September 2006 Mechanical Engineering Departments: Tech Focus, Pt. 2

This section was written by Associate Editor Alan S. Brown.

Technology Focus part 2:
Materials and Assembly

Massage Control

It's amazing how much work it takes to keep things simple. A case in point: the linear drive used to power water massage machines found in shopping malls. Massagers developed by Aqua Massage International of Groton, Conn., use pulsating water inside a heavy plastic barrier to pound tired shoppers.

The machine's 36 water jets attach to a nut assembly that zips along a 7-foot-long shaft, slowing when customers want it to work knotted muscles. Aqua Massage uses a lead screw to translate power into linear motion. Doing it smoothly inside a wet canister calls for the same type of slick engineering used to position factory equipment.

Roll forming produces complex threads that are precise enough for smooth, silent operation without whipping. A thread has 16 start locations.

Enter Kerk Motion Products Inc. of Hollis, N.H. The company makes nonball lead screws. It took sophisticated engineering to develop the screw used in the Aqua Massage, said Kerk's applications engineer, Bob Hawkins.

This started with thread manufacture. Conventional machining is accurate enough for single-start threads, but the Aqua Massage thread has 16 start locations. "You can't grind 16 starts and keep them perfect," Hawkins said. "There are always small variations, so the motion is not smooth and they can jam."

Instead, Kerk roll-forms rod stock into screws in molds. "It's expensive to make a perfect mold, but once you've got it, you can make perfect threads all day," Hawkins said. Rolling yields threads with up to 1/10,000 accuracy, more gradual slopes, rounded angles at thread crests and roots, and better surface finish. All of this improves precision and smoothness, reducing wear on the plastic nut holding the water jets.

The lead screw's 7-foot length and 0.75-inch diameter also posed problems. "The massage heads need to cover a lot of ground quickly, but you don't want it to reach a speed where it starts whipping," Hawkins said. Stretching the threads enables the nut to move 10 inches in four revolutions, or about a second.

The third key to the application was finding the right resin for the plastic nut. It had to match to 303 stainless steel thread's coefficient and could not contain glass or carbon reinforcement, which would grind the screw down over time. Nor could it absorb water, which would cause it to expand and bind. Finding the right resin enabled Kerk to create a fast-moving, long-lasting linear motion system that looks as simple as it was complex to manufacture.

Robots Next to Humans

Most industrial robots are so fast and powerful, they demand a safety barrier to keep people out of harm's way. Then there's Katana, which is designed to work cheek-and-jowl with humans.

"We're the only robot certified as inherently safe by the European Union," said Werner Klecka, sales director of Neuronics AG of Zürich, Switzerland.

Werner Klecka of Neuronics AG shows off a small robot designed to work next to humans without a safety barrier. The robot weighs about 4 kg.

The suitcase-size robot weighs in at 3 to 4 kilograms and has a 50-centimeter reach. Katana can lift up to 500 grams and place an object with 0.1-millimeter accuracy, thanks to its smooth harmonic drive. At only 1 meter per second, it moves too slowly to present much of a hazard and stops if it hits anything solid. Even its power supply is modest, resembling those used by laptop computers.

"We think it makes sense where humans are doing something stupid," Klecka said. "Take, for example, a quality control system where you have to position a part to visually inspect it under a microscope. The robot can move and position the slide faster and better than the inspector."

Katana robots have also been used to place and remove pressure sensors for ultrasonic welding, extract parts from spray casting molds, position inspection cameras, automate test tube placement in laboratories, and even pack easily torn, single-serving coffee filter bags.

Many of these applications require Katana to move fragile components. Its highly instrumented gripper makes this possible. It has nine infrared sensors, four force sensors, and a device for measuring conductivity. Neuronics also can equip the robot with ultrasonic sensors or video cameras to improve positioning and recognize shapes.

For a robot, Katana is easy to set up and operate. Users can steer it by hand and it will remember its positions. They can then improve their rough positions using its control software. Equally important, the robot's neuronal network learns from past actions and gradually improves its decision making.

Katana comes in a two- or three-joint configuration with four or five degrees of freedom. Users can mount it on a rail or trolley. According to Klecka, the robot costs about $25,000 with software. And there's no need to spend a penny to build a safety cage around it.

Lighter Metals Keep Surface Smooth

Imagine taking more than half the weight off a metal part without degrading its surface finish. That would yield aluminum parts with densities similar to polymers. Researchers at Germany's Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research (IFAM) in Bremen have done just that. They even won a gold medal from the International Forum of Design for their efforts.

IFAM researchers led by Jörg Weise lighten metals by adding hollow glass spheres to a melt prior to casting. The concept goes back several years, but Weise has found a way to improve the dispersion of his 60-micrometer spheres. "If the glass balls are evenly distributed, we get a smooth surface that feels absolutely smooth, like metal," he said. Allow less even distributions and the metal looks streaky.

The glass spheres slash density. According to Weise, zinc drops from 7.0 grams per cubic centimeter to 3.1 grams per cubic centimeter. Aluminum falls from 2.7 grams per cubic centimeter to 1.2 grams per cubic centimeter. Aluminum also retains its heat resistance and can withstand pressures to 1,000 bar. Its composite structure also absorbs impact energy, making it a contender for lightweight bumpers.

Despite micrometer-scale porosity, Weise says he can plate the surface using techniques similar to those used with conventional metals. He is currently working on chrome-plating sphere-loaded zinc with German metal finisher HDO Druckguss and Oberflächentechnik GmbH of Paderborn.