A foundational engineering lesson introducing first-year students to the Engineering Design Process (EDP) integrated with rigorous laboratory safety protocols. Students analyze historical engineering catastrophes caused by safety oversights and commit to ethical, safe engineering practices through a formal safety agreement.
Studio Challenge: Fail-Safe Redesign for a Benchtop CNC Circuit Mill
Your freshman engineering team is commissioned to build a high-speed (24,000 RPM) desktop PCB CNC milling machine for the open student makerspace. Apply fail-safe engineering principles to complete the safety design matrix below.
Hazard 1: High-Speed Debris / Broken Bit Proposed Engineering Control (Physical/Hardware):
Hazard 2: Enclosure Open While Spinning Proposed Fail-Safe Interlock Mechanism:
Hazard 3: Power Loss / Auto-Restart Proposed Magnetic No-Volt Release Control:
FMEA Test Verification Protocol: Describe one destructive or stress test your team will conduct to prove these fail-safes function prior to peer authorization:
Page 2 of 2 • Submit to your instructor for team design review and safety sign-off.
What Went Wrong?
✖ Designers removed electromechanical safety interlocks present on Therac-20.
✖ Relied solely on assembly software logic to verify beam collimator position.
✖ A race condition delivered massive, lethal radiation doses to patients.
Fail-Safe Hardware Principle
Safety-critical systems must possess independent, physically decoupled hardware interlocks that guarantee a safe state even if software crashes or enters an undefined branch.
Rule: Never allow software to be the sole guardian against physical harm.
EDP Takeaway: Hardware interlocks and physical fail-safes are non-negotiable.
ACTION
Studio Challenge & Lab Authorization
Today's Deliverables
1. Safety Contract Pledge
• Review lab SOPs and PPE requirements
• Initial all 5 conduct covenants
• Sign & submit for workshop badge approval
2. Design Flaw Forensic Lab
• Map safety checkpoints into the EDP cycle
• Complete Hyatt & Therac forensic analysis
• Redesign CNC mill fail-safe safety interlocks
"Safety is not what we do after we engineer. Safety is how we engineer."
BENCHTOP CNC MILL REDESIGN: MODEL BENCHMARK ANSWERS
Hazard 1: Bit Projectile
Exemplar: 6mm thick polycarbonate shatterproof enclosure surrounding entire spindle envelope with labyrinth seals to block ejected carbide fragments.
Hazard 2: Enclosure Open
Exemplar: Dual-channel magnetic safety interlock switch wired in series with the main spindle power relay, triggering an active DC injection brake within 0.5s.
Hazard 3: Power Loss
Exemplar: Magnetic No-Volt Release (NVR) switchgear that drops open upon power interruption, requiring manual push-to-start reset after power is restored.
FMEA Test Protocol Exemplar:
Conduct 50 consecutive door-open trigger cycles while spindle operates at max RPM (24,000) under zero-load, measuring spindle stop-time via optical tachometer to guarantee complete arrest under 1.0 second.
3. Engineering Design & Safety Rubric
Criterion
Exemplary (4 pts)
Proficient (3 pts)
Developing (1-2 pts)
EDP & Safety Integration
Accurately places safety checks in all 5 stages; deep understanding of iterative risk loop.
Places safety checks in most stages; minor gaps in feedback loop.
Views safety only as post-build testing or PPE constraint.
Explains general failure but lacks specific mechanical or software terminology.
Superficial summary; fails to connect root cause to design oversight.
Fail-Safe Machine Design
Proposes concrete physical engineering controls (NVR switches, interlocks, shields) with test protocols.
Proposes valid controls but lacks detailed specification or test plan.
Relies primarily on warnings, signs, or PPE rather than engineering controls.
STUDIO FACILITATOR TIP: PHYSICAL PROPS
Pass around physical examples of blown fuses, sheared bolts, and a working magnetic safety microswitch during the Case Study review. Allowing first-year students to touch the hardware significantly reinforces the concept that safety mechanisms are physical systems governed by physical laws.
Page 2 of 2 • Fail-Safe Engineering Instructional Resource Suite • Module 01
