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Lasers vs Waterjets | CNC Waterjets E-mail
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On the beams: lasers and abrasive waterjets - Emphasis: EDM and Lasers

Chip Burnham

A major benefit of cutting with a beam--[CO.sub.2] and YAG lasers, abrasive waterjets (AWJs) and plasma arc--is the ability to respond quickly to demands for small quantities of custom parts cut from a variety of materials. As tooling costs and just-in-time production demands increase, these alternative cutting methods become increasingly attractive for independent and manufacturers' machine shops.

The beam-cutting tools are usually integrated into CNC machines. Operators merely change programming and process parameters to respond to requirements for different parts and materials.

As with any new technology, much has been written about the comparative advantages and disadvantages between beam-cutting and traditional machine tools, and between one kind of beam cutting and another. Everyone would like to have a machine that can cut everything, and a lot of machine manufacturers would like you to believe that their machines will cut everything.

Material processors who consider purchasing new beam-cutting systems to upgrade, add to, or replace existing equipment need to analyze carefully the type and volume of work they want to do. Then they need to evaluate which of the methods will do the majority of the work within the boundaries of costs, productivity and profits they expects.

Lasers and abrasive waterjets are known for their speed, accuracy and versatility in cutting a wide variety of materials. Plasma arc cutting is fast as well, but is used primarily for metals. This discussion compares the capabilities and costs of lasers and AWJs.

Capabilities

In general, lasers do a superior job cutting many materials less than 0.25 inch (6.4 mm) thick. a survey of 156 job shops, which was reported in the September, 1990, Proceeding Of The Marketplace For Industrial Lasers, showed that 88 percent of them were using [CO.sup.2] laser to cut mild steel of 0.25 inch (6.4 mm) or less, and 65 percent were using [CO.sup.2] lasers to cut stainless steel of 0.125 inch (3.2 mm) or less. At these thicknesses, the heat-affected zones (HAZs) that lasers produce are small. At metal thicknesses greater than 0.25 inch (6.4 mm), the use of lasers designed to cut thin sheet metal becomes increasingly impractical, and AWJs may be a better choice, especially if the material is heat-sensitive.

Laser-cutting thicker metals tends to produce larger HAZs. Sometimes this is acceptable for the final part application, but at other times, costly grinding operations are required to remove these areas. Also, the laser power required to cut thicker metals is greater, and therefore, the cutting process is more expensive.

The AWJ cuts to tight tolerances without HAZs, mechanical stresses, or warping. The fine, sand-like abrasive removes material by high-speed erosion, and the AWJ-cut surface looks sand-blasted. Even for thin materials, the AWJ's "cold" cut can sometimes make a big difference in part production costs when it eleminates finishing steps.

The cutting speed of a laser for a given thin material is higher than the AWJ. Cutting speed, however, is not always determined by the cutting process itself. If the motion control positioning equipment can contour a complex shape to required tolerances at 50 inches per minute at best, then either process will do. Almost any gantry-type machine can travel fairly quickly in linear moves (over 300 inches per minute is not uncommon), but, to contour a small, intricate shape, the same machine may have to slow to 1/10 its linear speed.

Although sheet metal cutting is a major application for beam-cutting technology, lasers and AWJs are also being used in plate metal and custom material processing. Those who purchase beam-cutting technology for a specific application or customer base soon discover other areas where lasers or AWJs can reduce part production costs or allow diversification into new markets.

Not all these applications are for two-dimentional parts. Three-dimensional work requires five to six axes of programmable articulation provided by multi-axis machines with delicate wrists and low payload capacities. Both the AWJ and laser have relatively light cutting heads, which makes it easy to integrate them with these sophisticated CNC systems. The lighter the cutting head, the faster the manipulator can move with accuracy.

Both AWJs and lasers produce very low cutting forces. The AWJ usually exerts less than one pound of force onto the workpiece; the laser virtually none. When the workpiece and fixturing do not have to withstand high cutting tool forces, the result is simplified, less expensive and more flexible fixturing.

Material Examples

* Aluminum: Aluminum up to 0.25 inch (6.4 mm) can be cut with an inert assist gas (nitrogen), but its high thermal conductivity limits practical cutting to thicknesses less than 0.125 inch (3.2 mm). Any HAZ is small and quickly removed if necessary. AWJs cut aluminum up to 8 inches (203 mm) thick, although most production work is 1.5 inches (38 mm) or less. One-inch-thick materials can be cut at speeds up to 13 inches (330 mm) per minute. AWJs produce no HAZ and edge quality is in the range of 125 rms (on thicker materials), depending on cutting speed and other process parameters.

* Mild And Carbon Steels: Lasers cut mild carbon steels up to 0.5 inch (12.7 mm) thick with some HAZ, but mild steels (1010-type materials) do not harden, so the HAZ is easy to remove. Beyond 0.5 inch (12.7 mm), laser cutting requires higher power and slower cutting speeds and it becomes increasingly impractical. AWJs cut mild and carbon steels up to 6 inches (152 mm) thick, although most users are cutting in the range of 0.25 to 2 inches (6.4 mm to 50.8 mm). AWJ cutting speed for 1-inch (25.4-mm) steels is 5 inches (127 mm) per minute.

* Stainless Steel And Inconel: With a nitrogen assist gas, lasers cut stainless steel up to 0.5 inch (12.7 mm) thick and Inconel up to 0.125 inch (3.2 mm). At this thickness, the HAZ is small and relatively easy to remove with sanding wheels. AWJs cut stainless steel and Inconel in the same thickness ranges as any other ferrous metal--typically 0.25 to 2 inches (6.4 to 50,8 mm), 6 inches (152 mm) maximum. One-inch-thick stainless steel can be cut at speeds up to 4.5 inches (114 mm) per minute. With no HAZ, there is no need for any secondary removal processes.

