Hitco Carbon Composites was selected as one of the suppliers to Boeing 787 program for floor beams manufactured from CFRP. Hitco is one of hand full of manufacturers, worldwide that has the capability of manufacturing complex (laid up) CFRP (carbon fiber reinforced plastic) aerospace structural parts and final machining the parts for direct delivery to "point of use". Specific to the manufacture of floor beams, the process entails design and construction of detailed layup tools, using 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 in an airplane are those structural members that run crosswise down the length of the airplane. The length of the beam (the width of the airplane) varies as the airplane widens in width going from front to center of the airplane and then narrows again moving to the rear of the aircraft. While having a similar cross section the each beam is manufactured for different length and has a different hole drilling pattern dependant on the attaching connecting structural elements. In an effort to keep floor beam weight to an absolute minimum, the design of a floor beam is further complicated by gauge (or in this case layup thickness) reduction from end to center of the floor beam. The floor beam also has a recess section in the center third section which serves to route wiring and plumbing in the airplane form fore to aft.
To accomplish the complex manufacturing problems of machining trim and drilling these floor beams to the requisite tolerances and to meet the required production rates, HITCO turned to Bertsche to develop a complex 5-axis profile beam milling machine for this purpose. 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.
Over the course of several months, the project requirements were fully defined allowing Bertsche to then incorporate the many desired features into one full automatic machining system. The resultant full 5 axis 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 that they can be prepositioned as part of an automated CNC setup program as well as moved out 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 ore when measuring parts.
The work holding fixtures are designed to hold U, H, J cross sectional beams from 3.0" in width to 12.0" in width and can adjust for gage reduction (web ply thinning for weight reduction). The fixtures automatically adjust to varying web and flange thickness.
For added flexibility, the workholding fixtures are removable from the supporting base. New, different style fixtures can be attached to the support allowing the machine to be reconfigured for holding different style parts.
The machine entered service in August of 2009 as was subsequently qualified the rigorous standard of the application.
To enhance part positioning and flow, the machine incorporated. Large vacuum tools can be loaded into the machine supported by the same flexible clamping style work holding fixtures. Parts are now manufactured with either vacuum or mechanical clamping means as the process dictates.
Parts are machined dry. Dry machining is the preferred method of manufacture by the airplane manufacturers to avoid composite material swelling due to coolant absorption by material exposed from cutting. Coolants, especially oil base coolants can interact with the composite material causing the material to swell and the formation of residue that then must be cleaned after part machining. 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 forms a sludge that adheres to every crevice and nook in the machine.
For dust containment all machining is done in a fully enclosed machining compartment with integral dust collection hoods that capture air borne dust, directing it out of the machine into a dust filtration system. Composite slugs, chards and similar material are swept into a debris collection chute with a debris pusher device as part of an automatic cleanup cycle. A set of large HEPA filters cleans the air to a safe level and a dust collection drum makes for easy machine cleanup and dust disposal.
The thermal coefficient of expansion CFRP material is almost zero while the machine tool expansion is primarily governed by the coefficient of expansion of steel. Due to the long length of the parts manufactured, this difference becomes significant enough to adversely impact the hole location accuracy of features machined into the part. To insure manufacture of consistent length beams, thermal length compensation is used to adjust for length variation of the part based on the part and machine temperature.
To verify that parts are machined to tolerance and the end user's desire to measure the parts for dimensional accuracy in the same state in which they were machined, parts are 100% feature inspected in the machine. The inspection probe is used to verify the position of fixtures, datum surfaces of the fixture 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 and the inspection probe calibration software is used to insure 100% reliability of collected inspection data.
Now in full production, the machine drills, mills and saws complete floor beams on all sides in a single setup. The finish machined product is inspected using a full function metrology software package integral to the HMI front end PC. Parts are inspected for dimensional accuracy and statistical data are collected with complete tracking history created for each manufactured part.
The entire floor beam machining system is housed in a specially built facility that is temperature regulated. 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.
While current production rates are already four times faster than by previous methods, continued program optimization and process improvements are being implemented by Hitco with the expectation that floor beam manufacturing times will be reduced by at least another 50%, in the future.
For more information on this application, please contact:
BERTSCHE ENGINEERING CORPORATION
711 Dartmouth Lane
Buffalo Grove IL 60089
Phone: (847) 537-8757
Attention: Rich Bertsche, President