Model the structure
Create individually named components: wood members or open-web plywood side frames, transverse braces, local gussets, fittings and any M3 hardware.
POE Unit 2 • Bridge Capstone • Lesson 2.17
Detailed Bridge Design
Engineering question: Can another engineer manufacture and assemble your bridge correctly using only your CAD files, dimensions, parts list and instructions?
Project goals
Design it. Document it. Check it. Then request approval.
Communicate the design
Deliver an assembly drawing, fully dimensioned manufactured components, joint details, cut profiles and an exploded or annotated assembly view.
Demonstrate readiness
Verify bridge geometry and connections, estimate finished mass, and demonstrate that fabrication methods satisfy the rules before machines are used.
Build componentsDay 1
Assemble & inspectDay 2
Document & exportDay 3
Quality gate & submitDay 4
Suggested ~4 class periods (45–50 minutes each), not including actual printing or cutting time. Complex joinery may require additional design time.
01 / The design rules
Use the full specification, not just a weight target
≥ 400 mm
Clear span between support contact points
≤ 65 mm
Overall bridge width, including protruding fasteners
100 N
Centered qualification load
≤ 10 mm
Deflection at the 100 N load
≤ 150 g
Finished bridge, including glue and hardware
Wood-based only
Primary structural members: balsa, basswood, craft sticks or plywood
Prohibited: acrylic, solid plywood side panels, 3D-printed structural chords/diagonals/decks, screws, hot glue, epoxy, wire, staples, nails or bolts other than M3. Permitted: approved PVA/CA glue, interlocking wood joints, local plywood gussets, approved PLA/PETG connection fittings, and M3 nuts/bolts (washers permitted). Read all bridge rules before committing to a joint.
02 / Understand the full assembly
Two connected side frames make one bridge
Do not stop after drawing one side truss. Your Fusion assembly must represent the entire structure and the load path from the center loading contact through transverse members, two side frames, their joints, and finally the supports.
Complete component structure
- Side frame left: named members or single-piece open-web plywood panel
- Side frame right: independently positioned or mirrored components
- Transverse braces and cross-members to prevent sway/twist
- Center load-transfer component capable of transferring test force to sides
- Local connection details and all planned M3 hardware or fittings
Critical Fusion modeling habits
- Set document units to millimeters and name parameters (span, width, truss rise, thickness).
- Model manufactured parts as identifiable components rather than one unmanageable body.
- Use patterns/mirrors for repeated geometry and components where appropriate.
- Use rigid joints/rigid groups to represent an assembled structure without adding fictitious motion.
- Inspect for interferences, redundant overlapping parts, incorrect hole alignment and accessible tool/fastener locations.
03 / Guided Fusion manufacturing workflow
From the selected concept to a controlled CAD assembly
1 · Set user parameters
Create span = 400 mm minimum, target total width no more than 65 mm, truss rise, wood thickness, fitting clearances and bracing locations. Distinguish clear span from overall length.
2 · Model the side truss
Sketch the node geometry and consistent member centerlines. Make closed profiles of individual wood members or a single open-web plywood side panel; never use an uncut solid side plate.
3 · Make named components
Separate chords, webs, braces, gussets, loading crossmember and approved connection pieces into components. Use meaningful part IDs, dimensions and materials.
4 · Build the full assembly
Place two side frames; rigidly constrain braces and crossmembers; include holes, connection details and hardware where used. Show the intended load contact.
5 · Inspect and revise
Measure the total outside width across bolt heads/fittings. Verify the clear support span, side symmetry, bracing, interference-free joints and any 2.16 identified risks.
6 · Prepare manufacturing files
Create PDFs for assembly/part drawings; output clean millimeter-scale DXF profiles for laser operations; STL or 3MF for approved printed fittings; and a complete BOM.
Fusion workflow refresher: The Drawing workspace can generate orthographic and isometric views from the modeled assembly. Use Drawing → From Design and select millimeters, views, dimensions and parts lists. A drawing PDF communicates manufacture; it is not itself the correct laser-cut geometry. Autodesk's commands and labeling may vary by version. Open official Drawing from Design instructions ↗
04 / Instructional video library
Learn the skills directly from the tutorials
Watch each for the part of your workflow it supports. Stop and practice in your own Fusion file; do not simply copy the demonstration geometry into the bridge.
MrSeacrist • High-school CAD tutorial
Fusion: Assembling Parts with Rigid Joints
Watch for: Create separate components, align them with rigid joints, and check the bridge width after assembly.
