U.S. patent application number 14/430164 was filed with the patent office on 2015-10-01 for drilling tool and device for drilling with cryogenic cooling and method for drilling a stack of heterogeneous materials.
The applicant listed for this patent is EUROPEAN AERONAUTIC DEFENCE AND SPACE COMPANY EADS FRANCE. Invention is credited to Guillaume Abrivard, Daniel Aliaga, Dominique Schuster, Fernand Vinhas.
Application Number | 20150273597 14/430164 |
Document ID | / |
Family ID | 47594906 |
Filed Date | 2015-10-01 |
United States Patent
Application |
20150273597 |
Kind Code |
A1 |
Aliaga; Daniel ; et
al. |
October 1, 2015 |
DRILLING TOOL AND DEVICE FOR DRILLING WITH CRYOGENIC COOLING AND
METHOD FOR DRILLING A STACK OF HETEROGENEOUS MATERIALS
Abstract
A drill bit having an interior canal for liquid nitrogen to pass
longitudinally through the body of the drill bit. The canal has, on
the side of a cutting edge of the drill bit, at least one liquid
nitrogen ejection duct that opens near the cutting edge which is
formed by an insert made with polycrystalline diamond fixed to the
body of the drill bit. A device for drilling a metal-composite
stack includes the drill bit, a liquid nitrogen production unit and
a distribution network to distribute the liquid nitrogen. The
device drills through a metal-composite stack in a single pass of
the drill bit. The liquid nitrogen at cryogenic temperature is
conveyed close to the cutting edge, at least while the cutting edge
is in contact with the metallic material.
Inventors: |
Aliaga; Daniel;
(Aubervilliers, FR) ; Schuster; Dominique; (Saint
Germain En Laye, FR) ; Vinhas; Fernand; (Saint
Germain En Laye, FR) ; Abrivard; Guillaume; (Amiens,
FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EUROPEAN AERONAUTIC DEFENCE AND SPACE COMPANY EADS FRANCE |
Paris |
|
FR |
|
|
Family ID: |
47594906 |
Appl. No.: |
14/430164 |
Filed: |
September 19, 2013 |
PCT Filed: |
September 19, 2013 |
PCT NO: |
PCT/EP2013/069506 |
371 Date: |
March 21, 2015 |
Current U.S.
Class: |
408/1R ;
408/57 |
Current CPC
Class: |
Y10T 408/03 20150115;
B23B 2226/315 20130101; B23B 2222/88 20130101; Y10T 408/45
20150115; B23B 2228/36 20130101; B23Q 11/1053 20130101; B23B
2226/275 20130101; B23B 35/00 20130101; B23Q 11/1023 20130101; B23B
2250/12 20130101; B23Q 11/1061 20130101; B23B 51/0493 20130101 |
International
Class: |
B23B 51/04 20060101
B23B051/04; B23Q 11/10 20060101 B23Q011/10 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 21, 2012 |
FR |
1258901 |
Claims
1-12. (canceled)
13. A drill bit for drilling a stack comprising at least one layer
of a metal material and at least one layer of a composite material
comprising fibers held in a hardened matrix, the drill bit
comprising: a tail end configured to be held on a rotary driving
machine; at least one cutting edge at an end, along a length of the
drill bit, opposite the tail end, said at least one cutting edge
being formed by an insert made with polycrystalline diamond and
attached to a body of the drill bit; and a liquid nitrogen flow
channel passing internally through the body of the drill bit, the
liquid nitrogen flow channel comprising at least one liquid
nitrogen ejection duct that opens close to said at least one
cutting edge of the drill bit.
14. The drill bit as claimed in claim 13, further comprising a
plurality of cutting edges, each cutting edge comprising at least
one liquid nitrogen ejection duct that opens close to said each
cutting edge.
15. The drill bit as claimed in claim 14, wherein said each cutting
edge comprises at least one liquid nitrogen ejection duct that
opens on a cutting face of said each cutting edge and at least one
liquid nitrogen ejection duct that opens on a wear face of said
each cutting edge.
16. The drill bit as claimed in claim 13, wherein the liquid
nitrogen flow channel is substantially axial in the body.
17. The drill bit as claimed in claim 13, wherein the liquid
nitrogen flow channel is insulated internally by a layer of a
thermally insulating material over at least one portion of a length
of the liquid nitrogen flow channel.
