I have heard of lasers being used to cut metal, however all laser cutting machines I've found can't cut metal, I'm guessing because it reflects the light?
What is different in the designs of such machines between those that can and can't cut metal? Is it just accepted that energy is lost in reflection, and has a safe way of absorbing reflections?
A typical "hobbyist laser cutter" with about a 50W power rating won't get you very far cutting metal except for metal foil, but there is no big deal about cutting up to 1 inch thick mild steel plates or tubes with a few kilowatts of laser power. There are plenty of videos on YouTube.
Basically, you need enough power to cut quickly before the (high) thermal conductivity of the metal heats up with work piece and causes distortion, cracking, locked-in stress concentrations, etc. Aluminium needs higher power to cut than steel, because of its higher thermal conductivity.
Typical thermal conductivity of hobbyist materials like wood, plastics etc is less than 1 W/m.K. Stainless steel about 15, high and low carbon steels about 50, aluminium about 200, and copper about 400.
The price tag for say a 5kW laser cutter for "industrial-scale" metal cutting is in the £100,000 to £500,000 range, so you won't find many for sale on Ebay.
I know this is an old question, but I'd like to expand on the general info alephzero stated.
In my research for building my own metal cutting CNC laser, I've found that it takes at least a 150 watt CO2 laser to cut through even thin metals. This is also coupled with using a gas (N2 or O2) to either shield the material cut or to help it be cut. Thickness can also be a factor in whether a shield gas or a cutting gas is used.
http://am.co.za/laser/thickness
Due to the different reflective nature of different metals, not all metals can be cut at the same power. Thin steel and stainless steel can be cut with a 150 watt laser, but much more powerful lasers are needed for more highly reflective materials, like aluminum and silver.
ND or ND:YAG lasers are commonly used for metal cutting, although they are much more expensive laser systems, and therefore much more expensive machines.
https://en.wikipedia.org/wiki/Laser_cutting
For my own machine, I've made it large (4'x8' capacity) and found that a 180 watt CO2 laser tube is between 6.5' and 7' long. Since my CNC frame is so big, I don't have to worry about that, though. The tube, if gotten directly from China can be as "little" as \$900-\$1000, but a more local USA dealer has them for \$1500-\$1600.
I've spent around \$8000 on my own machine and still don't have the tube yet, but that's not the £100,000 to £500,000 alephzero mentions. I've seen machines (from China) that were around \$40,000 with power ratings to do metal, but that's still not something you are likely to get with the change in your couch.
The thing about the reflections is that they definitely need to be managed. In too small of a laser cutter, the reflections will go back into the tube and basically set up a standing wave that eventually burns out the tube, since not enough of the light is absorbed or dispersed, exceeding the rated power of the tube for too long.
In a laser that can cut through metal, the laser can still be reflected into the tube, but only until it pierces the material. At that point, the angle of reflection moves any reflected laser to a different angle. That usually means the laser is reflected into the bottom the of laser cabinet and handled/absorbed there, if not used to heat the metal. By the time the laser hits something at that point, it is usually not focused enough to do any damage.
Also, many laser cutters have a full cabinet with windows that absorb the laser wavelength. In the one I have access to, it uses specially coated glass windows to absorb the laser. In the other one, it's Plexiglas/acrylic windows. Acrylic goggles/glasses are used when a laser cutter doesn't have a cabinet.
