket roof starts
Add pause command at that layer (G-code: M0 or M600)
During print, insert nuts when printer pauses
Resume print, plastic grows over nuts

Slide-in method:

Design pocket open on one side
Print complete
Slide nut into pocket from side
Optional: Add small plastic tab to block side opening after insertion

Press-Fit Bearings

Ball bearings provide smooth rotation for wheels and joints:

Common types:
- 608 bearing (skateboard bearing): 8mm ID, 22mm OD, 7mm thick
- 688 bearing (miniature): 8mm ID, 16mm OD, 5mm thick
- MR105 bearing: 5mm ID, 10mm OD, 4mm thick

Design for press-fit:
- Hole diameter: Bearing OD - 0.1 to -0.2mm (tight press fit)
- Hole depth: Bearing thickness + 0.5mm (bearing sits fully in)
- Chamfer entrance: 45° chamfer makes bearing easier to press in

Installation:
- Press bearing in by hand (may need light mallet)
- Use vise or arbor press for very tight fits
- Heat plastic slightly with hot air gun (easier press)

Design tip: Add a flange or stop surface so bearing doesn't press through.

Shaft Collars and Set Screws

Hold shafts in place without glue:

Design:
- Shaft hole: Shaft diameter + 0.1-0.2mm (slip fit)
- Perpendicular M3 threaded hole intersecting shaft hole
- Tighten M3 screw to pinch shaft

Alternative: Split collar
- C-shaped collar wraps around shaft
- Bolt squeezes collar closed to grip shaft
- Allows shaft removal without loosening set screw

Part 5: Designing Assemblies

Multi-part assemblies require careful planning.

Snap-Fit Joints

Snap-fits allow parts to click together without fasteners:

Cantilever snap
- Flexible beam with hook at end
- Hook catches on mating part
- Requires TPU or thin PLA/PETG that flexes
- Design: Beam thickness 0.8-1.2mm, deflection ~10-20% of beam length

Annular snap
- Ring of material compresses to fit through opening
- Expands back to original size inside
- Example: Bottle cap mechanism

Limitations: PLA is brittle - snap-fits can break after few cycles. PETG is better.

Living Hinges

Living hinges are flexible joints printed in one piece:

Material: PETG or TPU (PLA breaks after few flexes)
Design:
- Hinge thickness: 0.4-0.6mm (thin enough to flex)
- Hinge width: 5-10mm (distributes stress)
- Layer orientation: Layers perpendicular to hinge axis (allows flexing between layers)
Print settings:
- 100% infill in hinge area
- Slow print speed (better layer adhesion)

Robotics use: Gripper jaws, sensor covers, cable management clips

Bolted Assemblies

Most reliable for robotics:

Through-bolts - Screw passes through both parts, nut on other side
- Advantage: Very strong, easy to assemble/disassemble
- Design: Clearance hole in both parts, hex nut pocket in one part

Standoffs - Threaded spacers between parallel plates
- Common: M3 brass standoffs, various lengths
- Design: M3 clearance holes in both plates

Threaded inserts (see above) - One part has insert, other has clearance hole
- Advantage: Reusable threads, no loose nuts

Part 6: Strength and Structural Design

Understanding Anisotropic Strength

Test data (varies by material and settings):

Tensile strength along layers - 40-60 MPa (very strong)
Tensile strength between layers - 10-20 MPa (weak)
Shear strength - 15-25 MPa (moderate)

Design implication: A part can be 3-5× stronger in one orientation than another.

Optimizing for Strength

Strategy 1: Perimeters Over Infill

Weak: 2 perimeters, 50% infill
Strong: 4 perimeters, 20% infill
Outer walls carry most load; infill just prevents wall collapse

Strategy 2: Reinforce Stress Points

Add fillets (rounded corners) to reduce stress concentration
Typical fillet radius: 2-5mm
Sharp corners crack under load

Strategy 3: Increase Wall Thickness

Minimum 2.4mm (6 perimeters with 0.4mm nozzle) for structural parts
Thicker = stronger, but diminishing returns above 4-5mm

Strategy 4: Add Ribs and Gussets

Vertical ribs increase bending stiffness without much added weight
45° gussets strengthen right-angle joints
Rib thickness: 1.2-2.0mm (3-5 perimeters)

Motor Mount Design Case Study

Requirements for SimpleBot motor mount:

Hold DC motor securely
Withstand motor vibration
Resist torque from wheel contact
Easy to print without supports

Design decisions:

Layer orientation - Mount flat on bed, motor axis parallel to bed

- Forces compress layers together (strong direction)

Motor pocket - Snug fit around motor body

- Designed diameter: Motor OD + 0.2mm (slip fit)

Mounting holes - M3 clearance holes for bolting to chassis

- Through-bolts with nuts on other side (strong, removable)

