JEC COMPOSITES MAGAZINE - Issue #92 - October/November 2014 - 60

tooling
coolant channels to provide lubrication
and to reduce the tremendous heat generated by titanium machining, due to its low
thermal conductivity. This makes MQL
absolutely necessary when drilling with
PCD tools, as otherwise excessive heat at
the cutting edge leads to graphitization or
TiC formation respectively. This reaction
leads to chemical wear in the flute region
and, finally, PCD chipping in the flutes.

Experimental study
The developed PCD tools were tested
experimentally with the purpose of evaluating the most suitable PCD grade and
tool geometries for the targeted applications. The following test setup and cutting
parameters were used:
- Test tools: Brazed PCD drills, diameter
11.113mm, with three different PCD
grades (G4, KD1415, and KD1425) and
an uncoated solid carbide drill with the
same geometry.
- Test coupon: 8.7mm thick commercially
purchased CFRP (Isocarbon 3K) plate
securely stacked with a 10.8mm thick Ti6Al-4V plate. The test tools drilled through
holes, entering the CFRP side and exiting
from the titanium side.

and exit surfaces, and two in the Ti layer
in the same manner). The hole exit burr
height on the Ti bottom surface was also
measured.

Results and analysis
A long, predictable, and consistent tool life
is the key factor deciding a tool's success.
When drilling a CFRP/Ti composite matrix, several requirements need to be met
simultaneously to form a good hole. The
hole size must be within tight tolerances
to fit a fastener, and the exit burr has to be
controlled within a certain height to ease
or eliminate deburring. To avoid potential
hole damage by the tool's catastrophic failure and try to keep the tool reconditionable, corner chipping should be minimized
and monitored. The following criteria
were chosen to decide where the test tool
reaches its end of life:
a) Hole size tolerance: 11.113 + 70µm
(H10);
b) Burr size: 0.2 mm;
c) Corner chipping occurrence.
The test results show that corner chipping
was the primary failure mode for the PCD
drills at the end of life. The carbide drill
failed by exceeding burr height specifica-

- Machine tool and coolant: CNC machining centre (Heckert CWK 400) with
a horizontal spindle and through-spindle
MQL coolant (Vascomill MMS FA2).
- Cutting parameters: 20m/min (65
SFM) cutting speed and 0.05mm/rev.
feed rate for both CFRP and titanium. No
pecking cycles were used.
- Tool inspection: To monitor tool wear
progression, test drills were inspected
under a microscope after drilling every 4
holes. A wear mechanism inspection was
conducted under SEM after drilling 24
holes.
- Hole measurement: After all the
machining tests, the test coupons were
cleaned and marked. All the drilled holes
were inspected. The hole diameter was
measured at four depths for each hole
(two in the CFRP layer near the entry

Fig. 2: Diamonds for CFRP Drilling

60 jec composites magazine / No92 October - November 2014

tion. Hole quality requirements could be
met for all drilled holes in this test.
Hole size
Figure 1 shows the hole diameters for a
tested PCD tool. For each inspected hole,
the hole diameter was measured at four
different positions: two in the CFRP section and two in the titanium section, close
to the hole entry surface and the bottom
exit surface respectively.
It can be seen that the hole diameter in the
titanium layer is well within the middle
of the specified tolerance, fluctuating in
a small range of 10μm. The hole size at
the entry of the Ti layer is pretty close
to the size at the hole exit. In the CFRP
layer, however, there is a relatively large
difference in hole size between entry and
exit. This difference is the result of chips
scratching the hole wall during evacuation. Improved chip control is essential to
reduce chip scratching and enhance hole
size consistency. It is difficult to solve the
chip control issue by tool design alone. It
is proven in practice that adding pecking
cycles or vibration-assisted drilling will
help to control chip length and decrease
the scratching effect.

JEC COMPOSITES MAGAZINE - Issue #92 - October/November 2014

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #92 - October/November 2014