One-Shot Dry Drilling of Composites/Titanium

TP13PUB77
One-Shot Dry Drilling of Composites/Titanium/
Aluminum Hybrid Stacked Materials in IT8 Quality
authors
PETER MUELLER-HUMMEL
ABDELATIF ATARSIA
MAPAL Inc.
Port Huron, MI
abstract
Low temperatures and intelligent chip removal are the most important requirements for dry
drilling. When drilling metal, such as aluminum or titanium, a lot of heat is required. When
cutting composites, the tool must be as cold as possible and drilling is without external or
internal coolant. A new drill design is now developed and patented for dry drilling of metals at
very low temperatures, giving a new, economical method for one-shot-and-clean drilling of a
hybrid metal and composite assembly.
terms
Dry drilling, Composites drilling, Metal-composite stacked hybrid, Stacked assembly
event
Composites Manufacturing 2013
SME
March 19–21, 2013
Long Beach, CA
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SME Technical Paper TP13PUB77
One shot - dry - drilling of Composites/Titanium/Aluminium
hybrid stacked Materials in IT8 quality
PETER MUELLER-HUMMEL, ABDELATIF ATARSIA*,
MAPAL Inc, 4032 Dove Rd
Port Huron, MI
ABSTRACT
This article describes the physical background and experience in the drilling of carbon fiber and
titanium-aluminum-carbon fiber stacks. Low temperatures and intelligent chip removal
technologies are the most important requirements for dry drilling or the avoid of minimum
quantity lubrication (MQL). The drilling in one shot and in IT8 quality becomes even more
reliable.
This presentation describes the main difference in drilling metal or composite. When drilling
metal, like aluminum or titanium, a lot of heat for the machining is required. When cutting of
composites, the tool must be as cold as possible and drill without external or internal coolant. A
new drill design is now developed and patented by MAPAL that allows the dry drilling of metals
at very low temperatures. We are now able to drill, all batches (composite / titanium / aluminum)
without MMS. The fast drilling and the shorter contact times between the tool and the material
stack also results in approximately twice the life of the tool. The new tool has already qualified
for drilling without MMS with a CPK of 2.4 for an application in the wing assembly. This gives
us new economic method for one-shot-and-clean drilling in the assembly. There are some
important criteria which the dry drilling permits at low temperature. That is what this
presentation of. Most of the tools on the market generate a lot of heat. This tool working
perfectly for cutting metal, but they destroy the integrity of the composite surface and create a lot
of burrs.
1. INTRODUCTION
High cost and residues of oils in the component are often the reason for the request of drilling
without MQL. So far, even with complex coatings systems, dry drilling it was not possible,
because the built-up edge is closing the flute of the tool spontaneously. These special drilling
SME Technical Paper TP13PUB77
tools from MAPAL, compared to other common drills, which were developed from the
traditional metal processing, are working with much less heat.
Another huge problem is the evacuation of carbon dust in combination with MQL. Like dry
powder of cement, the dust of carbon an easily sucked up under dry condition. Only a drop of
water hardens the cement and the MMS causes the same in the composite dust. Even polished
flutes are stuck with oil and dust and hinder evacuation.
2. Experimentation & Results
Dry Drilling of CFRP
Dry machining of CFRP materials has long been the state of the art. Besides lower costs for the
tool, the machine and cleaning peripheral processes, the process can be run much more secure.
So the dry carbon fibre dust can be even without additional ventilation in the tool, succeed out of
the hole.
2.2 Dry Drilling of CFRP/Aluminium Stacks
Out of the comparison between machine Composite and metal the most critical difference is the
heat resistance of the two materials. Composite starts to pyrolysis by more than 150°C. The next
main difference is the physical modelling of machining metal and machining composite.
Machining metal can be described with the formula of calculation on continuum mechanic.
Machining composite follows the rule of fracture mechanics. Comparing both physical models
we can see that continuum mechanic is always a function of the temperature. Fracture mechanics
is not like that.
E.g. Cutting the butter is also continuum mechanic. Cutting the cold butter with a cold knife is
hard to do. We have to heat up the butter or even the knife to lower the cutting force.
Now all tools we have developed for the aerospace in the past are tools for cutting the metal. The
more these tools are generating heat the better they are cutting the metal.
Using this kind of tools for cutting composite the generate heat will lead to a heat infected zone
on the composite part and let the tools been totally overheated.
The machining of composite is like peeling the eggs. The spoon is not sharp and the crack in the
shell comes from the impact of the spoon against the egg. There is no heat necessary to break the
shell of the egg therefore at all.