Reinforcement - Ribs connecting motor pocket to mounting points

- Distributes motor vibration, prevents cracking

Avoiding supports - All angles <45° from vertical

- Motor pocket has flat bottom (prints on bed, no support needed)

Testing:

Print first prototype
Test fit motor - too tight? Increase pocket 0.2mm
Bolt to chassis, run motor at full speed
Inspect for cracks or flexing
Iterate design if needed

Part 7: Print Settings for Mechanical Parts

Layer Height Selection

0.1mm - High detail, smooth surface (decorative parts)
- Slow, not necessary for most robotics parts
0.2mm - Standard, good balance (most common)
- Use for general purpose parts
0.3mm - Fast draft, rougher finish
- Use for large chassis parts, non-visible internals

Infill Strategy

10-15% - Light parts, minimal load (sensor brackets, covers)
20-30% - Standard structural parts (chassis, mounts)
40-50% - High-stress parts (motor mounts, wheel hubs)
100% - Small parts where mass doesn't matter (tiny gears, pins)

Infill pattern:

Grid - Fast, simple
Gyroid - Strong in all directions, recommended for structural parts
Honeycomb - Strong, but slower to slice and print

Perimeter Count

2 perimeters - Minimum for functional parts
3 perimeters - Standard for robotics parts
4+ perimeters - High-stress parts, thin walls

Rule: Wall thickness = perimeter count × line width (e.g., 3 perimeters × 0.4mm = 1.2mm wall)

Material Selection

PLA - Easy printing, rigid, brittle
- Good for: Chassis, brackets, low-stress parts
- Bad for: High-impact parts, outdoor use, hot environments
PETG - Moderate difficulty, strong, flexible
- Good for: Motor mounts, wheels, stressed parts, outdoor robots
- Bad for: Fine details (strings more than PLA)
TPU - Difficult printing, rubber-like
- Good for: Tires, grippers, shock absorption
- Bad for: Structural parts (too flexible)

Support Interface Layers

When supports are unavoidable:

Enable support interface layers (dense layer between support and part)
Set interface layer count to 2-3
Results in better surface finish where supports touch part

Part 8: Iterative Design Process

Professional designers rarely get parts perfect on first try. Embrace iteration.

Rapid Prototyping Workflow

Design initial version in CAD

- Use nominal dimensions, best-guess clearances

Print at 150% speed, 0.3mm layers (fast draft mode)

- Quality doesn't matter for first test

Test fit with actual hardware

- Measure with calipers, identify problems

Adjust design based on measurements

- Holes too small? Increase 0.2mm. Too large? Decrease 0.1mm

Print second version at normal settings

- Should be close to perfect

Final tweaks if needed

- Usually just minor adjustments

Print final version for documentation

- Take photos, measure, document design decisions

Time saved: Fast draft print in 1 hour vs. high-quality print in 3 hours. Get feedback 3× faster.

Design Validation Checklist

Before printing final version:

☐ All mounting holes have clearance (+0.3mm for M3)
☐ Hex nut pockets correct size (measure actual nut with calipers)
☐ Bearing press-fit dimensions calculated (bearing OD - 0.2mm)
☐ No overhangs >45° (or supports enabled)
☐ Minimum wall thickness 0.8mm (2 perimeters)
☐ Fillets on stress concentration points
☐ Layer orientation optimized for primary forces
☐ Parts split or oriented to avoid supports if possible

Documentation

When design is finalized:

Export STL files for distribution
Document design decisions (why you chose dimensions, orientations, materials)
Create assembly guide if multi-part
Share on SimpleBot wiki or create implementation page

Part 9: Example Project - Custom Sensor Bracket

Let's design a bracket to mount an ultrasonic distance sensor (Capability:Ultrasonic Sensing) on SimpleBot.

Requirements Analysis

Sensor: HC-SR04 ultrasonic sensor
- Dimensions: 45mm × 20mm × 15mm
- Mounting: 2× holes on either side (40mm spacing)
Mounting position: Front of SimpleBot chassis
- Must clear existing line sensors
- Point forward for obstacle detection
Constraints:
- Print without supports if possible
- Use M3 hardware (consistent with SimpleBot)
- Minimize material use

Design Process

Step 1: Sketch concept

L-shaped bracket: Vertical face for sensor, horizontal face for mounting to chassis
Sensor held with M3 screws through side holes
Bracket mounts to chassis with 2× M3 screws

Step 2: CAD model

Vertical face: 50mm × 25mm × 2mm thick
- 2× M3 clearance holes (3.5mm diameter, 40mm apart) for sensor mounting
- Center holes vertically on face
Horizontal face: 50mm × 15mm × 2mm thick
- 2× M3 clearance holes (3.5mm diameter, 40mm apart) for chassis mounting
- Position 5mm from edge
Connection: Vertical face perpendicular to horizontal face
- Add 5mm fillet for strength
- No overhang issues (prints flat on bed)

