Laser Cutting Theory
Focus assemblies provide support for the lens in order to image the beam. These assemblies generally provide means to adjust the focal point in or at the part. Height sensing devices can be incorporated to automatically maintain the proper focal point position regardless of undulations in the workpiece surface. These devices measure the lens-to-workpiece spacing either through contact probes riding on the workpiece surface or via a comparison of non-contact optical, acoustic, or electrical (inductance or capacitance measuring) signals bounced off the material. The feedback can trigger compensation of the vertical axis position.
For best results, the focal point of the beam must impinge on the surface of a workpiece. This factor is of greater or lesser importance, depending on the material; in general, materials which have a high intrinsic reflectivity to the laser beam will be most critical of the focal height setting (eg. mild steel 45% reflective; stainless steel 66%; aluminium 99%). The focal point on aluminium and stainless steel should be approximately 4/5 buried into the material. Thicker carbon steel will cut better when the focal point is 1-2 mm above the material.
The operator may find that the focal point needs to be “tweaked” occasionally during a job; the precise focal point can change slightly owing to thermal effects in the lens.