Approval & setup
Confirm instructor-signed design revision, machine certifications, permitted stock and accurate DXF/slicer files.
Principles of Engineering • Unit 2 • Lesson 2.18
Bridge Fabrication
Engineering question: Can we transform an approved CAD model into a physically accurate, well-documented bridge that is ready for safe qualification testing?
Learning outcomes
Five stages from CAD to finished bridge
Complete these stages in order, with teacher-approved access to the school's OMTech laser systems and Bambu printers where those tools are needed.
Manufacture components
Cut wood/plywood and print approved joint fittings under the FabLab's machine procedures; identify every part.
Dry fit & inspect
Count usable parts, verify thickness and hole fit, remove rejected parts, and match the approved assembly drawing.
Assemble & cure
Build two square side frames, connect cross-bracing and center load transfer, use only approved adhesive/hardware.
Measure & document
Check actual span, total outside width, complete mass, gusset and fitting limits, defects, and teacher review.
Suggested pacing: 3–5 45–50 minute periods, depending on machine queue, adhesive cure and team complexity. Students should alternate between file review, part organization, assembly preparation, documentation and inspection while machine access is scheduled. Teacher-selected curing requirements take precedence over class timing.
01 / Fabrication rules
What students may and may not build
≥400 mm
Clear span measured between real support contact positions
≤65 mm
Overall outside width including bolt heads and fittings
≤150 g
Actual finished bridge mass including glue and hardware
100 N load
Official test in 2.19, not during fabrication
≤10 mm deflection
At the 100 N test load; not proven by visual inspection
Approved wood
Primary structural members only
Approved construction
Balsa, basswood, craft sticks or plywood for primary members. One-piece laser-cut open-web plywood side trusses are permitted if manufacturing rules are met.
Connections may use PVA glue, CA glue, interlocking wood joints, local plywood gussets, approved PLA/PETG connectors or M3 nuts with bolts and permitted washers.
Prohibited during construction
No acrylic, solid plywood side plates, printed chords/diagonals/decks, hot glue, epoxy, staples, nails, screws, wire or non-M3 bolt sizes. No unauthorized strengthening after the design package is approved.
All corrections before the official test must be documented and, if the design changes, resubmitted for teacher approval.
Official rule: No repairs are allowed after official testing begins. Before the test, controlled revisions are allowed only with documented changes and teacher approval. Read the complete Unit 2 rules ↗.
02 / Before using any machine
FabLab safety and job release checklist
Machine and operator readiness
- Teacher-approved revision of CAD, cut profiles and print files is available.
- Current safety agreement and specific machine certification are completed by each operator.
- Material type, thickness, orientation and provenance are checked.
- Eye protection and other PPE are used according to classroom and machine SOP.
- Work area is clear, ventilation and machine safety devices are functioning.
- Teacher verifies the machine setup and decides who may run the operation.
- Know the stop procedure and alert the teacher immediately for a defect, flare-up, unusual smell or machine issue.
Specific hazards to recognize
Laser: fire and smoke, improper material, ventilation failure and cuts/burns after operation. Never operate with defeated interlocks or leave a running laser unattended. Only the trained supervisor handles an abnormal or emergency event.
3D printer: hot bed/nozzle, moving parts, unsuitable slicing settings, detached prints and sharp post-processing tools. Wait until the part and plate are cool enough to handle according to the machine SOP.
Wood/adhesives: dust, sharp cutters, wood splinters, CA bonding skin/eyes and poor ventilation. Keep fingers away from cutting paths, use teacher-approved clamping and follow each adhesive label and cure requirements.
Authoritative safety references: Georgetown laser pre/during/post-cut checklist ↗ and Illinois laser cutter safe-use guidance ↗. Classroom equipment SOPs and direct instructor directions govern the actual procedure.
03 / Video learning resources
Watch, identify the process controls, then build
Workshop Companion
Why Glue Alone Won't Make Strong Joints
Watch for: Surface contact, wood grain, force direction, fit and clamp pressure. Do not copy unapproved adhesives or joining methods.
Watch on YouTube ↗Planet 3DP
How to Use Bambu Studio — Beginner Tutorial
Watch for: Part orientation, slicer settings, preview and final mass estimates. Use this class's required print profile rather than generic presets.