Step 3: Print orientation

Lay horizontal face on bed (L-shape standing up)
Vertical face prints without support (perpendicular = vertical walls)

Step 4: First print (fast draft)

0.3mm layer height, 150% speed
Print time: 45 minutes
Test fit sensor - holes align correctly
Test fit to chassis - mounting holes correct

Step 5: Refinements

Add 2mm clearance between sensor and vertical face (prevents pressure on sensor PCB)
Increase horizontal face width to 20mm (more contact with chassis)
Add small chamfer to mounting holes (easier screw insertion)

Step 6: Final print

0.2mm layer height, 3 perimeters, 20% infill
PETG material (more durable than PLA)
Print time: 1 hour 15 minutes

Step 7: Assembly

Mount sensor to bracket with M3×10mm screws and nuts
Mount bracket to chassis with M3×8mm screws
Connect sensor wiring to microcontroller

Step 8: Documentation

Take photos of assembled bracket
Export STL file
Create wiki page: SimpleBot:Ultrasonic Sensor Bracket
Document design decisions and dimensions

Part 10: Advanced Techniques Preview

Once you've mastered intermediate skills, explore:

Parametric Design

Use variables in CAD instead of fixed dimensions
Adjust one parameter (wheel diameter), entire assembly updates
Example: Parametric chassis that scales to different motor sizes

Multi-Material Printing

Dual-extruder printers can print rigid and flexible materials
Example: Rigid wheel hub with TPU tire in one print

Topology Optimization

Software removes material from non-stressed areas
Creates organic-looking structures with minimal weight
Requires advanced CAD software (Fusion 360 Generative Design)

Functional Gears and Mechanisms

Print gears with proper involute tooth profiles
Calculate gear ratios for speed reduction
Design escapements, ratchets, and complex mechanisms

Composite Structures

Embed carbon fiber rods or metal inserts during printing
Creates parts stronger than solid plastic
Requires printer pause and careful alignment

Common Design Mistakes

Ignoring layer orientation - Part snaps because forces pull layers apart
Designing like metal parts - Sharp corners, thin walls, complex geometry that needs support
Forgetting shrinkage - Holes too small, shafts too big
Over-constraining assemblies - Parts can't fit together due to tolerance stack-up
Skipping test prints - Wasting 6 hours on full-quality print that doesn't fit
Using 100% infill - Wastes time and material, adds minimal strength
Not adding fillets - Stress concentrations cause cracking
Ignoring print direction visibility - Layer lines ugly on visible faces

Design Patterns for Common Robotics Parts

Motor Mount

Cylindrical pocket for motor body (motor OD + 0.2mm)
Flat bottom of pocket (no support needed)
M3 clearance holes aligned with motor mounting holes
Ribs connecting motor to mounting points
Layer orientation: Motor axis parallel to build plate

Wheel Hub

Press-fit hole for motor shaft (shaft diameter - 0.1mm)
Perpendicular M3 set screw to lock shaft
Outer rim for tire attachment
Spokes to reduce weight
Print flat on bed (strongest direction for torque)

Sensor Bracket

Flat mounting face perpendicular to sensor
M3 clearance holes for sensor screws
45° angle support to chassis
Thin walls (save material, print faster)

Cable Management Clip

C-shaped clip with slight interference fit
PETG material (flexes without breaking)
Mounting hole or adhesive pad for attachment
Layer orientation: Flex perpendicular to layers

Battery Holder

Cavity for battery with 1-2mm clearance (easy insertion)
Contact points for terminals
Strap or clip to retain battery
Ventilation holes (safety)

Next Steps

Apply Your Skills

Design custom parts for SimpleBot
- New sensor brackets for additional capabilities
- Modified chassis for different battery configurations
- Custom wheel designs for different terrains
Document your designs as implementation pages
Share STL files with the BRS community

Continue Learning

CAD Design - Master parametric modeling
Mechanics - Understand forces and structural analysis
3D Printing - Explore advanced competency overview
Advanced FDM Techniques (future tutorial) - Multi-material, optimization, complex mechanisms

Join the Community

Share your designs on Printables, Thingiverse
Get feedback on r/3Dprinting or r/functionalprint
Contribute STL files to BRS robot repositories
Create tutorials for techniques you discover

Resources

Online Tools

Fusion 360 (free for hobbyists) - Parametric CAD with simulation
FreeCAD (free, open-source) - Full-featured parametric CAD
Onshape (free for public projects) - Cloud-based CAD
Blender (free) - Organic modeling (less suitable for mechanical parts)

Reference Materials

Prusa Print Quality Guide
CNC Kitchen - Engineering tests of printed parts
Maker's Muse - 3D printing techniques
Design Prototype Test - Functional design

Community

Reddit r/functionalprint - Share functional designs
Printables / Thingiverse - Download and share STL files
BRS Discord (future) - Real-time help with designs