SME Technical Paper TP13PUB77
All traditional tools in the aerospace following the principle of the continuum mechanics for
cutting metal. This means that these tools generate a lot of heat. In order to reduce the
temperature, MAPAL discovered all heat generation zones. Only the heat on the very tip is
essential for the drilling. All the other zones can be optimized to remain cold. Under these
conditions no MQL is necessary at all.
Dry cutting of Composite / Metal hybrid stacks appears with a lot of new possibilities for making
the process more economic.
Dry dust of carbon can easily removed and the tools remaining clean. Using just a small quantity
of oil for the MQL carbon dust will change into a paste. Like powder of dry cement can easily be
removed by a vacuum cleaner. Putting some drops of water on it will get hard like stone.
We were able to qualify these tools on a power feed unit for aircraft components for flying
applications. The general CPK was better than 2.1.
Picture1: Results of dry drilling Composite/Aluminium stack
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2.3 Drilling of CFRP/Titanium
All know-how of today is required to dry drill CFRP/Titanium process safe in one operation.
Figure 2 shows the requirements for the process model. The solution leads to a drill geometry
with which all physical characteristics required can be met at high feed rates.
Figure 2:
Understanding the process leads to process-safe tools
While drilling of CFRP-titanium stacks beside the burrs and the chip flow is to be considered in
the design of the tool. The chips must not jam at the exit from the hole on the sensitive carbon
fibre surface. The necessary flute of the tool has to be designed significantly greater than the
single chip volume size, which prevents from scratches in the composite material. If the chip in
the flute of the tool area jammed, it comes to an extreme friction and heating of the component
and tool. Deep scratches in CFP and an increase in burr formation is the result. Figure 3
SME Technical Paper TP13PUB77
Figure 3: Drilling of CFRP/Titanium stacks in good quality
This model of burr generation shows us that also in traditional drilling of metal only low heat is
appreciated during the drilling.
In an extensive study, all the heat generating areas of the drilling tool has been determined.
Exclusively in the area in front cutting edge the heat for the continuous cutting is required. In all
other areas the heat gets generated by friction. We build a special the tool design to avid that
waste of heat (Figure 4).
SME Technical Paper TP13PUB77
Figure 4: Tools for dry milling of CFRP and dry drilling of CFRP/Titanium stacks
When drilling into metal, not the forces at the tip, but the temperatures at the edges of the drill,
causing the formation of burrs [1]. The maximum temperature occurs at the drill tip. The heat
wave in front of the drill accumulates at the exit hole and the material softens. The axial pressure
then is sufficient to generate the burr.
A tool which only heats up the relevant area at the tip independently and under minimal heat
generation, was not easy to develop. With such a tool the dry drilling of carbon fibre / metal
composites is possible now. Polished surface of the flutes and the part are remaining clean and
no sticky dust particles hinders the chip and dust removal. This will be similar like for the orbital
drilling [2]. This new tool concept has already qualified for a flying application.
To achieve an optimized tool life and process performance, the processing conditions should be
aligned with the requirements of the work piece material. To reduce the single chip volume size,
stutter-feeding strategy (Pecking / Vibration) has been tested successfully.
Another way to increase the lifetime and efficiency is the use of PCD or CVD diamond coatings.
This should be under cutting speeds lower then 30 m / min. Otherwise, the diamond will react
with titanium into TiC [3].
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3. Conclusions
The new generation of cutting tools are capable for dry cutting of CFRP/Titanium/Aluminum.
The continuous development of tools who are able to cut under very low heat conditions was
successful. A higher tool life and a clean environment emphasizes the economic benefit on this
technology. The paper shows also some results from the finalized qualification on a wing FAL
application
4. References
[1]
DORNFELD, D.; AVILA, M.; ARDNER, J.; REICH-WEISER, C.; VIJAYARAGHAVAN, A.: Strategies
for Burr Minimization and leanability in Aerospace and Atomotive Manufacturing. Consortium on Deburring
and
Edge
Finishing,
eScholarship
Repository,
University
of
California,
Berkeley,
http://repositories.cdlib.org/lma/codef/avila 06 1, 2006.
[2]
FANGMANN, S.; ET AL.: Orbitalbohren geschlossener Strukturteile erfordert optimierte Werkzeuge,
Maschinenmarkt, 06. 07. 2007.
[3]
MÜLLER-HUMMEL, P.; LAHRES, M.: Infrared Temperature Measurement on Diamond-Coated Tools
during Machining. In: Bachmann, P. K.; Bruckley-Golder, I.M.; Glass, I. T.; Kamo, M.: Proc. of 4th.
European Conference on Diamond, Diamond-like and Related Materials; Albufeira, Portugal, Elsevier
Sequoia S.A., Lausanne, SSDI 0925-9635(93)05111-0, S.: 765, 1993.

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