Watch on YouTube ↗Laser learning module and safety videos
The FLUX Laser Cutter 101 video curriculum ↗ includes laser fundamentals, safety, setup and parameter lessons. Its controls may differ from the classroom's OMTech machines, so use it for transferable concepts and the school certification for actual operating steps.
04 / Laser-cut plywood and wood pieces
Manufacture approved profiles, not unreviewed file exports
Before the cut
- Identify the team's approved revision, DXF, plywood thickness and part count.
- Confirm all closed exterior and interior profiles import at the correct 1:1 millimeter scale. Compare at least one physical dimension to the drawing.
- Confirm there is no unwanted text, duplicate cut, dimension line or construction geometry in the toolpath.
- Check grain direction, nesting, adequate part/web widths, bed size and kerf suitability against the material and approved design.
- Obtain teacher approval for the material and actual laser parameters. Do not use unverified power/speed settings from an unrelated machine.
During and after the cut
- Only a trained, authorized operator begins the job with functioning interlocks, extraction and any required air assist.
- Maintain direct active attendance for the full cut. Stop and call the instructor if there is persistent flame, unusual smoke or a malfunction.
- Wait for machine clearance and cool-down per the equipment SOP before opening/removing stock.
- Sort parts by ID. Measure a reference dimension and inspect for charring, warped edges, cracks, narrow webs, incomplete cuts and damaged corners.
- Mark faulty pieces REJECTED and retain them as evidence if needed. Do not quietly glue damaged parts into the bridge.
Manufacturing note: All laser-cut plywood stock must be approved and ≤3.2 mm thick. Local gussets must satisfy separate size, area, overlap and layer rules; these limits do not impose a 40 mm cap on primary members or entire open-web side trusses.
05 / Optional printed connection fittings
3D printing is for joints—not for structural bars
Teams using approved PLA/PETG connection fittings should print, cool, remove, measure and weigh each piece, then record quantity and location in the build log.
≤35%
Infill
4
Wall loops
5 / 5
Top / bottom layers
0.28 mm
Layer height
0.4 mm
Nozzle diameter
PLA / PETG
Permitted fitting plastics
Verify the sliced file
- Confirm P1S/X1C or other teacher-selected Bambu profile and 0.4 mm nozzle.
- Use the specified printed-connector profile, not the generic default.
- Preview the toolpath, supports, overhangs, layer count, predicted mass and fitting orientation.
- Confirm planned parts are each ≤50 mm in their longest dimension and predicted ≤15 g.
- Check that all printed parts combined are predicted ≤40 g.
Measure completed fittings
- Allow safe cooling and remove parts without prying near a hot nozzle.
- Inspect for warped holes, cracks, poor layer adhesion, over/under extrusion and damaged corners.
- Measure actual longest dimension and mass per fitting.
- Reject and document any part outside the approved limits; do not modify the design without review.
- Use M3 bolts/nuts where required; printed threads are prohibited.
06 / Joint fabrication and inspection
Build connections that match the approved drawing
Local plywood gusset check
- Plywood stock thickness ≤3.2 mm.
- Bounding size ≤40 × 25 mm; actual plan area ≤800 mm².
- Maximum 20 mm overlap on each connected structural member.
- At most one layer per side of a joint (two total).
- Wood surfaces fit and contact the approved design without gaps or accidental off-center force transfer.
A 40 × 25 mm rectangle has an area of 1,000 mm² and would violate the plan-area limit. A 40 × 20 mm rectangle is 800 mm²; both dimensions and overlap must still be checked.
Glue, interlocking and M3 assembly
- Dry-fit components, orient grain and check each part number before applying adhesive.
- Use only approved PVA or CA glue following label ventilation, handling, assembly/open time and full-cure instructions.
- Use a jig, clamp or temporary fixture if approved. Do not introduce bending or joint gaps by forcing pieces together.
- Where M3 fasteners are planned, check bolt/nut fit, edge clearance, access and even alignment.
- Remove temporary assembly jigs after cure. Weigh the actual finished object, not parts with jigs still attached.
07 / Complete bridge assembly
Two flat side frames, then square three-dimensional assembly
Step A · Side frame 1 & 2
Place drawings or approved nonbonding templates on the workbench. Assemble the two side frames using identical reference points and consistent joint overlap. Check straightness and dimensions before cure.
Do not remove the frame from its jig until it meets the adhesive manufacturer's and teacher's minimum handling requirements.
