JEC COMPOSITES MAGAZINE - Issue #112 - April/May 2017 - 38

AERONAUTlCS tooling
thermal conductivity means all the
heat generated at the point of cutting is
transferred into the tool. And of course
its high strength, retained even at high
temperatures, causes high forces during
cutting.
Each of the materials concerned with
stacks is inherently machined in completely different ways. Therefore, the
tool itself must be a mix of non-ideal
geometries for each material but a good
compromise overall. Then the tool is
to be programmed with different cutting parameters to suit each material,
introducing a pecking cycle through
the metals. This has been achieved with
good results and is the subject of two
studies commissioned by Element Six
and discussed later in this article.
Thermal expansion rates
Besides the different workpiece materials and their relative contradicting
cutting methods and geometries,
other issues also apply to stack material
drilling. Whichever metal material is
stacked below the CFRP layer requires
the metal chips to be drawn out of the
hole by the tool, often passing over the
inner bore of the CFRP and potentially
damaging it. Each workpiece material
exhibits different thermal expansion
rates, making it very difficult to maintain a given tolerance throughout the
entire stack. The use of a Minimum
Quantity Lubricant (MQL) when
drilling through the metal materials is
preferable, though even this is far from
desirable as the oil in the MQL causes
the CFRP dust to agglomerate, causing
further issues itself.
Tool wear when drilling triple stack
materials is often dominated by chipping and cutting edge rounding from
the highly abrasive CFRP element and
adhesion leading to plucking from the
titanium element. Aluminium does not
often influence the cutting tool significantly. Rather, it tends to produce hole
quality issues such as hole size and circularity, and significant exit burrs that
require reworking. Relatively high wear
rates are unavoidable when drilling

38

Fig.1: The Technical
University of
Hamburg, Germany

CFRP. However, PCD offers significant
abrasive wear resistance compared
to WC as a tool material. Chipping of
the extreme outer corner of the drill's
cutting edge is often seen. This is the
weakest area of a drill's geometry, combined with high mechanical loading
when transitioning between CFRP and
titanium elements. Tool makers use various geometrical features, such as radii
and chamfers, in order to protect the
corner. Because of titanium's low thermal conductivity, heat build-up at the
cutting edge often leads to a built-up
edge (BUE), the adhesion of titanium
material onto the tool. In some applications, this build-up of titanium material
can provide a protective barrier from
further abrasive tool wear. The negative
side of this build-up is that the cutting
action suffers greatly, causing high
thrust and cutting forces to be applied
to the workpiece materials. At the point
where the titanium material is forced
off the tool, it can pluck a small portion
of the cutting tool material from the
tool, leaving a crater in the cutting tool
surface.
3D geometries of PCD
Testing in stack materials was commissioned in order to evaluate the latest
3D geometries of PCD available in the
Aero-Dianamics™ range. This work
consisted in triple stack drill testing at
the University of Hamburg, Germany
(TUHH) (Figure 1), and CFRP/
Al stack drilling at the Advanced
Manufacturing Research Centre, UK
(AMRC) (Figure 2).
Testing consisted of parameter optimisation before tool life trials began.
In the triple stack testing, all AeroDianamics™ tools were of the same

jec composites magazine / No112 April - May 2017

Fig.2: The advanced Manufacturing Research
Centre, UK

geometry, and were then compared to
a commercially available veined PCD
tool and uncoated WC tool as a benchmark. In the CFRP/Al stack tests, all
the PCD tools had the same geometry
including the veined PCD product, so
that a direct comparison of the PCD
materials could be made. These were
compared to a CVD coated carbide
and uncoated carbide benchmark. Both
trials used aerospace-grade materials
and utilised typical aerospace quality
criteria.
Solid PCD solutions
During CFRP/Al stack drilling, it was
found that solid PCD solutions provided the best results. Hole quality issues
such as hole circularity, delamination
and aluminium exit burr height were all
used as potential criteria for tool failure.
As was catastrophic failure of the tool
in-process. The cutting conditions
used for CFRP/Al stack drilling with
PCD tooling were: 200m/min cutting
speed and 0.08mm/rev. feed rate; and
for coated WC: 100m/min cutting
speed and 0.06mm/rev. feed rate. A
pecking cycle was used when drilling
through aluminium, including an MQL
lubrication blast. Figure 3 shows the
total quantity of holes drilled by each
tool. Red bars indicate where a tool
has failed due to hole quality issues
Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #112 - April/May 2017
