Every used CNC listing tells you about the spindle hours. Almost none tells you whether the door interlock still works, and plenty of used machines arrive with the interlock bypassed, a window missing, or a chuck guard long gone. Federal law has an opinion about that machine, and it applies the day it lands on your floor. Here is the actual regulatory text, plus the federal injury data that explains why inspectors care.
The numbers behind the rule
From OSHA's Severe Injury Report seven-year lookback: "OSHA received 56,696 inpatient hospitalization SIRs and 18,559 amputation SIRs from 2015 to 2021," and the leading cause outranked even falls: "Body parts being caught in running equipment or machinery (12,930, 18%) and falls (10,485, 15%) were the most frequently cited causes of severe injuries from 2015 to 2021." Manufacturing was the top-reporting sector across the period.
OSHA's amputations publication puts machinery at the center of the older baseline too: BLS's 2005 survey counted 8,450 non-fatal amputation cases with days away from work, with machinery accounting for roughly sixty percent. This is the context for every guarding rule that follows: the thing machine guarding prevents is the single most common way American workers get severely hurt.
The law, verbatim
29 CFR 1910.212(a)(1): "One or more methods of machine guarding shall be provided to protect the operator and other employees in the machine area from hazards such as those created by point of operation, ingoing nip points, rotating parts, flying chips and sparks."
And the point-of-operation rule, (a)(3)(ii): the point of operation "shall be guarded," with the guard "so designed and constructed as to prevent the operator from having any part of his body in the danger zone during the operating cycle."
On a modern enclosed CNC, the enclosure is the guard. The interlocked door, the window, the sheet metal: that is the machine's compliance with 1910.212. Which is exactly why a bypassed interlock matters legally and not just practically: a machine whose door opens during the cycle no longer prevents the operator from reaching the danger zone, and the text above stops being satisfied. (We looked for an OSHA interpretation letter addressing CNC enclosures specifically and did not find one; what you just read is the standard's own text, which is the binding layer anyway.)
OSHA's hazard taxonomy, translated to the machine you are inspecting
OSHA's amputation guidance classifies hazardous motions in a way that maps cleanly onto CNC anatomy: rotating parts (spindle, chuck, cutter: "Amputation hazards result from rotating cutters at the point of operation and rotating spindles," says the milling-machine section), transversing motion (axis travel and gantries), in-running nip points (tool changers and belt drives), and cutting action at the point of operation itself. Each one corresponds to a physical guard or interlock you can put your hands on during a pre-purchase inspection.
Maintenance has its own statute
Guarding covers operation; servicing runs on 29 CFR 1910.147, lockout/tagout, which applies to "the servicing and maintenance of machines and equipment in which the unexpected energization or start up of the machines or equipment, or release of stored energy, could harm employees," and requires an energy-control program. For a used-machine buyer the practical question is simple: can this machine actually be locked out? Missing disconnect handles and jumpered circuits are maintenance-safety defects, not quirks.
The inspection checklist this creates
- Cycle the door interlock, in person. Command a spindle start with the door open; it must refuse. Zip-tied keys and taped sensors are the classic used-machine bypass, and undoing someone's bypass is your cost, not a discount.
- Every e-stop, every position. Press each one during a dry run. An e-stop that does not drop the machine is a guarding failure in the 1910.212 sense.
- Windows and enclosure panels present and intact. A missing window is a hole in the guard. Budget replacement before the machine cuts.
- Lathes: chuck guard and interlock. Rotating-part hazards are the top of OSHA's taxonomy, and open chucks are where they live: relevant to every lathe in our database, from the Haas ST-10 and ST-20 to the Mori Seiki NL2500.
- Open and semi-enclosed machines deserve extra thought. Older VMCs like the Fadal VMC 4020 and compact machines like the Tormach PCNC 1100 ship with less enclosure than a modern VF-2; whatever guarding configuration you run still has to satisfy the text quoted above.
- Verify the machine can be locked out. Disconnect present, lockable, and reachable. If the seller cannot show you, price the electrical work.
Sources: 29 CFR 1910.212 and 1910.147 (eCFR, fetched via proxy, quoted verbatim and independently re-verified), OSHA 3170, Safeguarding Equipment and Protecting Employees from Amputations, and OSHA Severe Injury Report: A Seven Year Lookback (2015-2021) (both official OSHA publications, quoted verbatim, independently re-verified). Per our methodology: BLS's own amputation-by-industry tables ship as spreadsheets our pipeline cannot yet parse, so the severe-injury figures here are OSHA-sourced and labeled as such.