Laser Cutting Theory
LASER CUTTING VARIABLES
Laser cutting feedrates have been found to fit empirical formulas based on the available laser power density and the properties of the material to be cut. Above a threshold amount, the feedrates are directly proportional to available power density which takes into account the laser’s performance features (eg power, mode) in addition to the focusing system’s characteristics (eg spot size). Cutting rates are likewise inversely proportional to the materials density and thickness.
Feedrates can be varied for a particular set of parameters in order to obtain different edge quality results, particularly for metals, the plot of cutting speed versus thickness for a material has two curves. The upper curve reflects the top speed at which through cuts are achieved while the lower curve shows the limit below which the material is self burning. The resultant window of acceptable cut speeds is usually wider at the thinner range of a material.
The assist gas is supplied coaxial with the focused beam to protect the lens and aid in the material removal process. Generally, compressed air or inert gas is used to purge melted and evaporated material from the cut zone while minimising any excess burning. For most metal cutting applications, a reactive gas assist can be employed to promote an exothermic reaction. The enhanced energy intensity from the use of oxygen can improve cutting speeds by 25% – 40% over the results obtained with use of air.
In addition to gas type, delivery pressure is an important consideration. Typically, pressures of 45-60 psi (3-4 bar) developed in the gas jet nozzle are used in cutting thin material at high speeds to help prevent the clinging of slag or dross to the back edge of the cut. The pressure is reduced as the material thickness increases or process speeds slow.