Step B · Transverse bracing
Use approved spacers, squares or temporary assembly fixtures to position the two side frames parallel and square. Attach the cross-bracing and lateral stabilization members shown in the Fusion assembly.
Check finished width including all protruding hardware before joints are locked in place.
Step C · Load-transfer crossmember
Install the approved central loading member to connect the two side trusses. Verify its position is at the intended midpoint and that it does not introduce an unintended eccentric load.
Allow approved full cure before any handling that introduces structural load. Do not perform the official qualification test in this lesson.
08 / Common defects and what they mean
Stop, record, and correct only with permission
Incomplete laser cut
Evidence: Part will not separate cleanly, narrow web fractures or edge is burned through.
Action: Quarantine the part, identify DXF/material/tool issue and request a corrected cut.
Poor printed fitting
Evidence: Hole is distorted, layers separate, fitting is oversized or overweight.
Action: Record printer/settings evidence, recheck the approved STL/3MF and request a new print.
Unsquare side frames
Evidence: The second frame is shifted, crooked or twisted relative to the first.
Action: Record diagonal/width measurements and return to an approved jig or remake the part before final cure.
Questionable glue joint
Evidence: An open gap, insufficient contact, wrong adhesive, missed overlap or visible fiber damage.
Action: Do not test it. Document the defect and obtain a teacher-approved repair/replacement plan before official testing.
Misplaced load crossmember
Evidence: The loading contact is offset from bridge midspan or does not transfer to both sides.
Action: Record the discrepancy and revise the model/build under instructor approval.
Unapproved substitution
Evidence: Material or fastener does not match the released CAD/BOM.
Action: Stop that assembly step and request an approved revision; do not hide the substitution.
Change control: Keep the part ID and CAD revision of every replaced or redesigned piece. Photograph the issue before changing it, write what caused the defect, and obtain teacher approval for any change that affects geometry, material, or connections. No repairs once official Lesson 2.19 testing has begun.
09 / Digital fabrication record
Track manufactured parts, waste and build decisions
Use actual quantities and observations. The table is intentionally blank: nothing has been manufactured yet. The digital record is temporary, so export your CSV before leaving the page.
Minimum photo evidence
- Parts laid out with identifiers, material and source file/revision.
- Laser-cut/printed quality close-up, including one measured feature.
- Two side frames before cross-bracing.
- Both critical joint assemblies before and after joining.
- Complete bridge with ruler/calipers and scale during final inspection.
This webpage records filenames and notes; it does not upload or store images. Submit actual photos through the teacher's normal assignment process.
Part production register
| Component category | Planned quantity | Usable finished | Rejected / scrapped | Inventory check |
|---|---|---|---|---|
| — | ||||
| — | ||||
| — | ||||
| — | ||||
| — | ||||
| — | ||||
| — | ||||
| — |
—Planned individual pieces/units
—Usable produced total
—Rejected total
For adhesives, a quantity of 1 can represent one application category; do not mistake its inventory count for adhesive mass. Component quantities are only a progress tool—final bridge weight must be measured on a scale.
Time-stamped build and deviation log
| # | Date | Stage | Status | Observation, revision or decision | Evidence filename |
|---|---|---|---|---|---|
| 1 | |||||
| 2 | |||||
| 3 | |||||
| 4 | |||||
| 5 | |||||
| 6 | |||||
| 7 | |||||
| 8 |
Enter the planned and usable part counts to identify any missing components.
10 / Physical inspection before load testing
Record real dimensions and teacher-review status
The inspection below uses actual measurements of the complete, unloaded bridge. A blank result means not yet inspected, not “pass.” Meeting these thresholds does not establish structural strength or approve the test.
Complete bridge measurements
Joint and fitting measurements (if used)
Construction choices
Witnessed evidence
Inspection interpretation
—Support span
—Outside width
—Finished mass
No physical results entered. The design is not yet ready for teacher review.
Important: “Ready for teacher review” is not the same as passing the bridge load test. Only the teacher can release the bridge for official testing. Do not apply trial loads or repair after official 2.19 testing begins.
11 / Independent self-check
Before you reveal the answer, justify your decision
Out-of-tolerance laser part
A plywood side truss is cut from a file imported at the wrong scale. Can you stretch the finished side truss by hand and still call it approved?
