Ultrasonic cutting matures into a viable machining process.
The DMS Ultrasonic Series system machines brittle materials up to 5 faster than conventional hard-material-cutting methods.
In 1955, AMERICAN MACHINIST editors reported on a technology that was "one practical remedy" to machining new, hard materials. That technology was ultrasonic machining, which, at the time, was performed by ultrasonically agitating grinding abrasives in a kerosene "carrier" that transported them to the workzone. While AM was impressed by the new technology's capabilities, editors didn't offer glowing reviews because, besides the hazards of the kerosene medium, the process was slow.
The passing of 47 years has done wonders for ultrasonic cutting technology. Still used for tackling advanced materials, the process is now far from slow, increasing productivity up to five fold as compared to conventional hard-material-cutting methods. And it has gone from an environmental nightmare to a dream — no longer requiring a hazardous carrier and needing only water as a coolant. The special tools used with the technology maintain a long life — because of reduced contact forces and low thermal stress — and produce surface finishes up to a Ra <0.2 µ.
The ultrasonic process begins with a high-frequency electrical signal that is converted to mechanical movement. A booster then amplifies the oscillation, which transfers to the tool.
These custom ultrasonic tools, whether a milling, boring, or grinding diamond cutter, essentially contract and expand in a pulsating manner at a rated frequency of around 20 kHz or 20,000 times/sec. At the highest point of oscillation, diamond kernels studded onto the tool collide with the workpiece surface, breaking it up into miniscule particles. The simultaneous rotation of the tool achieves accurate rotundity for the workpiece and, along with the water coolant, removes excess particles from the work area. As a result, materials like ceramics, glass, Stellite, silicon, graphite, composite materials, and precious stones are machined with low contact forces but high feed power.
These advancements in ultrasonics have been made possible because of companies like Deckel Maho Gildemeister (DMG), Germany. It has invested in ultrasonics because it feels the technology represents the future of machining — not just for the aerospace and medical industries, but for all industries as the development and implementation of advanced materials continues. In fact, DMG's chairman, Dr. Rüdiger Kapitza, believes ultrasonic technology will play a large part in his company's standard product offering.
"Customers are cutting more ceramics and exotic materials, and, because the process is so difficult, they are asking us to develop a way to better machine these materials. They need machines that can cut small, high-tolerance parts, so we have turned to incorporating ultrasonic technology into our machines. We can do ceramic 10 quicker and customers are interested. So for us, it is becoming a standard tool. We envision it as the first step when adding a high-tech extended component to a customer's machine."
Because of this belief in ultrasonic technology, DMG has acquired controlling interest in Herman Sauer GmbH & Co. KG (Sauer), Stipshausen, Germany, which has experience working on ultrasonic-machining techniques for over 30 years. DMG delivers two machine models, the DMS 350 mm and DMS 500 mm, in basic configuration to Sauer, where they are equipped with the high-tech ultrasonic components. These include the spindle, generator, oscillating sensors, and a custom diamond tool.
When Sauer is equipping the machines, component choice is essential. The efficiency and quality of the ultrasonic process hinges on not only the pulsating frequency and tool rotation, but also on the qualities of the diamond tool — the hardness, kernel size, and the chemicalmechanical connection of the tool carrier with the cutting material. That chemical-mechanical connection is important because, just as the electronic spindle is expanding, so is the binding material. Therefore, the material has to be one in which the kernels continue to adhere strongly enough to the tool at the highest point of expansion. On the other hand, a certain degree of flexibility is needed so that the kernels do not break off when contacting the workpiece.
The desired result is having only the kernel heads in the macro range split upon contact with the workpiece. Through this splitting, the number of single cutting edges increases, so the diamond tools sharpen themselves during operation. Consequently, the removal rates remain constant throughout the life of the machine.
Another important component in the ultra-sonic process is the tool mount. A cone-shaped thread provides a force-fit connection, guaranteeing the transfer of the oscillation from spindle to tool. The specific construction of the spindle, in a fashion similar to that of an unidirectional restrictor valve, ensures that no longitudinal oscillations bounce back into the spindle bearing or machine.
DMG's machines provide an efficient ultra-sonic procedure by optimizing controls for process safety and reproducibility. The CNC's algorithms let users continuously monitor the process and adjust the manipulated and controlled variables, including amplitude, feed, and rotational speed. Divergence from such variables is immediately known, making tool breakage almost impossible.
DMG machines have two types of process control. The first, adaptive control (German: "Adaptivregelung" - ADR), monitors the tool to maintain optimal feed. While the tool guides ADR, the Acoustic Control (ACC) monitors the acoustic process signals emitted by the workpiece to keep surface qualities high.
Both ADR and ACC constantly measure physical parameters and are deployable in combination. At the beginning of machining, for example, a maximum amount of feed can be made avail-beable of ADR. And during the finishing of surfaces, the control automatically switches over to the acoustic process control (ACC).
Satisfying global customersWhile machine tool companies worldwide have struggled with shaky markets, Deckel Maho Gildemeister (DMG) has thrived, increasing profits and revenue steadily over the past seven years. AMERICAN MACHINIST editor Tom Grasson talked to DMG chairman Dr. Rüdiger Kapitza about DMG's continual success in the global economy and the company's role in machining's future. Many tactics have contributed to DMG's success as a global supplier of machine tools. According to Kapitza, they include asking customers what they want, focusing on innovation and open controls for DMG's systems, building international technical centers, and having engaged employees and strong management. Evidently these tactics are working, and for Kapitza, the corresponding success means DMG is on its way to being a major global player in the machine tool market. "We want to be the number-one machine tool supplier in the world," says Kapitza. "When we started, we were number 40 or 43 in the world, one year later we were number 35, and that's when we decided we could do it. We want to be one of the biggest European play-ers first and then a large global player." DMG is closing the global gap by making its presence known throughout the world with over 5,200 employees. In the U.S. alone, DMG is working on building its fifth technical center, which will be located in New England. And the company further continues its emphasis on communication with its online machine-servicing program, which puts the manufacturer in touch with clients worldwide at the touch of a button. Currently, 720 customers are directly connected to DMG's maintenance network. Kapitza explains the ease-of-use and benefit of this system for remotely servicing machines worldwide. "If something happens, the customer wants quick service, not a guy coming to his shop floor. He wants a fix rate of seconds or minutes, not two to three hours. We use the Internet to connect directly to a customer's machine software, which means we can work on the simulation one to one. He gives us a mistake, tells us what's happening, and we can quickly make a diagnosis on the machine, the software, or the material to cheaply and quickly solve customer problems." When asked about the future of machining and DMG's role in the coming five to ten years, Kapitza commented on the increase in standard, high-powered machines — noting that the days of custom machines were lessening as technology improved and manufacturers could more affordably build standard, advanced machines that had once only served smaller markets. That's where ultrasonic technology comes into the DMG picture. Once used only for advanced applications in niche markets, Kapitza sees the technology blossoming into a standard option for his already advanced machining centers. He believes the reason for this will be as simple as supply and demand. "It is much easier to work with ultrasonics on new materials, and clients will ask us for this kind of advanced capability more and more, especially in aerospace and automotive. In the future, ultrasonic will not just be for niche markets. For us and for our clients, it will become a standard tool." |