18. The drill bit as claimed in claim 13, wherein the liquid
nitrogen flow channel of the drill bit comprises an enlarged cross
section that forms a reservoir in a region of the body located near
said at least one cutting edge.
19. The drill bit as claimed in claim 13, wherein the metal
material of the stack comprises a titanium-based alloy; and wherein
the liquid nitrogen flow channel and the liquid nitrogen ejection
duct are sized to provide a flow rate of liquid nitrogen,
substantially at a temperature of 77 kelvin to keep said at least
one cutting edge at a temperature below a temperature for
converting the polycrystalline diamond of the insert into a
graphite.
20. A drilling device, comprising the drill bit as claimed in claim
13, a liquid nitrogen production unit and a distribution system to
distribute liquid nitrogen from the liquid nitrogen production unit
to the drill bit.
21. A drilling method for drilling a stack comprising at least one
layer of a metal material and at least one layer of a composite
material comprising fibers held in a hardened matrix, comprising
the steps of: drilling the stack in one pass with a drill bit
comprising at least one cutting edge formed by an insert made with
polycrystalline diamond and attached to a body of the drill bit;
and transporting liquid nitrogen at cryogenic temperature close to
said at least one cutting edge in contact with the metal material
through a channel passing internally through the body of the drill
bit and through at least one ejection duct that opens close to said
at least one cutting edge.
22. The drilling method as claimed in claim 21, further comprising
the step of transporting the liquid nitrogen simultaneously on a
cutting face and on a wear face of said at least one cutting
edge.
23. The drilling method as claimed in claim 21, further comprising
the step of transporting the liquid nitrogen into the channel as a
function of a measured or estimated position of the drill bit with
respect to the stack.
24. The drilling method as claimed in claim 21, wherein the metal
material of the stack comprises a titanium-based alloy; and further
comprising the step of transporting the liquid nitrogen with a flow
rate sufficient to keep a temperature of the drill bit below a
temperature for converting the polycrystalline diamond into
graphite.
Description
[0001] The present invention belongs to the field of tools and
devices intended for drilling materials.
[0002] In particular, the invention relates to a drill bit and a
drilling device for drilling into stacks of metal materials and
composite materials such as those produced during the assembly of
aircraft structures.
[0003] The production of structures having high mechanical
performances and weights that are as low as possible leads the
designers of the structures to use titanium-based metal alloys and
composite materials comprising carbon, glass or aramid fibers held
in a hardened organic matrix, in general a polymer resin.
[0004] When these parts must be drilled or reamed in order to
produce assemblies, the drilling conditions must be adapted as a
function of the material machined.
[0005] Thus, titanium alloys, the thermal conductivity of which is
low, around ten times lower than that of an aluminum, are generally
drilled using tools having substrates made of tungsten carbide (WC)
with cobalt binder that withstand temperatures that may reach
1000.degree. C. during the drilling using an oily lubricant. These
high temperatures are the cause of accelerated wear of the drill
bits used for the drilling.
[0006] On the other hand, composite materials, which have high
abrasive characteristics, in particular in the case of carbon
fibers, which damage the drilling tool by abrasion, are generally
dry drilled with tools made of tungsten carbide with diamond or
diamond-coated inserts, preferably polycrystalline diamond (PCD)
inserts.
[0007] Owing to these very different drilling conditions and to the
need to use drill bits specific to each of these conditions, the
drilling of a stack of parts made of materials of a titanium alloy
and of a carbon fiber composite proves difficult.
[0008] In particular, owing to the fact that composite materials,
for the most common ones, should not locally exceed a glass
transition temperature of the resin, in the case of the most common
ones a temperature of the order of 180.degree. C., and that the
tools for drilling composite materials are not suitable for
drilling titanium alloys due to the temperature reached which would
damage the tool, the graphitization of the diamond taking place at
around 800.degree. C., the one-step drilling of a stack of these
various materials is in general carried out with a tool suitable
for drilling titanium while increasing as much as necessary the
amount of lubricant in order to limit the increase in
temperature.
[0009] The drilling of the part of the stack made of composite
material is then penalized by an accelerated wear of the drilling
tool via abrasion.
