In modern aircraft construction, large quantities of aluminum components are being replaced by more complex parts made of fiber reinforced materials - primarily carbon fiber reinforced plastics (CFRP). Most of these parts are structures on fuselage and wings, which reduces the weight and greatly simplifies assembly and logistics. For example, the fuselage of a Raytheon Hawker 4000, manufactured entirely of composite materials, is composed of a mere seven parts, whereas the version of conventional construction has around 21,000 metal parts. Until recently, most construction was performed manually; now the trend is increasingly leaning toward automation of the processes. Two of these processes are particularly important: automatic tape-laying for flat and slightly curved components such as wing cladding, and fiber placement for more curved components, such as fuselage segments.
In the tape-laying process, tapes 75mm to 300mm wide are rolled on a flat form with each one being cut off appropriately on the basis of the contour at the end of a web. If the configuration is too complex, separate precut tapes are employed. While laying out the tapes, each roll must be placed perpendicular to the surface. Consequently, the machine must possess at least five axes (three linear and two rotary axes).
The Multi-Head Tape Laying Machine affixes Carbon Fiber Reinforced Plastic (CFRP) adhesive tape to seal the fuselage. Perpendicularity is critical to achieve the proper placement and all machine motions are precisely controlled by the CNC onboard.
In addition to the five main axes, there are typically five more axes for the rolls and the cutting equipment. This process is only suitable for flat components; tapes would form wrinkles at bulges. For this reason, the fiber-placement process is utilized in the manufacture of curved components. Here, the "placement tape" usually consists of 32 parallel narrow tapes or cords, each of which can be cut off and threaded back individually in accordance with the contour to be covered. The placement speed is 20m/min to 30m/min. The placement roll is to be oriented perpendicular to the movement of the web, which necessitates at least 6º of freedom. For flatter components, such as wings, the machines employed are similar to those in tape-laying, but with an additional rotary axis in the placement head (three linear axes, three rotary axes). Components such as aircraft fuselage segments that are more curved are manufactured on 7-axis machines.
In addition to the three linear axes and the three rotary axes in the placement head, there is the "mandrel axis", onto which the component is clamped. This additional round table makes it possible to reduce the traversing range of the linear axes. The great weight of the mandrel (30t to 80t) ensures that the movement of the table be as uniform as possible when the tapes are placed in the circumferential direction of the component or diagonally. If the material is placed parallel to the vertical axis of the mandrel and the table is stationary, 6-axis machining must be possible.
When particles up to 6.5m in diameter and 10m long are involved, a slight error in the positions of the table axis compared to the linear axis results in big errors on the surface of the component. This requires a correction mechanism that is simple to handle. High weights and dimensions make it difficult to place the components exactly on the table. Any tipping and shifting must be measured and can be compensated for directly.
REQUIREMENTS AT A GLANCE
A ribbed section of the aircraft wing with the CFRP tape applied.
- Programming in workpiece coordinates for all machine types
- Easy correction of erroneous table positions
- Faulty measurement and on-line correction of erroneous workpiece positions
- Path-dependent control of the cutter and rethreading units of fiber-placement machines
PROGRAMMING IN WORKPIECE COORDINATES FOR ALL MACHINE TYPES
The 5-axis transformation concept has demonstrated its excellence in the milling of complex structural components. It enables the programming of components in workpiece coordinates independently of specific kinematics. The procedure with 5-axis tape layer machines is identical to that with 5-axis fork head milling machines. During this process, the roll is positioned relative to the workpiece, just as during 5-axis circumferential milling. All that has to be considered is that the tool tip customarily rest in the middle of the roll, and the basic orientation of the tool is perpendicular to the head orientation.
In the fiber-placement process, the situation is more complicated. The separate setting of the orientation of the head and the roll in the plane perpendicular to the head orientation require at least six axes and appropriate transformations. Two vectors are needed for the programming of the orientation. As with a 5-axis machine, the first (A3, B3, C3) defines the basic orientation of the placement head relative to the surface of the workpiece. The second (AN, B3, CAN) describes the orientation of the roll in the plane perpendicular to the first vector and normally points in the direction of the roll axis. Customarily, this makes it possible to program the coordinates of the tool tip and the orientations independently of the concrete machine axis configuration. As an alternative, the orientation can also be placed directly on the basis of axes C1, A and C2.
In the example at hand, the mandrel axis A is integrated into the transformation as the 7th axis. The rotation of the A-axis changes the orientation of the placement head relative to the workpiece surface. This is factored in by the transformation, which automatically calculates the correct position of the head relative to the surface whenever the A-axis is rotated. Naturally, this also applies for the tool tip. Just imagine that only the A-axis is being rotated: the machine follows in all linear axes of the A-axis. The tool tip remains in the same position on the surface of the cylinder. Turning the rotary axis also ensures that the orientation of head and roll relative to the cylinder surface remains constant. It is not necessary to forego the benefits of transformations on 7-axis machines either, i.e., for fuselage segments.
CORRECTION OF ERRONEOUS TABLE POSITIONS
An erroneous position of the table or the rotary axes in the placement head can be corrected by appropriately changing the configuration machine data of the transformation. In the case at hand, for example, it is readily possible to compensate for a situation in which the A-axes are not ideally parallel to the X-axis, or the C-axes are not exactly parallel to the Z-axis.
In 5-axis milling, erroneous clamps are corrected by frames that take into account any shifting or rotation of a Cartesian coordinate system compared to the ideal position on the machine. To correct an erroneous position, the real position is determined at the clamp on the basis of three precision balls. From the difference between the real and the ideal measuring points, the integrated function MEAFRAME automatically calculates the shift, rotation and tilt of the workpiece versus its ideal position.
With synchronous actions, it is possible to control the cutters as a function of position. These user-defined actions are processed in synch with the axis movements and handle the actual processing of workpieces in the background. The application times are defined by conditions and are not tied to NC record limits.
The mandrel applies the CFRP tape to the circumference on a rounded section of the fuselage.
Synchronous movement actions are always exerted in interpolation cycle, and it is possible to process several actions in the same cycle. During fiber-placement, it is possible to monitor each individual synchronous movement to ascertain whether a position for cutting or rethreading has been attained. If so, the exits for the cuts are switched directly in the same interpolation cycle, and the lace is cut or threaded in on the fly with positional accuracy. Likewise, during tape-laying, the cutting equipment can be started with positional accuracy.
The onboard Siemens SINUMERIK 840D CNC offers all functionalities required for costeffective tapelaying and fiber placement, and thus the following advantages:
- As with 5-axis milling, an optimal process chain has now also been established for the placement of compound materials
- The component samples are created in workpiece coordinates and processed by the control
- Synchronous actions replace additional hardware for the cutter control
- Erroneous axis positions can be corrected easily without having to create component programs
- The FRAME concept and measurement functions enable the simplest possible corrections of inaccurate component adjustment on the machine.