Watch directly on YouTube ↗Product Design Online
Export DXF for Laser Cutter from Fusion Drawing
Watch for: Explain the difference between a documented drawing sheet and a clean 1:1 cutting profile.
Watch directly on YouTube ↗What Make Art • 2026
Fusion Laser Cutting: 2D Profiles and DXF Setups
Watch for: Note how separate cutting operations, units, and inside/outside paths are managed.
Watch directly on YouTube ↗05 / Connection engineering
Design and dimension two critical joints
The correct joint depends on the force flow identified in 2.16. Your detail views must show geometry, member thickness, orientation, adhesive or M3 hardware, and assembly order—not just an artistic close-up.
Joint detail 1 · Center load transfer
- Identify the point where the 100 N loading nose acts.
- Model the crossmember, support surfaces and connectors carrying that load to the two side frames.
- Dimension the overall width and explain potential compression, bending, shear and connection risks.
- Check that the loading piece is a permitted structural material, not a continuous 3D-printed beam.
Joint detail 2 · Truss node or end support
- Choose one highly loaded joint or a connection supporting lateral bracing.
- Show the actual overlapping wood surfaces and how adhesive, a gusset, interlocking fit or M3 fastener transfers force.
- Dimension all local gussets and hole locations. Record plywood orientation/thickness.
- Explain risk of splitting, peel, bearing, interference, or buckling at the node.
Local plywood gussets
Approved plywood ≤3.2 mm thick. Each local gusset must stay ≤40 mm long, ≤25 mm wide, ≤800 mm² plan area; overlap onto each structural member is ≤20 mm. No more than one gusset layer on each side of the connection (two total).
Printed PLA/PETG fitting
Connection-only: maximum 50 mm length, ≤15 g per individual piece, ≤40 g of all printed pieces combined. No printed beams, chords, diagonals or decks; no printed threads or linked fittings forming prohibited continuous members.
M3 hardware / adhesive
Use M3 bolts with matching nuts; washers permitted. Provide usable holes or captive-nut pockets, appropriate clearance, and space to assemble. Only PVA or CA glue—not epoxy/hot glue. Model intended adhesive contact surfaces.
06 / Technical drawings
Produce an assembly drawing that can be inspected
Include an isometric view and dimensioned orthographic views. Students must dimension the support contact points for the clear span, not confuse them with overall printed/cut part length.
Assembly drawing sheet
- Side, top and end view, with appropriate scale, centerline and units
- Clear span ≥400 mm measured at actual supports and overall width ≤65 mm
- Actual height/rise and part spacing
- Position of 100 N load transfer point and two supports
- Two side frames, lateral braces, local fasteners and major assembly dimensions
- Ballooned components linked to a parts list
Individual manufactured-part sheets
- Full shape of each unique wooden/plywood component with sizes, angles and holes
- Material, stock thickness, quantities and grain orientation (when relevant)
- Gusset lengths, widths, plan area, overlap and layer count
- Printed fitting length, hole sizes, mass and slicer settings when used
- Revision, name/ID, part number, date and units
- Exploded view or clearly sequenced assembly sketch for joinery
For a part list in Fusion: Open the Drawing workspace and use a Parts List table associated with an assembly view; then verify quantities and names. Autodesk's official Parts List instructions ↗ show the steps. Your BOM must also include glue and consumables that may not appear as CAD components.
07 / Laser cutting workflow
Export manufacturing profiles—not drawing decorations
For plywood side frames, local gussets and braces, prepare 1:1 2D closed contours in millimeters and inspect them in the FabLab's approved laser software before any cut. Keep part IDs and cut groups separate from actual cutting geometry.
Prepare a correct DXF
- Create or project a flat sketch of the intended cut face, not a 3D drawing assembly.
- Include outside contours, interior openings and any holes that actually belong in the cut operation.
- Remove construction lines, dimension text, duplicated edges, drawing borders and other unwanted cuts.
- Confirm units are mm and the file imports at full 1:1 size.
- Inspect all paths for closure and check that narrow webs, corner radii, grain direction, kerf and usable stock are manufacturable.
- Have the instructor check nesting, cut settings and machine safety before operation.
Two valid Fusion approaches
Sketch export: Right-click a clean profile sketch and export it as DXF. For a modeled body, create a sketch on its planar face and project the edges before exporting.
Manufacture setup: Where available, a laser Fabrication setup can organize internal and external 2D profiles and export layers. The school's laser software must still validate imported scale, contours and operation order.