[0010] The invention provides a solution to these various problems
by means of a drill bit, equipped for drilling a stack comprising
at least one layer of a metal material, for example a
titanium-based alloy, and at least one layer of a composite
material comprising fibers held in a hardened matrix, for example
carbon fibers in a cured organic resin matrix. The drill bit
comprises one liquid nitrogen flow channel, at least, inside the
drill bit, that passes through a body of the drill bit along an
axis which for example corresponds substantially to a rotational
axis of the drill bit during a drilling operation, the channel
opening for example near a tail end via which the drill bit is
intended to be held on a rotary driving machine, the channel
comprising near at least one cutting edge of the drill bit, which
edge is located at an end opposite the tail end along a length of
the drill bit, at least one liquid nitrogen ejection duct opening
close to the at least one cutting edge, the at least one cutting
edge being formed by an insert made with polycrystalline diamond
and attached to the body of the drill bit.
[0011] The polycrystalline diamond tipped drill bit thus formed
makes it possible, by being combined with means for distributing
cryogenic liquid nitrogen into the channel, to drill metal
materials releasing a large amount of thermal energy that is
difficult to remove without causing excessive heating of the tool
that is capable of very rapidly damaging the tool.
[0012] The customary wear via abrasion of the composites is slowed
down by the use of PCDs.
[0013] In one embodiment, the drill bit comprises a plurality of
cutting edges and each cutting edge comprises at least one liquid
nitrogen ejection duct that opens close to the cutting edge in
question so that the liquid nitrogen is concentrated toward the
corresponding cutting edge during a drilling operation.
[0014] In one embodiment, each cutting edge comprises at least one
liquid nitrogen ejection duct that opens on a cutting face of this
cutting edge and at least one liquid nitrogen ejection duct that
opens on a wear face of this cutting edge which makes it possible
both to increase the flow rate of liquid nitrogen cooling the edge
in question and also to spread the liquid nitrogen that provides
the cooling of this cutting edge over the two faces of the insert
forming the cutting edge.
[0015] The liquid nitrogen flow channel is for example axial which
makes it possible, via a straight channel having a diameter that is
as wide as possible, to reduce the pressure drops and to facilitate
the circulation of the liquid nitrogen.
[0016] In order to improve the effectiveness of the liquid nitrogen
cooling by limiting the thermal losses before the liquid nitrogen
arrives close to the cutting edge(s), the channel of the drill bit
is insulated internally by a layer of a thermally insulating
material over at least one portion of a length of the channel.
[0017] In one embodiment, the channel of the drill bit comprises an
enlarged cross section that forms a reservoir in a region of the
body located near the cutting edge(s). Thus, during operation of
the device a reservoir of liquid nitrogen is formed that provides
an increased cooling mass close to the heating region and a buffer
volume of liquid nitrogen in the event of failure in the supply of
liquid nitrogen or in the event of premature cut-off of the liquid
nitrogen at the end of the drilling of a metal material.
[0018] Advantageously, the liquid nitrogen flow channel that passes
through the body and the liquid nitrogen ejection ducts are sized
so as to provide a flow rate of liquid nitrogen, substantially at
the temperature of 77 kelvin, sufficient for keeping the cutting
edge(s) at a temperature below a temperature for converting the
polycrystalline diamond of the insert(s) into graphite when the
drill bit is used for drilling into a titanium-based alloy
according to the intended cutting conditions so that the one-step
drilling of a stack comprising a titanium-based alloy is
industrially possible.
[0019] A drilling device, for drilling a stack comprising at least
one layer of a metal material and at least one layer of a composite
material comprising fibers held in a hardened matrix, comprises a
drill bit as claimed in one of the preceding claims, a liquid
nitrogen production unit and a system for distributing liquid
nitrogen from said production unit to the drill bit so that the
device ensures the cooling of the drill bit when drilling is in
progress.
[0020] The invention also relates to a drilling method, suitable
for drilling a stack comprising at least one layer of a metal
material, for example a titanium-based alloy, and at least one
layer of a composite material comprising fibers held in a hardened
matrix, for example carbon fibers in a cured organic resin matrix,
in which drilling is carried out in one pass of a drill bit
comprising at least one cutting edge, formed by an insert made with
polycrystalline diamond and attached to a body of the drill bit,
and in which liquid nitrogen at cryogenic temperature is
transported close to the cutting edge by a channel of the drill
bit, opening via at least one ejection duct close to the cutting
edge, when the cutting edge formed by the insert made with
polycrystalline diamond is in contact with the metal material
during the drilling of the stack.
[0021] The drilling of the stack comprising metal materials with a
tool suitable for drilling composite materials is thus made
possible by the method.
