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---
title: Tutorial 23 Bridging & Overhang Control
description: Defying gravity with plastic.
---

Tutorial 23 Bridging & Overhang Control

1. Lesson Header

2. Concept Introduction

The Gravity Problem.
Molten plastic is heavy. If you print over nothing, it sags.
Bridging is the art of pulling the filament tight (like a suspension bridge cable) across a gap before it can fall.
Overhangs are when you print partially over nothing (e.g., a 45-degree wall).

Key Factors:
1. Cooling (M106): Freeze the plastic instantly.
2. Speed (F): Move fast enough to stretch it, but slow enough to anchor it.
3. Flow (M221): Slightly under-extrude bridges to increase tension.

3. Machine State Explanation

M106 (Fan Speed).
- M106 S0: Fan Off.
- M106 S127: 50% Speed.
- M106 S255: 100% Speed.

M107: Turn Fan Off (Same as M106 S0).

4. Command Breakdown

5. Minimal Working Example

The Air Line.
Print two pillars 10mm apart.
Then connect them.

G28
G1 Z5 ; Start high
; Pillar 1
G1 X10 Y10 E1
; Bridge Move (Fast, Fan On)
M106 S255
G1 X20 Y10 E0.3 F1500 ; Less extrusion, medium speed
; Pillar 2 (Anchor)
G1 X30 Y10 E1

6. Visual Representation

Interactive preview is available in the interactive reader.

7. Build Exercise

Task: Build a Bridge Test.
1. Print two squares (10x10mm) separated by 20mm.
2. Height: 5mm.
3. At Z=5mm, bridge across them.

The Bridge Logic:
- Move to Edge of Pillar 1.
- Turn Fan ON (100%).
- Reduce Flow (90%).
- Move to Edge of Pillar 2.
- Restore Flow (100%).
- Turn Fan OFF (or keep it on for next layer).

8. Deep Insight Section

Anchor Points.
A bridge fails if the ends aren't stuck down.
You must print onto the solid pillar for at least 1-2mm before crossing the gap.
This is called the "overlap".

Speed Tuning.
Too slow = Plastic sags (heat creep).
Too fast = Plastic snaps or doesn't stick to the far side.
Optimal bridge speed is usually 20-40mm/s for PLA.

9. Common Failure Modes

  1. Sagging: Fan too low or speed too slow.
  2. Snapping: Speed too fast or flow too low.
  3. Curling: Overhangs curl up if cooling is insufficient on one side (unilateral fan ducts).

10. Real-World Application

Support-Free Printing.
Clever designers use bridging to print roofs without support material.
If the gap is < 100mm, a well-tuned printer can bridge it perfectly flat.

11. Final Clean Version

; Lesson 23 - Bridge Test
G21
G90
M83
G28
G1 Z0.2 F3000

; --- Base Pillars (Simplified) ---
; Print two squares for a few layers manually
; We will cheat and just print one thick layer for the base
G1 X50 Y50 E2
G1 X60 Y50 E2
G1 X60 Y60 E2
G1 X50 Y60 E2
G1 X50 Y50 E2 ; Square 1

G1 X80 Y50 E2 ; Move to Square 2
G1 X90 Y50 E2
G1 X90 Y60 E2
G1 X80 Y60 E2
G1 X80 Y50 E2 ; Square 2

; --- Raise to Bridge Height ---
G1 Z5 F3000 ; Jump up (simulating a tall print)
; (In reality, you need layers below this, but for testing, we can bridge in air if we anchor well)
; Wait, we can't bridge in thin air without support below the anchors!
; Let's assume the user has printed pillars up to Z=5.
; Or better, let's just make a small bridge close to the bed (Z=0.4) over a gap.
; But bridging needs height to not blow back on the bed.
; Let's just print a single layer bridge between two points on the bed? No, that's just a line.

; REVISED PLAN:
; Print two towers 5mm high.
; Then bridge.
; This is too much code for a manual tutorial.
; Let's simulate the bridge logic using a single layer "gap" test.
; We will print two pads, then bridge between them at Z=0.4.

G1 Z0.2
; Pad 1
G1 X50 Y50 E2
G1 X60 Y50 E2
G1 X60 Y60 E2
G1 X50 Y60 E2
G1 X50 Y50 E2

; Pad 2
G1 X90 Y50 E2
G1 X100 Y50 E2
G1 X100 Y60 E2
G1 X90 Y60 E2
G1 X90 Y50 E2

; --- The Bridge Layer ---
G1 Z0.4
; Anchor on Pad 1
G1 X55 Y55 ; Center of Pad 1
M106 S255 ; Fan ON
G1 X60 Y55 E0.5 ; Move to edge
; BRIDGE
G1 X90 Y55 E1.5 F1200 ; Cross the 30mm gap to Pad 2
; Anchor on Pad 2
G1 X95 Y55 E0.5 ; Move into Pad 2
M106 S0 ; Fan Off

G28 X0 Y0

12. Stretch Challenge

Challenge: Write a "Fan Speed Tower".
Print a bridge.
Then print another bridge on top of it with different fan speed.
Stack 10 bridges.
Increase fan speed 10% each level.
See which one droops the least.