MACHINE TOOLS

September 1996

Putting the hex(apod) on machining
The hexapods are coming and they'll likely change your manufacturing paradigm.

You won't have to look far for that next unusual machine tool out on your shopfloor. It might just be the first truly commercial version of a concept that has captured the fancy of machine tool designers and users alike over the last few years--the hexapod.

The latest firm to enter the hexapod development race is Hexel Corp, Portsmouth, NH. But unlike early starters, Ingersoll Milling Machine Co and Giddings & Lewis, Hexel president Michael D Fortier reports the sale of several of its new Hexel Tornado 2000 hexapod machines. One reason for its success: Hexel has been able to bring the price down into the half-million-dollar range, near the cost of a high precision horizontal machining center. Beyond that, the firm is unveiling ambitious plans to develop the concept for virtually every machine tool application.

Hexel is a company jointly formed in June 1995 by Kingsbury Corp, Keene, NH, Renaissance Design Inc, Hampton, NH, and Crossley Associates, Chichester, England, to apply the hexapod technology to a broad range of metalworking applications.

Kingsbury is a manufacturer of rotary machines, widely used in the automotive and other industries for high production application, and machining centers and other metalworking equipment; Renaissance Design is an advanced engineering company with experience in machine design, 3D CAD, two-, three-, and five-axis CNC programming, and rapid prototyping; Crossley Associates is the original engineering team that developed the patents the Hexel machines are being based on. It first showed its machine under the name of Geodetics at IMTS 94 in Chicago, at the same time Giddings & Lewis unveiled its Variax.

Roderick Whitehouse, vice president, explains that Hexel's mission is to develop and expand the hexapod technology into a wide range of markets. Those targets include workcell-size machines for milling, welding, lasing, grinding, turning, and EDMing, among others; very large format machines for automotive and aerospace applications; small format machines; CMMs; robotics; automated worktables; and platform positioners.

A short history of hexapods
It's one thing to have a good idea, however strange and unconventional; another to develop a working model or prototype from it; and still a third to successfully bring the brainchild to market as a commercially viable and attractive machine tool.

Such is the short and not altogether uninteresting saga of the hexapod machine. The hexapod machining center has its roots in the so-called Stewart Platform, successfully used as a flight simulator to train pilots. As a multiaxis machine, it is unconventional. In the hexapod, all six degrees of freedom are enabled with a unified parallel arrangement of struts. For any set of strut lengths, there is a single rigid position for the mechanism; this position changes as the strut lengths are varied. By contrast, conventional multiaxis machines use separate linear or rotary mechanisms to perform each degree of freedom.

A number of hexapods have been announced in varying stages of development. The Variax Hexacenter milling machine introduced at IMTS 94 for high speed milling of aluminum took its design impetus from the flight simulator, with the struts acting as the frame and crossing over each other with the spindle pointing downward from the platform toward a working volume enclosed by the mechanism. At $1 million-plus per copy, none has, to this point, been sold, though there have been some near takers. The Variax has been adopted by a European consortium for further study.

In the early 1990s, Ingersoll Milling announced and showed its own "octahedral hexapod" prototype at its headquarters and subsequently built one at the headquarters of the National Institute for Standards and Technology. Ingersoll's machine utilized a 12-node hexapod suspended from an octahedral framework, with the spindle pointing down toward the workpiece.

Hexel dramatically simplifies its approach by utilizing a nine-node hexapod in which six nodes are in the workcell and three are on the platform. Its spindle also points down to the workpiece. The advantages of Hexel's approach are said to be:

• reduction in setup time because of the intelligent find-the-workpiece capability;
• flexibility to improve machine utilization by using standard hexapods as "soft machines" with interchangeable head units.
• Hexapods in general offer basic advantages in stiffness, accuracy, speed, dexterity, and scaling.
• Stiffness. When the hexapod is loaded, strut forces act only longitudinally either in tension or under compression. These can be accommodated more robustly and with a lighter structure than the bending moments experienced by conventional orthogonal beam machines. A conventional multiaxis machine has a series of connected movable linkages that each contribute some slack; on a five-axis machine there will be a minimum of six. In the hexapod, the links act in parallel, leaving two in the chain.
• Accuracy. Manufacturing errors in parallel structures are subject to averaging; whereas errors in serial structures are additive. The lightweight and low friction hexapod is less prone to hysteresis on movement reversal and can follow a tighter machining path. Most errors in conventional multiaxis machines originate at the rotary stages because of the complication of the gearbox. A hexapod platform can effectively rotate simply by altering strut lengths.
• Speed. A hexapod can be designed such that none of the heavy motors and servos need do more than gently oscillate. The moving mass is therefore comparatively small, minimizing inertia and machine power needed to overcome it. Also because all the necessary freedom of movement is enabled by the mechanism, the workpiece and its generally heavy bed can be stationary.
• Dexterity. A good hexapod with well-designed linkages can articulate farther than the usual tilt stages. The mechanism can also reach over a larger area than its own framework and is therefore very compact for its working volume. Multiaxis stages are available to increase the dexterity even further.
• Scaling. Struts, which carry only axial loads, can be scaled up economically. Doubling their diameter allows a fourfold increase in length with a sixty-fourfold increase in working volume, or a sixteenfold increase in maximum loading. This can be achieved with only a modest increase in weight and cost.