[0022] In order to ensure sufficient cooling of the cutting edges
and to enable relatively rapid drilling of metal materials, the
liquid nitrogen is sent close to the cutting edge simultaneously on
a cutting face of the cutting edge and on a wear face of the
cutting edge, and this being the case for each cutting edge.
[0023] In order to limit the liquid nitrogen consumption of a
drilling operation if necessary, the liquid nitrogen is sent into
the channel as a function of a measured or estimated position of
the drill bit with respect to the stack when the position
determines that the drilling of the metal material is in progress
or likely, it being possible for such a condition to be obtained
from a knowledge of the characteristics of the drilled stack and
for measuring or estimating the position of the end of the drill
bit with respect to a frame of reference of the stack in the course
of drilling.
[0024] In order to ensure a satisfactory service life of the drill
bit used for drilling into metals having poor heat conduction, the
liquid nitrogen is transported with a flow rate sufficient for
keeping the temperature of the drill bit below a temperature for
converting the diamond into graphite when said drill bit drills a
titanium-based metal alloy.
[0025] The present invention is described with reference to the
figures which, nonlimitingly, schematically represent:
[0026] FIG. 1: a drilling device according to the invention;
[0027] FIG. 2: along a longitudinal cross section, an example of a
drill bit used in the drilling device of FIG. 1
[0028] FIG. 3: the steps of a method for drilling a stack using the
drilling device of FIG. 1;
[0029] FIG. 4: an example of fitting a drilling device to a
portable drilling unit in position on a drilling grid.
[0030] The figures are not to scale, both among the figures and
among the parts of one and the same figure, in order to facilitate
the understanding thereof and similar parts of different figures
bear identical references.
[0031] The device 100 represented in FIG. 1 comprises a drill bit
101, a unit 102 for producing low-temperature nitrogen in the
liquid state and a system 103 for distributing liquid nitrogen from
the production unit 102 to the drill bit 101.
[0032] The production unit 102 consists of any means that can
produce liquid nitrogen substantially at ambient pressure, that is
to say cryogenic nitrogen at a temperature of the order of 77
kelvin, with the flow rate desired as a function of the operating
conditions of the drill bit 101. In a simple embodiment, the
production unit 102 consists of a reinforced thermally insulated
tank of cryostat type containing a stock of liquid nitrogen. In
another embodiment, the production unit consists of an apparatus
for producing liquid nitrogen by condensation of atmospheric
nitrogen, for example using a Stirling cycle machine.
[0033] The distribution system 103 consists of any means capable of
conveying the liquid nitrogen from the production unit 102 to the
drill bit 101 and of controlling the flow rate thereof.
[0034] In particular, the distribution system 103 comprises at
least one liquid nitrogen delivery duct, a portion of said duct
being advantageously incorporated into a rotary driving machine,
not represented in FIG. 1, which rotates the drill bit 101.
[0035] Such a rotary driving machine is for example a fixed
drilling machine, to which parts forming a stack that has to be
drilled are clamped, or a portable drilling unit used at the
assembling stations of large structural assemblies, a station for
joining an aircraft wing to the fuselage for example.
[0036] The term "stack" will be used here generically to denote a
region of a structure comprising two or more parts comprising
different materials, and/or two or more different materials,
including at least one metal material and at least one composite
material, in the path of a drilling operation to be carried
out.
[0037] The drill bit 101 comprises a body 10 of overall cylindrical
shape having an axis 11 corresponding to a longitudinal axis of the
drill bit about which said drill bit is rotated during drilling
operations.
[0038] A first end of the body 10 forms a tail end 12 via which the
drill bit is attached to a rotary driving machine, where
appropriate by means of a mounting cone not represented, and a
second end of the body, opposite on the body 10 to the first end,
comprises one or more cutting edges 13 as illustrated in the detail
a) of FIG. 1 of a drill bit with two cutting edges.
[0039] As illustrated in FIG. 2, schematically representing the
drill bit in axial cross section, a channel 20 inside the body 10
passes through said body along a substantially axial length and
opens, on the one hand, in a region of the tail end 12 at at least
one liquid nitrogen inlet opening 21 and on the other hand, close
to the cutting edge(s) 13 at the nitrogen ejection ducts 22.
[0040] In one preferred embodiment, each cutting edge 13 is
associated with at least one ejection duct opening onto a cutting
face 131 of said cutting edge and with at least one ejection duct
opening onto a wear face 132 of said cutting edge, detail a) of
FIG. 1.
