Machinery's Handbook, 31st Edition
1348 Electro-Thermal Processes conducted away from the cut zone. This diverted energy performs no useful work and only serves to heat the material. As a result, efficiency may dictate reduced cutting speeds and cut depths for highly thermally conductive metals, such as aluminum, brass, and copper. Laser Cutting Performance Characteristics: Modern laser systems in the 6–8 kW range can cut through 1.5 in. (38 mm) steel, 0.75 in. (19 mm) aluminum, or 0.38 in. (9.6 mm) cop- per. The depth of cut is proportional to laser power and inversely proportional to speed. While fiber lasers are much more efficient and usually faster for cutting thin materials, CO 2 lasers usually are as fast as fiber systems when oxygen (flame) cutting materials over 0.2 in. (5 mm) thick. The cutting tolerance of a typical system on 0.5 in. (12.7 mm) thick carbon steel is ±0.005 in. (0.13 mm). Higher precision is possible in some cases. Table 1, Table 2, and Fig. 2 provide some examples of expected performance characteristics. In thin workpieces, laser systems can create extremely small holes and fine features with a great deal of precision. Cut patterns can be as close as one beam diameter apart, if the cutting process is accurate and material thin. Kerf width will depend on wavelength, setup (spot size), process, and workpiece characteristics. A fiber laser has a wavelength ten times smaller than that of a CO 2 laser. As a result, a fiber laser can focus on a much smaller spot size, and kerf widths of 0.001 in. (0.025 mm) are achievable. Deeper cuts require a larger beam and wider kerf. Beam shape is not cylindrical, so cuts will tend to have a wider entrance than exit. A 2 degree taper angle is typical. Reduction of this taper angle is possible by changing head angle, process parameters, standoff distance, or material thickness. Zero taper can be achieved in some cases.
Cut Face
Roughness (striations)
Heat-Affected Zone (HAZ)
Cut Face Squareness Kerf
Dross (burr)
Fig. 2. Characteristics of Laser Cuts Laser Hole Cutting and Drilling: Laser drilling can create holes that are blind or pierce through. The holes will tend to have taper, with the entrance being larger than the exit. When drilling, molten material rapidly expands in the hole and ejects upward. Some of this material can cling to the sides of the hole and form a recast layer. In some cases, a single-shot direct drilling process can rapidly create a shallow drilled hole with minimal recast. A low-power pulsed beam can create holes of greater depth, smaller holes, and higher precision holes. Called percussion drilling, this method is slower than direct drilling and more prone to form a recast layer. Helical drilling and trepanning can produce large diameter holes with high precision, mini- mal recast, and controllable taper. These methods require more sophisticated equipment and take longer than direct or percussion drilling. Helical drilling involves moving a pulsing laser along the circumference of the desired hole, while increasing the cut depth on each pass. Trepanning starts with percussion drilling a pilot hole, and then moving the pulsing laser in a spiral pattern, until it reaches the target diameter. Unlike the other drilling methods, trepan- ning begins with a through hole, allowing downward ejection of molten material. Plasma Arc Machining (PAM).— Conductive metals, including expanded metal, cast iron, and rusted or painted parts, can be plasma cut. In this electro-thermal process, an
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online