Tripod-mounted spindle targets monolithic parts

DN Staff

March 12, 2001

8 Min Read
Tripod-mounted spindle targets monolithic parts

Augsburg, Germany-Airframe engineers are designing new types of floor beams, wing ribs, and fuselage frames, thanks to a new spindle carrier that helps streamline production of large parts. Now, instead of building up structural components from an assembly of plates, angles, channels, and fasteners, airframe engineers can design larger, single-piece, monolithic structures. Using larger parts yields stronger, more reliable, and lower cost designs. And the spindle carrier may find applications in other industries as well.

Landing its first production application at the Augsburg plant of the European Aeronautics Defense and Space Company (EADS), the patented three-axis

spindle carrier from DS Technologie (DST) GmbH (Monchengladbach, Germany) has been busy making center-section components for the new Eurofighter fuselage. After a year of operation, EADS Leader of NC Programming Alfred Lilla says that the new spindle carrier's parallel kinematics contributed to a 30-50% reduction in production time compared to gimbal-type heads. Other advantages he sites include improved surface finish, greater accuracy, better reliability, and a more compact design.

In its first production application, the tripod-mounted spindle is busy making center-section components for the Eurofighter fuselage.

Last year, Cincinnati Machine formed a strategic alliance to incorporate the DST head into its HyperMach(TM)line of high performance machines for the aerospace industry. Through the strategic alliance, the giant U.S. machine tool manufacturer sells and supports DST technology including the new spindle carrier, which it calls the Z 3 Head. "In the end, the Z 3 Head improves cutting speed, part accuracy, and surface finish for our customers," says Randy Kappesser, product manager at Cincinnati Machine.

But the Z 3 Head differs from gimbal or contouring-style heads typically used in 5-axis milling, in that it doesn't rely on its host machine to provide Z-axis travel. Instead, it supplies its own Z-axis motion using three parallel ballscrew drives connected to the spindle housing by universal joints from INA (Herzogenaurach, Germany). Moving all three ballscrews simultaneously creates the traditional Z path, while moving them differentially creates A and B-axis motion.

"By eliminating the whole-head rotation, which typically slows down fork-type [gimbal] head operation, the Z 3 head moves the tool to any point within the workspace more quickly. And it eliminates rotary couplings and slip rings to simplify the design, reduce costs and maintenance, and improve reliability," says Dr. Norbert Hennes, design and development chief at DST.

The high-speed spindle offers speeds ranging from 7,000 to 24,000 rpm and power up to 100 hp (75 kW). Providing angular velocities of 1,968 inches/min (50 m/min), 32.2ft/sec 2 (1g) linear acceleration, and rotary acceleration of 685 degrees /sec 2 , the Z 3 head tilts plus or minus 40 degrees around the A/B axis within a 14.6-inch (370-mm) Z stroke. Its maximum Z-axis travel is 26.3 inches (670 mm).

Moving all three ballscrews on the Z3 Head simultaneously creates the traditional Z path, while moving them differentially creats A and C-axis motion.

The need for speed. Machining large parts from a plate of material instead of assembling them from hundreds of pieces offers enormous benefits to airframe manufacturers: less tooling to hold the parts together during assembly, less rivet-pounding labor, and less distortion of the part after loading the rivets into it. But the economics of hogging all of that aluminum out of huge billets ultimately depends on cutting speed. So the need for speed has been somewhat of a tradition at EADS Augsburg.

The High-Speed Cutting (HSC) evolution has continued at Augsburg ever since engineers first rebuilt and modified a large surface-milling machine to create its first monolithic structure out of aluminum back in the 1980s. One significant step according to Lilla, was in 1998 when they installed a Droop + Rein HSC system that had a new 40 kW (67 hp) spindle with 24,000 rpm capability.

Although the machine achieved accelerations of 1.5 m/sec 2 and a rapid traverse rate of 20 m/min, it couldn't meet the production demands of the planned Eurofighter aircraft. Its center section fuselage components have many thin-walled pockets and ribs. Approximately 90% of the metal had to be removed during machining. And cutting the large number of contours would require simultaneous 5-axis machining.

