CNC Safety & Interlocks

CNC machines spin sharp metal at thousands of RPM, plasma torches throw molten steel, lasers blind. This page is reference-style: the practices and the hardware that make hobby CNC less likely to maim you. Not a substitute for OSHA or NFPA training; not legal advice. Cross-reference with Laser Firmware & Controllers for laser-specific concerns.

Hard E-stops & interlocks

• ★ Mushroom-head E-stop button — required on every machine. Wire it to a normally-closed contactor cutting power to the spindle / steppers — not just a software signal. Schmersal, IDEC, Pilz, Allen-Bradley all sell hobby-friendly switches.
• ★ Safety contactor / safety relay — paid (~$30–100); a real safety relay (Pilz PNOZ X1, Allen-Bradley 440R) latches the E-stop until manually reset. The "right way" vs. "just a button into the controller."
• Door interlocks — magnetic / mechanical interlocks (Schmersal AZ16, Omron D4N) on enclosures; cut motion when door opens. Standard on Pocket NC, Tormach 770MX, retrofit Bridgeports with enclosures.
• Light curtains — paid, expensive (Banner / Sick / Allen-Bradley); pro-grade non-contact safety. Overkill for hobby unless you're running production.
• Two-hand control / dead-man pendants — for operations like manual jog where you need both hands clear of the work area.

Spindle / motion safety

• Spindle braking — VFDs (Hitachi WJ200, Delta MS300) support DC injection braking; bring a 24k-RPM spindle to a stop in seconds. Critical: an unbraked spindle coasts for 30+ seconds.
• Soft limits — controller-level G-code travel limits ($20=1 in GRBL/FluidNC; LIMITS in LinuxCNC). Stops the machine before it rams a limit switch.
• Hard limits — physical limit switches at end of travel; final backstop. Mechanical (Honeywell), inductive (M12 NPN/PNP), or optical.
• Home + index switches — required for repeatable WCS. See Wiring & Electrical.

Fire safety (router / laser / plasma)

• ★ Class B/C fire extinguisher — paid (~$30); ABC dry chemical extinguisher within arm's reach. Dust + electronics + spinning metal = real fire risk on routers; lasers ignite Plexi/MDF; plasma is constant fire risk.
• Fire watch on lasers — never run a laser unattended; the rule. Cellulose acetate (acrylic), wood, MDF can ignite mid-cut.
• Smoke detectors in the shop (battery + interconnected to home).
• Spark-proof dust collection — ducted dust on a router cutting MDF can carry embers; DustOpo / Oneida fire-suppression systems exist for ducted shops.
• Sprinkler systems — beyond hobby scope, but production sign shops with multiple routers + dust ducts often need them per NFPA 654.

Personal protective equipment (PPE)

• ★ Safety glasses (ANSI Z87.1+) — required, always. Side shields. Cheap.
• Face shield — for lathe work, parting cuts, and any operation where chips fly toward the face.
• Hearing protection (NRR 25+ dB) — routers + Makita = sustained 95+ dB. Earplugs at minimum; over-ear muffs better. 3M Peltor X-series, Howard Leight Impact Sport (electronic, lets conversation through).
• Respirator (P100) — for MDF / plywood cutting, finishing, and sanding. 3M 6200 + 7093 cartridges, Sundström SR-100. Disposable N95s are minimum; not suitable for continuous use.
• Closed-toe shoes / steel-toe for plasma / mill work where chips can drop.
• No gloves on rotating tooling — the rule that catches everyone. Gloves are great for material handling; lethal in a spinning spindle.
• No loose clothing, hair, or jewelry near anything that rotates.

Laser-specific PPE

• ★ Laser safety glasses (OD 4+ at the wavelength) — required for direct exposure; even reflections from a CO2 laser hitting metal can damage retinas. Glendale, Honeywell, Phillips Safety — buy specifically for your wavelength (455nm diode, 808nm IR, 1064nm fiber, 10.6μm CO2). Generic glasses won't help.
• Enclosure with proper interlock — Class 4 lasers should be interlocked.
• See Laser Firmware & Controllers.

Plasma-specific safety

• ★ Plasma face shield + welding helmet (Shade 5–8 for plasma) — UV/IR from plasma arc damages eyes; even brief unprotected exposure causes "arc eye." Auto-darkening helmet is fine.
• Leather gloves + jacket — for changing consumables and probing; sparks fly.
• Ventilation — plasma generates ozone, NOx, and metal-fume particulates. Downdraft or water table mandatory; respirator backup. See Dust & Coolant.
• Compressed-air quality — water in plasma air shortens consumable life and can backflash; use a coalescing filter and dryer.

Electrical / wiring safety

• GFCI / RCD on workshop circuits — coolant + water-cooled spindles + electricity; required.
• Proper earth bonding — VFDs need real ground (not chassis-only); skipping this generates EMI and electrocution risk. See Wiring & Electrical.
• Disconnect switches — service-rated lockable disconnects for the spindle / VFD circuit. Lockout-tagout for any maintenance.
• ESD wrist straps + mats — for electronics work near PCB mills, breakout boards, controllers.

Documentation / training

• NFPA 5000 / NFPA 654 — fire codes for woodworking shops with dust collection. Free read-only access on NFPA's site.
• OSHA 1910 Subpart O — machine guarding (US). General reference.
• ISO 13849-1 / EN ISO 12100 — machine safety standards (EU); reference for serious DIY builds.
• Tormach / Haas / Avid CNC manuals — vendor-supplied safety sections; often the most accessible starting point.

Pick this if…

• Bare minimum: ANSI Z87.1 glasses + ABC fire extinguisher within reach + a real E-stop button wired through a contactor.
• Hobby router on a sub-panel: add P100 respirator for MDF, hearing protection, and a smoke detector overhead.
• Hobby plasma: auto-darkening helmet, leather gloves, downdraft or water table, and never run unattended.
• Diode/CO2 laser: wavelength-matched OD4+ glasses, fire watch, fire extinguisher, ventilation. Don't run unattended.
• DIY enclosure on a benchtop mill: door interlock + light tower + Pilz safety relay.