Nuclear Fusion and Unlimited Electricity
ROMER portable coordinate measuring machine

J. P. Pattern Inc., a machining and foundry-pattern shop near Milwaukee, is helping to build machines that may eventually harness nuclear fusion to solve the world's energy problems. However, the company's day-to-day concerns are more mundane--create a pattern for 10 identical aluminum-alloy castings, rough- and finish-machine them, and then verify that their dimensions meet all specifications.

The castings are the winding frames for 10 coils that generate a helical magnetic field. The field encloses and insulates plasma that reaches in excess of 100 million° C. Components built by J.P. Pattern fit over the ends of a doughnut-shaped vacuum vessel that encloses the plasma and its magnetic field.

The machine for which J.P. Pattern built these components is the compact toroidal hybrid (CTH) at the Auburn University physics department. The device advances research into fusion power, and relates to a larger effort to harness nuclear fusion for the production of electrical power. The CTH winding frames consist of two sets of five castings that make up five helical mandrels. "The biggest dimensional challenge was measuring and inspecting warpage in the castings due to the way the castings cool and the forces of machining," says Gerald "Gary" Puhl, J.P. Pattern vice president.

These critical measurements were done with a portable coordinate measuring machine (CMM) from ROMER Inc., a Hexagon Metrology company. The flexibility of the ROMER arm allowed Puhl to measure the two fully assembled sets of castings while standing in the toroidal center openings.

The dimensional data were processed by PowerINSPECT from Delcam Inc., software which ROMER resells as an integral part of its measuring system. "Without the ROMER portable CMM and PowerINSPECT, we probably would have contracted out the measurement work," says John Puhl, J.P. Pattern president. "Contract inspections can be complicated and time-consuming to manage. If you are uncomfortable with a reported dimension or tolerance, you either have to call the inspection people back or take the casting to them for a remeasure."

"We measured each of the 10 castings at least twice," says Gary Puhl. "The first time was to make sure the rough castings had enough stock so we could machine to final dimensions." Each casting was measured again after machining to final dimensions. A third set of inspections was done during assembly. Each set of five castings is nearly 9 ft in diameter and stands about 18 in. high.

Had the castings or the measurement process been off in any way, months of planning and work might have been wasted due to a production cycle that could have lasted more than a year.

The tightest dimensional specifications J.P. Pattern had to meet--and the toughest measuring challenges--were in the 10 curved and twisting coil troughs. The five troughs in each of the two sets of five castings control the magnetic wire windings. "Each trough is curved in two axes so it wraps helically around a torus," explains Gary Puhl. "The distance between the trough sidewalls was designed to be a constant value if the helix was unwrapped."

Measuring each trough required about 180 contact-probe points--60 on each sidewall and the trough's U-shaped bottom. The troughs also were precisely located in relation to each other, part of 17 discrete features to be measured and documented. The windings determine the power of the magnetic field and, thus, the temperature of the plasma.

Small deviations in the magnetic field can have big effects on the way the plasma behaves. Learning to better control plasma is the whole point of the Auburn CTH. "With older methods, the troughs would have taken four to five times longer, half a day apiece, and so a total of 10 man-days," says Gary Puhl. "There would have been a lot more chance for error, too."

With all magnetic-coil winding completed and energized, the CTH is now in operation. To verify the winding frames' measurements, Gregory Hartwell, an Auburn assistant research professor, visited J.P. Pattern in the spring of 2004 to witness the assembly and gather dimensional-measurement reports. "The measurements indicated that the [devices] were within specifications, and nothing since then has given us cause for any doubt," says Hartwell.

ROMER portable coordinate measuring machine

Benefits:

• Integrated Wi-Fi wireless communication lets the operator position the computer where it's most convenient.
• Infinite rotation of principle axes allows inspection in difficult-to-reach areas.
• An integrated low-profile counterbalance reduces fatigue and provides ergonomic handling.
• Integration with Delcam's Power- INSPECT eases data analysis.

www.jppattern.com or www.romer.com