I. MECHANISM Laser beams, even high-power ones, do not exert substantial forces on materials. The cutting process normally used in manufacturing is called gas-assisted laser cutting, where the actual removal of material is done by a stream of gas. Figure 4.7 is a schematic showing the elements of the process. A lens focuses a laser beam on the surface of a workpiece since the need for pressurized assist gas makes it difficult to make a laser cutting head with reflective focusing optics. The power density of the focused spot is enough to alter the physical state of the workpiece in the area of the spot. An assist gas, introduced coaxially with the focused beam, blows away the heated material. When the cutting head or workpiece is moved, a kerf is formed. Laser cutting with carbon dioxide (C02) or neodymiumdoped yttrium aluminum garnet (Nd:YAG) lasers is primarily a thermal process: materials are cut because the laser beam heats them until they melt, decompose, or vaporize. In many cases, the assist gas reacts chemically with the material being cut to either enhance or retard the process. We can characterize materials for laser cutting by examining their optical, thermal, and chemical response to the process.