Virtual axis machining: the shape of things to come?

I came away from the 1988 machine tool show in Chicago with the image of a football field-sized room full of machine tools which were indistinguishable except for color. People were talking about the size of the bolts they used to hold down the machine ways...Product differentiation was absolutely minimal.

From that sense of Paul Sheldon's frustration was born the quest to find a better way; a search that started six years ago and will end in September, where it began, at the International Machine Tool Show in Chicago as Giddings & Lewis Inc unveils the Variax, a virtual axis machining center that may eventually make the on-going arguments over box ways vs linear guides and cast vs welded bases passe.

The Variax represents a paradigm shift in machine tool design as it does not have a separate supporting structure, claims its chief inventor, Paul Sheldon, currently Giddings & Lewis' vice president for research and innovation. "The machine is the structure. Supported and moved by six legs, driven by individual ball screws and servo motors, the entire upper portion of Variax moves. This section, which holds the cutting tool, aligns itself to the part.

"The Variax is a virtual axis machine, which has control over all six degrees of freedom. Therefore, sources of error seen in conventional machines, which occur because of machine or structure geometry, are nonexistent. It uses the mathematically aligned virtual axes to adjust itself to the part, simplifying fixturing and part set-up," explains Mr Sheldon.

Pioneering Patent

Although Paul Sheldon and Ed Kirkham, co-inventors, were granted pioneering patents and describe the technological leap as "revolutionary," rather than "evolutionary", Mr Sheldon claims, "the research days are over. We are prepared to start selling this machine. It will be at IMTS running and making chips and we'll be taking orders."

The price has not yet been finned up but it will be priced competitively with five-axis machines. It'll come with a tool changer and table shuttle and a 630 mm work cube.

Mr Sheldon feels initial interest will come from people currently using five-axis machines. "If you are working with difficult material, want to contour faster, or need extreme accuracies, the Variax leap-frogs what can be done in any other way," he claims.

His confidence is based on experience. Mr Sheldon tells Tooling & Production, they have been cutting chips on the Variax since 1991 when the patents were issued. Although it is new, to Mr Sheldon it is already established technology, having worked on it since 1988 in a skunk works atmosphere where he, Mr Kirkham and a small team did the development, and proved out the analysis and mathematical algorithms.

The Variax adventure started during a conversation Messrs Sheldon and Kirkham were having. Mr Kirkham said he always envisioned a spindle floating out in space. At the time both were at Kearney & Trecker, a company since acquired by Giddings & Lewis.

"At that point the proverbial light bulb went on and I pictured this parallel kinematic link mechanism," the machine tool innovator declared. "We ended up working evenings and weekends on that idea to see where it would lead. Initially, I unknowingly reinvented the Stewart platform (a flight simulator is an example) which has been around since the 1960s," Mr Sheldon said.

He hastens to explain that the current Variax is not a Stewart platform. "It has six legs and operates in six-dimensional space but is a generation beyond that," he adds. The Stewart platform, he says, has limitations in construction and usable work space as it is moved around.

Culture Shock

Mr Sheldon admits that introducing such a revolutionary approach to building machine tools has an element of risk that is rooted in culture and perception.

"We got rid of all the classic aspects of machine tool design. Historically machine tools are sold by weight. If you want it to be rigid it has to be heavy. Now we come with a machine tool you can see through claiming it is more rigid than conventional machine tool structures," he claims, adding: "we did all the analysis and truly measured it to be five times more rigid." The rigidity is inherent in the design, which includes the six crossed active supporting members that counteract the stresses through either compression or tension.

The innovator claims that technologically the risk is minimal, explaining that the machine's components include conventional ball screws and bearings. "All that changed is the way we put them together."

Although the machine is mechanically simple, Mr Sheldon claims it is very rugged. He explains that there are no ways or way wipers to get contaminated and the bearings are sealed. "Consequently, where on a conventional machining center, grinding, for instance, may be frowned upon because of the grit that might contaminate the ways, on the Variax, with sealed rotary bearings, grit and dirt are less troublesome," Mr Sheldon claims.

The unique structure also makes wear issues less of a problem. As Mr Sheldon explains: "For example, normally in spade drilling in the Z axis, there is one set of ways and one ball screw that are shouldering the full load of that operation. On the Variax, all loads are shared by six motors and six ball screws."

Software is key

Mr Sheldon admits that "many people, when they hear terms like leading-edge technology, immediately assume the system is complex. While the original mathematics may have been complicated, the operation of the machine is not." The Variax uses a Giddings & Lewis CNC to control the six-axis machine and existing machine tool part programs and conventional part programming language are used. The movement required of each of the six legs, in order to position the spindle and curing tool to achieve a specific curing position, has all been built into the software. "The operator is isolated from the concerns of the complex motion of the machine. The computer does all the calculations," he explains.

The lack of a massive structure also improves response time. Acceleration on the Variax has been clocked at greater than 1 G, Mr Sheldon reports, compared to a typical machine tool rated at 0.1 or 0.2 Gs. He explains that the improvement in acceleration, or responsiveness, permits vastly improved contouring speed and precision.

"The accuracy of the Variax has to be considered volumetrically. It is out of the world of CMMs," he claims, adding: "Historically machine tools have been characterized in terms of single-axis accuracy. But the parts that people want to make live in volumetric space. Because there are no ways--it's just virtual axes--the Variax inherently exhibits volumetric accuracy. It has the accuracy in the part space rather than in the machine space.

New-found freedom

"In addition, because it is a six-axis machine, it has the dexterity to do very complex parts. It has full control of those six degrees of freedom, much like an airplane moves through space."

The virtual axis freedom that Variax enjoys, Mr Sheldon contends, places the entire aspect of fixturing into a new light. On a conventional machine, parts have to be lined up to the machine, often requiting expensive fixturing. "On the Variax, because the ways are determined mathematically, we can put the part on the machine, probe it to determine location and then realign the machine to the part. That is, we can redefine the axis system to match the part," he claims.

That same dexterity permits controlling and changing the orientation of the tool relative to the work piece. That, Mr Sheldon contends, presents new opportunities in designing the machining process and reducing the need for custom tools. To eliminate spring back and distortion in cutting a thin wall between two pockets, often a custom-ground, tapered tool is called for. With the Variax, Mr Sheldon claims, you can accomplish the same end by programming in a number of arc-seconds of tilt using a standard tool.

Quantum leap 

The thrust, speed, acceleration, and accuracy of this machine make machining difficult materials a lot less troublesome and could open an entire new world of possibilities to designers. Very often, Mr Sheldon says, the advances in software, computing, motors, controls, and tooling have not been fully utilized because of the mechanical limitations of the traditional machine tool's structure. "Now with the Variax we have leapt ahead of all that," the 45-year-old innovator contends.

The new structure's portability will also facilitate equipment reorganization on the factory floor to accommodate the rapid change in product mix, Mr Sheldon foresees. Because the Variax doesn't need any foundation, it can be easily moved to accommodate the production flow through a factory, he adds.

"You can no longer look at a machine as just being a machine or a control being a control. What really matters is how all the components and disciplines tie together," Mr Sheldon philosophizes. He feels the Variax is only the beginning. "This pioneering patent opens up a whole new world and the tough decision we face is just where to start putting this new technology to work."

Paul Sheldon's enthusiasm indicates he feels he has a good start with the Variax. The market will tell him what's next when Giddings & Lewis unveils its "new baby" in Chicago in September.