Do not assume: a PDF or a 3D DXF generated from a whole assembly is automatically a laser-ready 2D file. Export a clean planar profile and inspect its actual geometry.
08 / Optional additive fabrication
Printed connection fittings: document the slicer settings
If you choose approved 3D-printed joints, they must be individual PLA or PETG connection fittings, not structural members. Document model geometry, build orientation, slicer preview, and measured mass for each physical printed part before the build.
35% maximum
Infill percentage
4 wall loops
Perimeter count
5 top layers
Top solid layers
5 bottom layers
Bottom solid layers
0.28 mm
Layer height
0.4 mm
Nozzle diameter
Fitting mass is not just an estimate: use the slicer estimate for planning and measure actual printed part masses for the final record. All printed parts together ≤40 g, each ≤15 g and maximum fitting length ≤50 mm. Settings above reflect the Unit 2 required profile; if your printer cannot use those settings, request instructor direction rather than silently substituting another profile.
09 / Interactive design review
Build your BOM, estimate mass, and catch rule violations
Enter the real or carefully estimated quantity and mass per item. All cells begin blank because your design quantities and materials are not known. When a component category is not used, set quantity to 0. Repeated printed connector types can be entered separately. Then run dimension and connection checks below.
| Part / category | Type | Qty | Mass each (g) | Total mass (g) |
|---|---|---|---|---|
| Wood primary | — | |||
| Wood primary | — | |||
| Wood primary | — | |||
| Wood primary | — | |||
| Plywood gusset | — | |||
| Adhesive | — | |||
| M3 hardware | — | |||
| Printed fitting | — | |||
| Printed fitting | — |
For cured adhesive and mixed M3 hardware, quantity 1 may represent the total package mass used in this bridge. Mark unused categories with zero; add costs or specific purchased part IDs to your submitted BOM if your teacher requires them.
—Estimated finished mass
—Remaining under 150 g
—Estimated printed mass total
Enter quantities and unit masses to calculate a complete estimate.
Bridge geometry and connection compliance checks
Assembly envelope
If the design has no plywood, leave stock thickness blank and explain. A value is only relevant when plywood is used.
Sample local gusset
The bounding length/width and actual plan area are separate checks; irregular shapes may have smaller area than their bounding rectangle. Apply limits to every local gusset, not just the sample.
Sample printed fitting
A single fitting example only. Check every 3D-printed part's separate mass, max length, and construction purpose.
Slicer profile
Only required if printed parts are included in your bridge.
Allowed construction
Manufacturing readiness result
Complete your real design's measurements. Blank fields remain unverified.
This automated check is screening, not a permit to fabricate. It cannot establish real joint load capacity, kerf suitability, side-truss strength, printed purpose or 100 N / 10 mm performance. Instructor approval remains required.
10 / Final pre-fabrication quality gate
Review before anyone operates the laser or printer
Geometry & structural design
- Clear span ≥400 mm between true support contacts
- Total outer width ≤65 mm including fittings/bolts
- Center loading contact and load transfer to both sides modeled
- Compression risks and two critical joints from 2.16 addressed
- Lateral bracing and connection accessibility shown
Manufacturing & safety
- Laser-ready 1:1 DXF verified in intended tool/software
- Approved wood/plywood stock and material thickness
- Cut paths closed; web widths / kerf checked
- Approved adhesives / M3 only if used
- Approved printing settings / connection-only geometry if used
Documentation
- Native editable CAD file exists and is saved
- Assembly drawing + individual part drawings in PDF
- 2D DXFs for actual laser-cut profiles, not screenshot
- STL/3MF + slicer screenshots for printed fittings if needed
- BOM with masses, quantities, material and manufacturing process
- Documented build order and instructor approval request
Hold point: Students should not begin bridge fabrication until the teacher has approved drawings, cut files, component limits, material choices and the manufacturing method. Lesson 2.18 is where students actually build and inspect the bridge.
11 / Independent checks
Evaluate the mistake before revealing the answer
DXF file format
A student exports the whole 3D assembly as DXF and sends it straight to the laser. What's missing?
Reveal explanation
A clean planar 1:1 cut profile for the intended material, inspected for scale, contours, layers, kerf, and duplicate/unwanted geometry. A 3D DXF is not automatically a safe 2D toolpath.
Gusset area
A rectangular gusset is 40 mm × 25 mm. Does it satisfy the plan-area restriction?
