Beam reflection when cutting metals

[Mike Lysov]

Hi guys,

finally got my new 280W laser fixed. It was delivered with one tube cracked(careless packing for the whole system) and it took almost three months to get it replaced.
It cuts thick MDF(tried up to 18mm) and even 18mm thick OAK quite nice and clean.
I asked the laser mfg if it can cut through metal with a proper gas and they told me it is not designed for it. They explained that laser beam reflection from shinny metal surface can damage laser tubes.
Ok, I thought I did not bought it for metals and that's ok it cannot do it.

But curiousity won and I decided to try with just normail air from my air compressor and some non shinny metal.

I found a piece of 1mm thick zink plate that is used to join wooden beams. I put it on a laser bed, focused, set speed to 400mm/m, set power to 90% and initial power to 65% and hit a start button. and .. it worked. The plate was cut through with just regular air. Wow!
Ok, it has not been as smooth as it sounds. I figured out that I did not set initial power to a proper number so initial pulse did not pierce through. But since the plate had a lot of holes for nails when the beam went through one of these wholes and hit the plate again it started cutting through.

Now, I am wondering, is it really a problem with some metals that beam can be reflected, sent back to the tube and damage it? But how do these metal laser cutting machine work if this problem exists?
I have tried to google but information I have found on a few websites is different. Some articles say that only 10% is reflected, some say that only specific metails reflect the beam.

[Bruce Boone]

Hi Mike.
The 280 watts is at the low end of the spectrum on what it takes to get through metal. Because that's the case, the beam will do a lot of reflection rather than piercing straight through. Some metals like copper and gold reflect almost all the infrared from a CO2 laser. When cutting metal, the industrial lasers do a dwell in one spot to turn the metal molten before the gas assist can blast it away or through the hole. The time varies with wattage and thickness of metal. I dwell for about 1/3 of a second when going through .080" thick titanium and for about 1 second going through .200" titanium for example. Once through, the cut goes easily.

The very high power industrial lasers will have special optics that allow the beam through but will cancel it out when it's being reflected back up the optical path. It's done with polarizing the beam. Even though my fiber laser's wavelength is about 1/10th as long as a CO2 laser's, I still don't attempt to cut through copper because of it's reflecting properties. A CO2's mirrors are often made from copper. They say that if you are going to try a metal like that, you need to start with way too much power then back down until the cut is good. If you start with too little power to get through, the beam will be reflected and cause problems.

Cutting zinc is supposed to cause poisonous gasses, so avoid that. Thin stainless steel should be a good trial for it. Many amateur CO2 laser builders measure their output in "Gillettes" or how many razor blades they can blast through.

[Mike Lysov]

Thank you Bruce for your detailed answer. I am not going to use it for metals then.
I can recall now that the manufacturer has mentioned that their mirrors are made of copper and it can cause a problem with the beam reflected back to the tubes.

[Scott Shepherd]

I'm pulling this from very old memory, so take that for what it's worth, but in the early days of lasers cutting metal, it was aluminum that caused the major problem. Molten aluminum is like a mirror. That caused major problems for lasers in the early days. It was a given that you should never cut aluminum. It didn't take long for them to figure out how to get around that problem, but the problem was never much wider than aluminum and a few other softer metals. That issue was probably resolved 20 years ago. Cutting aluminum with lasers is really very common practice these days. Having said all that, I'd put your laser in the boat with the early metal cutting lasers and I'd stay away from things like aluminum. You machine probably doesn't have the safe guards in place to stop that reflection. Just my opinion from so old information. I could be wrong.

[Richard Rumancik]

. . . I found a piece of 1mm thick zink plate that is used to join wooden beams. I put it on a laser bed, focused, set speed to 400mm/m, set power to 90% and initial power to 65% and hit a start button. and .. it worked. . . .

Do you mean that it was zinc plated steel? I doubt that you had zinc sheet, as that is not really used for much of anything and certainly not as as joiner for wooden beams. Also zinc has minimal tensile strength so I assume that it was steel that you were cutting . . .

Just the same the fumes from zinc plate are also hazardous so proper venting would be needed.

[AL Ursich]

I will jump in here with a WAG... Wild AL Guess.. Having worked at a Lumber Yard, the New Pressure Treated with the copper based Preservative needs Hot Dipped Zinc Metal fastners.... Regular Steel will react and rust... So Metal brackets used for Pressure Treated Lumber. Hence the Holes....

AL

[Dan Hammerstron]

Mike, what machine do you have with a 280 watt tube? Thats the kind of tube I want..we do furniture and a clean cut of 18mm would be nice.

[Mike Lysov]

Dan, I have just sent you a personal message.

[Mike Lysov]

Sorry I meant zinc. That's the one I was talking about
http://www.bovanail.com/technology/img/stycnikova-deska.jpg