February 2011 / Volume 63 / Issue 2|
Boosting beam production
By CTE Staff
HITCO Carbon Composites Inc. is one of a handful of manufacturers in the world able to manufacture laid-up, or complex, carbon fiber-reinforced plastic aerospace structural parts and to final machine the parts for direct delivery to “point of use.” As such, The Boeing Co. selected HITCO, Gardena, Calif., as one of the suppliers for CFRP floor beams for the 787 program.
Manufacturing floor beams entails designing and constructing detailed layup tools and using CNC tape laying machines to form the structures, which are then cured in autoclaves. HITCO then saws, edge routes, cuts to length and drills the cured floor beam profiles.
HITCO began by machining prototypes of the floor beams on a large gantry mill, but needed a machine optimized for the process. To produce the beams at the required rate to the specified tolerances, Boeing requested that HITCO purchase a P5 5-axis profile beam milling machine from Bertsche Corp., Buffalo Grove, Ill. That’s because Bertsche Engineering previously built one for another aerospace structural parts manufacturer that was producing floor beams for Boeing, noted Richard Bertsche, company president.
Courtesy of Bertsche Engineering
According to Bertsche, the previously built machine was designed for maximum flexibility but not maximum throughput. “We supplied it with incremental improvements to the original machine,” he said. “It’s hard to call the second machine ‘the second generation,’ but in many ways it was.”
The resultant machine provides automated part setup, 100 percent part inspection, the ability to “wave mill” and control of all workholding fixtures through the part program.
The mechanical fixtures can clamp beams with varying web widths and flange lengths, and are individually controlled or operated as coordinated groups that open and close, lift and retract and reposition in a synchronous manner when loading, machining or measuring parts. For example, the first, third and fourth clamps act like a single clamp even though they’re at different locations, Bertsche explained. “So when you open group 1, all group 1 fixtures open while the other groups stay closed,” he said. “That way you never release the beam and never lose position.”
Having the fixtures work as a coordinated group helps perform wave milling, where clamps are programmed to move out of the toolpath when the portion of the part they were holding is being cut and swing back into place after the cutter passes. “It’s kind of like at a football game, where people stand up and sit down and stand up and sit down,” Bertsche said.
Every floor beam from the front to the back of the plane differs in terms of features like drilled hole pattern and length. Also, part changes occur frequently. The coordinated workholding system provides the flexibility needed to accommodate these changes.
In addition, large vacuum tools can be loaded into location on the machine as needed and supported by the same flexible fixtures. “The vacuum tooling is nice because it’s quick, dead-nuts on and always the same,” Bertsche said. “But when they make changes to the beam, say, new drilled hole patterns, then you must modify your vacuum tool at those positions.”
Machining the floor beams generates composite dust because the parts are machined dry. Dry machining is preferred to avoid coolants, especially oil-based ones, from interacting with the material and causing it to swell and form residue that must be removed after machining. Also, dust is easier to remove than coolant-laden dust sludge, which adheres to every crevice and nook in the machine, according to Bertsche.
HITCO’s machine has a fully enclosed machining compartment with integral dust collection hoods, which capture airborne composite dust and direct it out of the machine into a filtration system. Composite slugs, shards and similar debris are swept into a collection chute with a debris pusher device as part of an automatic cleanup cycle.
After the P5 entered service and was subsequently qualified for the rigorous application standard, the milling machine enabled HITCO to boost CFRP floor beam production 400 percent by reducing the number of setups and machining parts complete in a single setup.
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