Laser Cutting Theory
Condition of the nozzle
The copper nozzle may become damaged or blocked in time, usually as a result of hot metal spatter thrown up from the work surface. The orifice can be cleared by a conventional oxy tip-cleaner.
After some usage, the orifice may become “out of round”; this causes swirling or vortex action in the assist gas jet which usually produces highly directional effects in the cutting. Often, this can be rectified by carefully drilling the orifice; if the orifice is of large diameter then excessive assist gas will be consumed. Eventually, the nozzle will need replacement.
External Optical Alignment
The optical system (set of mirrors) external to the laser cavity (including the mirror mounted at the top of the Beam Control Unit) should normally be checked and adjusted on a routine basis (say, once per month). However, if the integrity of the alignment is under suspicion in the meantime, it can be checked by using the cross-wire method, allowing the raw beam to pass through the system onto a target. The image produced by the cross-wire (its shadow) will indicate whether the beam passes through the position centrally.
Misalignment of the external beam will generally cause haphazard cutting results, with highly noticeable directionality.
Assist Gas Pressure
Generally oxygen is used for metal cutting, and air is used for non-metal cutting. High pressure nitrogen can be used to cut mild steel, stainless steel and aluminium. Using nitrogen as an assist gas leaves the cut edges clean and free of dross but is expensive because up to 25 bar is needed. The general rule with assist gas is: there must be sufficient flow (pressure) in each case, but an excessive amount is wasteful. Normally, the thicker the material, the greater the required pressure. Of course, the type of material is also an influence; very low carbon steel, for example, will be adversely affected by excessive oxygen flow since it is highly reactive.
In any particular case, the pressure used will be experimentally determined, and is usually not highly critical. Note that no material can be cut without any assist gas.