Reveal explanation
No. Area = 40×25 = 1,000 mm², above the 800 mm² limit. Both bounding dimensions may pass while plan area fails. A 40×20 mm rectangle equals 800 mm² and can satisfy the geometric size criteria.
Width includes hardware
Two side frames measure 59 mm outside-to-outside, but the fasteners project 4 mm on both sides. Does it pass 65 mm overall?
Reveal explanation
No. Total width is 59 + 4 + 4 = 67 mm, exceeding 65 mm.
Printed mass total
A design has four printed brackets at 12 g each. Is it acceptable?
Reveal explanation
Each 12 g bracket is below the 15 g individual cap, but together 4×12 = 48 g, above the 40 g aggregate printed-part cap.
Fastener choice
Can you use an M3 machine screw without a nut or printed threads because the opening is tight?
Reveal explanation
No. The rules allow M3 nuts and bolts, with washers allowed. Printed threads and unapproved screw fastening are not permitted. Design access to a genuine nut-and-bolt connection if mechanical fastening is used.
Full bridge versus side model
Why must the Fusion file include a center crossmember rather than just two symmetric truss sides?
Reveal explanation
The load must be applied to the whole bridge and transferred to both side frames. Its crossmember, connections and lateral braces affect load sharing, torsion, total width and mass.
Stock thickness
Your model says 3.2 mm plywood, but calipers measure 3.4 mm. Can the teacher approve using it?
Reveal explanation
Not under the current ≤3.2 mm plywood stock rule. Model the actually approved material instead; do not treat nominal labels as exact measurements.
Editable manufacturing package
Why isn't a single screenshot of the finished CAD bridge sufficient evidence?
Reveal explanation
It does not provide precise part dimensions, editable geometry, 1:1 tool paths, assembly sequence, mass estimates, material specifications or the fabrication details another maker needs.
Reference library
Resources to solve problems without guessing
Autodesk · Create drawing from design
How to create engineering drawings from the active Fusion assembly, select components, dimensions, views and document units.
Read Drawing workflow ↗Autodesk · Create a parts list
Add numbered parts to a drawing table and confirm its quantities and naming, then supplement consumable masses in the BOM.
Read Parts List workflow ↗Autodesk · Export a sketch to DXF
Export a flat sketch and specify units; for an existing solid, project its planar face into a new sketch first.
Read DXF export steps ↗Fusion truss design visual tutorial
Photo-based example of using consistent member centerlines and creating a complete bridge model; compare its methods to the class's different fabrication requirements.
Explore Fusion bridge design example ↗Student submission
Exactly what to submit for POE 2.17
Submit one organized Bridge Detailed Design & Fabrication Package. This is a manufacturing approval checkpoint, not the physical build. Label all files with your team and a revision identifier (e.g., Team03_Bridge_R1).
Required deliverables
- Fusion editable native design: all parts and the completed assembly, properly named and constrained/positioned.
- Assembly PDF: side/top/end/isometric views showing the 400 mm clear span, actual rise, overall width, load point, support contacts and lateral bracing.
- Manufactured-part drawing PDF: dimensions, materials, thicknesses, hole positions and quantities for every unique custom component.
- Two detailed connection drawings: center load-transfer joint and another critical node/support, with adhesive and/or M3 fastener information.
- 1:1 DXF cutting profiles: every intended laser-cut component, with safe contours and verified units; include nest/stock plan if assigned.
- Approved printed connector files if used: STL/3MF and screenshots of slicer settings, per-part mass and aggregate printed mass.
- Itemized bill of materials and mass estimate: include adhesives, all nuts/bolts/washers, printed fittings and every brace or gusset.
- Assembly sequence and inspection checklist: brief numbered build steps, critical dimensions and safe machine-approval request.
- Revision note: identify at least two design decisions or changes that respond to the engineering analysis in Lesson 2.16.
Quality-control submission check
- The CAD represents the whole bridge, not one floating side-truss sketch.
- Dimension units and DXF scale are explicitly millimeters and 1:1.
- All custom parts can actually be cut, printed or assembled using available tools.
- Local gusset dimensions, overlaps and layers follow their separate rules.
- Printed parts, when used, are only connectors and meet all documented size/mass/settings limits.
- Overall width accounts for hardware protrusion and mass includes cured adhesive.
- All restricted materials and fastening methods are absent.
- No claim of actual 100 N capacity or ≤10 mm deflection is made solely from CAD or a mass budget.
- The package is teacher-reviewed before fabrication begins in 2.18.
Due date: Consult the Daily Agenda and teacher's submission instructions.