Yep, all true. My niece was familiar with CNC lasers from her architecture degree labs, and bought a Full Spectrum as soon as she could afford it. She told me a lot about her learning curve.
Her point is a variant of what you just said: people buy these things thinking they’re an appliance, like a coffee maker or refrigerator, not knowing that they are really automated tools that need technically-accurate setup, alignment, maintenance, software setup, and surrounding-area modifications to run for any significant amount of time. People who don’t have either the technical background or the willingness to learn quickly should not be buying them thinking that they’ll just go make easily.
My reasoning on the heat/flame sensors was much like yours. There is a vaporization, minor flames, and a very, very hot spot inside the K40 cabinet all the time. Sensing vapor and fumes (ala smoke detector) won’t work, because there is supposed to be vapor and smoke in there. Sensors that look at the beam target spot will of course see high levels of both near and far infrared - that’s how it works. I went after excess heating in the exhaust stream.
We know that the laser delivers (at most) 40W to the air in the enclosure. The tube’s cooling water removes about 160-200W of heating (I came up with this in the cooling files I haven’t yet posted here), and there is some cross-ventilation from sucking air from the power supply section, maybe another 20-30W. One ought to be able to figure the air mass flow rate, calculate the temperature rise of air in the throat of the exhaust vent, and set a temperature threshold where getting hotter than that requires burning fuel in side the laser cab - a fire.
I’ve also speculated about doing a two stage extinguisher, the first being a water mister, coming from web sites from colleges where they require not only watching their lasers in labs, but also keeping a water spray bottle ready to extinguish small flames before they can grow. After that, you’d valve in CO2 to smother the flames.