Reveal explanation
No. Quarantine it, compare the DXF scale and a known dimension, document the failed cut, and recut using a reviewed file. A distorted part is not a substitute for the approved geometry.
Gusset size
A local gusset measures 40 × 25 mm and is rectangular. Is it legal just because neither dimension exceeds its stated maximum?
Reveal explanation
No. Its plan area is 1,000 mm², over the 800 mm² limit. Both dimensions and plan area must be checked.
Printed fitting mass
Four fittings each weigh 12 g. Do they meet both individual and aggregate printed-mass limits?
Reveal explanation
No. Each is ≤15 g, but four total 48 g, violating the 40 g aggregate cap.
Final width with hardware
The two trusses measure 60 mm outside-to-outside, and bolt heads protrude 4 mm on each side. What is the total width?
Reveal explanation
60 + 4 + 4 = 68 mm, exceeding the 65 mm maximum.
Uncured adhesive
The two side trusses look straight, but the glue has not reached the required cure condition. Can the team load it to check if it feels stiff?
Reveal explanation
No. Follow the actual adhesive product's cure requirements and teacher instructions before handling the structure under load. Official testing occurs later.
Change after fabrication
A team discovers an M3 bolt interferes with lateral bracing and wants to relocate the hole. What is the correct process?
Reveal explanation
Document the interference, propose a CAD/manufacturing revision, and obtain teacher approval before altering the physical bridge. Record the new revision and replacement part if applicable.
Actual versus predicted mass
Fusion estimates 145 g, but the fully assembled bridge weighs 153 g on the scale. Which value governs the physical limit?
Reveal explanation
The measured finished mass: 153 g exceeds the 150 g limit. The design must be revised and approved before official testing.
After testing begins
The bridge has just started the official qualification test and a gusset starts peeling. May the team add CA glue and retry?
Reveal explanation
No. The Unit 2 rules prohibit repairs after official testing begins. Record the failure evidence and follow the teacher's test protocol.
12 / Lab resource library
When students need more guidance
Stanford · Laser cutter setup and safe use
Preparation, permitted materials, interlocks, exhaust, air assist, continuous attendance and post-cut housekeeping.
Open Stanford laser safety guide ↗University of Illinois · Bambu slicing walkthrough
Step-by-step slicer preparation and printer setup. Use the classroom's bridge-specific PLA/PETG profile rather than unrelated generic values.
Open Bambu preparation tutorial ↗Woodworkers Journal · PVA glue basics
Correct glue selection, surface preparation, assembly time and holding/clamping strategy.
Read and watch woodworking reference ↗POE Lesson 2.17 — Detailed Bridge Design
Return to dimensioned Fusion files, 1:1 laser profiles, joint details, slicer settings and the approved bill of materials.
Reopen the manufacturing package →Student assignment and approval gate
Exactly what to submit for POE 2.18
Submit a single organized Bridge Fabrication & Pre-Test Inspection Record with real measurements and evidence. This is not the Lesson 2.19 load-test report.
Required submission evidence
- Team and approved revision of the 2.17 manufacturing package (file names, not necessarily re-uploading unchanged CAD).
- Manufacturing job record listing machine/method, materials, usable and rejected part counts.
- Chronological build log with any errors, replacements, reprints, recuts or approved design deviations.
- At least five labeled photographs: parts, fabrication detail, side frames, critical joints and completed bridge with measured dimensions/mass.
- Two critical connection inspections documenting permitted joint methods, dimensions and any defects.
- Actual assembled clear span (mm), maximum outside width (mm), and finished mass (g).
- Actual printed fitting masses/dimensions and slicer profile evidence if connectors were printed.
- Local plywood gusset and stock-thickness compliance evidence if gussets were used.
- Short reflection about one fabrication challenge, its root cause, corrective action and an engineering question for the load test.
- Teacher-reviewed inspection form or teacher-directed sign-off before 2.19 testing.
What a complete record demonstrates
- The real finished bridge, not just CAD, has been measured.
- All components match approved wood-based manufacturing and connection rules.
- Every rejected part and design deviation is visible in the record.
- Both side frames, lateral braces and center loading crossmember are complete.
- All adhesive joints have satisfied required cure instructions.
- The bridge is unloaded, undamaged to the extent visible, and submitted for teacher review.
- The official 100 N and 10 mm test results are not invented or claimed in advance.
Due date: See the Daily Agenda and teacher's submission directions.