September 4, 2001
The various cutting methods available to fabrication shops today can be both a little daunting and very beneficial. Choice is good--learn how to make the most of the diversity all your choices offer to you.
Diversification is the linchpin of running a successful business. The key to producing quality work and investing in equipment wisely is knowing which cutting method—laser, plasma, electrical discharge machining, waterjet, or another—to use for a particular application.
While waterjet cutting is perceived as competing with laser, plasma, and EDM, many shops use more than one of these processes and view them as complementary to each other.
Lasers cut materials that bear the right thermal properties to be melted away. In general, lasers are designed for cutting mild steel from 1/50 to 1/2 inch, stainless to 1/3 in., and aluminum to 1/5 in. Most of the time, a laser is used on 20-gauge mild steel.
Lasers perform best when cutting high-quality, homogeneous materials. Impurities and inclusions in a target material may have adverse effects on cut quality and might cause a reaction that sprays molten product in the direction of the lens or produces defects on the cut surface.
On single layers of thin sheet metal, lasers cut at fast speeds with tolerances of ±0.001 to ±0.006 in. On this material, smaller, lower-kilowatt lasers have a fine wave pattern to allow for a precise cut.
While lasers have been used primarily for sheet metal cutting, their use has increased for cutting increasing thicknesses of stainless steel and aluminum. However, cutting thick materials requires powerful lasers and can require secondary operations to remove the heat-affected zone (HAZ), which can cause microcracking if not removed. New high-power lasers (3 to 6 kW) can be tuned down to cut thin material with a precision equal to that of a lower-powered laser.
Initial setup time for a laser is fairly quick. Different materials often require different gases, so material changes also require system setup changes.
If a shop is cutting only mild steel sheet metal parts in medium to large batch sizes, a laser may be the best choice. However, a gray area occurs with materials from about 0.250 to 0.50 in. thick. The waterjet becomes more viable for thicker materials.
Also, shops using both laser and waterjet find a recurring need to process materials other than typical mild steel. Although the laser can cut a variety of metals, the waterjet is usually found to be a more productive method for many materials.
For example, waterjets are usually the most productive method of cutting aluminum. Titanium, nickel alloys, copper, brass, glass, and stone also can be cut productively with a waterjet. The quick setup time with a waterjet makes it suitable for small to medium batch runs. The part geometry, batch size, and material type must be considered before the cutting method is chosen.
Plasma is a thermal process that cuts electrically conductive materials. Plasma cutting involves a near-supersonic jet of ionized gas that leaves a negatively charged electrode inside a torch tip. The ionized gas then cuts the positively charged metal. In other words, the plasma stream cuts by heat—about 20,000 to 50,000 degrees F—essentially melting the material being cut.
Assist gases are used to prevent the superheated surface from reacting with air (which can cause oxides or nitrides to form on the surface). They also help blow away the molten material, cool the part, and minimize double arcing.
For cutting mild steel, the assist gas usually is oxygen or air. Stainless steel up to a thickness of 5 in. often is cut with oxygen, air, or argon-hydrogen. Aluminum is cut using air as the primary gas, and methane sometimes is used as the shield gas. Methane is more expensive but produces a better edge, and aluminum up to 6 in. thick can be cut with this process.
Plasma cutting is a good choice for cutting mild steel between 0.300 and 1.25 in. thick when the HAZ does not need to be removed before the part is used in a final application. Most of the time, hand grinding is required to remove HAZ before the part can be used in the final application. Secondary operations such as this often add unexpected costs to part production. A majority of the time, plasma is used on mild or carbon steel up to 1.25 in. thick. Beyond this thickness, oxygen acetylene torch cutting also can be used.
Plasma cuts at high speeds and generates heat that leaves rough edges. Dross sometimes builds up on the bottom side of the cut. Accuracies are in the range of ±0.030 to ±0.060 in., depending on the thickness of the material. Kerf width typically is 0.125 to 0.250 in. and can be much smaller with precision plasma.
Wire electrical discharge machining (EDM) uses spark erosion to remove material from only electrically conductive materials, the wire being negative and the workpiece being positive. Direct-current electric pulses are generated between the wire electrode and the workpiece. During cutting, material is melted away by the lightning bolt and flushed out of the kerf area by the dielectric solution.
Wire EDM is used to cut steels, INCONEL® alloys, carbide, graphite, aluminum, copper, brass, and titanium. Wire material varies with the application. For instance, zinc-coated brass wires cut quickly, while stronger wires such as molybdenum cut more accurately.
EDM is extremely accurate—rough cutting accuracy is ±0.0015 in., and precise cutting accuracy is ±0.0001 in. EDM can cut almost any material thickness, although it is used most typically on materials up to 6 in. thick. EDM leaves no burr, little HAZ, and an excellent surface finish. However, it is slow, up to 10 times slower than waterjet cutting.
Accuracy is the main difference between EDM and waterjet cutting. EDM is the more accurate process and may be the best choice for applications that require extremely tight tolerances. However, EDM is also limited to cutting only conductive materials.
Waterjets cut faster than EDM and can cut nonconductive materials. If the application allows accuracy to be sacrificed for increased speed, or if a part can tolerate a 100-root- mean-square (rms) finish and ±0.003- to 0.005-in. tolerance, a waterjet may be suitable for the job. Kerf width with EDM is smaller than with waterjet—0.08 to 0.015 in. compared to 0.030 in. to 0.050 in.
While waterjets are perceived as competing with laser, plasma, and EDM, many shops have more than one of these types of machines and consider them complementary processes.
Waterjets use cold supersonic erosion to cut almost any material, both metals and nonmetals. It can cut metals ranging from thin shim stock to more than 10 in. thick with accuracies of ±0.0003 to ±0.015 in. Seventy-five percent of waterjets are used to cut material less than 4 in. thick. Repeatability is ±0.001 in. Thick cutting (more than 4 in.) with a waterjet loosens tolerances by at least two times.
The same parameter set (water pressure, abrasive flow rate, cutting nozzle, etc.) is used for nearly all cutting; only the cut speed varies from material to material. The fact that the parameters usually do not change from one material to another means that setup is fast, and the opportunity for operators to select improper parameters for a particular job is greatly reduced.
With EDM, plasma, and laser, various parameters, gases, or wires must be used to process different materials. Operator expertise and proper setup are essential with these other processes, while they are of less consequence with a waterjet. The productivity of this machine can be improved by cutting stacked material and running with multiple waterjet heads when cutting sheet metal. For materials 1 in. or more thick, users typically put all the power through one head and do not stack layers.
Materials most often cut with waterjet are aluminum, stainless steel, and high-strength metals such as titanium, Hastelloy®, INCONEL alloys, nickel alloys, composites, and metal laminates.
Waterjets often are used to cut short-run prototypes that require minimum tooling or fixturing. They also are used for net-shape cutting, in which the final part is produced without the need for secondary operations to achieve required tolerance or surface finish; or for hogging out, in which the abrasive waterjet rough-cuts a part to within about 0.015-in. tolerance and it is finished on a milling machine to achieve a tighter tolerance and smoother surface.
Diversifying cutting capabilities can help improve a fabricator's productivity, and the available cutting technologies must be considered carefully for specific applications. When deciding whether to select lasers, EDM, plasma, or waterjet, knowing customers' needs and looking at their future needs are critical. Strength and improved operations often can be found in diversification.
The FABRICATOR® is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971. Print subscriptions are free to qualified persons in North America involved in metal forming and fabricating.