JEC COMPOSITES MAGAZINE - Issue #108 - October 2016 - 57

Automated repair preparation and post-machining of composites

Fig. 9: Sacrificial machining for optimized
bond thickness
Fig. 6: Special scarfing geometries for boundary areas

Fig. 7:
Fibre-oriented scarf

Tests were also conducted for geometrically challenging repair applications, where standard scarfing geometries cannot be applied.
Damages near the edges of FRP components (e.g. trailing edges)
were a common problem. Frequently, the available surface area is
not large enough to accommodate a uniformly-shaped patch. A
special scarfing geometry was developed that satisfies the structural
requirements in these areas while maintaining minimum material
thicknesses and taking account of other factors. Such scarfing geometries are shown in Figure 6.
New geometries for fibre-oriented joints and repairs are becoming
possible. Figure 7 shows a fibre-oriented scarf in a CFRP plate. With
such improved scarfing geometries, repair areas could shrink since
the bonding area is reduced significantly without losing structural
strength.
For components of reasonable size, e. g. in the case of reworking
during the manufacturing phase or for the stationary repair of helicopter rotor blades or aircraft-related components such as radomes,
flaps, etc., regular stationary machine tools or robots can be used as
a basis for an automated scarfing system.
For on-aircraft repairs or very large components, a mobile machine
is the most suitable solution. Figure 8 shows the "mobileBLOCK"
machine from DMG MORI SAUER ULTRASONICS GmbH,
which is equipped with BCT software for automated scanning, tool
path generation, adaptation and 5-axis milling.

Adaptive post-machining applications
The necessity to compensate for geometrical variances does not only
occur in the repair and reworking of composite components. Another situation where precise surfaces are needed is the joining of composite components. Currently, the manual shimming process often
required when bonding CFRP components constitutes a bottleneck
in the production of such components. To achieve minimum weight
and maximum structural performance, the goal is to produce a bond
that is as thin as possible.
This is extremely difficult to achieve, owing to the aforementioned
shape deviations displayed by composite components. For this reason, shims are commonly used to permit bonding of such components.
Adaptive machining and measuring processes can be utilized in two
different ways to optimize the bonding process. One of these consists in scanning the two surfaces to be bonded, and then calculating
a shim geometry that will be milled, created with additive manufacturing, or produced with any other suitable technology.
The other approach is sacrificial machining, where one or both surfaces to be bonded are manufactured with some additional material. When this material thickness is known and both surfaces are
scanned, it is possible to calculate the ideal matching geometry,
which can then be milled into the components, as illustrated in
Figure 9.
As described in the previous sections, limitations and requirements
related to the geometry of the shim or the components can also be
considered. For example, the maximum milling depth into the components can be defined to ensure structural integrity, or a minimum
wall thickness can be specified.

Conclusion
During all phases of FRP manufacturing and repair, deviations of
composite components from the nominal shape play a decisive role.
Adaptive machining compensates for both individual deviations
from the nominal shape and inaccurate clamping positions. This
makes it possible to automate geometrically-critical NC processes such as the post-machining, reworking and repair of composite
structures. The process of adaptive machining in composite applications has been successfully demonstrated on both mobile and
stationary machines as well as on robots. Additional steps such as
surface treatment have been integrated and tested successfully, facilitating subsequent process steps. It can thus be said that adaptive
machining demonstrates great potential for improved efficiency and
quality in the post-machining, reworking and repair of composite
components.
More information: www.bct-online.de
Contact: c.bremer@bct-online.de

Fig. 8: Mobile
milling machine

No108 October 2016

/ jec composites magazine

http://www.bct-online.de

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