FabShop Magazine - March 2012 - 31

››› lasER MaintEnancE
into trouble. Most inexperienced operators will do the exact opposite. cornering Some machines have dynamic power or a variation of this feature. Dynamic power lowers the overall cutting power when part geometry requires a change in feedrate, such as when you turn a corner on a part. The slower the feedrate, the lower the power. Think of it this way, when you’re cutting at 200 IPM you need X amount of power to maintain a cut, but when you make a corner, the feedrate actually goes to 0 IPM, as the drives change direction. If full power is used, the corner will burn. For the machines that don’t have this capability, they will need to be stopped, have the laser beam turned off but will have the assist gas left on to cool down in the corners. This, of course, adds cycle time, but you really have to choose. It’s either burned corners or added cycle time. The other option is to do a loop in the corners to transfer the burn to the inside of the loop, but this only works on external corners. cutting tip gap This is the distance from the cutting head tip to the material during the cut cycle. When cutting with nitrogen, the tip is typically closer to the material to help direct the high gas pressure down through the kerf. When cutting with oxygen on thicker mild steel, the tip is raised to aid in widening the kerf. A raised tip will also allow extra oxygen to flow near the surface of the material so that as gas goes through the kerf and creates a venturi affect, oxygen is pulled in and not surrounding air that contains impurities and nitrogen that will affect the cut edge – and not in a good way. nozzle diameter Along with gas pressure, the amount of gas volume that goes through the kerf needs to be determined. On thin mild steel, we need a higher PSI, but a smaller volume. Therefore, we would use a small orifice diameter and conversely just the opposite on thicker mild steel. On thin stainless steel, we need a lower PSI of nitrogen and a smaller volume. Therefore, a small orifice diameter is used. On heavier stainless, we need a higher volume and higher PSI. Think of it as blowing the molten material away from the cut. Kerf Kerf is an offset value or the width of the beam. This number is changed to give you an exact part dimension. A larger kerf value produces a smaller hole diameter or feature. This parameter does not affect the cut edge, but it does affect the part size. A laser’s adjustable cutter compensation is the offset of the actual width of the cut left behind by the laser beam. If we say the kerf or beam width is 0.010 in., the beam will stop 0.005 in. short of the inside of a hole. The edge of the beam is contained in the hole. lens A lens with a smaller focusing distance will create a smaller laser-beam-spot size. This in return offers more power at the cutting point, and it can be used as an advantage for extra cutting speed. However, the focal point at the material will need to be more precise. As we go up in focal distance, we need longer focal points, because they create a longer beam waist. This waist reduces power a bit, but on

thicker materials it’s invaluable for keeping more of the material in focus and creating a slightly larger kerf to help expel more molten material out the bottom of the cut. Focus I have saved the most important item for last. If the focal point is incorrect, then every other value above will probably change to compensate for the error in focus. I can’t stress enough the importance of this item. Close is not close enough, unless you prefer to cut at slower feedrates and have dross on your parts. You would think that every operator that had to grind their parts to get them clean would take the time to make sure this was set correctly. After all, you could take a couple of

seconds and have clean parts or spend all your time grinding parts while the laser creates more parts to grind. On mild steel, we typically set the focus at the top of the material or slightly above on heavier material, and as we discussed, stainless and aluminum are just the opposite. The thicker the stainless material, the further we lower the focus into it. As a rough guide about 3/4 of the material thickness from the top. Along with training, truly understanding a laser’s adjustments can make an old laser cut well or keep a new laser cutting like it should. You bought a laser for it’s precision and great edge quality, but a lot of companies don’t run them that way.

FabShop Magazine - March 2012

Table of Contents for the Digital Edition of FabShop Magazine - March 2012