FabShop Magazine - March 2012 - 30

››› lasER MaintEnancE
Just because they’re a good operator doesn’t necessarily make them a good trainer. I’ve seen some operators leave key items out of a training session, and because of this, the machine will not perform as productively as when they first ran it to make themselves look good in the eyes of the managers or other workers. I think they call that job security. It’s impossible to cover the terminology for every type of machine in this short article. So we’ll use common names that can be compared to each manufacturer’s own terminology. After all, most cutting conditions come down to a few common adjustments, once you understand which ones and when to adjust them. Piercing To begin cutting, materials require a starting hole. Depending upon the type of laser or options on your machine, you might select a soft pierce made by a series of laser pulses that remove small amounts of material until the cutting beam finally exits out the bottom of the material. Controlling factors for this type of cutting are the frequency (number of laser pulses per second), the duty cycle (percent of on time versus off time of each frequency pulse) and the assist gas pressure. If you use oxygen at a high PSI, it will add extra heat to each laser pulse. When the preceding parameters values are too high, blowouts occur and this might add splattered material onto the bottom of your cutting lens while overheating the material, making it difficult to cut smaller features in your parts. If the preceding parameters are too low, the cycle time will be extended, the material will overheat and small part features will be difficult to cut. The balance here is to go as fast as possible without causing a blowout. Some machines are capable of a fast pierce that basically lifts the cutting head up and changes the focus to the top of the material but pops a larger hole through it. This offers a short cycle time, and with the head raised, there’s less chance of splattering the lens. The downside is a larger hole in the material, making it difficult to cut smaller features in the parts. Always use the maximum amount of power without causing a blowout pierce, as most of the heat in a part is developed during the pierce. ramping the pierce While piercing some materials, it might be necessary to vary the laser power over time. To do this, the laser is switched on at a lower starting power. Then the power is normally ramped up over time going from low to maximum in a short period. Once at full power, the level is held for the remainder of the pierce cycle until the beam exits through the material. On thicker material, a cool down period might be required that switches off the laser then blows assist gas on it to cool it. Pierce assist gas The type of gas used for piercing usually matches the cut assist gas used for cutting. When piercing with oxygen, keep the pressure down under 30 PSI. Oxygen adds an exothermic reaction. When piercing with nitrogen, keep the pressure between 35 to 50 PSI. Excessive nitrogen will cause a loss of piercing ability, as the nitrogen is an inert gas. Pierce nozzle standoff Pierce nozzle standoff is the distance from the cutting head tip to the material during the pierce cycle. With auto focus and a fast pierce, the tip distance would be raised to protect the lens. For a soft pierce, the tip will be closer to a typical cutting height. cutting For many materials, the selection of cutting parameters is typically predetermined by the manufacturer. However, the parameters can change based on material quality. Lower feedrates can produce excessive heating and poor edge quality. Adversely, if the feedrate is too high, the cut will be lost and sparks will exit from the top of it. Controlling factors for this type of cutting are familiar, as they are a repeat of the controlling factors for the pierce, which are the frequency, the duty cycle and the assist gas pressure. The only exception to these controlling factors is how the feedrate is controlled for cutting. Power Cut power is the maximum power level used to process the part. The maximum amount of power doesn’t always yield a quality cut edge. Mild steel can only accept so much heat before the heat runs through the material rather than out the bottom of the part destroying the material composition. Stainless and aluminum can handle the heat, but if your feedrate is slow, you’ll see dross. The balance here is feedrate and laser power. Frequency Frequency determines how many pulses per second the laser outputs during the cutting cycle. Typically, the higher the frequency the better. Think of frequency as a control of heat. Each pulse heats the material and each cycle off allows the material to cool or stabilize. You need cooling on mild steel, otherwise the power will destroy the material. Stainless and aluminum are typically cut with nitrogen, so we need heat. duty cycle Duty cycle is the on time versus the off time of the laser beam for every frequency pulse of the cut. The higher the duty cycle, the more power you deliver before you switch off to cool or stabilize the material. For mild steel, a lower duty-cycle number is typical. For stainless and aluminum, usually the maximum cycle is used to get as much power as possible. For example, an 85-percent duty cycle at 1,000 watts will yield an average of 850 watts, because the beam is off 15 percent of the time. Or conversely, the material is cooling 15 percent of the time. cutting assist gas When cutting with oxygen, the pressure will range from 8 PSI to 65 PSI on mild steel. The thicker the material, the lower the pressure. Yes, you read that correctly. We are trying to control the heat in heavy material, and oxygen adds an exothermic reaction. Plus, the thicker material will hold heat. Cutting with nitrogen is completely different. Typically, gas pressure starts around 85 PSI to 250 PSI depending on material thickness. In this case, the thicker the material, the higher the gas pressure. Excessive nitrogen will cause a loss of cut, as the nitrogen is an inert gas and will cool the material, just the opposite of oxygen. Not enough gas or the feedrate is too slow, and you have dross. This is where most operators stumble with adjustments and get

FabShop Magazine - March 2012

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