Hi Jeshua, I spent a number of years feeding a Trumpf 4kW industrial laser. It has been a while but I'll dump what I know.
1. The output of the laser beam changes width as it exits the laser tube. The industrial machines use a patented fixed length beam delivery system. Think of this as a trombone arrangement with 2 90 degree mirrors mounted in the slider. As the carriage moves from left to right and front to back, the trombone slider moves in and out to keep the lens at the same distance from the laser tube throughout the travels of the machine. The beam waist (beam diameter) does not necessarily disperse to infinity like a solid state laser pointer. A CO2 will have nodes and anti-nodes where the waist diameter gets slightly larger and smaller as you move away from the laser head. This visual looks like the exhaust from a fighter jet when the afterburners are on - pockets of blue flame spaced about 1 foot apart. In the jet engine this is due to the resonance of the engine and in the laser due to the resonance of reflection, power supply frequency, etc. Most hobbyists use an adjustable length beam delivery because it is easier and takes fewer mirrors. 2. The beam delivery system is pressurized with clean moisture free air or other cutting/purge gas to keep splatter and fumes away from the cutting lens and any mirrors near the cutting lens. You can cut with pure O2 as this helps to oxidize the material as you cut. We used large nitrogen tanks for purge air as we were cutting 10-12 hours/day. Shop air can also be used if it is oil-less and dry. If no purge air is used deposits will build up on the lens. If you can see smoke or splatter and do not have a purge system the particulates will build up of the optical surfaces and degrade the output. 3. CO2 lenses have fixed focal lengths based on the beam delivery head. The beam delivery head has the output lens recessed back into it a bit with purge air passages around the lens. The air exits like a air gun but with a larger opening to allow the beam to exit. If you want to work with a longer focal length you need a lens specific to that focal length. Many industrial laser manufacturers have replacements lenses for under $100. We had several sets of lenses with different focal lengths. Stay away from Edmund Scientific or similar companies because they charge an arm and a leg for scientific lenses. They are good lenses but not priced as consumables. For your power level you should never have to replace a good high power $100 lens. Each lens was tailored (and prescribed by the manufacturer) for different materials. We ran mild steel, stainless and aluminum. Two of the three could get by with the same lens. Don't remember which two at the moment. A long focal length provides a small beam waist for a greater distance and is less subject to distance variation between the output lens and material. A shorter focal length was better for cutting steel as the bulk of the beam was able to disperse more completely into the material from what I remember. The long focal length has its advantages but once you pierce through the material the narrow beam keeps going into the support grating in the machine's bed (for a 1/2" or so). 4. All of our mirrors were mounted on machined aluminum blocks and water cooled. Controlling the temperature of the mirrors helps to stabilize thermal expansion and unwanted beam steering. As I remember the first surface mirrors were 99.5% or 99.75% reflective. A mirror that absorbed 0.25% of the beam's energy at 4kW resulted in 10W of dissipation. Not a lot of power but 10W builds up if not dissipated. A smaller system would only dissipate 0.5W at 0.5% loss & a 100W tube. 5. Our system had a capacitive head to maintain a constant distance from the lens to the material. Some of the hot rolled we cut was not the flattest material in the world. If not cutting large pieces this is not as important. Capacitive sensing heads only work when cutting metals. If trying to cut wood or plastic with a capacitive head, the head would keep moving down until it crashes into the material. From the DIY standpoint you can use laser triangulation with a linear position sensing device, PSD, or a distance sensor like the Vishay VCNL4000 (http://www.newark.com/jsp/search/productdetail.jsp?SKU=37T8990&&CMP=AFC-SF-T11). The VCNL4000 has an I2C interface and sensing distance of 1mm to 200mm. It is most sensitive to distances in the 1-5mm range and has a non-linear output. I have thought about rolling my own high power C02 system but just never had the time. I would use multiple tubes at right angles from one another for higher power and scalability. Start with one tube and add later. The tubes would use pyrex or similar tubing with machined end caps to support beam delivery and laser gas cycling with a vacuum pump. Direct electrode tubes like the one on ebay have a limited lifetime. The electrodes eventually sputter themselves away and coat the inside on the tube. At 300W input power and 100W beam delivery I don't know how long that would take. My system would use an RF exciter coil around the tube. I believe the frequency for RF excited CO2 is in the 50MHz range. Exciting the laser gas through the glass tube prevents contaminants from electrodes being deposited within the tube. A discussion for another time. CO2 is 25-30% efficient so this is my system of choice these days from a cost/operating stand point. I have a 12" YAG laser but the reflector needs to be rebuilt. Also a project for another time. The YAG is about 4% efficient. For the DIY solution you can make your own beam delivery bellows using a wood mandrel shaped like the inside of the bellows, coat it with silicone spray and spread a thin sheet of black silicone caulk over the mandrel. After it dries, peal it off and turn it inside out. It will look and function like a perfect bellows to keep dust out and stray laser scatterings contained. My thoughts were targeted to a higher power system but the same concepts apply to lower powers as well. A couple of decades back I visited a laser marking company in Wisconsin. They were close to the Minnesota border as I recall. They started as a laser marking job shop. Their client base grew so fast that they got into the business of building their own lasers to save costs. Since that point they began selling their own systems. Their lasers used galvo servo systems with programmable X-Y mirrors. They were amazingly fast. The thing I remember is if this shop could roll their own lasers it must not be all that difficult. Just some thoughts. Dennis > -------Original Message------- > From: Jeshua Lacock <jes...@3dtopo.com> > To: Enhanced Machine Controller (EMC) <emc-users@lists.sourceforge.net> > Subject: [Emc-users] CO2 Laser Control > Sent: Jun 20 '12 00:41 > > > Greetings, > > I am thinking about making a laser attachment for my CNC machine with > something like this: > > > http://www.ebay.com/itm/80W-CO2-Laser-Tube-Power-Supply-Engraver-Cutter-New-/150552923492?pt=LH_DefaultDomain_0&hash=item230da74d64 > > I am tempted to go with the 130W: > > > http://www.ebay.com/itm/130W-CO2-Laser-Tube-Power-Supply-Engraving-Cutting-/160470882820?pt=LH_DefaultDomain_0&hash=item255ccf5a04 > > I bet that might even be enough power for selective aluminum sintering. > > It states that those power supplies can be controlled from a TTL PWM, so I > assume that it will be easy to control from a standard parallel port with HAL > and LinuxCNC. Is that correct? > > Has anyone set up a servo or stepper drive to adjust the laser's focal > point? Seems easy enough, guess I am just wondering how to tie it into g-code. > > Before I embark on this adventure, I would appreciate any advice you might > have. Thanks! > > > Cheers, > > Jeshua Lacock > Founder/Engineer > 3DTOPO Incorporated > <http://3DTOPO.com> > Phone: 208.462.4171 ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
