Laser

Why do you think cut wood would make the laser catch on fire?

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There are two kinds of people on this earth: The crazy, and the insane. The first sign of insanity is denying that you're crazy.

this

burning

Would does not burn. It is the gasses emitted by hot wood that burn. The laser does not heat up a sufficient area of the wood to cause the entire piece to go up in flames.

In my wasted youth, I once worked with products that required making steel rule die blanks for cutting patterns out of cloth, rubber, cardboard, and other sheet material. Something like this:

The blanks and backing were made from 3/4" maple plywood. The traditional method was to use a router, but that was inefficient. Trying to cut a 0.1" dia (or less) kerf into incredibly hard maple plywood and heavy plywood (at 50 lb/ft^3) would wreck a carbide router bit every 10-20ft. So, we used a CO2 laser. The problem was we didn't want to cut all the way through the plywood, just enough to hold the steel rule cutter blade. That meant the hot gasses and and carbonized wood tended to remain inside the cutting area, where the accumulation of heat in a small area would tend to create some localized burning. The solution was to simply blow lots of air into the hole to remove both the heat and the waste. The trick was also to keep heat, gases, and waste away from the laser lens.

The only way that plywood is going to burst into flames is if the localized heating is allowed to accumulate. Wood is a fairly good thermal insulator, but far from perfect. Anything that dense, heavy, and massive is going to act as a heat sink, no matter how lousy a thermal conductor. The glue and filler used in plywood are not so good. They tend to spread the heat into rest of the plywood sheet. The laser keeps pouring more and more heat into the kerf and the plywood slowly spreads it out, thus spreading the heat affected zone, and cooling the kerf. The ejection of gasses and carbonized fiber also cools the kerf area. The only way the wood will combust is if more heat is dumped into the area, than the plywood sheet and air can remove.

If you wanna try a crude simulation at home, grab a sheet of heavy plywood, and hit it with a common hand propane torch with the smallest diameter flame possible. After igniting the torch, but before applying it to the plywood, weigh the torch and tank. After reaching ignition, record the elapsed time. Turn off the torch and weigh it again. The difference is that propane consumed. Propane is about

71,000 Btu/gal, so you can calculate how much energy is required to initiate combustion. Now do the same calcs with the energy delivered by a random commercial CO2 laser and see how close you get.

Incidentaly, it's difficult to tell from the original question, whether the OP wants to prevent the plywood from burning, or is building some kind of Star Wars death ray designed to set fire to the neighborhood.

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This is what happens when I try to apply my half-baked, uneducated logic to a situation. Thanks for the clue.

Mark Z.

It may catch on fire. Some solutions:

(0.5) Cool the wood first down to -250F

(1) Use a pulsed laser to ablate off the wood.

(2) Squirt water on the wood, just enough to stop the flame area from heating up.

(3) Douse the area with some non-flammable gas, such as Xenon, Helium, or CO2.