[0041] Diamond inserts 30, in practice inserts made with
industrially produced polycrystalline diamond (PCD), are attached
to the body 10 so as to form the cutting edges 13.
[0042] In one embodiment, the liquid nitrogen inlet opening 21 is
arranged on one face of the first end of the drill bit 10
substantially centered on the axis 11.
[0043] In one embodiment, the channel 20 has an enlarged cross
section over one portion at least of a length of said channel so as
to form a reservoir 23.
[0044] In one embodiment, the channel 20 comprises a thermally
insulating coating 24, for example a sheath made of an alloy having
a high content of chromium and nickel, such as Invar.RTM., or a
non-metallic material, for example cork or a polymer material such
as polytetrafluorocarbon, so that the material constituting the
body 10 is not in direct contact with the liquid nitrogen passing
through said channel and limits thermal exchanges at the body 10.
In the exemplary embodiment illustrated, the thermally insulating
coating 24 only affects a portion of the channel 20 from the inlet
opening 21 to an uninsulated region 25, it being possible for said
region for example to correspond to the reservoir 23 when the drill
bit is provided with such a reservoir.
[0045] A transverse cross section of the channel 20 is in practice
as big as possible, without however weakening the mechanical
strength of the drill bit 101 to a point that could result in
rupture of said drill bit under forces expected during a drilling
operation, in order to favor a high flow rate of liquid nitrogen
passing through the body 10 in order to keep the temperature at the
cutting edges at an acceptable value for the drill bit. The
transverse cross section of the channel 20, in the same way as a
cross section of the reservoir 25, limited by the depth of the
flutes of the drill bit, is for example determined by drill bit
mechanical strength calculations.
[0046] The advantages of the device 100 which has just been
described will be better understood from the description of the
drilling method 500, FIG. 3, using said device.
[0047] In a first step 510, the device 100 is mounted 510 on a
drilling unit 40, for example an independent drilling unit as
represented in FIG. 4, that can be transported to an assembling
station and moved to various locations where drilling must be
carried out through a stack 41 of parts 411, 412 comprising metal
materials, in particular based on titanium such as an alloy
Ti6A14V, and composite materials with mineral fibers, in particular
carbon fibers. In practice, the drill bit 101 is attached to a head
for rotating and advancing the drilling unit suitable for enabling
a flow of liquid nitrogen to the inlet opening 21 of the drill bit
and the drilling unit 40 is connected to the distribution system
103 for example by means of a flexible connector 104 in order to
deliver the liquid nitrogen.
[0048] At the end of this first step, the device 100 and the
drilling unit 40 form a cryogenic drilling unit comprising a source
of liquid nitrogen 102 at a pressure close to ambient pressure,
means for rotating, where appropriate for advancing the drilling, a
drill bit in accordance with the drill bit 101 described above, and
the liquid nitrogen distribution system 103 transporting liquid
nitrogen from the tank to the drill bit, passing through a head of
the drilling unit via which the drill bit is attached to said
drilling unit.
[0049] In a second step 520, the drilling unit 40 and the stack 41
through which drilling must be carried out, are clamped in the
desired relative position, for example by means of a drilling grid
42.
[0050] In a third step 530, the drilling is initiated, that is to
say that the drill bit 101 is rotated and that an advance, that is
to say an axial displacement movement in the direction of the
assembly to be drilled, is started.
[0051] During this third step 530, liquid nitrogen at cryogenic
temperature is sent 531 into the channel 20 from the production
unit 102 via the distribution system 103 at least into the
positions of the drill bit 101, the advance of which corresponds to
drilling into a metal.
[0052] In practice, the knowledge of the assembly to be drilled
makes it possible to determine for which penetration depths of the
drill bit the cutting edges of the second end of the drill bit are
in contact with a metal material.
[0053] The position of the drill bit along the direction of advance
corresponding to the penetration depth is for example obtained by a
signal from a sensor used to activate a valve 105 of the
distribution system 103.
[0054] The flow of nitrogen transported to the cutting edges 13 is
determined in order to maintain a temperature of the drill bit 101
at said cutting edges of below 800.degree. C. when the material
drilled is a titanium alloy, the case considered here to be the
most constraining, in practice below a temperature for which the
diamond of the drill bit could be converted into graphite.
