On the beams: lasers and abrasive waterjets

Modern Machine Shop,  Feb, 1992  by Chip Burnham

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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.