Conventional gimbal-type heads didn't have the required speed to mill the radii in the very small pockets of these large parts. And available parallel systems were all too inaccurate, according to Lilla. So EADS started searching for a new spindle unit with plus or minus 40 degrees of movement, that could meet the highest quality standards with respect to dimensional accuracy and surface finish.

It's no surprise that the Z 3 Head's first production application was at EADS Augsburg. In fact, says DST's Hennes, EADS' production requirements were a key driver in its development. "Nothing on the market could meet Augsburg's requirements. So we had to come up with something completely different."

EADS wanted higher speeds, higher feeds, and more acceleration, according to Hennes. "With traditional two-axis heads, the limiting factor is the rotary axis-especially when machining the tapered corner radii on the deep pockets." So DST engineers knew to increase speed they had to eliminate the rotary/tilt operation of conventional heads that slows down the sequence.

The three legs. "One of our sales engineers first came up with the idea of the three legs forming a tripod," explains Hennes. "We made a wooden model, showed it to our customer. And they liked it. None of us knew if or how it would work at that point."

Nonetheless, DST patented its tripod-mounted spindle design in 1998. "It's nothing new to have a tripod," Hennes explains, "but combining a hinge joint on one end of each linear axis with a universal joint on the other end creates an extra degree of freedom."

While DST may hold the patent, the Z 3 's remarkable 14-month development time from concept to full production was a team effort. EADS, DST, together with Siemens AG (Erlangen and Chemnitz, Germany) and subsidiary company RTS (Real Time Software GmbH) of Schwieberdingen, developed a Siemens 840D CNC-based control concept that uses Heidenhain (Schaumburg, IL) encoders for position feedback.

In total EADS Augsburg has four Z 3 Head-equipped machines connected in two flexible manufacturing systems. Each cell consists of two linked HSC machines, two tool magazines, one pallet transportation system, a tilt load/unload, and a buffer station. The horizontal spindles (72 kW/24,000 rpm) together with a minimal spray coolant system provide optimal chip removal, according to Lilla.

Both cells contain a tilting loading station that allows the operator to load and clamp the parts to the pallet in the horizontal position. From there, the rail guided transport system brings the pallets automatically to the machines. One person at the buffer station operates each cell using a central control station. The operator monitors the machining process with the help of a video camera. Needless to say, Lilla explains, manual over-ride once the NC program has started is not possible due to the high dynamics.

Consequently, NC program simulation plays a major part, with 100% simulation of cutter path and collision considerations for every part. NC data is compared, point by point, to the 3D-CAD-models (IBM Catia Version 4.1.9).

At last year's Parallel Kinematic Seminar in Chemnitz, Germany, experts agreed that advances in automatic calibration, electronic control, and software development tools were key to processing the real time algorithms that this tripod-mounted spindle requires for quick and exact movements.

&HEADLINE>How to achieve 5-axis machining&/HEADLINE>

Four common 5-axis machining methods stand out for aerospace applications: horizontal and vertical machining centers, vertical profilers, and vertical ram-type machines with contouring heads.

Horizontal machining centers normally achieve X and Z-axis motion from the table and column. Y-axis motion comes from the spindle carrier moving vertically on the column. The horizontally mounted spindle tilts up and down for A-axis motion, while the table rotates for the B axis.

Vertical machining centers use a two-axis head that tilts up to 40 degrees in A (forward and back) and B (side to side). A compound table moving in and out and left to right gives X and Y motion. The Z-axis comes from the head traveling up and down on the column.

Vertical profilers may house multiple spindles each mounted in a spindle carrier. X-axis motion comes from the crossrail traversing the length of the machine, with the carrier moving along the crossrail providing the Y-axis. The carrier traveling up and down provides the Z-axis. The spindle carrier tilts in and out for the A-axis, while a trunnion on each spindle allows it to pivot left and right for the B-axis.

Vertical ram-type machines use a gimbal or contouring-style head mounted vertically on the ram. Whole-head rotation provides the C-axis, while the spindle pivots for the A-axis. X-axis motion normally comes from the table moving back and forth. The saddle travels along the crossrail for Y, and the ram raises and lowers as the Z-axis.

Three-axis heads, an emerging trend, supply their own Z-axis motion. Heads with a Z-axis, such as the Z 3 Head, allay the machine structure of this requirement. The result is a lighter more compact machine that can achieve higher acceleration and deceleration rates.

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