* Titanium: It is possible to laser-cut titanium as thick as 3/16 inch (4.8 mm), but it is expensive. Laser-cutting of titamium requires argon or argon/helium assist gases. One user reports that he routinely cuts 0.060-inch (1.5-mm) material at 120 to 150 inches (3048 to 3810 mm) per minute. Without assist gases, the AWJ cuts 0.060-inc (1.5-mm) titanium at 50 to 80 inches (1270 to 2032 mm) per minute, and AWJs can cut titanium up to 3 inches (76.2 mm) thick. With no HAZ to remove, the AWJ cut is often the final cut, which also saves material costs.

* Stack Cutting: It usually is not practical to cut stacked materials with a laser because it is diffuclt to prevent hot gases and molten material from blowing between the layers. AWJs, on the other hand, easily cut stacked sheets of material such as shim stock.

Costs

A 1.2- to 1.5-Kw X-Y laser system that will cut thin sheet metal fast and accurately can be purchased for as little as $ 249,000. For the ability to cut thicker materials--up to 0.5 inch (12.7 mm) for some metals and up to 1.0 inch (25.4 mm) for non-metals--with reasonable speed and accuracy, costs range between $ 325,000 and $ 375,000 for 1.2- to 1.5-Kw systems. For routine cutting of thicker (0.5 to 1 inch) material such as plate metal, prices start at $ 450,000 for 1.5- to 2.5-Kw systems. Sophisticated multi-axis systems for prototyping start at $500,000 and can exceed $1 million for trim-line die cutting systems such as those used by automobile manufacturers.

In general, a laser system consists of the laser, a cooler, a fume extractor, and various assist gases (oxygen, nitrogen or argon, depending on the material being cut). The laser system may also require a separate foundation--usually a 10-inch (254-mm) "floating" concrete slab.

Equivalent AWJ systems start at $100,000 for a small system. Prices for a turnkey two- or three-axis NC-controlled shapecutting system that is 6 feet by 9 feet (1.8 by 2.7 meters) range from $130,000 to $220,000. Five-axis gantry systems range from $280,000 to $1 million.

An AWJ system consists of an intensifier pump to pressurize water to 55,000 psi, a water booster and filtration system to supply clean water to the pump, an abrasive delivery system, and an AWJ cutting head. The AWJ system may require a 6-inch (152 mm) rebar-reinforced concrete pad.

For either system, a large percentage of the cost is in the motion control equipment.

Lasers and AWJs use similar techniques for material support. As mentioned above, the low cutting forces do not require elaborate part support structures.

The supports themselves are often sacrificial--they wear out druing cutting and must be replaced occasionally. Examples of support media (both sacrificial and permanent) include pins, slats, grating and rollers. Special applications may use other techniques.

The laser does not require an elaborate catcher system because the beam's intense heat is concentrated at its focal region. Typically, a small catcher or tray is needed to collect the molten globules of material for disposal. A bigger concern is the fumes that are given off when melting (cutting) the material. Most laser systems are completely enclosed and vented to remove fumes from the cutting area.

AWJs do require some form of a catcher device to dissipate the residual kinetic energy of the stream and collect the slurry. The slurry consists of spent water, abrasive and the kerf material of the cut part. Most X-Y systems use a tank catcher that also supports the workpiece. These tank catchers may be "self-cleaning," where the slurry is automatically removed from the tank. The remaining waste and process water is easily disposed of. Multi-axis systems use a point catcher that follows the AWJ nozzle as it cuts. The virtually heat-free cutting produces no fumes, so no ventilation is required.

Although AWJs generate no fumes, they can be noisy. Some systems are partially enclosed to reduce the noise level. A properly designed catcher also decreases the noise to levels that are well within state and federal requirements. Many such noise-suppressing designs are available for two-and five-axis systems.

Operating costs vary, depending on the process parameters (including assist gases, if any) for a given material, desired cutting speed and edge quality. When a laser system is operating at power levels greater than 2 Kw (the level needed for most exotic metals), costs can run as high as $90 per hour, excluding labor and fixed costs. For most applications, however, the operating costs range from $25 to $40 per hour. Operating costs for AWJ systems range from $15 to $35 per hour, with most applications running about $18 per hour.

Downtime and maintenance are other costs to consider. Most laser installations experience downtimes of 10 to 20 percent per year. AWJ systems typically experience 5 to 10 percent annual downtime. This is because AWJ technology is relatively simple. In-house maintenance people quickly learn everything they need to know about the filtration unit, pump and abrasive delivery system. The mixing tube (sometimes called a focusing nozzle) on the cutting head must be replaced every 100 hours or so. Pump seals, valves and water filters also require occasional replacement.

Laser technology is less familiar to most machine tool maintenance technicians. As a result, the user often is dependent on the manufacturer for maintenance and repairs. The mirrors, lenses and tips must be cleaned or replaced periodically.

Conclusion

The primary advantages of lasers over AWJs are faster cutting speeds and tighter tolerances in cutting materials less than 0.125 inch (3.2 mm). The primary advantages of AWJs over lasers are the ability to cut thicker materials, the ability to cut heat-sensitive materials, and the absence of a HAZ on the cut surface. Part tolerances are a product of manufacturers such as positioning equipment and cutting process accuracy, fixturing and material flatness. Startup costs for AWJs are less than those for equivalent laser systems. Operating, downtime and maintenance costs are generally less for AWJs than for lasers. In the final analysis, the method to choose is the one that provides the most flexibility for the least overall cost for a given volume of material and customer requirements.

COPYRIGHT 1992 Gardner Publications, Inc.
COPYRIGHT 2004 Gale Group


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