As one of a handful of manufacturers worldwide that has the capability of making complex (laid-up) CFRP (carbon-fiber-reinforced plastic) aerospace structural parts and final-machining the parts for direct delivery to “point of use,” Hitco Carbon Composites was selected by Boeing to supply floor beams for soon-to-be-in-service 787 commercial airplanes.
The floor-beam process entails design and construction of detailed layup tools, with the use of CNC-controlled tape-laying machines to form the structures that are then cured in large autoclaves. The cured floor beam profiles are sawed, edge-routed, cut to length, and drilled. They are manufactured as ship sets in a demand-driven on-off manufacturing environment, each ship set corresponding to an airplane order.
Floor beams run from one side of an airplane to the other and so vary in length to accommodate an aircraft’s shape (planes are wider in the middle than at the ends). The 787 beams have different hole-drilling patterns, depending on the connecting structural elements. The design of a floor beam is further complicated by gauge (or in this case layup thickness) reduction (for weight-reduction purposes) from end to center of the floor beam. The beams also have a recess section in the center third section to accommodate fore-to-aft wiring and plumbing paths. The beams have similar cross sections.
Hitco turned to Bertsche to develop a five-axis profile beam milling machine to make the beams. Bertsche has a long history designing and building composite milling and drilling machines beginning with the 777 program, the first commercial airplane to use CFRP floor beams.
The machine features automated part setup, part program control of all work-holding fixtures, a wave milling feature, and 100% part inspection. The complex work-holding fixtures can clamp beams of varying web width, varying flange length, and programmable web thickness datum surfaces. The fixtures themselves are repositionable so they can be prepositioned as part of an automated CNC setup program as well as moved out of the way during machining operations to provide better access to certain areas of the floor beam for cutting and drilling operations. The fixtures are individually controlled but also operate as coordinated groups that open and close, lift and retract, and reposition in synchronous fashion for part-loading sequences when machining or when measuring parts.
Dry machining is the preferred method of manufacture by airplane manufacturers to avoid composite material swelling due to coolant absorption, according to Hitco. Coolants, especially oil-based ones, can cause the material to swell and residue to form. Dry machining also has an advantage because dust containment and removal is an easier problem to solve than preventing dust sludge buildup when the plastic material and cutting fluids combine and adhere to every crevice and nook in the machine. For dust containment, all machining is done in a fully enclosed compartment with integral dust collection hoods.
The thermal coefficient of expansion of CFRP material is almost zero, while the machine tool expansion is primarily governed by the coefficient of expansion of steel, Hitco says. Due to the long length of the beams, this difference is significant enough to potentially adversely impact the location of holes drilled into the parts. For that reason, thermal length compensation technology is used. The entire floor beam machining system is housed in a specially built facility that is temperature-regulated.
Parts are 100% feature-inspected inside the machine. The inspection probe is used to verify the position of fixtures, datum surfaces of the fixtures, and dimensional inspection data for all drilled and milled features. The machine is also equipped with a calibration sphere that allows the machine to quickly check itself for any position deviation.
The finished machined product is inspected using a full function metrology software package integral to the HMI front-end PC.
Engineering and manufacturing offices surround the machine, giving manufacturing engineers constant oversight of complete operations and allowing them to stay fully involved at all times with the manufacturing process.
Hitco claims that current production rates are four times faster than by previous methods and that program optimization and process improvements are being implemented with the expectation that beam manufacturing time will be reduced by at least another 50%.