The rotating / reciprocating mass may appear stationary at some RPM.
Just a reminder to only ever use incandescent or halogen lights around machinery you can touch while in operation: drills, lathes, mills, slotting machines, etc.
Before you start a tool you should act out your planned motion. Then start the tool and only perform this motion. If something changes and you cannot complete the motion, stop the tool immediately. Following this and a few other safety tips I've operated power tools for years without incident and I'm legally blind.
Okay, but, say you've configured the tool wrong and now it's destroying your work. You stop the tool. How soon is it safe to grab your work away from the work surface—i.e. when will it go from "it's too dangerous, let it keep destroying the work" to "it's now slow enough to pull the work off of the tool while it's still—slowly—running, to save the work from further damage"? That's a thing you have to figure out with your eyes (and ears).
So, evidently, I may have over stated my initial claim about only using incandescent lights.
Still, it’s something to me conscious of.
Not saying it can't happen but it does seem stupidly hard to achieve.
Many AC motors are synchronized to the frequency of the power line, by the same principal that turbines in the power grid are. So 60hz AC often means 30hz = 1800 rpm motor, and 59.8hz AC means 29.9hz motor. So if you have LEDs with a half bridge rectifier and your machinery is 180 degree rotationally symmetric, it will appear to not be moving, even if the line frequency fluctuates.
Fluorescent tubes are most noticeable, LEDs a little less. Tungsten or Halogen are the only sensible option for workpiece illumination.
For my hobby stuff, I just bought a couple of spare halogens that should see me out.
If the light is flickering it's absolutely guaranteed that it's derived from this.
But it's hard to imagine a lightbulb that emits a short enough pulse of light to make something look stopped. Even an absolute garbage one-way rectifier will be emitting light more than a quarter of the time. That can make a tool look odd, but it won't make it look still.
Not necessarily. If it's a 50/60 or 100/120 Hz flicker then yes, but LED lights with a cheap switching power supply might still flicker at the switching frequency, which could be say 400Hz or something.
Even after 20 years in the metal fabrication industry that scenario still spooks me.
Most will flicker at a multiply of the power line frequency (60 or 50 Hz depending where you live).
It's so common, that most digital cameras have a setting to set the power line frequency, so they can reduce the flickering in the footage.
Incidentally - many alternating current engines are also working with multiples of the powerline frequency, because it's easier that way.
Edited to add: Have you ever heard of a timing light for an engine? This works exactly the way you are describing.
The timing light is then pointed at the flywheel on an engine, which has numbers or marks stamped into it. Each time the spark plug fires the timing light (which is hooked into that same current via induction) lights up for a brief amount of time to show at what timing offset the engine is currently at. (this all happens at hundreds of rpms a minute).
so I'm not sure it's the same
Never underestimate the ability of machinery to rapidly render you dead.
Never underestimate the ability of otherwise intelligent people to intentionally disable safeguards, or guninely make mistakes.
So far, the discussion sounds like it's a theoretical possibility. If it's a real possibility, then it's something that should have happened a number of times.
The rotation frequency would have to be close to a relatively simple multiple (or fraction) of AC frequency but the way electric motors are contstructed if the piece is directly connected to a motor it is very likely it is true.