by btboone » Sat Oct 06, 2012 3:08 pm
I have built one on a Tormach mill with a fiber laser. A CO2 laser's wavelength is not optimal for cutting metals. Industrial ones do it with the brute force technique, and usually have thousands of watts. They need a continuous supply of nitrogen, CO2, and helium and usually take 460 volt three phase power. Big ones have a radio frequency resonator, and gasses are shot in the vacuumed out chamber at nearly the speed of sound by a compressor running at something like 300,000 rpm. You can cut thin metal with about 400 watts with an oxygen assist, but you are at the lower threshold of being able to cut metal at all.
A fiber laser is a whole different animal. It works similar to a ruby or yag laser in that the laser medium is a solid that's pumped by light. The difference is that the lasing medium is only about 50 microns wide, but is extremely long. This means that is is extremely efficient because all the photons are already traveling in exactly the right direction, but also the pump light is introduced from one end and can only bounce around the fiber core and can't get out due to total internal reflection angles. The light that excites them is done by diodes, and is exactly the right wavelength to get the fiber to lase. The diodes are extremely efficient and are glued to a plate of aluminum. Because the fiber is so long and thin and because the diodes are glued to the plate and their power combined, the laser is extremely efficient, with a wall efficiency of around 40%. This means a fiber capable of cutting through metal plugs into the wall and needs no cooling! Because of a trick of optics, fiber lasers also don't use external mirrors; they are "built in" to the fiber itself! The beam does need to go through a collimator and then focused to a spot. A separate laser head serves this function. The laser head is also plumbed with a high pressure air or other assist gas. I am cutting titanium, so I am using argon. Steel could use nitrogen, oxygen, or air.
My laser is 450 watts continuous, but due to the diodes pumping the fiber, they can be pulsed and the laser can produce a peak power of 4500 watts. I had some trials run, and we went through 7mm of titanium. The piece was a thick ring blank and had a hole through the middle. The beam went through one side and actually went through the opposite side as well. This was a total of 14mm of titanium using "only" 3000 watts. I went with a special laser head with a long and short lens option to be able to do very fine stuff. Theoretical kerf with the shorter lens is 50 microns. That's about .002", which is less than a hair. The longer lens is meant to go through thicker material, and has a theoretical spot size of 100 microns, although I haven't yet gotten that small in practice. I am still doing trials, and have a kerf on .070" stainless sheet in the order of .006". It's a very clean cut from the front, but I still have a little slag on the back. I have a technician coming in next week to help dial it in.
Honestly, for metal applications, fiber will be replacing CO2 in factories everywhere. They have beam switches where several machines can be run from a single laser. One can be cutting while another is welding. It's pretty amazing technology.