Make it simple
Hexapods usually have 12 geometric nodes--one at the end of each of six struts. According to Mr Whitehouse, this has proved to be a challenging problem for most earlier attempts at developing a hexapod platform machine tool. Hexel, however, has developed a unified joint (bifurcated ball) between pairs of struts meeting at the working platform. This development, he adds, reduces the number of nodes to nine, improves the stiffness, simplifies the control mathematical transforms, and allows for automated calibration. In short, adds Mr Fortier, the Hexel concept embodies a design philosophy that strives for precision to be derived from software, not the individual component accuracies. This flexibility will cut manufacturing costs and time, thus giving Hexel the first truly "soft" machine.

The Hexel strut is a high strength, highly reliable, roller screw extending from the unified joint (bifurcated ball) through the drive node sphere at the other end of the strut. This type of mechanism is extremely stiff and reliable because it does not require any telescoping mechanism. It also has a large extension-to-compression ratio. These nodes are all designed as low pressure, spherical universal joints facilitating accurate and repeatable calibration while inherently providing excellent strut damping.

The workcell cabinet also is extremely stiff, says Mr Whitehouse. It is fully enclosed and shear-damped with access on three sides. The workcell features the Shear Damper base invented and patented by Professor Alex Slocum of the Massachusetts Institute of Technology. It uses a secondary internal sheet-steel frame coated with a shear-damping layer and then coupled rigidly to the outer skin by a concrete grout. Vibrations are forced to microslip in the viscoelastic shear plane, dissipating their energies. As a result, the cabinet is significantly immune to vibrations, particularly those compromising low frequencies.

The single-axis, high power, high speed (20 kW; 25,000 rpm) spindle motor is built into the platform for rapid metal removal. A tilt-and-turn spindle motor will be added in 1997.

As well as offering industry standard GM codes for compatibility with existing manufacturing techniques, the Hexel controllers have advanced metrology features that in the initial offering enable partial automation of machine setup procedures. These features are the foundation of the machine's ability to go from CAD to finished part by pushing the "make" button in a way similar to the use of the "print" button in word processing.

Engineering solutions
Hexel uses roller screws as the struts with planetary roller nuts. Compared with ball screws they have a longer life, are more reliable, are stiffer, have less stiction, and are able to run faster. The strut is rigidly attached to the unified ball joint. The tubular nature of the strut improves its stiffness-to-mass ratio. A side-acting cam lock and quick release allow for rapid and highly accurate strut replacement. Error control in strut length is facilitated by pitch variations along the length of the strut being held in software for real-time error correction by the machine controller. Thermal expansion is similarly controlled by a real-time, one-dimensional, finite element, error-correcting technique.

The joint connecting each pair of struts to the platform replicates a ball and socket. The ball has two struts attached to it which can rotate about a common axis passing through the center of the ball. The socket is magnetic and retains the ball while holding less than half its surface. This, in turn, enables a wide degree of three-axis movement of the ball within the socket. The ball-and-socket interface is coated with a heavy shear lubricant that contributes to damping out strut vibrations. This bifurcated ball is part of the licensed patents that have made the hexapod technology viable, Mr Whitehouse claims.

The spindle units use high efficiency water-cooled DC motors with drives enabling seamless control from zero to maximum output. Optical encoders provide high resolution radial control for slow speed requirements; ceramic hybrid bearings provide high speed capability and long life. Spindles for the single- and three-axis heads operate at a DN (rpm spindle diameter ratio in mm) of 1.5m, placing this in the high speed machining range. Metals can be removed for less than 20% of the usual energy, significantly reducing heat transfer to the part and providing cleaner cutting and thin wall delicate section capability.

The head unit uses standard HSK50E quills reducing the cost of ownership of the machine. Added to this is a compatible touch-trigger probe available for selection for metrology and setup functions.

The control system
The hexapod machine controller is PC-based with signal co-processors on separate boards. They run their control transforms in real-time, interpreting the moves into second-order Bezier smoothed paths. Feedforward makes the best use of processing power and ensures adequate sampling through rapid changes in direction. All errors and offset data from the mechanical assemblies are summed with the dynamic thermal data to maintain an error-correcting lookup table.

High level interface. The Hexel machine may be treated as an industry standard five-axis milling machine and controlled in the conventional way through provided G and M codes. A Windows 95 interface is available to provide a user-friendly environment. These capabilities include features such as diagnostics, service history, cutters and coolant, and calibration; job management and job costing.

A significant advance offered by hexapod technology is the automation of the process of removing and then replacing a partly completed workpiece to the working environment, Mr Fortier says. To achieve this, the software maintains a virtual model of the evolving shape of the workpiece as it is machined. Features in the partly machined model or fixture can then be identified and used to subsequently determine any offsets and rotation in repositioning the workpiece with an autoprobing routine. Setup and programming can be accomplished off-line on a conventional PC.

The Advanced Machine Controller (AMC) developed for the Hexel and still evolving is a radical evolution of the conventional machine controller, claims Mr Whitehouse. As well as supporting the machine's conventional functionality, it has features automating the setup of jobs, including those requiring changes in holding position; maintains a current model of its working environment permitting spatial awareness for collision avoidance and smart movement strategies; and is packaged in a flexible PC Windows multiuser environment with an intuitive Graphical User Interface (GUI) configurable to a wide variety of user profiles.

According to Mr Whitehouse, it is intended that AMC V1.0 will itself evolve into a complete manufacturing system in a number of stages, including CMM functionality, integration with CAD/CAM systems such as through the Pro-E developers program, automation of multiaxis rough cutting, and support an increasing range of machine head options for grinding, polishing, turning, wire EDM, and the like.

Hexel believes that the AMC will open metalworking paradigms for prototyping and short-run applications and will package tailored variations for the OEM market.