JEC COMPOSITES MAGAZINE - Issue #108 - October 2016 - 56

TECHNOLOGY repair
is used to detect the topology of the area, from which the adaptive
milling software calculates a tool path for the laser line scanner. In a
next step, the line scanner is used to detect the 3D geometry of the
component with a high degree of accuracy.
For easy-to-scan surfaces, a red-light laser line scanner is used. This
scanning equipment is sufficient for most CFRP applications. For
surfaces that are more difficult to scan, an enhanced blue-light laser
line scanner is preferable. This scanning method has shown greatly
improved scanning precision compared with a red-light laser scanner operating on semi-transparent surfaces such as GFRP.

Fig. 2: Generic
algorithm for the
generation of nominal NC tool paths for
stepped and tapered
scarfings of any
shape

Geometrically-adaptive machining
The scanning data collected in previous process steps constitutes
the basis for adaptive processing of the component. First, a 3D surface model, reproducing the actual geometry of the damaged area, is
generated from the scan data. As a next step, the target geometry is
calculated. Multiple factors can be considered when calculating this
geometry, which describes the shape of the part after milling. Besides the actual part geometry, the nominal part geometry and other parameters for the geometry can be used to influence the target
geometry. These can include, for example, factors such as minimum
wall thickness or aerodynamic parameters that must be maintained
to ensure the functionality of the finished part.
Various methods can then be used to generate a master milling program that describes the NC tool path without reference to actual
component geometry. It can be calculated on the basis of the CAD
model of the component - e.g. for adaptive post-machining - or using a dedicated software for the generation of master NC programs
- e.g. for scarfing applications (see next section). In this case, the
3-axis master milling program is then transferred to the 3D surface
geometry of the scanned part. The geometrically-adapted milling
programs have 5 axes and run parallel to the contours of the actual
geometry of the damaged area.
The final result of the adaptation process is an NC program adapted
to the actual scanned geometry. In contrast to a fixed NC program,
the milling tool thus follows the actual component contours and not
a fixed path. To calculate the geometrically-adapted NC program,
the tool centre point is computed according to the displacement
of the corresponding tool touch point. This is possible through the
knowledge of the actual part geometry as well as tool geometry and
orientation. This step eliminates the need for machine-internal tool
calculations and offsets. The NC code for a point to be milled can
consist of a single line of code, making the adaptive milling process
compatible with virtually all NC machines and independent of path
strategies or machine technologies.

imported, it allows for the calculation of a patch geometry as needed. Both stepped and tapered scarfings can be generated, as illustrated in Figure 2.
Based on the calculated geometry or an imported geometry, the
master milling program is created. As described in the previous sections, this master milling program is then adapted to the part geometry, machine kinematics and transferred to the machine or robot
for milling. Some examples of scarfing geometries and their application on FRP components are shown below (Figures 3-7).

Fig. 3: Circular scarfing on a commercial aircraft fuselage skin

Fig. 4: Elliptical scarfing on a helicopter rotor blade

Automated scarfing for composite repair
Based on the adaptation process described above, a repair process
for FRP components was developed. The adaptation software package runs on a PC and is connected via LAN to the NC control of the
multi-axis stationary or mobile machine, or robot.
Within the software package for composite repair, a generic patch
geometry generator was implemented. If no scarfing geometry is

jec composites magazine / No108 October 2016

Fig. 5: Rectangular scarfing on a helicopter fuselage

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #108 - October 2016