[0055] In this case, care will be taken to take into account all
the uncertainties of measurement by the sensor(s) and the deviation
of the thicknesses of the various materials of the stack to be
drilled so that the liquid nitrogen arrives from the start of the
drilling of a metal material, preferably at least a short time
before the start of the drilling of the metal material, and
continues to the end of this drilling, preferably at least a short
time after this end of drilling, in order to prevent heating of the
drill bit.
[0056] In one embodiment, less economic in terms of liquid
nitrogen, liquid nitrogen is sent throughout the drilling operation
of the stack from the start of the advance to the withdrawal of the
drill bit, or at least up to a step 540 in which the advance
movement is reversed in order to free the drill bit from the
drilling made.
[0057] When the drilling operation 500 is finished, the drilling
unit is stopped and is separated 550 from the drilled assembly.
[0058] Another drilling operation can then be carried out in which
the step 510 is not necessarily carried out when the drilling unit
40 is simply moved, for example to another position of the same
drilling grid 42, without having been separated from the device
100.
[0059] Thus, contrary to the accepted principles for drilling metal
materials, in particular titanium alloys, it has been made possible
to use a polycrystalline diamond (PCD) tipped drill bit, the
temperature of which during the drilling is kept well below
temperatures which would have led to the destruction of the
tool.
[0060] Such a polycrystalline diamond tipped drill bit is very
suitable for drilling highly abrasive composite materials, such as
carbon fiber composite materials, and consequently the drilling of
the stack of various metal and composite materials is carried out
in a single pass with the same drill bit.
[0061] This possibility results in a significant time saving,
estimated at around 40%, over a drilling cycle time.
[0062] Moreover, the service life of the drill bit is also
increased by a factor of three on average.
[0063] The use of a cooling of the drill bit with liquid nitrogen
transported as close as possible to the cutting edges, that is to
say as close as possible to the creation of thermal energy during
the drilling, proves to be much more effective than with known oily
lubricants. The service life of the drill bits 101 of the invention
are in particular increased by an estimated factor of three under
industrial drilling conditions compared to drill bits having a
tungsten carbide substrate that are used with oily lubricants for
drilling such stacks.
[0064] The liquid nitrogen used is an inert substance which
presents no problems with respect to the environment, which is not
the case for oily lubricants.
[0065] The cost of the liquid nitrogen is also much lower than that
of the oily lubricants, especially since for the latter is
necessary to take into account the costs linked to the reprocessing
of the lubricants that have been used.
[0066] Liquid nitrogen, besides its chemical inertness, is
vaporized almost instantaneously during a drilling operation and
the result of this is increased safety for the operators.
[0067] In the case of a drill bit 101 comprising a reservoir 23
close to the second end of said drill bit, the reservoir is filled
with liquid nitrogen as soon as the liquid nitrogen is sent into
the channel 20 and, by remaining supplied with nitrogen, guarantees
that the end of the drill bit will be kept at a low temperature by
serving as a heat sink. In the event of a momentary rise in
temperature, a partial vaporization of the liquid nitrogen will
have the effect, on the one hand, of absorbing more thermal energy
and of promoting the flow of low-temperature nitrogen through the
ejection ducts 22.
[0068] The reservoir 23 also acts as a buffer in the event of a
momentary interruption in the supply of liquid nitrogen and delays
a rise in temperature to a damaging level for the drill bit.
[0069] When the channel 20 comprises a thermally insulating
coating, a risk of premature vaporization of the nitrogen in the
channel and also a risk of external condensation and/or icing of
the body 10 are reduced and, by limiting the thermal losses, the
cooling is better concentrated in the region of the cutting edges
13.
[0070] Another advantage of the device 100 is its safety with
respect to the drilled stack.
[0071] Indeed, the cost of the parts forming the stack is
generally, at the stage of the final assembly thereof, very high,
in any case disproportionate with respect to the cost of a drill
bit, and damaging the parts at this stage may have significant
economic consequences both with respect to the parts themselves and
implications regarding the production cycle of the products
manufactured.
[0072] In the case of the device 100, the drill bit is suitable for
dry drilling without the need for cooling in the composite material
but requires significant cooling in the metal material. However, a
loss of the liquid nitrogen cooling function, for example by
running out of the liquid nitrogen or by a breakdown of the
distribution system 103, during the drilling of the metal material
will result only in the destruction of the tool, which is not
suitable in the absence of cooling, without damaging the metal
part.
[0073] The parts forming the stack in the process of being drilled
are therefore protected in the event of failure of the device, at
the expense of a drill bit, the cost of which in general remains
much lower than that of the parts worked.
* * * * *