12/1/1998 | 3 MINUTE READ

A Different Approach

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Ordinarily, we cover existing products on this page. But this is not an ordinary machine. Admittedly, it isn't even a bona fide machine yet. Indeed, Paul Sheldon, chairman and CEO of Sheldon/Van Someren Inc., a technology brokerage, proffers a model of the Triax that he put together with Popsicle sticks, pop rivets, scrap plywood, and a CD case from Walgreen's. "I'm a big believer in rapid prototyping," the inventor quips.

Sheldon should not be underestimated. While working at Giddings & Lewis—where he had a title not often seen among executives of machine tool companies: "vice president of Research and Innovation"—Sheldon spearheaded the work that led to the Variax six-axis machining center. Sheldon left G&L in 1995 and began a consultancy, SheldonWorks, which continues.

Although Sheldon garnered recognition for his work on the Variax, he admits that the machine didn't take off as he'd hoped. But he learned some les-sons along the way.

One of the concerns that people have with things with an unusual configuration like the Variax—and the Triax—is the unusual configuration. It is officially categorized as a "parallel kinematic machine" (PKM). The visible strangeness of the machine can cause pause among those who might benefit from the use of the equipment (e.g., "What would you do with that!?!?!") Another problem is that because there is new functionality provided (e.g., the ability to get at different points in space with high rigidity), there needs to be an alteration of status quo process plans. With different machines it is necessary to do things differently.

One of the concerns that Sheldon now has is that during a visit to Milan this past fall, where he was invited by the National Science Foundation to make a presentation to an international conference on PKMs, he discovered that there were more than 10 different PKMs under development and that there is plenty of research being undertaken in both Europe and Asia. Remarking on what he sees not happening in the U.S., he comments, "It is stunning how many people are not very far along." But he admits that although the Variax, which he developed 10 years ago, was meant to be a machining device, much of the activity in PKMs is in the area of robotics. He thinks that has something to do with the history of robots as compared with the history of machine tools: Robots are a whole lot younger. As he puts it, "Robotics is a newer field. There is less history. The paradigm is more malleable compared with that of machining centers."

Still, he continues to plug along on what he thinks is a configuration that can be so beneficial to users that the benefits will outweigh the, well, weirdness of the machine.

As he recalls of the days of the hexapod, "We'd go into a company and make a presentation. Everyone would be impressed with the performance that the machine provides. But then at the end they would ask, `Can you make a three-axis version?'" He adds, "People in the auto industry have spent years designing parts so that they can be made with no more than three axes." And he thinks that had his first machine been even a four-axis machine, rather than six, there would be more success. After all, there are costs associated with having the motors, drives and actuators necessary for all of those axes.

All of which has led to the Triax, which is, as its name implies, a three-axis machine. Ticking off the benefits that can be achieved, he cites extreme rigidity, mechanical simplicity, low cost, and high speed. The design includes three pairs of stacked four-bar linkages. Using a finite element analysis package, he calculates that the rigidity of a 600 mm machine—one that uses 3-in. square tubes and Timken angular contact roller bearings—is on the order of 600,000 lb./in. "It's the math not the mass that makes it stiff," he notes. The structure resists loads because the axes are in tension and compression. There are no ways on the machine as, Sheldon remarks, "In automotive machines, the first thing to wear out are the ways." He doesn't have that concern with what are essentially truck bearings.

There are ballscrews for each of the axes. And as a result of the combination of the three working to move the spindle in space, although the ballscrews each move at 1,500 ipm, through what he describes as "a squirting effect," which has to do with "a vectoral gain in speed at an angle," the spindle actually moves through space at 2,800 ipm.

Sheldon admits that Sheldon/Van Someren has no interest in becoming a machine tool manufacturer. Rather, they are looking for people who are interested in licensing the technology.

Hand holding a crystal ball

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