Editor’s Note: This is the fourth of a series of articles on design for manufacturability. We recommend that you read the series to refresh your own knowledge, and also share them with younger engineers.
In this installment on how machine tools work, we’ll consider alternative ways of cutting metal parts, including electric discharge machining (EDM), laser cutters and plasma cutters. These represent the slowest (EDM) and fastest (plasma) ways of cutting metals.
EDM is an irreplaceable tool for creating high-precision internal geometries, sharp inside corners and very long holes. EDM comes in two varieties: block and wire EDM.
EDM is a controlled metal-removal process that works by means of electric spark erosion. It is also known as spark machining, spark eroding, burning, die sinking, wire burning or wire erosion. In this process, an electric spark is used as the hard, conductive, cathode cutting tool to cut (erode) the workpiece to produce a finished part of the desired shape.
Block EDM typically uses a block of electrically charged, machined copper or graphite to eat away an exact, but reversed, copy of itself in a metal part. Block EDM is commonly used to make molds, because it is easy to create sharp inside corners and tapered walls with a mirror finish.
Wire EDM is more common than block EDM because of its versatility in making three dimensional objects and tiny, long holes. Wire EDM uses the same electrode erosion methods, but the cathode is a wire, usually of tiny diameter. To control heat, the parts are often submerged in a cooling bath. This machining process can readily produce precise holes of more than a foot in length. Like block EDM, wire EDM is very slow, and a complex machining operation can take over a day to complete. The wire erodes during use, so fresh wire is fed from a spool as it operates.
Rapid machining of sheet material is achieved using laser or plasma cutters. Laser cutting works by directing the output of a high-power, optical laser. The focused laser beam is directed at the material, which can be anything from wood to acrylic to metal. The laser beam then either melts, burns or vaporizes the material, blowing away the debris with a jet of gas and leaving an edge with a high-quality surface finish.
There are three types of laser cutters: gas, crystal and fiber. Gas laser cutting, most often known as CO2 laser cutting, uses a carbon dioxide-mixed laser. The carbon dioxide mixture is electrically stimulated to then cut the material. For CO2 laser cutters, the materials tend to be thin, nonmetal sheets, such as wood or plastic, and since the power is low, CO2 laser cutters are excellent tools for wood burning, engraving and etching surfaces to create beautifully patterned designs. Gas laser cutting can also use nitrogen, which works well with metals such as steel and aluminum. Nitrogen gas laser cutters can cut through thicker materials, too.
Crystal laser cutting uses lasers made from nd:YAG (neodymium-doped yttrium aluminum garnet) and nd:YVO (neodymium-doped yttrium orthovanadate). These crystals allow for extremely high powered cutting and can be used with both metals and nonmetals, and have a huge range of applications from medical and dentistry to aerospace manufacturing. Unfortunately, crystal laser cutting machines have a shorter life expectancy than other machines at around 8,000 to 15,000 hours, and they tend to be more expensive to buy because of their pump diodes.
Fiber laser cutters have similarities to crystal laser cutters. They use continuous wavelengths of light or pulsed wavelengths, and they use the same general wavelength of light of 1.064 micrometers. The biggest benefit of a fiber laser cutter is that it has a much longer service life of around 25,000 hours. It also requires very little maintenance and features inexpensive replacement parts.
Plasma cutters work by sending an electric arc through a gas in a constricted opening nozzle. The nozzle that the gas passes through causes it to squeeze by at a high speed, like air passing through a venturi, and this high-speed gas cuts through the metal by melting it. Unlike laser cutters, plasma cutters are a low precision machining technique, but they are very fast (reaching speeds of 8 meters or 26 feet per minute) and can cut through up to 160 mm (over 6 inches) of thickness in steel!
For parts that will be finish machined in a second operation or where tolerances are not critical, plasma cutting is a rapid, cost-effective technology. New gases are being developed all the time that increase the power and range of plasma cutters. They are also merging with laser cutters to create interesting hybrid machines with even more capabilities.
Dr. Deborah Munro has worked in the orthopedic medical device field for almost 20 years and holds numerous patents, mostly in the area of spinal fusion. She founded a Master’s in Biomedical Engineering at the University of Portland in Oregon and has since moved to the University of Canterbury in Christchurch, New Zealand, where she is developing a Minor in Biomedical Engineering within the Department of Mechanical Engineering. One of her areas of expertise is automated manufacturing, a course she created and taught for six years.