POWER TOOLS: Into Painless Piercing? Try It With Microwaves - by Mark SincellScience, 298, 587-589, (2002)Anyone unfortunate enough to remain awake in the dentist's chair may be acutely aware of at least two of the three primary drawbacks to using a spinning mechanical drill to grind a hole: noise, vibration, and flying debris. The drill bit feels the pain, too, eventually wearing out or breaking under the repeated stress. Now, a Tel Aviv University team led by mechanical engineer Eliyahu Jerby reports on page 587 of this issue that it has developed a drill that uses microwave energy to excavate solids. The new microwave-powered drill suffers from none of the problems that plague mechanical drills. It is silent, steady, and dust-free, and the bits almost never wear out.
Drilling with electromagnetic radiation is nothing new. For years, engineers and scientists have been using the tightly focused light beams from laser drills to punch tiny holes as small as 1 micrometer in everything from semiconductor circuit boards to human bone. But laser drills are expensive, and a several-hundred-thousand-dollar laser drill might not always be the right tool to quietly put a 1-millimeter-wide hole in a concrete block.
So Jerby's team cooked up a low-cost alternative in the kitchen. "We pulled the magnetron from a domestic microwave oven," Jerby says. "It cost about $20." To focus the microwaves, radiation from the magnetron is directed into a rectangular metal box that guides the microwaves into one end of a piece of coaxial cable--"just like the cable going to your TV, except ours is a little stiffer," Jerby explains. The other end of the cable is placed near the surface where the hole will be drilled.
By adjusting a mirror at one end of the metal box, the researchers can match the impedance of the coaxial cable to the surface being drilled. That tuning allows microwave energy to travel into the surface instead of being reflected, thus concentrating the energy of the microwaves into a spot just below the surface. As the spot starts to heat up, changes in the material cause a peculiar thing to happen: Instead of cooling more rapidly to shed the excess heat, the spot starts to soak up even more energy than before. A molten hot spot forms beneath the surface of the material, and a drill bit passing down the center of the coaxial cable can easily scoop out the molten material.
Jerby's team has already used a prototype microwave drill to put holes with diameters ranging from about 1 millimeter to 1 centimeter in ceramics, concrete, basalt, glass, and silicon. Because regions near the hot spot stay relatively cool, even brittle materials don't build up enough thermal stress to shatter, Jerby says. "The cool thing is that you can drill without wear, breakage, or cracking the tool bit," says electromagnetic scientist John Booske of the University of Wisconsin, Madison, who believes that the microwave drill will be particularly useful for drilling ceramics such as those used to mount semiconductor devices on a circuit board. "It would also be great for drilling jewelry and pottery," says Booske. Jerby adds another low-tech application of his silent drill: concrete construction. "If you have ever had a neighbor drill into a concrete wall next to your apartment at midnight, you know what I mean," he says.
But don't expect to see the microwave drill during your next dental checkup. Although the lab model emits less radiation than a typical home microwave oven, Jerby says, "safety is still a big concern." To keep stray microwaves from cooking the internal organs of an unwary drill operator, production models of the microwave drill either will be completely enclosed, like an oven, or will use a shielding plate.
Main Article:
https://www.thevespiary.org/rhodium/Rhodium/pdf/microwave.drill.pdf
